1. Standard Enthalpy Changes of Reaction 15.1

2. 15.1.1 – Define and apply the terms standard state, standard enthalpy change of formation (ΔH f ˚) and standard enthalpy change of combustion (ΔH c ˚).

3.  A – 298K (25˚C) around room temp  1.00 x 10 5 Pa (101.3 kPa) around room pressure Q – What are “standard” conditions?

4. ΔH c ˚ Heat of Combustion  ΔH c ˚= the enthalpy change that occurs when 1 mol of a substance burns completely (excess oxygen) under standard conditions (see table 12 of IB data booklet) Balance these equations for one mole of the reactant instead of the product. Always exothermic CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (l)  H  comb = -891 kJ/mol

5. 15.1.2  Determine the enthalpy change of a reaction using standard enthalpy changes of combustion. ΔH˚ reaction =  ΔH c ˚ reactants -  ΔH c ˚ products  They are “flipped” because formation focuses on the energy used in creation and combustion focuses on the energy used in destruction.

6. Practice Problem ΔH˚ reaction =  ΔH c ˚ reactants -  ΔH c ˚ products Calculate  H  rxn of the hydration of ethene, C 2 H 4 + H 2 O  CH 3 -CH 2 -OH given the following:  H  combustion ethene (C 2 H 4 ) = -1409 kJ/mol  H  combustion ethanol (CH 3 -CH 2 -OH) = -1371 kJ/mol  H  combustion water (H 2 O) = 0 kJ/mol

7. Practice Problem ΔH˚ reaction =  ΔH c ˚ reactants -  ΔH c ˚ products ΔH˚ reaction = [(-1409 + 0) - (-1371)] kJ/mol = -38 kJ/mol

8. Practice Problems with Heats of Combustion  Read Section 15.1 pp 147-149  Do Ex 15.1 # 4, 5, 9, 12 ------------------------------------------------------- More Practice – Try Talbot – HL Practice  Heat of Formation: MC 2, 14, 19, 21, OR 3a  Heat of Combustion: OR 3bcd

9. Summing up  So now we have several ways to predict the enthalpy changes of a chemical reaction Experimentally – Heat Energy = m  c  T Using Hess’s Law Average Bond Enthalpies ○ ΔH˚=  bonds broken-  bonds formed Standard Enthalpies of formation ○ ΔH˚ reaction =  ΔH f ˚ products -  ΔH f ˚ reactants Standard Enthalpies of combustion ○ ΔH˚ reaction =  ΔH c ˚ reactants -  ΔH c ˚ products