C5 - Energy Changes

Why do all chemical reactions involve energy changes?

Because energy is needed to break bonds and is released when bonds are made

Exothermic if the reaction gets…

Hot

Endothermic if the reaction gets…

Cold

Energy changes practical

1. React an acid with an alkali, and measure the max. temp reached. Repeat with increasing volumes of alkali

2. The max. temp will eventually decrease as the alkali is in excess (same energy released but larger volume)

3. Plot graph and where two lines of best fit meet is the volume needed to neutralise

1?

Activation energy

2?

Net energy out, increases temp, then lost to surroundings

In exothermic reactions, why must there be a net decrease in potential energy?

So there can be a net increase in kinetic energy (increase in temp)

Activation energy (from surroundings)

In endothermic reactions, why must there be an increase in potential energy?

So there is a decrease in kinetic energy (temp. decreases)

Bond energy

The amount of energy required to break bonds, the same as the amount of energy released when a bond is formed

Bond energy for C-H

413 kJ/mol

Bond energy for O=O

495 kJ/mol

Bond energy for C=O

799 kJ/mol

Bond energy for O-H

467 kJ/mol

How to work out bond energies for a reaction

1. Write out equation

2. Draw structure to see bonds

3. Calculate energy (number of bonds x bond energy) for each molecule

4. If energy before is lower, reaction is exothermic. If energy before is higher, reaction is endothermic

5. Net potential energy change = start energy - end energy