Breaking Bonds Requires Energy

What students typically write

“When the bonds in methane break, energy is released.” Or: “Breaking the C-H bonds gives out heat.” Or: “The fuel releases energy when its bonds are broken.”

This is arguably the single most damaging misconception in IGCSE Chemistry energetics. It reverses the fundamental principle that drives all bond energy calculations.

Why it is wrong

A chemical bond is a force of attraction holding atoms together. To break that attraction, you must put energy in, just as you must do work to pull two magnets apart. Breaking bonds is always endothermic.

When atoms come together and form new bonds, energy is released. The attraction pulls the atoms together, and the energy of that process is given out to the surroundings. Making bonds is always exothermic.

In a combustion reaction, two things happen: bonds in the fuel and oxygen break (energy in), and bonds in carbon dioxide and water form (energy out). The reaction is exothermic overall because the energy released by making C=O and O-H bonds in the products is greater than the energy absorbed by breaking C-H, C-C, and O=O bonds in the reactants.

If breaking bonds released energy, then every substance would spontaneously fall apart, releasing heat as it did so. That does not happen, which is immediate evidence that breaking bonds costs energy.

What the mark scheme actually wants

A typical question: “Explain, in terms of bond energies, why the combustion of methane is exothermic.” [3]

Marking points:

1. Energy is needed/absorbed to break bonds in the reactants (1 mark)
2. Energy is released when bonds are made/formed in the products (1 mark)
3. More energy is released making bonds than is absorbed breaking bonds, so the overall reaction is exothermic (1 mark)

An answer that says “energy is released when bonds break” directly contradicts marking point 1 and scores 0 for it.

Worked example: wrong vs right

Question: Use bond energies to explain whether this reaction is exothermic or endothermic: H2 + Cl2 goes to 2HCl. [Bond energies: H-H = 436, Cl-Cl = 242, H-Cl = 431 kJ/mol] [4]

Wrong answer: “Breaking H-H releases 436 kJ and breaking Cl-Cl releases 242 kJ. Total released = 678 kJ. Making 2 H-Cl absorbs 2 x 431 = 862 kJ. So 862 - 678 = 184 kJ absorbed. Endothermic.” This scores 0/4. The energy direction is reversed at every step, giving the wrong conclusion.

Right answer: “Energy absorbed breaking bonds: H-H = 436, Cl-Cl = 242. Total = 678 kJ (1). Energy released making bonds: 2 x H-Cl = 2 x 431 = 862 kJ (1). Overall energy change = 678 - 862 = -184 kJ (1). More energy is released than absorbed, so the reaction is exothermic (1).” This scores 4/4.

How to avoid this mistake

Memorise this pair and never swap them:

• Breaking bonds = energy IN (endothermic)
• Making bonds = energy OUT (exothermic)

A physical analogy: tearing a piece of paper requires effort (energy in). You do not get energy from ripping it. Similarly, breaking a chemical bond requires energy input.

In bond energy calculations, always label two columns clearly: “energy absorbed (breaking)” and “energy released (making)”. If the total released exceeds the total absorbed, the reaction is exothermic. If the total absorbed exceeds the total released, it is endothermic.

For full worked examples of bond energy calculations, see bond energy calculations. The exothermic and endothermic reactions page covers energy-level diagrams and activation energy.