5.1.3 The energy change of reactions
Breaking bonds takes energy in; making bonds gives energy out
Definition
Reactant
A reactant is a substance that is changed or used up during a chemical reaction.
- During a chemical reaction, atoms are rearranged as bonds in the reactants break and new bonds in the products form.
- Energy must be supplied to break existing covalent bonds, so bond breaking is endothermic.
- Energy is released when new covalent bonds form, so bond making is exothermic.
- The supplied bond energies, measured in kJ/mol\text{kJ/mol}kJ/mol, let you calculate these energy transfers.
Common Mistake
- Do not write that breaking bonds releases energy.
- Bond breaking takes energy in, while bond making gives energy out.
The overall energy change compares the energy taken in with the energy given out
Definition
Product
A product is a substance formed during a chemical reaction.
- Add the bond energies of every bond broken in the reactants to find the total energy taken in.
- Add the bond energies of every bond made in the products to find the total energy given out.
- Find the overall energy change from energy change=energy to break bonds−energy released making bonds\text{energy change} = \text{energy to break bonds} - \text{energy released making bonds}energy change=energy to break bonds−energy released making bonds.
- The reaction is exothermic if making the product bonds gives out more energy than breaking the reactant bonds takes in.
- The reaction is endothermic if breaking the reactant bonds takes in more energy than making the product bonds gives out.
Key Idea
- A negative overall energy change means the reaction is exothermic.
- A positive overall energy change means the reaction is endothermic.
Working through a bond-energy calculation step by step
Definition
Bond energy
The energy required to break one mole of a particular covalent bond, measured in kilojoules per mole.
- Start from the balanced symbol equation so you know how many molecules of each substance react.
- Use the displayed formulae, or sketch the structures, to see the bonds in each molecule.
- Count every bond in the reactants, including the extra molecules shown by the numbers in front of a formula.
- Multiply the number of each bond by its supplied bond energy, then add the results for the bonds broken total.
- Repeat for the bonds in the products to find the bonds made total.
- Subtract the bonds-made total from the bonds-broken total, add the unit kJ/mol\text{kJ/mol}kJ/mol and read the sign.
Note
- Keep the bonds-broken and bonds-made totals separate until the final subtraction.
- You do not memorise bond energies; the values you need are given in the question.
- A number in front of a formula multiplies every bond inside that molecule.
Example
- Hydrogen burns in oxygen to make water vapour: 2H2(g)+O2(g)→2H2O(g)2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(g)}2H2(g)+O2(g)→2H2O(g).
- Supplied bond energies: H-H=436 kJ/mol\text{H-H} = 436\ \text{kJ/mol}H-H=436 kJ/mol, O=O=498 kJ/mol\text{O=O} = 498\ \text{kJ/mol}O=O=498 kJ/mol and O-H=463 kJ/mol\text{O-H} = 463\ \text{kJ/mol}O-H=463 kJ/mol.
- Bonds broken: two H-H\text{H-H}H-H bonds and one O=O\text{O=O}O=O bond, so (2×436)+498=1370 kJ/mol(2 \times 436) + 498 = 1370\ \text{kJ/mol}(2×436)+498=1370 kJ/mol.
- Bonds made: four O-H\text{O-H}O-H bonds in the two water molecules, so 4×463=1852 kJ/mol4 \times 463 = 1852\ \text{kJ/mol}4×463=1852 kJ/mol.
- Overall energy change: 1370−1852=−482 kJ/mol1370 - 1852 = -482\ \text{kJ/mol}1370−1852=−482 kJ/mol.
- The negative sign shows the reaction is exothermic, releasing 482 kJ/mol482\ \text{kJ/mol}482 kJ/mol.
Exam technique
- Always start from the balanced equation and count bonds from the displayed formulae; a number in front of a formula multiplies every bond in that molecule.
- Subtract in the right order, bonds broken minus bonds made: a negative answer is exothermic and a positive answer is endothermic.
- Quote the final value with its sign and kJ/mol\text{kJ/mol}kJ/mol, then state the reaction type to secure the last mark.
Self review
- What happens to energy when bonds in the reactants are broken?
- What calculation gives the overall energy change of a reaction?
- What does a negative overall energy change tell you about a reaction?
- A reaction takes in 920 kJ/mol920\ \text{kJ/mol}920 kJ/mol to break bonds and releases 760 kJ/mol760\ \text{kJ/mol}760 kJ/mol making bonds; what is the overall energy change and reaction type?
- Why do you not need to learn bond energies by heart for these calculations?
