# Enthalpy Changes and Calorimetry

*30 minutes, 3 stages*

Calorimetry is the experimental method chemists use to measure heat released or absorbed during a chemical reaction. Understanding how to calculate enthalpy changes from calorimetry data connects the abstract concept of energy to measurable quantities you can determine in the lab ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23641)). You will apply the heat equation and interpret enthalpy notation to solve problems at the level you will encounter on the diploma examination ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23634), [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23638)).

## Stage 1: Heat Transfer Calculations (10 min)

Recall that the heat transferred in a calorimeter is calculated using Q = mcΔt, where Q is heat in joules, m is mass in grams, c is specific heat capacity in J/(g·°C), and Δt is the temperature change in °C ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23634)).

**Task 1:** A calorimeter contains 150 g of water. The temperature rises from 22.5°C to 31.8°C when 2.5 g of a food sample burns inside it. Calculate the heat absorbed by the water. Use c = 4.18 J/(g·°C) for water.

Show your work:
- Write the temperature change (Δt)
- Substitute all values into Q = mcΔt
- Report your answer in joules and kilojoules

**Task 2:** In a second experiment, 200 g of water in a calorimeter increases in temperature by 8.2°C when a different fuel burns. Calculate the heat absorbed. Use the same specific heat capacity.

## Stage 2: Enthalpy and Molar Enthalpy (12 min)

Enthalpy is a measure of the heat energy available in a chemical system at constant pressure ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636)). When you burn a fuel or run a reaction in a calorimeter, the enthalpy change (ΔH) tells you how much energy the reaction released or absorbed per mole of reactant ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636)).

**Task 3:** In a calorimetry experiment, 0.80 g of ethanol (C₂H₅OH, molar mass 46 g/mol) burns completely. The heat released is 29.6 kJ. Calculate the molar enthalpy of combustion for ethanol.

Show your work:
- Calculate the number of moles of ethanol burned
- Divide the heat released by the moles
- Write your answer as ΔH_comb = ___ kJ/mol and explain whether the sign is positive or negative

**Task 4:** A reaction absorbs 18 kJ of heat and consumes 0.50 mol of a reactant. What is the molar enthalpy change for this reaction? Write your answer using ΔH notation ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23638)).

## Stage 3: Calorimetry Data Analysis (8 min)

You now combine the heat equation and molar enthalpy to analyze real calorimetry data.

**Task 5:** A student burns 1.2 g of sugar (C₁₂H₂₂O₁₁, molar mass 342 g/mol) in a bomb calorimeter containing 500 g of water. The water temperature rises from 20.0°C to 26.8°C. Calculate the molar enthalpy of combustion for sugar. Use c = 4.18 J/(g·°C).

Show your work in order:
- Calculate Q using Q = mcΔt
- Convert Q to kilojoules
- Calculate the moles of sugar burned
- Divide Q by moles to find ΔH_comb
- Write the final answer with units and sign

## Self-check

1. In your own words, explain why the sign of ΔH is negative for a combustion reaction and positive for a melting or dissolving process.

2. If you doubled the mass of fuel burned in a calorimeter but kept everything else the same, how would the heat released (Q) change, and how would the molar enthalpy (ΔH) change? Explain the difference.

3. Look back at Task 5. If the student's calorimeter lost 5% of the heat to the surroundings, would the calculated molar enthalpy be higher or lower than the true value? Why?

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*AILI student homework · language en · model claude-haiku-4-5-20251001 · generated 2026-09-17 · id 6969d31c-b83f-4b37-b489-19425e748727*

### Sources

- node:n1: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636)
- node:n2: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23634)
- node:n3: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23641)
- node:n4: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23634)
- node:n5: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636)
- resource:r1: Resources › type#studentsupport, type#teachersupport › SCN3796 (https://goa-cc-uat-aili-app-001.azurewebsites.net/library/resource/YlIUl7k3G0y6R-b8XyPAGw)
- resource:r2: Resources › type#activity › SCN3796 (https://goa-cc-uat-aili-app-001.azurewebsites.net/library/resource/9ykNTynh306g6EnZlkTITQ)
- node:n6: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23638)
- resource:r3: Resources › type#activity › SCN3796 (https://goa-cc-uat-aili-app-001.azurewebsites.net/library/resource/0uxhxD914kqzmssVwCK4tg)