Measuring Enthalpy Changes

1. Measuring Enthalpy Changes • We can’t measure energy changes directly • We can only measure temperature changes • Using the specific heat capacity of a substance we can calculate energy changes 12 Chemistry 2.6 rxn rates, eqm, acids and bases CR 07

2. Specific Heat Capacity • Specific Heat Capacity is the energy required to heat 1 g of a substance by 10 C • Symbol: c Unit: J per g0C 12 Chemistry 2.6 rxn rates, eqm, acids and bases CR 07

3. Enthalpy Change Formula • H = m c t Specific heat capacity Change in temperature Mass in grams 12 Chemistry 2.6 rxn rates, eqm, acids and bases CR 07

4. Worked Example • Water has c = 4.23 J g-1 0C-1 • Calculate enthalpy (amount of energy used) to heat 3L (3kg) of water from 200 C to 950 C 12 Chemistry 2.6 rxn rates, eqm, acids and bases CR 07

5. H = mct • = 3000x4.23x(95-20) • =3000x4.23x75 • =951 750J • =951.75 kJ 12 Chemistry 2.6 rxn rates, eqm, acids and bases CR 07

6. Calorimeters • A calorimeter (calorie is the old unit of energy) is an insulated container with a thermometer (and sometimes, a stirrer) 12 Chemistry 2.6 rxn rates, eqm, acids and bases CR 07

7. Enthalpy changes are expressed as heat released or absorbed or -H or + H • You can express H for mass or amount of a substance. You can then use multiples of H for varying amounts of substance 12 Chemistry 2.6 rxn rates, eqm, acids and bases CR 07

8. Unit of H • H can have the unit Jmol-1 (J/mol) or Jg-1 (J/g) • OR if calculating energy needed or released: J or kJ (because you have used g or mol in the calculation) 12 Chemistry 2.6 rxn rates, eqm, acids and bases CR 07