21. The change in internal energy (ΔU) is defined as:

• A) The heat added to the system minus the work done by the system
• B) The work done on the system minus the heat added
• C) The heat absorbed plus the work done on the system
• D) The heat lost plus the work done by the system
• Answer: A

22. In Hess’s Law, the enthalpy change for a reaction can be calculated using:

• A) The average bond enthalpies
• B) The standard enthalpy of formation
• C) The enthalpies of all intermediate steps
• D) The heat capacity of the reactants
• Answer: C

23. Which of the following is an example of an isochoric process?

• A) Heating a gas in a closed container
• B) Expanding gas in a piston
• C) Cooling a gas while it expands
• D) Heating water at constant pressure
• Answer: A

24. The change in enthalpy (ΔH) is equal to:

• A) ΔU + PΔV
• B) ΔU – PΔV
• C) ΔU + W
• D) ΔU – Q
• Answer: A

25. In an exothermic reaction, the enthalpy change (ΔH) is:

• A) Positive
• B) Negative
• C) Zero
• D) Independent of the reactants
• Answer: B

26. When calculating the heat of reaction using Hess’s Law, what is true about the reaction’s pathway?

• A) It must be a direct reaction
• B) It can occur through multiple steps
• C) It cannot involve any intermediate reactions
• D) It must be at standard conditions
• Answer: B

27. The heat capacity of a substance is defined as:

• A) The heat required to raise the temperature of 1 mole of a substance by 1°C
• B) The heat required to raise the temperature of a given mass by 1°C
• C) The heat lost when the substance cools by 1°C
• D) The total heat content of the substance
• Answer: B

28. What is the internal energy change for an ideal gas during an isothermal process?

• A) Positive
• B) Negative
• C) Zero
• D) Depends on pressure
• Answer: C

29. Which law states that the total energy of an isolated system remains constant?

• A) First Law of Thermodynamics
• B) Second Law of Thermodynamics
• C) Third Law of Thermodynamics
• D) Hess’s Law
• Answer: A

30. For a reaction represented by: A + B → C + D, the standard enthalpy change (ΔH°) can be calculated using:

• A) ΔH° = Σ(ΔH° products) – Σ(ΔH° reactants)
• B) ΔH° = Σ(ΔH° reactants) – Σ(ΔH° products)
• C) ΔH° = ΔU + W
• D) ΔH° = Q/P
• Answer: A