Measuring heat changes

1. Measuring heat changes Section 11.2

2. After reading Section 11.2, you should know: • How to write or interpret a thermochemical equation • How to calculate heat changes in a chemical or physical process

3. Calorimetry • Calorimetry – the accurate and precise measurement of heat changes which occur during chemical and physical processes.

4. Enthalpy • Enthalpy (H) – measure of the heat content of a system. • ∆H – change in enthalpy at constant pressure • For reactions which take place at constant pressure, the terms heat and enthalpy are used interchangeably, that is, q = ∆H. • q = ∆H = (m)(Cp)(∆T)

5. Calorimeters • Coffee cup calorimeter – an insulated device used to measure the absorption or release of heat of a reaction at constant pressure. • Bomb calorimeter – used for measuring heat released when a compound is burned at constant volume.

6. Thermochemical Equations • Thermochemical equation – an equation which includes the heat of the reaction • The physical state of all reactants and products must also be stated • The energy absorbed or released in a chemical equation can be expressed in two different ways: • As a reactant or product or • Using the symbol ΔH

7. Thermochemical Equations Endothermic reaction (heat is a absorbed, so it is a reactant): 2 NaHCO3(s) + 129 kJ → Na2CO3(s) + CO2(g) + H2O(g) OR 2 NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g) ∆H = +129 kJ

8. Thermochemical Equations Exothermic reaction (heat is given off, so it is a product): CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ OR CaO(s) + H2O(l) → Ca(OH)2(s) ∆H = − 65.2 kJ

9. After reading Section 11.2, you should know: • How to write or interpret a thermochemical equation • How to calculate heat changes in a chemical or physical process