Thermochemistry — Practice Quiz

A Chemistry cheat sheet for Thermochemistry — every key formula with its symbols defined — plus a medium-level practice quiz to test recall.

Practice quiz

1. A $2.0 \text{ kg}$ object is moving at a velocity of $5.0 \text{ m/s}$. What is its kinetic energy?

   • $10 \text{ J}$
   • $25 \text{ J}$
   • $50 \text{ J}$
   • $100 \text{ J}$

   Answer: $25 \text{ J}$

2. A system absorbs $150 \text{ J}$ of heat from its surroundings and performs $50 \text{ J}$ of work on the surroundings. What is the change in the internal energy of the system?

   • $-200 \text{ J}$
   • $-100 \text{ J}$
   • $100 \text{ J}$
   • $200 \text{ J}$

   Answer: $100 \text{ J}$

3. A gas expands from an initial volume of $2.0 \text{ L}$ to a final volume of $5.0 \text{ L}$ against a constant external pressure of $1.5 \text{ atm}$. Calculate the work done by the gas in Joules. (Given: $1 \text{ L} \cdot \text{atm} = 101.3 \text{ J}$)

   • $-4.5 \text{ J}$
   • $-456 \text{ J}$
   • $4.5 \text{ J}$
   • $456 \text{ J}$

   Answer: $-456 \text{ J}$

4. How much heat is required to raise the temperature of $100.0 \text{ g}$ of liquid water from $25.0^\circ \text{C}$ to $75.0^\circ \text{C}$? Use the provided specific heat table.

   • $418 \text{ J}$
   • $2090 \text{ J}$
   • $20900 \text{ J}$
   • $41800 \text{ J}$

   Answer: $20900 \text{ J}$

5. A $50.0 \text{ g}$ sample of an unknown metal absorbs $1125 \text{ J}$ of heat when its temperature increases from $20.0^\circ \text{C}$ to $70.0^\circ \text{C}$. What is the specific heat capacity of the metal?

   • $0.225 \text{ J/g} \cdot \text{K}$
   • $0.45 \text{ J/g} \cdot \text{K}$
   • $0.90 \text{ J/g} \cdot \text{K}$
   • $1.125 \text{ J/g} \cdot \text{K}$

   Answer: $0.45 \text{ J/g} \cdot \text{K}$

6. For a process occurring at constant pressure, which of the following correctly describes the relationship between the enthalpy change ($\Delta H$) and the internal energy change ($\Delta E$)?

   • $\Delta H = \Delta E$
   • $\Delta H = \Delta E - P\Delta V$
   • $\Delta H = \Delta E + P\Delta V$
   • $\Delta H = P\Delta V - \Delta E$

   Answer: $\Delta H = \Delta E + P\Delta V$

7. Calculate the standard enthalpy of reaction ($\Delta H^\circ_{rxn}$) for the combustion of methane: $\text{CH}_4(\text{g}) + 2\text{O}_2(\text{g}) \rightarrow \text{CO}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l})$. Use the provided standard enthalpies of formation table.

   • $-890.3 \text{ kJ/mol}$
   • $-802.3 \text{ kJ/mol}$
   • $-74.80 \text{ kJ/mol}$
   • $890.3 \text{ kJ/mol}$

   Answer: $-890.3 \text{ kJ/mol}$

8. Under what condition is the change in internal energy ($\Delta E$) approximately equal to the change in enthalpy ($\Delta H$) for a chemical reaction?

   • When the reaction is endothermic.
   • When the reaction is exothermic.
   • When there is no significant change in volume ($\Delta V \approx 0$).
   • When the pressure is extremely high.

   Answer: When there is no significant change in volume ($\Delta V \approx 0$).

9. Based on the provided table of specific heats, which substance requires the most heat to raise the temperature of a $1 \text{ g}$ sample by $1 \text{ K}$?

   • Aluminum ($\text{Al}(\text{s})$)
   • Iron ($\text{Fe}(\text{s})$)
   • Water ($\text{H}_2\text{O}(\text{l})$)
   • Methane ($\text{CH}_4(\text{g})$)

   Answer: Water ($\text{H}_2\text{O}(\text{l})$)

10. If the velocity of an object is doubled while its mass remains constant, how does its kinetic energy change?

    • It remains the same.
    • It doubles.
    • It triples.
    • It quadruples.

    Answer: It quadruples.