Catalysts lower the activation energy

A lower pass through the hills

A catalyst gives the reactants a different path, one with a much lower activation energy. With a lower hill, many more molecules have enough energy to get over it, so more products form each second: the rate increases.

The book's definition: a catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.

In Fig 5.6 the reactants and products stay at the same energies; only the peak is lower. So the catalyst changes Ea, not the start or finish.

Examples from the book: nickel in turning oil into banaspati ghee, platinum in making sulphuric acid, and chlorine, which speeds up the breakdown of ozone. In our cells, enzymes are the catalysts, for example in glycolysis.

A catalyst is like a tunnel through a mountain: same start, same destination, less climbing.

Key terms

Catalyst

Faster industrial and biological reactions.

Enzyme

Reactions in living cells.

Reaction pathway

What a catalyst changes.

Rate of reaction

What a lower Ea raises.

Short questions with model answers

1. Q1. Without a catalyst a reaction has Ea = 120 kJ; with one, 70 kJ. ΔH is −50 kJ. What does the catalyst change? (idea of Fig 5.6, p. 77)

   • 120 − 70 = 50 kJ
   • ΔH = −50 kJ in both
   • More molecules get over: rate ↑

   Only Ea (and so the rate) changes; ΔH stays −50 kJ.

2. Q2. Nickel is added to vegetable oil and hydrogen to make banaspati ghee. After the reaction, what has happened to the nickel? (p. 77)

   • Ni is the catalyst
   • No permanent chemical change

   It is still nickel: it sped up the reaction without being used up.

Common mistakes

• ✗ A catalyst makes a reaction give out more heat.

  ✓ Reactants and products keep their energies, so ΔH is the same (Fig 5.6).

• ✗ The catalyst is used up like a reactant.

  ✓ It undergoes no permanent chemical change (p. 77).

• ✗ A catalyst works by giving the molecules extra energy.

  ✓ It changes the path so less energy is needed.

MCQs

1. 1. A catalyst increases the rate of reaction by:

   1. (a) Decreasing the activation energy
   2. (b) Increasing ΔH
   3. (c) Raising the products' energy
   4. (d) Being used up
   Show answer

   (a) Key point 11.

2. 2. Which catalyst is used to make banaspati ghee?

   1. (a) Pt
   2. (b) Ni
   3. (c) Cl
   4. (d) Fe
   Show answer

   (b) p. 77: Ni in the hydrogenation of oil.

3. 3. Platinum acts as a catalyst in the production of:

   1. (a) H₂SO₄
   2. (b) NaCl
   3. (c) Banaspati ghee
   4. (d) Ozone
   Show answer

   (a) p. 77.

4. 4. Adding a catalyst to an exothermic reaction makes ΔH:

   1. (a) More negative
   2. (b) Positive
   3. (c) Unchanged
   4. (d) Zero
   Show answer

   (c) Fig 5.6: reactants and products stay at the same level.

5. 5. Biological catalysts in our cells are called:

   1. (a) Lipids
   2. (b) Enzymes
   3. (c) Glycogen
   4. (d) Pyruvate
   Show answer

   (b) p. 77: glycolysis is catalysed by enzymes.

Quick revision

• A catalyst speeds up a reaction without any permanent chemical change to itself.
• It opens a new path with lower activation energy; ΔH stays the same.
• Examples: Ni (ghee), Pt (H₂SO₄), Cl (ozone breakdown), enzymes in cells.