To consider the heat (enthalpy) of chemical reactions

1. Objectives • To consider the heat (enthalpy) of chemical reactions • How do we describe the heat generated or absorbed by chemical reactions? • To understand Hess’s Law • Energy as a state function

2. A. Thermochemistry (Enthalpy) • Enthalpy, H • H is equal to the energy that flows as heat (at constant pressure) • H = q (at constant pressure) Enthalpy change for a reaction is usually measured in kJ and is the heat flow associated with the completion of a “whole” reaction: Fe2O3(s) + 2Al (s) → 2Fe (l) + Al2O3(s) H = -852kJ Exothermic reactions

3. H Calculations For the reaction below, what is H when the following substances take part? 4Fe (s) + 3O2(g) →2Fe2O3(s) H = -1652k 4 moles of Fe 3 moles of O2 1 mole of Fe 28 g of Fe 48 g of O2 117 g of Fe2O3

4. A. Thermochemistry (Enthalpy) • Heat of Formation Hf is the enthalpy of formation of onemole of a substance from its constituent elements • 2Na + Cl2 2NaCl H =  822kJ/mol • Na + 1/2Cl2 NaCl Hf =  411kJ/mol NaCl Formation

5. A. Thermochemistry • Calorimetry • Change in Enthalpy, ΔH is measured using a simple calorimeter. • Change in Enthalpy is specific heat times change in temperature times mass • ΔH = S.m.ΔT

6. Calorimetry Bomb Calorimeter

7. B. Hess’s Law • For a particular reaction, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps. Would you expect that? Germain Hess 1802 - 1850 Example: N2(g) + 2O2(g) 2NO2(g)H1 = 68 kJ N2(g) + O2(g) 2NO(g) H2 = 180 kJ 2NO(g) + O2(g)  2NO2(g)H3 = -112 kJ ---------------------------------------- ------------------