In every chemical reaction, bonds in the reactants are broken (energy absorbed) and new bonds in the products are made (energy released). The two happen in the same reaction but they are separate energy events, and exam answers earn marks for keeping them apart.
This lesson closes energy changes and bonds by joining the ideas from estimating an energy change using bond energies to the diagrams in energy profiles.
Why do the two processes go opposite ways?
Holding two atoms together in a bond is a lower-energy arrangement than having them apart. To pull them apart you must supply energy. When atoms join to form a bond, that same amount of energy is released.
So there are two totals in any reaction: the energy absorbed to break all the reactant bonds and the energy released in forming all the product bonds. The overall energy change is the difference between them.
How do you separate them cleanly?
- Draw out the molecules (or list the bonds in each formula).
- Make a table with two columns, “bonds broken” and “bonds made”.
- Fill the first column from the reactants only, and the second from the products only.
- Multiply by the coefficients.
- Total each column separately before doing any subtraction.
- Compare totals: made greater than broken means exothermic; broken greater than made means endothermic.
Worked example
The bond energies are supplied values for practice, in kJ/mol: H–H 436, O=O 498, O–H 464.
Consider 2H₂ + O₂ → 2H₂O.
| Bond | How many | Energy per mole | Total (kJ/mol) | |
|---|---|---|---|---|
| Broken | H–H | 2 | 436 | 872 |
| Broken | O=O | 1 | 498 | 498 |
| Made | O–H | 2 × 2 = 4 | 464 | 1856 |
Step 1, broken total: 872 + 498 = 1370 kJ/mol.
Step 2, made total: 4 × 464 = 1856 kJ/mol.
Step 3, compare: 1856 is greater than 1370, so more energy is released than absorbed.
Step 4, energy change: 1370 − 1856 = −486 kJ/mol. The reaction is exothermic.
Check: the table gives 2 × 436 = 872 and 4 × 464 = 1856. Both totals agree with the working, and the sign matches the comparison in step 3.
The reverse reaction, 2H₂O → 2H₂ + O₂, breaks the 4 O–H bonds (1856) and makes the H–H and O=O bonds (1370). Its energy change is 1856 − 1370 = +486 kJ/mol. Same numbers, opposite sign.
The mistake to watch for
The typical slip is to put a product bond in the “broken” column.
Mistaken answer: “Broken: 2 × 436 + 498 + 4 × 464 = 3226; made: nothing.”
The student listed every bond in the equation as broken and did not separate reactants from products.
The correction is that only reactants have bonds broken and only products have bonds made. Using two columns prevents this.
Check yourself
Use these supplied values for practice: H–H 436, Cl–Cl 242, H–Cl 431.
1. For H₂ + Cl₂ → 2HCl, how many of each bond are broken and made?
Show answer
Broken: 1 H–H and 1 Cl–Cl. Made: 2 H–Cl. The totals are 436 + 242 = 678 broken and 2 × 431 = 862 made.
2. Is the reaction in question 1 overall exothermic or endothermic, and why?
Show answer
The energy released in making bonds (862) is greater than the energy absorbed in breaking bonds (678), so it is exothermic. 678 − 862 = −184 kJ/mol.
3. Complete the sentence: “Breaking bonds ______ energy and making bonds ______ energy.”
Show answer
Breaking bonds absorbs (needs) energy and making bonds releases energy.
Where this leads next
Now gather everything in the energy changes and bonds practice set. For the wider picture of how reactions proceed, look at the rates of reaction module.
If you can calculate but struggle to explain, a teacher in online one-to-one Chemistry tuition can coach your written explanations until each clause says what you mean.