Reversible and irreversible reactions

Can the products turn back?

Some reactions run one way until a reactant is used up. Mix sodium chloride and silver nitrate solutions and silver chloride forms at once; with exact amounts, no reactant is left. This is an irreversible reaction: forward only.

In most reactions, though, the products react with each other to give back the reactants under the same conditions. Such a reaction runs both ways and is written with the double half-arrow ⇌. It is a reversible reaction.

Nitrogen and hydrogen in a closed container at 400 °C and 200 atm with iron make ammonia, but ammonia also breaks back down. The reaction never goes to completion: nitrogen, hydrogen and ammonia are all present at every moment.

A reversible reaction can be forced to completion by removing a product. Heat calcium carbonate in an open container and the carbon dioxide escapes, so all of it turns into calcium oxide. In a closed container the CO₂ stays and reacts with CaO to give back calcium carbonate.

Open or closed matters: the same CaCO₃ reaction finishes in an open tube and stops short in a sealed one.

Key terms

Irreversible reaction

Reactions that use up a reactant.

Reversible reaction

Reactions that never finish in a closed vessel.

⇌

Marking a reversible reaction.

Closed system

Letting products react back.

Short questions with model answers

1. Q1. CaCO₃ is heated (a) in an open container, (b) in a closed container. What is in each at the end? (p. 84)

   • CaCO₃ ⇌ CaO + CO₂
   • (a) CO₂ escapes
   • (b) CO₂ stays

   (a) Only CaO: it goes to completion. (b) CaCO₃, CaO and CO₂ together.

2. Q2. In the Haber process, how does liquefying ammonia help? (p. 88)

   • N₂ + 3H₂ ⇌ 2NH₃
   • Liquid NH₃ is taken out

   It removes the product, driving the reaction towards completion.

Common mistakes

• ✗ A reversible reaction in a closed flask finishes if you wait long enough.

  ✓ It never goes to completion; all species stay present (p. 84).

• ✗ Open or closed container makes no difference.

  ✓ If a gas product escapes, the reaction can go to completion.

• ✗ A reversible reaction is written with a single arrow →.

  ✓ It uses ⇌, two half-arrowheads pointing both ways.

MCQs

1. 1. NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq) is:

   1. (a) Irreversible
   2. (b) Reversible
   3. (c) At equilibrium
   4. (d) Endothermic
   Show answer

   (a) p. 83: it goes to completion, forward only.

2. 2. To produce the most CaO from CaCO₃, heat it:

   1. (a) In a closed vessel
   2. (b) In an open vessel
   3. (c) Without heating
   4. (d) With extra CO₂
   Show answer

   (b) CO₂ escapes, so it goes to completion (p. 84; MCQ iv, p. 90).

3. 3. The symbol for a reversible reaction is:

   1. (a) →
   2. (b) ⇌
   3. (c) =
   4. (d) ↑
   Show answer

   (b) p. 83.

4. 4. In N₂ + 3H₂ ⇌ 2NH₃ in a closed container, which are present at any time?

   1. (a) Only NH₃
   2. (b) Only N₂ and H₂
   3. (c) N₂, H₂ and NH₃
   4. (d) Nothing
   Show answer

   (c) p. 84: all three species are present.

5. 5. A reversible reaction can be made to go to completion by:

   1. (a) Sealing the container
   2. (b) Withdrawing a product
   3. (c) Adding more product
   4. (d) Nothing can do it
   Show answer

   (b) Key point 2.

Quick revision

• Irreversible reactions go forward only, to completion.
• Reversible reactions (⇌) run both ways and never complete in a closed vessel.
• Removing a product (gas escaping, liquid taken out) forces completion.