Calculating energy changes - Higher
Using bond energies
The energy change in a reaction can be calculated using bond energyThe amount of energy needed to break one mole of a particular covalent bond.. A bond energy is the amount of energyThe capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. needed to break one moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 ×10²³ number of particles). of a particular covalent bondA bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons..
Different bonds have different bond energies. These are given when they are needed for calculations.
To calculate an energy change for a reaction:
- add together the bond energies for all the bonds in the reactantA substance that reacts together with another substance to form products during a chemical reaction. - this is the 'energy in'
- add together the bond energies for all the bonds in the productA substance formed in a chemical reaction. - this is the 'energy out'
- energy change = energy in - energy out
Example
Hydrogen and chlorine react to form hydrogen chloride gas:
H−H + Cl−Cl → 2 × (H−Cl)
Use the bond energies in the table to calculate the energy change for this reaction.
| Bond | Bond energy |
| H−H | 436 kJ mol-1 |
| Cl−Cl | 243 kJ mol-1 |
| H−Cl | 432 kJ mol-1 |
| Bond | H−H |
|---|---|
| Bond energy | 436 kJ mol-1 |
| Bond | Cl−Cl |
|---|---|
| Bond energy | 243 kJ mol-1 |
| Bond | H−Cl |
|---|---|
| Bond energy | 432 kJ mol-1 |
Energy in = 436 + 243 = 679 kJ mol-1
Energy out = (2 × 432) = 864 kJ mol-1
Energy change = in - out
= 679 - 864
= -185 kJ mol-1
The energy change is negative. This shows that the reaction is exothermic.
Example 2
Hydrogen bromide decomposes to form hydrogen and bromine:
2 × (H−Br) → H−H + Br−Br
Use the bond energies in the table to calculate the energy change for this reaction.
| Bond | Bond energy |
| H−Br | 366 kJ mol-1 |
| H−H | 436 kJ mol-1 |
| Br−Br | 193 kJ mol-1 |
| Bond | H−Br |
|---|---|
| Bond energy | 366 kJ mol-1 |
| Bond | H−H |
|---|---|
| Bond energy | 436 kJ mol-1 |
| Bond | Br−Br |
|---|---|
| Bond energy | 193 kJ mol-1 |
Energy in = 2 × 366 = 732 kJ mol-1
Energy out = 436 + 193 = 629 kJ mol-1
Energy change = in - out
= 732 - 629
= +103 kJ mol-1
The energy change is positive. This shows that the reaction is endothermic.
Question
Hydrogen reacts with oxygen to form water:
2 × (H−H) + O=O → 2 × (H-O-H)
Use the bond energies in the table to calculate the energy change for this reaction.
| Bond | Bond energy |
| H−H | 436 kJ mol-1 |
| O=O | 498 kJ mol-1 |
| O-H | 464 kJ mol-1 |
| Bond | H−H |
|---|---|
| Bond energy | 436 kJ mol-1 |
| Bond | O=O |
|---|---|
| Bond energy | 498 kJ mol-1 |
| Bond | O-H |
|---|---|
| Bond energy | 464 kJ mol-1 |
Energy in = (2 × 436) + 498
= 872 + 498
= 1370 kJ mol-1
Energy out = 2 × 2 × 464
= 1856 kJ mol-1 (there are two O-H bonds in each water molecule)
Energy change = in - out
= 1370 - 1856
= -486 kJ mol-1
The energy change is negative. This shows that the reaction is exothermic.