A reversible reaction is written with the symbol ⇌, which shows that the forward reaction and the reverse reaction can both happen. You meet this at the start of reversible changes and equilibrium, and every later lesson relies on writing the equation correctly first.
What makes a reaction reversible?
Most reactions you meet early are treated as one-way: magnesium burns, and you do not expect the magnesium oxide to turn back into magnesium. In a reversible reaction the products can react together to re-form the reactants under the same conditions.
Three standard school examples:
| Reversible change | Equation |
|---|---|
| Hydrated copper(II) sulfate and water | CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l) |
| Dinitrogen tetroxide and nitrogen dioxide | N₂O₄(g) ⇌ 2NO₂(g) |
| Hydrogen and iodine forming hydrogen iodide | H₂(g) + I₂(g) ⇌ 2HI(g) |
Another is ammonium chloride breaking into ammonia and hydrogen chloride: NH₄Cl(s) ⇌ NH₃(g) + HCl(g). Some of these are shown in teacher demonstrations. Treat them as things to understand from the equation, not to try yourself.
How do you write one, step by step?
- Write the formulas of all substances, from the names or the description.
- Balance the atoms using coefficients only. Count each element on both sides.
- Join the two sides with ⇌, not →.
- Add state symbols if the question provides them.
- Keep the energy information separate. If the forward reaction is endothermic, the reverse is exothermic, and the equation is the same either way.
Worked example
Nitrogen gas and hydrogen gas react reversibly to form ammonia gas. Write the balanced equation.
Step 1, formulas: nitrogen is N₂, hydrogen is H₂, ammonia is NH₃.
Step 2, unbalanced skeleton: N₂ + H₂ ⇌ NH₃
Step 3, count: left has 2 N and 2 H; right has 1 N and 3 H. To get 2 N on the right, use 2NH₃. Now the right has 2 N and 6 H. To get 6 H on the left, use 3H₂.
Step 4, check: left: N₂ gives 2 N, 3H₂ gives 6 H. Right: 2NH₃ gives 2 N and 6 H. Balanced.
Answer: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Notice the gas counts: 1 + 3 = 4 gas molecules on the left and 2 on the right. That number becomes important in predicting a shift under a stated change.
The mistake to watch for
A typical slip is to balance by changing a formula instead of a coefficient.
Mistaken answer: N₂O₄ ⇌ NO₂, then “fixed” by rewriting NO₂ as N₂O₄.
Changing the subscript makes a different substance, so the equation no longer describes the reaction.
The correction: the formulas stay as they are, and you put a coefficient in front. N₂O₄ has 2 N and 4 O, so the right needs 2NO₂. The balanced equation is N₂O₄(g) ⇌ 2NO₂(g).
Check yourself
1. Write the balanced reversible equation for hydrogen and iodine forming hydrogen iodide.
Show answer
Skeleton: H₂ + I₂ ⇌ HI. Left has 2 H and 2 I; right has 1 H and 1 I. Use 2HI.
H₂(g) + I₂(g) ⇌ 2HI(g)
2. A student writes CuSO₄·5H₂O ⇌ CuSO₄ + H₂O. Is it balanced? Correct it if not.
Show answer
Not balanced. The left has 5 water molecules in the crystal, the right only 1. Use 5H₂O.
CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O
Check oxygen: 4 + 5 = 9 on the left; 4 + 5 = 9 on the right. Hydrogen: 10 on each side.
3. The forward reaction in N₂O₄ ⇌ 2NO₂ is endothermic. What can you say about the reverse reaction?
Show answer
The reverse reaction is exothermic. The same amount of energy taken in by the forward reaction is given out by the reverse reaction.
Where this leads next
Once the equation is reliable, move on to explaining a dynamic state. The equation balance reasoning trainer and mole and equation-ratio tutor let you check your atom counts.
If equations are the point where your chemistry starts to slip, that is the kind of pattern a teacher in our online one-to-one Chemistry tuition looks for.