Here below some basic MCQ’s about enthalpy, entropy, gibbs free energy with answer and explanation. Which is given below.
- Which of the following is a state function?
a) Work
b) Heat
c) Enthalpy
d) None of these
Answer: c) Enthalpy
Explanation: Enthalpy (H) is a state function because it depends only on the state of the system, not on the path taken to reach that state.
- The enthalpy change of a system is negative for:
a) Endothermic reactions
b) Exothermic reactions
c) Isothermal processes
d) Adiabatic processes
Answer: b) Exothermic reactions
Explanation: In exothermic reactions, heat is released to the surroundings, causing a negative enthalpy change.
- Which of the following describes entropy correctly?
a) Measure of heat
b) Measure of disorder
c) Measure of work done
d) Measure of temperature
Answer: b) Measure of disorder
Explanation: Entropy (S) measures the disorder or randomness of a system. Greater entropy indicates more disorder.
- The second law of thermodynamics states that in any spontaneous process:
a) The entropy of the system remains constant
b) The entropy of the universe increases
c) The energy of the system decreases
d) The enthalpy of the system increases
Answer: b) The entropy of the universe increases
Explanation: According to the second law of thermodynamics, the total entropy of the universe always increases in a spontaneous process.
- For a spontaneous process at constant temperature and pressure, the sign of Gibbs free energy (ΔG) should be:
a) Positive
b) Negative
c) Zero
d) Undefined
Answer: b) Negative
Explanation: For a process to be spontaneous, the Gibbs free energy change (ΔG) must be negative.
- Which of the following is true for an endothermic reaction?
a) ΔH is negative
b) ΔH is positive
c) ΔS is always negative
d) ΔS is always positive
Answer: b) ΔH is positive
Explanation: In an endothermic reaction, the system absorbs heat, resulting in a positive enthalpy change (ΔH).
- In a reversible process, which statement is correct?
a) Entropy of the universe increases
b) Entropy of the system increases
c) Entropy of the system remains constant
d) Entropy of the surroundings increases
Answer: c) Entropy of the system remains constant
Explanation: In a reversible process, there is no net change in entropy for the system and surroundings.
- Which of the following conditions are necessary for a reaction to be non-spontaneous?
a) ΔG > 0
b) ΔG = 0
c) ΔG < 0 d) ΔS > 0
Answer: a) ΔG > 0
Explanation: If Gibbs free energy (ΔG) is positive, the reaction is non-spontaneous under the given conditions.
- Gibbs free energy (G) is related to enthalpy (H) and entropy (S) by the equation:
a) ΔG = ΔH + TΔS
b) ΔG = ΔH – TΔS
c) ΔG = TΔH – S
d) ΔG = ΔH + ΔS/T
Answer: b) ΔG = ΔH – TΔS
Explanation: This is the Gibbs free energy equation, which relates the change in Gibbs energy to enthalpy (H), entropy (S), and temperature (T).
- The entropy change for the evaporation of water at 100°C is:
a) Positive
b) Negative
c) Zero
d) Undefined
Answer: a) Positive
Explanation: During evaporation, water molecules move from a more ordered liquid phase to a less ordered gaseous phase, thus increasing entropy.
- Which of the following processes results in a decrease in entropy?
a) Melting of ice
b) Dissolution of salt in water
c) Freezing of water
d) Evaporation of alcohol
Answer: c) Freezing of water
Explanation: Freezing causes water molecules to become more ordered as they transition from liquid to solid, resulting in a decrease in entropy.
- The term ‘spontaneous process’ refers to a process that:
a) Occurs rapidly
b) Requires external energy input
c) Occurs without external intervention
d) Always increases enthalpy
Answer: c) Occurs without external intervention
Explanation: A spontaneous process occurs naturally without the need for continuous external energy input.
- At equilibrium, the value of Gibbs free energy (ΔG) is:
a) Positive
b) Negative
c) Zero
d) Undefined
Answer: c) Zero
Explanation: At equilibrium, the Gibbs free energy change (ΔG) is zero, indicating no net change in the system’s energy.
- Which of the following statements is true about Gibbs free energy (ΔG)?
a) It is always positive for spontaneous reactions
b) It is always negative for non-spontaneous reactions
c) It is zero at equilibrium
d) It is independent of temperature
Answer: c) It is zero at equilibrium
Explanation: ΔG = 0 at equilibrium, indicating that there is no driving force for the reaction to proceed in either direction.
- Entropy of a perfectly crystalline substance at absolute zero is:
a) Zero
b) Positive
c) Negative
d) Infinite
Answer: a) Zero
Explanation: According to the third law of thermodynamics, the entropy of a perfect crystal at absolute zero is zero.
- Which statement is true for an exothermic process?
a) Heat is absorbed from the surroundings
b) Heat is released to the surroundings
c) Entropy always decreases
d) Gibbs free energy is always positive
Answer: b) Heat is released to the surroundings
Explanation: In exothermic processes, the system releases heat to the surroundings, making ΔH negative.
- A reaction is spontaneous if:
a) ΔH is positive and ΔS is negative
b) ΔH is negative and ΔS is positive
c) ΔH is negative and ΔS is negative
d) ΔH is positive and ΔS is positive
Answer: b) ΔH is negative and ΔS is positive
Explanation: A reaction is most likely to be spontaneous when enthalpy decreases (ΔH < 0) and entropy increases (ΔS > 0).
- The entropy change of a system when ice melts is:
a) Positive
b) Negative
c) Zero
d) Constant
Answer: a) Positive
Explanation: Melting increases the disorder of the system, so entropy increases.
- For a reaction to be non-spontaneous at all temperatures, the correct condition is:
a) ΔH > 0, ΔS > 0
b) ΔH > 0, ΔS < 0 c) ΔH < 0, ΔS > 0
d) ΔH < 0, ΔS < 0 Answer: b) ΔH > 0, ΔS < 0
Explanation: If ΔH is positive and ΔS is negative, ΔG will always be positive, making the process non-spontaneous at all temperatures.
- Which thermodynamic quantity remains constant in an isothermal process?
a) Temperature
b) Pressure
c) Volume
d) Enthalpy
Answer: a) Temperature
Explanation: In an isothermal process, the temperature of the system remains constant.
- When water freezes, what happens to its enthalpy and entropy?
a) Both increase
b) Both decrease
c) Enthalpy increases, entropy decreases
d) Enthalpy decreases, entropy increases
Answer: b) Both decrease
Explanation: Freezing releases heat (enthalpy decreases) and decreases randomness (entropy decreases).
- A process is spontaneous if:
a) ΔG > 0
b) ΔG < 0 c) ΔH > 0
d) ΔS < 0
Answer: b) ΔG < 0
Explanation: For a process to be spontaneous, the Gibbs free energy change (ΔG) must be negative.
- The change in entropy (ΔS) for the dissolution of a solid in water is generally:
a) Zero
b) Positive
c) Negative
d) Unpredictable
Answer: b) Positive
Explanation: Dissolution increases the randomness or disorder of the system, leading to a positive change in entropy.
- The third law of thermodynamics states that:
a) The entropy of the system is zero at absolute zero
b) The enthalpy of the system is zero at absolute zero
c) The energy of the system is zero at absolute zero
d) The temperature of the system is zero at absolute zero
Answer: a) The entropy of the system is zero at absolute zero
Explanation: The third law states that the entropy of a perfectly crystalline substance becomes zero as the temperature approaches absolute zero.
- If the value of ΔG for a reaction is zero, the reaction is:
a) At equilibrium
b) Spontaneous
c) Non-spontaneous
d) Impossible
Answer: a) At equilibrium
Explanation: A ΔG of zero indicates that the system is at equilibrium, meaning no net change is occurring.
- In a spontaneous process, the total entropy of the system and surroundings:
a) Decreases
b) Increases
c) Remains constant
d) None of these
Answer: b) Increases
Explanation: According to the second law of thermodynamics, the total entropy of the system and surroundings increases in a spontaneous process.
- For a reaction to be spontaneous at low temperatures, which condition must be satisfied?
a) ΔH < 0 and ΔS < 0 b) ΔH > 0 and ΔS > 0
c) ΔH < 0 and ΔS > 0
d) ΔH > 0 and ΔS < 0
Answer: a) ΔH < 0 and ΔS < 0
Explanation: If both ΔH and ΔS are negative, the reaction will be spontaneous at low temperatures since ΔG = ΔH – TΔS.
- The enthalpy change (ΔH) for a chemical reaction is the difference between:
a) The energy of products and reactants
b) The internal energy of products and reactants
c) The total heat absorbed and released
d) The heat content of products and reactants
Answer: d) The heat content of products and reactants
Explanation: Enthalpy change (ΔH) is the difference between the enthalpy of the products and the reactants.
- Entropy of the surroundings decreases in which of the following processes?
a) Exothermic reaction
b) Endothermic reaction
c) Reversible process
d) Isothermal expansion
Answer: b) Endothermic reaction
Explanation: In an endothermic reaction, heat is absorbed from the surroundings, decreasing their entropy.
- At constant pressure, the heat change in a process is equal to:
a) Change in internal energy
b) Change in enthalpy
c) Change in entropy
d) Change in Gibbs free energy
Answer: b) Change in enthalpy
Explanation: At constant pressure, the heat absorbed or released in a process is equal to the change in enthalpy (ΔH).
- Which of the following correctly describes a spontaneous reaction at all temperatures?
a) ΔH > 0, ΔS > 0
b) ΔH < 0, ΔS < 0 c) ΔH < 0, ΔS > 0
d) ΔH > 0, ΔS < 0 Answer: c) ΔH < 0, ΔS > 0
Explanation: A reaction is spontaneous at all temperatures if it releases heat (ΔH < 0) and increases entropy (ΔS > 0).
- The relationship between Gibbs free energy and equilibrium constant (K) is given by:
a) ΔG = RT ln K
b) ΔG = -RT ln K
c) ΔG° = RT ln K
d) ΔG° = -RT ln K
Answer: d) ΔG° = -RT ln K
Explanation: The equation ΔG° = -RT ln K relates the standard Gibbs free energy change (ΔG°) to the equilibrium constant (K), where R is the gas constant and T is the temperature in Kelvin.
- The term ‘enthalpy’ refers to:
a) The sum of internal energy and pressure
b) The sum of internal energy and volume
c) The sum of internal energy and the product of pressure and volume
d) The sum of internal energy and temperature
Answer: c) The sum of internal energy and the product of pressure and volume
Explanation: Enthalpy (H) is defined as the sum of the internal energy (U) of the system and the product of pressure (P) and volume (V), i.e., H = U + PV.
- A reaction is spontaneous when:
a) ΔG < 0 b) ΔG = 0 c) ΔG > 0
d) ΔH > 0
Answer: a) ΔG < 0
Explanation: A reaction is spontaneous if the change in Gibbs free energy (ΔG) is negative, which indicates that the process can occur without external energy input.
- The entropy of an isolated system always:
a) Decreases in a spontaneous process
b) Increases in a spontaneous process
c) Remains constant in a spontaneous process
d) Becomes zero in a spontaneous process
Answer: b) Increases in a spontaneous process
Explanation: According to the second law of thermodynamics, the entropy of an isolated system always increases in a spontaneous process.
- In which of the following processes is entropy likely to decrease?
a) Sublimation of dry ice
b) Condensation of steam
c) Melting of ice
d) Evaporation of alcohol
Answer: b) Condensation of steam
Explanation: Condensation results in a transition from the gaseous phase (more disordered) to the liquid phase (more ordered), thus decreasing entropy.
- In an exothermic reaction, which of the following is true?
a) ΔH is positive, ΔS is positive
b) ΔH is positive, ΔS is negative
c) ΔH is negative, ΔS is positive
d) ΔH is negative, ΔS is negative
Answer: d) ΔH is negative, ΔS is negative
Explanation: In an exothermic reaction, heat is released (ΔH < 0), and if the reaction leads to a more ordered system, entropy decreases (ΔS < 0).
- In which condition will a reaction always be spontaneous?
a) ΔH > 0, ΔS < 0 b) ΔH < 0, ΔS > 0
c) ΔH < 0, ΔS < 0 d) ΔH > 0, ΔS > 0
Answer: b) ΔH < 0, ΔS > 0
Explanation: A reaction is always spontaneous if enthalpy decreases (ΔH < 0) and entropy increases (ΔS > 0).
- The entropy change when water vapor condenses into liquid water is:
a) Positive
b) Negative
c) Zero
d) Undefined
Answer: b) Negative
Explanation: Condensation causes water vapor molecules (high disorder) to form a liquid (lower disorder), resulting in a decrease in entropy.
- A process is non-spontaneous if:
a) ΔG > 0
b) ΔH < 0 c) ΔS > 0
d) ΔG = 0
Answer: a) ΔG > 0
Explanation: If the Gibbs free energy change (ΔG) is positive, the process is non-spontaneous under the given conditions.
- The term ‘enthalpy of formation’ refers to:
a) The heat change when a compound is formed from its elements in their standard states
b) The heat change when a compound is formed from its gaseous ions
c) The heat change when a compound is formed from its atoms in the ground state
d) The heat change when a compound is formed from another compound
Answer: a) The heat change when a compound is formed from its elements in their standard states
Explanation: The enthalpy of formation (ΔHf°) is the heat change when one mole of a compound is formed from its constituent elements in their standard states.
- A reaction at constant temperature and pressure is spontaneous if:
a) ΔH < 0, ΔS < 0 b) ΔH > 0, ΔS > 0
c) ΔG < 0
d) ΔH = 0
Answer: c) ΔG < 0
Explanation: At constant temperature and pressure, a reaction is spontaneous if the change in Gibbs free energy (ΔG) is negative.
- At equilibrium, the entropy of the universe:
a) Increases
b) Decreases
c) Remains constant
d) Becomes zero
Answer: c) Remains constant
Explanation: At equilibrium, there is no net change in entropy, and the total entropy of the universe remains constant.
- Which of the following is correct about entropy (S)?
a) It is a measure of work done
b) It is a measure of temperature
c) It is a measure of disorder or randomness
d) It is a measure of internal energy
Answer: c) It is a measure of disorder or randomness
Explanation: Entropy (S) is a thermodynamic quantity that measures the disorder or randomness of a system.
- In a process where entropy decreases and enthalpy increases, the process will be:
a) Spontaneous at all temperatures
b) Spontaneous at low temperatures
c) Non-spontaneous at all temperatures
d) Spontaneous at high temperatures
Answer: c) Non-spontaneous at all temperatures
Explanation: When entropy decreases (ΔS < 0) and enthalpy increases (ΔH > 0), the process is non-spontaneous because both factors oppose spontaneity.
- A spontaneous process at high temperature is most likely to have:
a) ΔH < 0 and ΔS > 0
b) ΔH > 0 and ΔS > 0
c) ΔH > 0 and ΔS < 0 d) ΔH < 0 and ΔS < 0 Answer: b) ΔH > 0 and ΔS > 0
Explanation: A process can be spontaneous at high temperatures if the entropy change (ΔS) is positive, even if the enthalpy change (ΔH) is positive.
- Which of the following is a correct representation of the second law of thermodynamics?
a) ΔH = TΔS
b) ΔS ≥ 0
c) ΔG = 0 at equilibrium
d) ΔG < 0 for spontaneous processes
Answer: b) ΔS ≥ 0
Explanation: The second law of thermodynamics states that the total entropy of an isolated system always increases (ΔS ≥ 0) in spontaneous processes.
- Which of the following reactions will have the greatest increase in entropy?
a) N₂(g) + 3H₂(g) → 2NH₃(g)
b) CaCO₃(s) → CaO(s) + CO₂(g)
c) 2SO₂(g) + O₂(g) → 2SO₃(g)
d) H₂(g) + Cl₂(g) → 2HCl(g)
Answer: b) CaCO₃(s) → CaO(s) + CO₂(g)
Explanation: The decomposition of CaCO₃ results in the formation of a gas (CO₂), leading to a significant
- The change in Gibbs free energy (ΔG) is related to the equilibrium constant (K) of a reaction by the equation:
a) ΔG = RT ln K
b) ΔG° = RT ln K
c) ΔG = -RT ln K
d) ΔG° = -RT ln K
Answer: d) ΔG° = -RT ln K
Explanation: The relationship between standard Gibbs free energy change (ΔG°) and the equilibrium constant (K) is given by ΔG° = -RT ln K, where R is the gas constant and T is the temperature.
- For which of the following reactions is entropy (ΔS) likely to decrease?
a) Liquid water freezing into ice
b) Dissolution of salt in water
c) Evaporation of alcohol
d) Decomposition of hydrogen peroxide
Answer: a) Liquid water freezing into ice
Explanation: Freezing causes the molecules of water to become more ordered, which results in a decrease in entropy.
- For a process at constant pressure, the heat absorbed or released is equal to:
a) Change in internal energy
b) Change in enthalpy
c) Change in entropy
d) Change in Gibbs free energy
Answer: b) Change in enthalpy
Explanation: At constant pressure, the heat absorbed or released by the system is equal to the change in enthalpy (ΔH).
- A reaction that absorbs heat from the surroundings has a:
a) Positive ΔG
b) Negative ΔS
c) Positive ΔH
d) Negative ΔH
Answer: c) Positive ΔH
Explanation: A reaction that absorbs heat from the surroundings is endothermic, and thus the enthalpy change (ΔH) is positive.
- If ΔH < 0 and ΔS > 0, a process will be:
a) Non-spontaneous at all temperatures
b) Spontaneous at all temperatures
c) Spontaneous at low temperatures only
d) Spontaneous at high temperatures only
Answer: b) Spontaneous at all temperatures
Explanation: When ΔH is negative and ΔS is positive, ΔG will be negative at all temperatures, indicating the process is spontaneous at all temperatures.
- The standard Gibbs free energy change (ΔG°) of a reaction is negative, which means:
a) The reaction is at equilibrium
b) The reaction is spontaneous under standard conditions
c) The reaction is non-spontaneous under standard conditions
d) The reaction is slow
Answer: b) The reaction is spontaneous under standard conditions
Explanation: A negative standard Gibbs free energy change indicates that the reaction is spontaneous under standard conditions.
- Which of the following is true for an irreversible process?
a) Entropy of the system decreases
b) Entropy of the system remains constant
c) Entropy of the universe increases
d) Entropy of the universe decreases
Answer: c) Entropy of the universe increases
Explanation: In an irreversible process, the entropy of the universe always increases according to the second law of thermodynamics.
- The entropy of a perfectly crystalline substance at absolute zero is:
a) Zero
b) Infinite
c) Negative
d) Positive
Answer: a) Zero
Explanation: According to the third law of thermodynamics, the entropy of a perfectly crystalline substance approaches zero as the temperature approaches absolute zero.
- Which condition would result in a non-spontaneous process?
a) ΔH < 0, ΔS < 0 at low temperatures b) ΔH > 0, ΔS > 0 at high temperatures
c) ΔH > 0, ΔS < 0 at all temperatures d) ΔH < 0, ΔS > 0 at high temperatures
Answer: c) ΔH > 0, ΔS < 0 at all temperatures
Explanation: A positive ΔH (endothermic process) and negative ΔS (decreasing entropy) will result in a positive ΔG at all temperatures, making the process non-spontaneous.
- If ΔG = 0 for a process, it means that:
a) The process is spontaneous
b) The process is non-spontaneous
c) The system is at equilibrium
d) The process is impossible
Answer: c) The system is at equilibrium
Explanation: A ΔG of zero indicates that the system is at equilibrium, meaning there is no net change in the system.
- Which of the following statements about Gibbs free energy is correct?
a) It is always negative for non-spontaneous reactions
b) It is positive for spontaneous reactions
c) It is the maximum amount of useful work obtainable from a process
d) It is the total energy of the system
Answer: c) It is the maximum amount of useful work obtainable from a process
Explanation: Gibbs free energy represents the maximum amount of useful work that can be obtained from a process at constant temperature and pressure.
- Which of the following reactions has an increase in entropy?
a) 2H₂(g) + O₂(g) → 2H₂O(l)
b) CaCO₃(s) → CaO(s) + CO₂(g)
c) 2SO₂(g) + O₂(g) → 2SO₃(g)
d) N₂(g) + 3H₂(g) → 2NH₃(g)
Answer: b) CaCO₃(s) → CaO(s) + CO₂(g)
Explanation: The decomposition of calcium carbonate produces a gas (CO₂), which increases the disorder of the system, hence increasing entropy.
- When a gas expands in an isolated system, the entropy:
a) Decreases
b) Increases
c) Remains constant
d) Becomes zero
Answer: b) Increases
Explanation: The expansion of a gas increases the number of accessible microstates, resulting in an increase in entropy.
- A process with ΔH > 0 and ΔS > 0 will be:
a) Spontaneous at high temperatures
b) Spontaneous at low temperatures
c) Non-spontaneous at all temperatures
d) Non-spontaneous at high temperatures
Answer: a) Spontaneous at high temperatures
Explanation: When both ΔH and ΔS are positive, the process is spontaneous at high temperatures because the TΔS term will dominate the ΔG = ΔH – TΔS equation.
- If ΔH is negative and ΔS is negative, the process will be spontaneous at:
a) High temperatures
b) Low temperatures
c) All temperatures
d) No temperatures
Answer: b) Low temperatures
Explanation: When both ΔH and ΔS are negative, the process is spontaneous at low temperatures, where the enthalpy term dominates.
- Which statement is true for a reaction that is non-spontaneous at standard conditions but becomes spontaneous at high temperatures?
a) ΔH < 0, ΔS < 0 b) ΔH > 0, ΔS < 0 c) ΔH < 0, ΔS > 0
d) ΔH > 0, ΔS > 0
Answer: d) ΔH > 0, ΔS > 0
Explanation: If both enthalpy and entropy are positive, the process becomes spontaneous at high temperatures because the TΔS term will dominate the equation for Gibbs free energy.
- For an endothermic reaction with an increase in entropy, the reaction will be:
a) Spontaneous at all temperatures
b) Non-spontaneous at all temperatures
c) Spontaneous at high temperatures
d) Spontaneous at low temperatures
Answer: c) Spontaneous at high temperatures
Explanation: For an endothermic process (ΔH > 0) with an increase in entropy (ΔS > 0), the process is spontaneous at high temperatures, where the TΔS term becomes large enough to make ΔG negative.
- At absolute zero, the entropy of a perfect crystal is:
a) Zero
b) One
c) Infinite
d) Constant
Answer: a) Zero
Explanation: According to the third law of thermodynamics, the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero.
- Which of the following is correct about a spontaneous process?
a) It always requires energy input.
b) It increases the entropy of the system.
c) It increases the entropy of the universe.
d) It decreases the entropy of the universe.
Answer: c) It increases the entropy of the universe
Explanation: According to the second law of thermodynamics, a spontaneous process always increases the entropy of the universe, even if the system’s entropy decreases.
- If the reaction N₂(g) + 3H₂(g) → 2NH₃(g) is exothermic, what will happen to the equilibrium constant as temperature increases?
a) It increases
b) It decreases
c) It remains constant
d) It becomes zero
Answer: b) It decreases
Explanation: For an exothermic reaction, increasing the temperature decreases the equilibrium constant because the reaction shifts towards the reactants to counteract the added heat (Le Chatelier’s principle).
- The third law of thermodynamics states that:
a) The entropy of the universe is always increasing.
b) The entropy of a perfect crystal at absolute zero is zero.
c) The total energy of an isolated system is constant.
d) No process can occur if ΔG is positive.
Answer: b) The entropy of a perfect crystal at absolute zero is zero
Explanation: The third law of thermodynamics states that the entropy of a perfect crystalline substance is zero at absolute zero because there is no disorder in the crystal at that temperature.
- Which of the following conditions will favor a reaction to be spontaneous at high temperatures?
a) ΔH > 0, ΔS > 0
b) ΔH < 0, ΔS < 0 c) ΔH > 0, ΔS < 0 d) ΔH < 0, ΔS > 0
Answer: a) ΔH > 0, ΔS > 0
Explanation: When both enthalpy (ΔH) and entropy (ΔS) are positive, the process becomes spontaneous at high temperatures where the TΔS term dominates the Gibbs free energy equation.
- Which of the following processes will have a positive entropy change?
a) Freezing of water
b) Condensation of steam
c) Evaporation of water
d) Formation of ice from water
Answer: c) Evaporation of water
Explanation: Evaporation involves the transition from liquid to gas, which increases the disorder of the system, resulting in a positive change in entropy.
- Which statement is true regarding an endothermic process at equilibrium when the temperature is increased?
a) The equilibrium shifts towards the products.
b) The equilibrium shifts towards the reactants.
c) The equilibrium does not change.
d) The entropy of the system decreases.
Answer: a) The equilibrium shifts towards the products
Explanation: According to Le Chatelier’s principle, increasing the temperature in an endothermic process shifts the equilibrium towards the products to absorb the added heat.
- Which of the following thermodynamic quantities is a state function?
a) Work
b) Heat
c) Entropy
d) Distance
Answer: c) Entropy
Explanation: Entropy (S) is a state function, meaning its value depends only on the current state of the system, not on how the system reached that state. Work and heat are path functions.
- In a reaction where ΔH < 0 and ΔS < 0, the reaction will be spontaneous:
a) At high temperatures
b) At low temperatures
c) At all temperatures
d) At no temperatures
Answer: b) At low temperatures
Explanation: When both ΔH and ΔS are negative, the process is spontaneous at low temperatures where the enthalpy term dominates the Gibbs free energy equation.
- For which process does the entropy of the system decrease?
a) Expansion of a gas
b) Melting of ice
c) Freezing of water
d) Boiling of water
Answer: c) Freezing of water
Explanation: Freezing of water involves the transition from liquid to solid, which is a more ordered state, resulting in a decrease in entropy.
- The enthalpy change for a reaction is positive, and the entropy change is negative. This reaction will be:
a) Spontaneous at high temperatures
b) Spontaneous at low temperatures
c) Non-spontaneous at all temperatures
d) Spontaneous at all temperatures
Answer: c) Non-spontaneous at all temperatures
Explanation: When ΔH is positive and ΔS is negative, the Gibbs free energy will always be positive, making the reaction non-spontaneous at all temperatures.
- Which of the following will increase the entropy of the system?
a) Freezing of water
b) Dissolution of sugar in water
c) Condensation of steam
d) Compression of a gas
Answer: b) Dissolution of sugar in water
Explanation: Dissolution increases the disorder in the system as the sugar molecules disperse in the water, leading to an increase in entropy.
- If ΔH > 0 and ΔS > 0 for a reaction, the reaction will be spontaneous at:
a) Low temperatures only
b) High temperatures only
c) All temperatures
d) No temperatures
Answer: b) High temperatures only
Explanation: When both ΔH and ΔS are positive, the reaction is spontaneous at high temperatures because the TΔS term dominates and makes ΔG negative.
- For a reaction to be spontaneous at all temperatures, which condition must be met?
a) ΔH < 0, ΔS > 0
b) ΔH > 0, ΔS < 0 c) ΔH < 0, ΔS < 0 d) ΔH > 0, ΔS > 0
Answer: a) ΔH < 0, ΔS > 0
Explanation: A negative enthalpy change (exothermic process) and a positive entropy change make the reaction spontaneous at all temperatures.
- At constant pressure, the heat absorbed by a system is equal to:
a) The change in Gibbs free energy
b) The change in internal energy
c) The change in enthalpy
d) The change in entropy
Answer: c) The change in enthalpy
Explanation: At constant pressure, the heat absorbed by the system is equal to the change in enthalpy (ΔH).
- For which of the following reactions will ΔS be positive?
a) N₂(g) + 3H₂(g) → 2NH₃(g)
b) CaCO₃(s) → CaO(s) + CO₂(g)
c) 2SO₂(g) + O₂(g) → 2SO₃(g)
d) H₂(g) + Cl₂(g) → 2HCl(g)
Answer: b) CaCO₃(s) → CaO(s) + CO₂(g)
Explanation: The decomposition of calcium carbonate produces a gas (CO₂), which increases the disorder of the system, leading to a positive entropy change.
- At equilibrium, the Gibbs free energy change (ΔG) is:
a) Positive
b) Negative
c) Zero
d) Undefined
Answer: c) Zero
Explanation: At equilibrium, there is no net change in the system, and ΔG is zero.
- The entropy of a system decreases when:
a) Water evaporates
b) Ice melts
c) A gas condenses
d) A solid dissolves in a liquid
Answer: c) A gas condenses
Explanation: Condensation involves the transition from a gas to a liquid, which is a more ordered state, resulting in a decrease in entropy.
- For an exothermic reaction with an increase in entropy, the reaction will be:
a) Spontaneous at all temperatures
b) Spontaneous at low temperatures only
c) Spontaneous at high temperatures only
d) Non-spontaneous at all temperatures
Answer: a) Spontaneous at all temperatures
Explanation: For an exothermic reaction (ΔH < 0) with an increase in entropy (ΔS > 0), the process is spontaneous at all temperatures because ΔG will always be negative.
- If a chemical reaction has ΔH < 0 and ΔS < 0, the reaction will be spontaneous:
a) At high temperatures
b) At low temperatures
c) At all temperatures
d) At no temperatures
Answer: b) At low temperatures
Explanation: When both ΔH and ΔS are negative, the reaction will be spontaneous at low temperatures because the enthalpy term (ΔH) dominates the TΔS term in the Gibbs free energy equation.
- Which of the following processes will always result in a positive change in entropy?
a) Freezing of water
b) Melting of ice
c) Condensation of steam
d) Sublimation of dry ice
Answer: d) Sublimation of dry ice
Explanation: Sublimation is the transition from a solid to a gas, which increases the disorder of the system and results in a positive entropy change.
- In an isolated system, which of the following always increases for a spontaneous process?
a) Enthalpy
b) Gibbs free energy
c) Internal energy
d) Entropy
Answer: d) Entropy
Explanation: According to the second law of thermodynamics, in an isolated system, the entropy of the system will always increase for a spontaneous process.
- The dissolution of NaCl in water results in an increase in entropy because:
a) NaCl is a solid.
b) Water molecules become more ordered.
c) Na⁺ and Cl⁻ ions become dispersed in the water.
d) The solution cools down.
Answer: c) Na⁺ and Cl⁻ ions become dispersed in the water
Explanation: When NaCl dissolves in water, the Na⁺ and Cl⁻ ions spread throughout the solution, increasing the disorder and thus increasing the entropy of the system.
- Which of the following reactions will have a positive entropy change?
a) 2H₂(g) + O₂(g) → 2H₂O(l)
b) 2SO₂(g) + O₂(g) → 2SO₃(g)
c) NH₄Cl(s) → NH₃(g) + HCl(g)
d) N₂(g) + 3H₂(g) → 2NH₃(g)
Answer: c) NH₄Cl(s) → NH₃(g) + HCl(g)
Explanation: The decomposition of solid ammonium chloride into two gases increases the number of gas molecules, which leads to an increase in entropy.
- Which of the following statements about Gibbs free energy (ΔG) is correct?
a) If ΔG > 0, the reaction is spontaneous.
b) If ΔG < 0, the reaction is non-spontaneous.
c) If ΔG = 0, the system is at equilibrium.
d) If ΔG = 0, the reaction is non-spontaneous.
Answer: c) If ΔG = 0, the system is at equilibrium
Explanation: When ΔG = 0, the system is at equilibrium, meaning no net change in the reaction occurs.
- A reaction that has a negative ΔH and a positive ΔS will be:
a) Spontaneous at all temperatures
b) Non-spontaneous at all temperatures
c) Spontaneous at low temperatures only
d) Spontaneous at high temperatures only
Answer: a) Spontaneous at all temperatures
Explanation: A reaction with a negative ΔH (exothermic) and a positive ΔS (increasing disorder) will always have a negative ΔG, making it spontaneous at all temperatures.
- The entropy change for the reaction 2H₂(g) + O₂(g) → 2H₂O(l) is likely to be:
a) Positive
b) Negative
c) Zero
d) Infinite
Answer: b) Negative
Explanation: The reaction results in the formation of a liquid from gases, which leads to a more ordered system and a decrease in entropy.
- For an exothermic reaction with a decrease in entropy, the reaction will be:
a) Spontaneous at high temperatures only
b) Spontaneous at low temperatures only
c) Non-spontaneous at all temperatures
d) Spontaneous at all temperatures
Answer: b) Spontaneous at low temperatures only
Explanation: For an exothermic reaction (ΔH < 0) with a decrease in entropy (ΔS < 0), the reaction is spontaneous at low temperatures where the enthalpy term dominates.
- Which of the following is true for a reaction where ΔG < 0?
a) The reaction is at equilibrium.
b) The reaction is spontaneous.
c) The reaction is non-spontaneous.
d) The reaction will not proceed.
Answer: b) The reaction is spontaneous
Explanation: A negative ΔG indicates that the reaction is spontaneous under the given conditions.
- Which of the following conditions indicates a non-spontaneous process?
a) ΔG = 0
b) ΔG < 0 c) ΔG > 0
d) ΔH < 0 and ΔS > 0
Answer: c) ΔG > 0
Explanation: A positive ΔG means that the process is non-spontaneous under the given conditions.
- The entropy change for the process of freezing water is:
a) Positive
b) Negative
c) Zero
d) Infinite
Answer: b) Negative
Explanation: Freezing water involves the transition from liquid to solid, which is a more ordered state, resulting in a decrease in entropy.
- For a reaction where ΔH > 0 and ΔS < 0, the reaction will be:
a) Spontaneous at high temperatures
b) Spontaneous at low temperatures
c) Non-spontaneous at all temperatures
d) Spontaneous at all temperatures
Answer: c) Non-spontaneous at all temperatures
Explanation: When ΔH is positive and ΔS is negative, ΔG will always be positive, making the reaction non-spontaneous at all temperatures.
- A spontaneous reaction is one where:
a) ΔG is positive
b) ΔG is negative
c) ΔH is positive
d) ΔS is negative
Answer: b) ΔG is negative
Explanation: A spontaneous reaction occurs when the Gibbs free energy (ΔG) is negative.
- The change in entropy when a solid dissolves in a solvent is usually:
a) Positive
b) Negative
c) Zero
d) Infinite
Answer: a) Positive
Explanation: Dissolution usually increases disorder in the system, resulting in a positive entropy change.
- The second law of thermodynamics states that:
a) Energy can be created or destroyed.
b) The entropy of the universe is always increasing.
c) The entropy of a system decreases in a spontaneous process.
d) Enthalpy and entropy are independent of each other.
Answer: b) The entropy of the universe is always increasing
Explanation: The second law of thermodynamics states that in any spontaneous process, the total entropy of the universe increases.
- Which of the following processes is endothermic?
a) Combustion of methane
b) Melting of ice
c) Freezing of water
d) Condensation of steam
Answer: b) Melting of ice
Explanation: An endothermic process absorbs heat from the surroundings. In the melting of ice, heat is absorbed to convert solid ice into liquid water.
- Entropy is a measure of:
a) Energy stored in a system
b) Heat content of a system
c) Disorder or randomness in a system
d) Work done by a system
Answer: c) Disorder or randomness in a system
Explanation: Entropy (S) measures the randomness or disorder of the particles in a system. Higher entropy indicates greater disorder.
- Which of the following is true for a spontaneous process?
a) ΔG > 0
b) ΔG = 0
c) ΔG < 0
d) ΔH = 0
Answer: c) ΔG < 0
Explanation: A process is spontaneous if the Gibbs free energy (ΔG) is negative. This indicates that the process can occur without external energy input.
- For an exothermic reaction, which of the following is true?
a) ΔH > 0
b) ΔH = 0
c) ΔH < 0
d) ΔS < 0
Answer: c) ΔH < 0
Explanation: In an exothermic reaction, heat is released into the surroundings, which means the change in enthalpy (ΔH) is negative.
- The unit of enthalpy change (ΔH) is:
a) Joule (J)
b) Kilocalorie (kcal)
c) Joule per mole (J/mol)
d) Calorie (cal)
Answer: c) Joule per mole (J/mol)
Explanation: Enthalpy change is the heat change per mole during a process and is measured in Joules per mole (J/mol).
- For which process is the entropy change positive?
a) Freezing of water
b) Condensation of steam
c) Sublimation of dry ice
d) Deposition of iodine
Answer: c) Sublimation of dry ice
Explanation: Sublimation involves a solid directly converting into gas, which increases the disorder of the system, hence increasing entropy.
- What happens to the Gibbs free energy of a system at equilibrium?
a) ΔG > 0
b) ΔG = 0
c) ΔG < 0
d) None of the above
Answer: b) ΔG = 0
Explanation: At equilibrium, the Gibbs free energy change (ΔG) is zero because there is no net change occurring in the system.
- Which of the following conditions favors a spontaneous reaction at all temperatures?
a) ΔH > 0, ΔS > 0
b) ΔH < 0, ΔS > 0
c) ΔH > 0, ΔS < 0 d) ΔH < 0, ΔS < 0 Answer: b) ΔH < 0, ΔS > 0
Explanation: A negative ΔH (exothermic) and a positive ΔS (increasing disorder) always favor spontaneity, regardless of temperature.
- The entropy of a perfectly crystalline substance at absolute zero is:
a) Positive
b) Negative
c) Zero
d) Infinite
Answer: c) Zero
Explanation: According to the third law of thermodynamics, the entropy of a perfect crystal at absolute zero is exactly zero because there is no randomness or disorder in the arrangement of particles.
- Which of the following processes decreases entropy?
a) Evaporation of water
b) Mixing of gases
c) Freezing of water
d) Dissolution of salt in water
Answer: c) Freezing of water
Explanation: In the freezing process, water molecules become more ordered as they arrange into a solid structure, which decreases entropy.
- The change in Gibbs free energy for a reaction is given by the equation:
a) ΔG = ΔH – TΔS
b) ΔG = ΔS – TΔH
c) ΔG = ΔH + TΔS
d) ΔG = ΔS + TΔH
Answer: a) ΔG = ΔH – TΔS
Explanation: The Gibbs free energy equation is given by ΔG = ΔH – TΔS, where ΔG represents the Gibbs free energy change, ΔH is the enthalpy change, T is the temperature, and ΔS is the entropy change.
- For a process to be spontaneous at high temperature, which of the following must be true?
a) ΔH < 0, ΔS < 0 b) ΔH > 0, ΔS < 0 c) ΔH > 0, ΔS > 0
d) ΔH < 0, ΔS > 0
Answer: c) ΔH > 0, ΔS > 0
Explanation: A positive ΔH and positive ΔS can lead to spontaneity at high temperatures because the TΔS term may become larger than ΔH, making ΔG negative.
- Which of the following is a characteristic of an exergonic reaction?
a) ΔG > 0
b) ΔG = 0
c) ΔG < 0 d) ΔH > 0
Answer: c) ΔG < 0
Explanation: An exergonic reaction is a spontaneous reaction where the Gibbs free energy (ΔG) is negative.
- If ΔG for a reaction is positive, the reaction is:
a) Spontaneous
b) Non-spontaneous
c) At equilibrium
d) Exothermic
Answer: b) Non-spontaneous
Explanation: A positive ΔG means that the reaction requires energy to proceed and is non-spontaneous under the given conditions.
- For an endothermic process with increasing entropy, the reaction will be:
a) Always spontaneous
b) Never spontaneous
c) Spontaneous at high temperatures
d) Spontaneous at low temperatures
Answer: c) Spontaneous at high temperatures
Explanation: If a reaction is endothermic (ΔH > 0) and entropy increases (ΔS > 0), the process can become spontaneous at high temperatures because the TΔS term will outweigh ΔH.
- Which of the following correctly defines Gibbs free energy?
a) It is the total energy of a system
b) It is the maximum amount of work a system can perform
c) It is the energy stored in chemical bonds
d) It is the heat energy released during a reaction
Answer: b) It is the maximum amount of work a system can perform
Explanation: Gibbs free energy represents the maximum amount of work that can be done by a thermodynamic system during a reversible process.
- At constant temperature and pressure, which condition will guarantee a spontaneous process?
a) ΔH > 0, ΔS > 0
b) ΔH < 0, ΔS > 0
c) ΔH > 0, ΔS < 0 d) ΔH < 0, ΔS < 0 Answer: b) ΔH < 0, ΔS > 0
Explanation: A process with negative ΔH (exothermic) and positive ΔS (increase in disorder) is always spontaneous at constant temperature and pressure.
- Which of the following is true for an ideal gas undergoing an isothermal process?
a) ΔS = 0
b) ΔG = 0
c) ΔH = 0
d) ΔU = 0
Answer: d) ΔU = 0
Explanation: In an isothermal process, the temperature remains constant, and for an ideal gas, the change in internal energy (ΔU) is zero.
- What is the value of ΔG when a system is at equilibrium?
a) Positive
b) Negative
c) Zero
d) Infinite
Answer: c) Zero
Explanation: At equilibrium, the system has no net tendency to change, and therefore the Gibbs free energy change (ΔG) is zero.
- The second law of thermodynamics states that:
a) Energy is conserved in an isolated system
b) The total entropy of an isolated system can only increase
c) The total energy of an isolated system is constant
d) Entropy of a perfect crystal at absolute zero is zero
Answer: b) The total entropy of an isolated system can only increase
Explanation: The second law of thermodynamics states that the entropy (disorder) of an isolated system will either increase or remain constant, but it cannot decrease.
- Which of the following processes is associated with a decrease in entropy?
a) Evaporation of alcohol
b) Mixing of gases
c) Crystallization of a supersaturated solution
d) Melting of ice
Answer: c) Crystallization of a supersaturated solution
Explanation: During crystallization, particles in a disordered state (solution) become ordered (solid crystals), which decreases entropy.
- If a reaction is exergonic, which of the following is true?
a) ΔG > 0
b) ΔG = 0
c) ΔG < 0 d) ΔH > 0
Answer: c) ΔG < 0
Explanation: An exergonic reaction is spontaneous, meaning the Gibbs free energy (ΔG) is negative.
- Which of the following would increase the entropy of a system?
a) Freezing of water
b) Formation of NaCl from Na and Cl
c) Vaporization of liquid water
d) Dissolution of sugar in water
Answer: c) Vaporization of liquid water
Explanation: Vaporization increases entropy because the water molecules move from a more ordered liquid phase to a more disordered gas phase.
- In which of the following reactions is ΔS expected to be negative?
a) N₂(g) + 3H₂(g) → 2NH₃(g)
b) 2H₂O₂(l) → 2H₂O(l) + O₂(g)
c) NaCl(s) → Na⁺(aq) + Cl⁻(aq)
d) C(s) + O₂(g) → CO₂(g)
Answer: a) N₂(g) + 3H₂(g) → 2NH₃(g)
Explanation: In this reaction, there is a decrease in the number of gas molecules (from 4 moles to 2 moles), leading to a decrease in entropy.
- Which factor affects the Gibbs free energy change of a reaction?
a) Only temperature
b) Only pressure
c) Both temperature and entropy
d) Only the reaction enthalpy
Answer: c) Both temperature and entropy
Explanation: The Gibbs free energy change (ΔG) depends on both temperature (T) and entropy (ΔS) in the equation ΔG = ΔH – TΔS.
- For a reaction with ΔH > 0 and ΔS < 0, the reaction will be:
a) Always spontaneous
b) Always non-spontaneous
c) Spontaneous at high temperatures
d) Spontaneous at low temperatures
Answer: b) Always non-spontaneous
Explanation: If ΔH is positive (endothermic) and ΔS is negative (decreasing entropy), the reaction will never be spontaneous because ΔG will always be positive.
- For a process to be spontaneous at low temperatures, which of the following must be true?
a) ΔH < 0, ΔS > 0
b) ΔH < 0, ΔS < 0 c) ΔH > 0, ΔS < 0 d) ΔH > 0, ΔS > 0
Answer: b) ΔH < 0, ΔS < 0
Explanation: A process with a negative ΔH (exothermic) and a negative ΔS (decreasing entropy) will be spontaneous at low temperatures since the TΔS term is minimized.
- Which of the following is correct for the enthalpy change of a reaction (ΔH)?
a) It is negative for endothermic reactions
b) It is zero for all reactions
c) It is positive for exothermic reactions
d) It is negative for exothermic reactions
Answer: d) It is negative for exothermic reactions
Explanation: In exothermic reactions, heat is released, so the change in enthalpy (ΔH) is negative.
- At constant temperature, the spontaneity of a process is governed by:
a) ΔS only
b) ΔH only
c) ΔG
d) TΔS only
Answer: c) ΔG
Explanation: At constant temperature, the Gibbs free energy (ΔG) determines the spontaneity of a process. If ΔG < 0, the process is spontaneous.
- The dissolution of ammonium chloride in water is endothermic, but it is spontaneous. This implies that:
a) ΔH > 0 and ΔS > 0
b) ΔH < 0 and ΔS > 0
c) ΔH > 0 and ΔS < 0 d) ΔH < 0 and ΔS < 0 Answer: a) ΔH > 0 and ΔS > 0
Explanation: The dissolution of ammonium chloride is endothermic (ΔH > 0), but the increase in entropy (ΔS > 0) makes the process spontaneous, especially at room temperature.
- Which of the following is true for a reaction in which ΔH is negative and ΔS is negative?
a) The reaction is always spontaneous
b) The reaction is never spontaneous
c) The reaction is spontaneous at low temperatures
d) The reaction is spontaneous at high temperatures
Answer: c) The reaction is spontaneous at low temperatures
Explanation: If both ΔH and ΔS are negative, the reaction will be spontaneous at low temperatures when the TΔS term is small.
- Which of the following laws is used to determine the direction of spontaneous change?
a) First law of thermodynamics
b) Second law of thermodynamics
c) Third law of thermodynamics
d) Zeroth law of thermodynamics
Answer: b) Second law of thermodynamics
Explanation: The second law of thermodynamics states that spontaneous processes occur in a direction that increases the overall entropy of the universe.
- If a process is spontaneous, what can be said about its entropy change?
a) ΔS = 0
b) ΔS > 0
c) ΔS < 0 d) ΔS is positive or negative depending on temperature Answer: b) ΔS > 0
Explanation: A spontaneous process typically increases the entropy of the system and its surroundings, resulting in a positive overall entropy change.
- For a reaction to be non-spontaneous at any temperature, which condition must be met?
a) ΔH > 0, ΔS > 0
b) ΔH > 0, ΔS < 0 c) ΔH < 0, ΔS > 0
d) ΔH < 0, ΔS < 0 Answer: b) ΔH > 0, ΔS < 0
Explanation: If ΔH is positive (endothermic) and ΔS is negative (decrease in entropy), the reaction will always be non-spontaneous since ΔG will always be positive.
- Which of the following is true regarding a reversible process?
a) It increases the entropy of the universe
b) It decreases the entropy of the system
c) It has no effect on the entropy of the universe
d) It increases the entropy of the surroundings
Answer: c) It has no effect on the entropy of the universe
Explanation: A reversible process occurs in such a way that the system and surroundings are always in equilibrium, resulting in no net change in the entropy of the universe.
- In a chemical reaction, if ΔG is positive, the reaction:
a) Will occur spontaneously
b) Will not occur spontaneously
c) Is at equilibrium
d) Releases heat
Answer: b) Will not occur spontaneously
Explanation: A positive Gibbs free energy (ΔG) indicates that the reaction is non-spontaneous and requires external energy to proceed.
- Which of the following processes has a negative Gibbs free energy?
a) Rusting of iron
b) Decomposition of water into hydrogen and oxygen
c) Electrolysis of NaCl
d) Freezing of water
Answer: a) Rusting of iron
Explanation: Rusting of iron is a spontaneous process, which means that its Gibbs free energy (ΔG) is negative.
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