Energy change calculations
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 broken 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 formed in the productA substance formed in a chemical reaction. - this is the 'energy out'
The 'energy in' is an endothermic change, as the energy is being used to break bonds. The 'energy out' is an exothermic change, as the energy is released as new bonds are formed. Therefore the energy change is:
- 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 | Energy |
| H−H | 436 kJ mol-1 |
| Cl−Cl | 243 kJ mol-1 |
| H−Cl | 432 kJ mol-1 |
| Bond | H−H |
|---|---|
| Energy | 436 kJ mol-1 |
| Bond | Cl−Cl |
|---|---|
| Energy | 243 kJ mol-1 |
| Bond | H−Cl |
|---|---|
| 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, as the 'energy out' is larger than the 'energy in'.
Example
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 | Energy |
| H−Br | 366 kJ mol-1 |
| H−H | 436 kJ mol-1 |
| Br−Br | 193 kJ mol-1 |
| Bond | H−Br |
|---|---|
| Energy | 366 kJ mol-1 |
| Bond | H−H |
|---|---|
| Energy | 436 kJ mol-1 |
| Bond | Br−Br |
|---|---|
| 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, as the 'energy out' is smaller than the 'energy in'.
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 | Energy |
| H-H | 436 kJ mol-1 |
| O=O | 498 kJ mol-1 |
| O-H | 464 kJ mol-1 |
| Bond | H-H |
|---|---|
| Energy | 436 kJ mol-1 |
| Bond | O=O |
|---|---|
| Energy | 498 kJ mol-1 |
| Bond | O-H |
|---|---|
| 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.
