Breaking and Forming Bonds Revision Notes | GCSE AQA Higher Chemistry

1. Why Chemical Reactions Need Energy

Picture trying to pull apart two magnets stuck together - you need to put in energy to separate them. Chemical bonds work the same way. Atoms in molecules are held together by strong attractive forces, and breaking these bonds always requires energy input.

Think of it like this: when you heat methane gas, you're giving the $CH_4$ molecules enough energy to break their C-H bonds. This energy input is essential - without it, the bonds would stay intact and no reaction could happen.

2. The Energy Payback - Bond Formation

Here's where it gets interesting. While breaking bonds costs energy, forming new bonds gives energy back. When atoms come together to form new chemical bonds, they release energy - like magnets snapping together.

In our methane example, when the broken C and H atoms form new bonds with oxygen atoms to make $CO_2$ and $H_2O$, energy is released. The key question becomes: do we get back more energy than we put in?

3. The Energy Balance Sheet

Every chemical reaction is like a financial transaction. You spend energy breaking old bonds, then earn energy back forming new ones. The overall energy change depends on which amount is bigger.

Different bonds have different strengths. A C-H bond needs 413 kJ/mol to break it, while an O=O bond needs 498 kJ/mol. These values are your 'price list' for breaking bonds.

4. Calculating Energy Changes

Let's see how this works with real numbers. We'll calculate the energy change for methane burning - a reaction that clearly releases energy since it's used for heating.

Worked example: Calculating Energy Change for Methane Combustion

Calculate the overall energy change for: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ Bond energies: C-H = 413 kJ/mol, O=O = 498 kJ/mol, C=O = 805 kJ/mol, O-H = 464 kJ/mol

1. In $CH_4$: 4 × C-H bonds In $2O_2$: 2 × O=O bonds — Count each bond in the reactant molecules
2. Energy in = (4 × 413) + (2 × 498) Energy in = 1652 + 996 = 2648 kJ/mol — This is the energy cost to break all reactant bonds
3. In $CO_2$: 2 × C=O bonds In $2H_2O$: 4 × O-H bonds — Count each bond in the product molecules
4. Energy out = (2 × 805) + (4 × 464) Energy out = 1610 + 1856 = 3466 kJ/mol — This is the energy gained from forming new bonds
5. Overall change = 2648 - 3466 = -818 kJ/mol — Negative value means exothermic - energy is released

Answer: -818 kJ/mol (exothermic reaction)

The negative result confirms what we know - burning methane releases energy. We get back 818 kJ more energy than we put in, which is why methane makes such a good fuel.

5. Exam Success - Bond Energy Questions

Bond energy questions follow a predictable pattern in exams. Master the method and you'll pick up clear and systematic marks every time.

The most common mistake is mixing up which bonds you're breaking and which you're forming. Always start with the reactants (bonds to break) then move to products (bonds to form).

Exam precision note: In Breaking and Forming Bonds - The energy change of reactions (HT only) - Exothermic and endothermic reactions, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

The key comparison for this GCSE topic is whether the reaction takes in more energy to break bonds than it gives out when new bonds form. If energy in is greater than energy out, the overall change is positive and the reaction is endothermic. If energy in is less than energy out, the overall change is negative and the reaction is exothermic. Students should say this explicitly rather than just quoting a single worked-example value such as -818 kJ/mol from methane combustion.