[] - Head Start to A-Level Chemistry (CGP A-Level Chemistry) (2021, CGP Books)

Page 48

Different bonds have different bond energies.
- Bonds between different atoms require varying amounts of energy to break.
- Same two atoms bonding in the same way require a consistent amount of energy.
Average bond energy values for common bonds:
- C–H: 413 kJ mol–1
- O=O: 498 kJ mol–1
- C=O: 743 kJ mol–1
Calculating energy change in a reaction:
- Energy required to break old bonds minus energy released by new bonds formed.
Example reaction: CH4 + 2O2 Æ CO2 + 2H2O
- Energy change: -690 kJ mol–1 (exothermic reaction)

Example calculation for energy change in reactions:
- Burning 1 mole of propane and ethanol.
- Hydrogenation of 1 mole of ethene.

Planning experiments:
- Select appropriate equipment.
- Control variables for a fair test.
Variables in experiments:
- Independent variable: Quantity changed.
- Dependent variable: Thing measured.
Presenting data:
- Record results in a table or graph.
- Use mean for reliable results.
Anomalous results:
- Spot anomalies in data.
- Ignore anomalies when calculating the mean.