1. ENERGY EXCHANGES IN CHEMICAL REACTIONS

2. CHEMICAL REACTIONS Produce (lose) energy = exothermic reaction
Absorb (require) energy = endothermic reaction

3. ENERGY
The energy (in a reaction) is linked to either MAKING or BREAKING bonds

4. BOND BREAKING
energy absorbing
endothermic

5. BOND MAKING
energy releasing
exothermic

6. The big BUT…
In most reactions the bonds of the reactants are broken and the new bonds of the products are formed (so we have both exo and endo reactions

7. ex. Combustion of methane
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
All the C-H bonds and O-O bonds are broken
This requires energy – call this Ein (ACTIVATION ENERGY)
C=O bonds and O-H bonds need to be formed
Energy is released – call this Eout

8. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
The ΔH can therefore be thought of as
ΔH = Ein – Eout
When methane burns ΔH is negative because it is an exothermic reaction. This means that forming bonds releases more energy than what was absorbed to break the bonds in the reactants Eout > Ein

9. In general
If the total energy input is greater than the output ΔH is positive and the reaction is endothermic

10. If the total energy output is greater than the input ΔH is negative and the reaction is exothermic

11. ex. 2 Formation of Hydrogen Iodide
H2(g) + I2(g)  2HI(g)
The reaction is endothermic, therefore the products possess more total formation enthalpy than the reactants

12. In general: exothermic reactions
ΔH < 0 kJ
The energy input is less than the energy output

13. In general: endothermic reactions
ΔH > 0 kJ
The energy input is greater than the energy output

14. Catalysts
A catalyst is a chemical that, when added to a reaction, speeds up the reaction and also remains when the reaction is over (it is not INVOLVED in the reaction)

15. Catalysts and Hess’s Law
Hess’s Law states that the enthalpy of a reaction depends ONLY on the enthalpies of the initial and the final states, no matter what series of steps exist in between.
A catalyst has NO effect on the enthalpy change of a reaction

16. Catalyzed reactions lose heat more rapidly if reaction is exothermic
gain heat more rapidly if reaction is endothermic

17. Catalysts speed up a reaction by providing an alternate reaction pathway that has lower activation energy