Calculating energy changes
Chemical bonds
The forces that hold together atomThe smallest part of an element that can exist. in a moleculeA collection of two or more atoms held together by chemical bonds. are called bondThe chemical link that holds molecules together.. During a chemical reaction the bonds within the reactantA substance that reacts together with another substance to form products during a chemical reaction. break. This requires the transfer 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. from the surroundings. This is called the activation energyThe minimum energy required for a chemical reaction to take place..
The formation of bonds within the productA substance formed in a chemical reaction. is the opposite process. Energy is released back to the surroundings.
If the energy required to break bonds is greater than the energy released when new bonds are formed then overall energy is taken in from the surroundings. This reaction is endothermicReaction in which energy is taken in..
If the energy required to break bonds is less than the energy released to the surroundings when new bonds form then overall energy is released back into the surroundings. This reaction is exothermicReaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases..
Calculating energy change - Higher
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 energy 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:
| Step | Action |
| 1 | Write the balanced chemical equation for the reaction. |
| 2 | Look up the bond energy of each bond that breaks and each bond that forms. |
| 3 | Use the chemical equation to work out how many of each type of bond must break. |
| 4 | Calculate the total energy of all the bonds that must break - this is the 'energy in'. |
| 5 | Use the chemical equation to work out how many of each type of bond must form. |
| 6 | Calculate the total energy of all the bonds that must break - this is the 'energy out'. |
| 7 | Calculate the overall energy change (energy in - energy out). |
| Step | 1 |
|---|---|
| Action | Write the balanced chemical equation for the reaction. |
| Step | 2 |
|---|---|
| Action | Look up the bond energy of each bond that breaks and each bond that forms. |
| Step | 3 |
|---|---|
| Action | Use the chemical equation to work out how many of each type of bond must break. |
| Step | 4 |
|---|---|
| Action | Calculate the total energy of all the bonds that must break - this is the 'energy in'. |
| Step | 5 |
|---|---|
| Action | Use the chemical equation to work out how many of each type of bond must form. |
| Step | 6 |
|---|---|
| Action | Calculate the total energy of all the bonds that must break - this is the 'energy out'. |
| Step | 7 |
|---|---|
| Action | Calculate the overall energy change (energy in - energy out). |
If the answer is negative, more energy was released by bond formation than was needed to break the bonds. The reaction is exothermic.
If the answer is positive, the reaction is endothermic.
Example
Hydrogen and chlorine react to form hydrogen chloride gas. What is the energy change of this reaction?
Use the bond energies in the table to calculate the energy change for this reaction.
| Bond | Bond energy (kJ mol-1) |
| H-H | 436 |
| Cl-Cl | 243 |
| H-Cl | 432 |
| Bond | H-H |
|---|---|
| Bond energy (kJ mol-1) | 436 |
| Bond | Cl-Cl |
|---|---|
| Bond energy (kJ mol-1) | 243 |
| Bond | H-Cl |
|---|---|
| Bond energy (kJ mol-1) | 432 |
| Step | Result |
| 1 | H2 + Cl2 → 2HCl |
| 2 | Bond energies are given in the question. |
| 3 | Bonds that must break: 1 × H-H, 1 × Cl-Cl |
| 4 | Energy in = 436 + 243 = 679 kJ mol-1 |
| 5 | Bonds that must form: 2 × H-Cl |
| 6 | Energy out = (2 × 432) = 864 kJ mol-1 |
| 7 | Overall energy change = 679 - 864 = -185 kJ mol-1 |
| Step | 1 |
|---|---|
| Result | H2 + Cl2 → 2HCl |
| Step | 2 |
|---|---|
| Result | Bond energies are given in the question. |
| Step | 3 |
|---|---|
| Result | Bonds that must break: 1 × H-H, 1 × Cl-Cl |
| Step | 4 |
|---|---|
| Result | Energy in = 436 + 243 = 679 kJ mol-1 |
| Step | 5 |
|---|---|
| Result | Bonds that must form: 2 × H-Cl |
| Step | 6 |
|---|---|
| Result | Energy out = (2 × 432) = 864 kJ mol-1 |
| Step | 7 |
|---|---|
| Result | Overall energy change = 679 - 864 = -185 kJ mol-1 |
The energy change is negative showing that the reaction is exothermic.
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 (kJ mol-1) |
| H-H | 436 |
| O=O | 498 |
| O-H | 464 |
| Bond | H-H |
|---|---|
| Bond energy (kJ mol-1) | 436 |
| Bond | O=O |
|---|---|
| Bond energy (kJ mol-1) | 498 |
| Bond | O-H |
|---|---|
| Bond energy (kJ mol-1) | 464 |
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.