Thermochemistry Practice Questions (PDF)

1. What is the de nition of enthalpy change (ΔH)? a) The change in temperature of a reaction. b) The heat energy change measured under conditions of constant pressure. c) The total energy of a system. d) The rate of a reaction. Answer: b) The heat energy change measured under conditions of constant...

1. What is the de nition of enthalpy change (ΔH)? a) The change in temperature of a reaction. b) The heat energy change measured under conditions of constant pressure. c) The total energy of a system. d) The rate of a reaction. Answer: b) The heat energy change measured under conditions of constant pressure. 2. What are the standard conditions represented by the symbol "Θ"? a) 0 °C and 1 atm b) 25 °C and 1 atm c) 100 kPa and 298 K d) 0 K and 100 kPa Answer: c) 100 kPa and 298 K 3. In an exothermic reaction, is ΔH positive or negative? a) Positive b) Negative c) Zero d) It depends on the reaction Answer: b) Negative 4. In an endothermic reaction, does the temperature of the surroundings increase or decrease? a) Increase b) Decrease c) Stay the same d) It depends on the reaction Answer: b) Decrease 5. What is the de nition of standard enthalpy of formation (ΔHfΘ)? a) The enthalpy change when 1 mole of a substance is burned completely in oxygen. b) The enthalpy change when 1 mole of a compound is formed from its fi fi constituent elements in their standard states. c) The energy required to break 1 mole of bonds. d) The enthalpy change when 1 mole of a substance reacts under standard conditions. Answer: b) The enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states. 6. What is the ΔHfΘ value for any element in its standard state? a) +1 kJ mol⁻¹ b) -1 kJ mol⁻¹ c) 0 kJ mol⁻¹ d) It depends on the element Answer: c) 0 kJ mol⁻¹ 7. What is the de nition of standard enthalpy of combustion (ΔHcΘ)? a) The enthalpy change when 1 mole of a compound is formed from its elements. b) The enthalpy change when 1 mole of a substance is burned in excess oxygen. c) The energy required to break 1 mole of bonds. d) The heat energy released when a reaction occurs. Answer: b) The enthalpy change when 1 mole of a substance is burned in excess oxygen. 8. What is the ΔHcΘ value for oxygen (O₂)? a) +1 kJ mol⁻¹ b) -1 kJ mol⁻¹ c) 0 kJ mol⁻¹ d) It depends on the substance Answer: c) 0 kJ mol⁻¹ 9. What does Hess's Law state? a) Energy cannot be created or destroyed. b) The enthalpy change for a reaction is independent of the route taken. fi c) The rate of a reaction is proportional to the temperature. d) Entropy always increases. Answer: b) The enthalpy change for a reaction is independent of the route taken. 10. When using ΔHfΘ values in a Hess's Law calculation, are the arrows in the cycle pointing up or down? a) Up b) Down c) Both up and down d) Neither Answer: a) Up 11. When using ΔHcΘ values in a Hess's Law calculation, are the arrows in the cycle pointing up or down? a) Up b) Down c) Both up and down d) Neither Answer: b) Down 12. In a Hess's Law cycle using ΔHfΘ values, what are the "intermediates"? a) Products of combustion b) Elements in their standard states c) Reactants in their standard states d) Activated complexes Answer: b) Elements in their standard states 13. In a Hess's Law cycle using ΔHcΘ values, what are the "intermediates"? a) Elements in their standard states b) Reactants in their standard states c) Oxides of the elements in the equation d) Activated complexes Answer: c) Oxides of the elements in the equation 14. What is the de nition of mean bond enthalpy? a) The enthalpy change when 1 mole of a fi  User con nue ti WJEC Okay, let's complete the multiple-choice options: 14. What is the de nition of mean bond enthalpy? a) The enthalpy change when 1 mole of a compound is formed from its elements. b) The energy required to break 1 mole of covalent bonds in the gaseous state. c) The enthalpy change when 1 mole of a substance is burned completely in oxygen. d) The average energy released when a bond is formed. Answer: b) The energy required to break 1 mole of covalent bonds in the gaseous state. 15. Are mean bond enthalpies generally positive or negative values? a) Positive b) Negative fi  c) Zero d) They can be either positive or negative Answer: a) Positive 16. How do you calculate the overall enthalpy change of a reaction using mean bond enthalpies? a) (Sum of bond enthalpies broken) - (Sum of bond enthalpies formed) b) (Sum of bond enthalpies formed) - (Sum of bond enthalpies broken) c) (Sum of bond enthalpies broken) + (Sum of bond enthalpies formed) d) Product of bond enthalpies Answer: a) (Sum of bond enthalpies broken) - (Sum of bond enthalpies formed) 17. What is the equation used to calculate the heat energy change (Q) in calorimetry? a) Q = mcΔT b) Q = mΔT/c c) Q = cΔT/m d) Q = m/cΔT Answer: a) Q = mcΔT 18. What does 'c' represent in the calorimetry equation (Q = mcΔT)? a) Concentration b) Speci c heat capacity c) Change in temperature d) Molar mass Answer: b) Speci c heat capacity 19. What are the units of ΔH? a) J g⁻¹ °C⁻¹ b) kJ mol⁻¹ c) kJ d) mol Answer: b) kJ mol⁻¹ fi fi 20. In a calorimetry experiment for a combustion reaction, is a polystyrene cup or a metal can typically used? a) Polystyrene cup b) Metal can c) Either can be used d) Neither can be used Answer: b) Metal can 21. If a reaction's enthalpy change cannot be measured directly, which principle/law can be used? a) Newton's law b) Hess's Law c) Le Chatelier's Principle d) Boyle's Law Answer: b) Hess's Law 22. What process is described as measuring the enthalpy change of reaction from experimental data? a) Titration b) Chromatography c) Calorimetry d) Distillation Answer: c) Calorimetry 23. In an exothermic reaction, is heat energy released or absorbed? a) Released b) Absorbed c) Neither released nor absorbed d) It depends on the speci c reaction Answer: a) Released 24. In an exothermic reaction pro le, are the products higher or lower in energy compared to the reactants? a) Higher b) Lower c) The same fi fi d) It depends on the activation energy Answer: b) Lower 25. In an endothermic reaction pro le, is the activation energy typically larger or smaller? a) Larger b) Smaller c) The same d) No correlation can be determined Answer: a) Larger fi