--- title: "Enthalpy Change, Standard Conditions and Calorimetry" description: "Enthalpy change is the heat a reaction gives out or takes in at constant pressure, quoted per mole under agreed conditions. It is measured by tracking a temperature change and dividing by the amount t" canonical: https://lightmysky.com/learn/science/enthalpy-change-standard-conditions-and-calorimetry-mt_GPgwgyqyvN source: https://lightmysky.com/learn/science/enthalpy-change-standard-conditions-and-calorimetry-mt_GPgwgyqyvN.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Enthalpy Change, Standard Conditions and Calorimetry Enthalpy change is the heat a reaction gives out or takes in at constant pressure, quoted per mole under agreed conditions. It is measured by tracking a temperature change and dividing by the amount that reacted. Subject: Science · Area: Chemistry · Ages 16 to 17 Page: https://lightmysky.com/learn/science/enthalpy-change-standard-conditions-and-calorimetry-mt_GPgwgyqyvN ## Ready when they can - States what standard conditions fix and why a quoted value means nothing without them - Uses the mass of the solution, its specific heat capacity and the temperature change to find the heat transferred - Divides by the amount in moles to get a value per mole and attaches the right sign - Gives two reasons a bench value comes out well below the data book value ## Lesson: Catching the heat of a reaction Enthalpy change is the heat a reaction gives out or takes in at constant pressure, quoted per mole. Reactions that warm the mixture are exothermic and take a negative sign, while ones that cool it are endothermic and positive. Values mean nothing without standard conditions: solutes at 1 mol per cubic decimetre, gases at 100 kilopascals, and a stated temperature, usually 298 K. The little circle in the symbol is the promise that these apply. A coffee cup calorimeter is gloriously simple: mix, stir, and watch the thermometer. Multiply the mass of solution by its specific heat capacity, 4.18 joules per gram per degree for dilute aqueous mixtures, and by the temperature change to get q. That q belongs to the whole beaker, so divide by the moles of limiting reactant to reach kilojoules per mole. **Example.** Burn ethanol under 100 g of water and watch it rise 10 degrees. Then q is 100 times 4.18 times 10, which is 4180 joules. If 0.05 mol burned, each mole gave 4180 over 0.05, or 83.6 kilojoules. Heat left the fuel, so delta H is minus 83.6 kilojoules per mole. **Tip.** Bench values always undershoot data books. Heat leaks into the air and the apparatus, and reactions rarely run to completion. Insulate well, stir fast, and quote the conditions, or the number is orphaned. **Recap.** Measure the temperature rise, turn it into joules, divide by moles, and mind the escaping heat. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Reaction Profiles and Activation Energy](https://lightmysky.com/learn/science/reaction-profiles-and-activation-energy-mt_6LtFkXn_xQ) - [Gas Volumes and the Ideal Gas Equation](https://lightmysky.com/learn/science/gas-volumes-and-the-ideal-gas-equation-mt_I1fz_6wjuD) - [Specific Heat Capacity as a Material Property](https://lightmysky.com/learn/science/specific-heat-capacity-as-a-material-property-mt_mXC9IKom3P) ## Opens up - [Hess's Law and Enthalpy Cycles](https://lightmysky.com/learn/science/hesss-law-and-enthalpy-cycles-mt_530V9czLOb) - [Uncertainty in a Measurement and Where It Comes From](https://lightmysky.com/learn/science/uncertainty-in-a-measurement-and-where-it-comes-from-mt_JXw4D9DReI)