# Enthalpy Changes and Calorimetry: Heat Transfer Calculations *30 minutes, 3 stages* Calorimetry is the experimental method chemists use to measure the heat absorbed or released during chemical reactions. Understanding how to calculate heat transfer using the relationship Q = mcΔt and how to connect these measurements to enthalpy changes allows you to quantify energy in reactions ([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/23641)). This skill is essential for analyzing real combustion and dissolution processes and for predicting whether reactions release or absorb energy. ## Stage 1: Heat Transfer Calculations (10 min) Use the relationship Q = mcΔt to solve these problems. Remember that Q is heat in joules, m is mass in grams, c is specific heat capacity in J/(g·°C), and Δt is the change in temperature in °C. **Problem 1:** A 250 g sample of water is heated from 20°C to 65°C. The specific heat capacity of water is 4.18 J/(g·°C). Calculate the heat absorbed by the water. **Problem 2:** In a calorimetry experiment, 150 g of water cools from 45°C to 28°C. How much heat is released? (Use c = 4.18 J/(g·°C).) **Problem 3:** An unknown metal sample with a mass of 75 g absorbs 2250 J of heat and its temperature increases by 12°C. Calculate the specific heat capacity of the metal. Show all your work, including the formula, substitution, and units. ## Stage 2: Enthalpy and ΔH Notation (12 min) Enthalpy (H) is the total heat content of a system, and molar enthalpy is the heat change per mole of reaction ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636)). The notation ΔH represents the change in enthalpy. A negative ΔH indicates an exothermic reaction (heat released), and a positive ΔH indicates an endothermic reaction (heat absorbed) ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23638)). **Task 1:** For each scenario, write whether the reaction is exothermic or endothermic and explain your reasoning. - A combustion reaction in which ΔH = −890 kJ/mol - A dissolution process in which ΔH = +25 kJ/mol **Task 2:** In a calorimetry experiment, 2.5 mol of ethanol is burned and releases 3100 kJ of heat. Calculate the molar enthalpy of combustion (ΔH) for ethanol. Include the sign and units. **Task 3:** A reaction absorbs 450 J of heat. Express this as ΔH in kilojoules and state whether the reaction is exothermic or endothermic. ## Stage 3: Connecting Calorimetry Data to Enthalpy (8 min) Calorimetry data, mass, temperature change, and specific heat capacity, allow you to calculate the heat transferred in a reaction, which you then convert to molar enthalpy ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23641)). **Problem:** In a calorimetry experiment, 3.0 g of magnesium is burned in oxygen inside a calorimeter containing 500 g of water. The water temperature increases from 22°C to 54°C. The specific heat capacity of water is 4.18 J/(g·°C). The molar mass of magnesium is approximately 24 g/mol. 1. Calculate the heat absorbed by the water using Q = mcΔt. 2. Convert your answer to kilojoules. 3. Calculate the number of moles of magnesium that reacted. 4. Calculate the molar enthalpy of combustion (ΔH) for magnesium in kJ/mol. Is this reaction exothermic or endothermic? ## Self-check 1. In a calorimetry experiment, you measure that 200 g of water increases in temperature by 8°C. Without calculating, explain whether the heat absorbed by the water is greater or less than the heat absorbed when 200 g of water increases by 15°C. 2. A reaction has ΔH = −45 kJ/mol. What does the negative sign tell you about whether energy is released or absorbed? 3. If 2.0 mol of a substance reacts and releases 280 kJ of heat, what is the molar enthalpy change for the reaction? --- *AILI student homework · language en · model claude-haiku-4-5-20251001 · generated 2026-09-17 · id a53c2f69-6912-408e-a89b-e54893406ae3* ### 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) - node:n7: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23648) - resource:r3: Resources › type#activity › SCN3796 (https://goa-cc-uat-aili-app-001.azurewebsites.net/library/resource/0uxhxD914kqzmssVwCK4tg)