# Enthalpy Changes and Calorimetry

*Time: 20 min. 6 questions.*

## Instructions

Answer all six questions. For multiple choice, select the best answer. For short answer, write a complete sentence or brief explanation. For true or false, state whether the statement is correct. For the numeric question, show your calculation and include units in your final answer. You may use a calculator.

## Questions

1. A student burns 2.5 g of ethanol in a bomb calorimeter. The temperature of 1000 g of water surrounding the bomb rises by 8.2 °C. Calculate the heat released by the combustion, using Q = mcΔt. (Use c = 4.18 J/g·°C for water.)
([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23634))

2. Which of the following statements best describes enthalpy?
A) The total kinetic energy of all particles in a system
B) The heat content of a substance at constant pressure
C) The energy required to break all bonds in a molecule
D) The rate at which a reaction proceeds
([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636))

3. A reaction has ΔH = −156 kJ/mol. Explain what this notation tells you about the reaction and whether it is endothermic or exothermic.
([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23638))

4. True or false: An endothermic reaction absorbs heat from the surroundings, resulting in a negative ΔH value.
([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23643))

5. Using standard enthalpies of formation, calculate ΔH for the reaction 2H₂(g) + O₂(g) → 2H₂O(l), given that ΔH°f[H₂O(l)] = −286 kJ/mol, ΔH°f[H₂(g)] = 0 kJ/mol, and ΔH°f[O₂(g)] = 0 kJ/mol.
([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23639))

6. A student performs a calorimetry experiment to measure the enthalpy of combustion of a candle. Explain why the measured value is typically lower than the theoretical value found in reference tables.
([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23641))

## Curriculum Coverage

| Question | Outcome Alias | Specific Outcome |
|---|---|---|
| 1 | [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23634) | Apply Q = mcΔt to heat transfer analysis |
| 2 | [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636) | Define enthalpy and molar enthalpy |
| 3 | [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23638) | Use and interpret ΔH notation |
| 4 | [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23643) | Classify reactions as endothermic or exothermic |
| 5 | [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23639) | Predict enthalpy change using standard enthalpies of formation |
| 6 | [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23641) | Use calorimetry data to determine enthalpy changes |

## Answer Key

1. Q = (1000 g)(4.18 J/g·°C)(8.2 °C) = 34,276 J ≈ 34.3 kJ; this represents the heat absorbed by the water and released by the combustion.

2. **B) The heat content of a substance at constant pressure.** Enthalpy is defined as the heat content of a system at constant pressure and is a state function used to describe energy changes in reactions.

3. **The negative sign indicates that 156 kJ of energy is released per mole of reaction; this is an exothermic reaction.** Negative ΔH values always correspond to exothermic processes in which heat flows out of the system.

4. **False.** Endothermic reactions absorb heat from the surroundings, which results in a positive ΔH value, not negative.

5. **−572 kJ.** Using ΔH°rxn = Σ(ΔH°f products) − Σ(ΔH°f reactants) = [2(−286)] − [0 + 0] = −572 kJ; the reaction is highly exothermic.

6. **Heat is lost to the surroundings during the experiment through the calorimeter walls and to the air, so less heat is captured than the theoretical value predicts.** Calorimeters are not perfectly insulated, and incomplete combustion or other experimental losses also reduce the measured enthalpy.

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*AILI quiz · language en · model claude-haiku-4-5-20251001 · generated 2026-09-17 · id aec04d6f-fb94-4b30-94cc-ced9e625334f*

### Sources

- node:n1: Unit A: Thermochemical Changes (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23632)
- node:n2: General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23633)
- node:n3: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23634)
- node:n4: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636)
- node:n5: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23638)
- node:n6: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23639)
- node:n7: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23640)
- node:n8: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23641)
- node:n9: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23643)
- node:n10: General Outcome 2 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23650)
- node:n11: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23652)