The Catalyst
Kinetics, Never Thermodynamics

Effect of a catalyst

Mechanism of the rate increase

A catalyst alters the pathway, not the molecules' energies.

k = A e^(−Ea/RT) — a smaller Ea gives a larger k

1. A catalyst forms temporary bonds with reactants, creating an alternative route.
2. That route has a lower energy barrier than the uncatalysed one.
3. A lower Ea makes the exponential factor larger, so k and the rate increase.
4. It does not change molecular kinetic energy, which is set by temperature — that would be option (b), describing heating instead.

Effects of a catalyst

The unchanged quantity

A catalyst affects kinetics but never thermodynamics.

ΔG = −RT ln K, and ΔG is unchanged

1. A catalyst lowers Ea and therefore raises k, so (b) and (c) change.
2. It accelerates the backward reaction as well, so (d) changes.
3. NCERT states that a catalyst does not alter ΔG, and K depends only on ΔG.
4. So the equilibrium constant is unchanged.

Scope of catalytic action

Which reactions a catalyst can catalyse

A catalyst cannot change ΔG, so it cannot make an unfavourable reaction happen.

ΔG unchanged by a catalyst

1. Spontaneity is determined by ΔG, which the catalyst does not alter.
2. A reaction with positive ΔG will not proceed regardless of catalysis.
3. So a catalyst can only accelerate reactions that were already spontaneous.
4. NCERT states this directly: it catalyses spontaneous reactions but does not catalyse non-spontaneous reactions.

A substance that slows a reaction

Its name

NCERT reserves the word catalyst for rate-increasing substances only.

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1. NCERT states that the word catalyst should not be used when the added substance reduces the rate.
2. Such a substance is then called an inhibitor.
3. Some texts say 'negative catalyst', but NCERT's terminology is what exam options follow.

Intermediate complex theory

How the catalyst acts

The catalyst participates chemically, then is released.

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1. The catalyst forms temporary bonds with the reactants.
2. This produces an intermediate complex with only a transitory existence.
3. The complex then decomposes to yield the products and the catalyst.
4. Because the catalyst is returned, a small amount can process a large quantity of reactant.

A reversible reaction with a catalyst

Effect on both directions

The same lowered barrier serves both directions equally.

ΔH = Ea(f) − Ea(b) is unchanged, so both barriers drop equally

1. The catalyst creates one new, lower peak that both directions must cross.
2. Since ΔH cannot change and ΔH equals the difference of the two barriers, both must fall by the same amount.
3. Equal reductions mean both rate constants rise by the same factor.
4. So equilibrium arrives sooner but its position is unchanged — NCERT's exact wording is 'to the same extent'.

2KClO₃ →(MnO₂) 2KCl + 3O₂

Role of MnO₂

MnO₂ increases the rate and is recovered unchanged.

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1. MnO₂ appears above the arrow, not among the reactants or products.
2. It increases the rate of the decomposition considerably.
3. It undergoes no permanent chemical change and is recovered afterwards.
4. That fits the definition of a catalyst exactly. NCERT uses this as its standard illustration.

Four statements about catalysts

The incorrect one

Enthalpy depends only on reactants and products, which the catalyst does not touch.

ΔH = H(products) − H(reactants)

1. Statements (b), (c) and (d) are all standard textbook facts.
2. ΔH depends only on the energies of reactants and products.
3. A catalyst changes the route between them, not their energy levels.
4. So statement (a) is incorrect.

Comparison of catalyst and temperature increase

The distinguishing effect

Both increase k, but only temperature shifts the equilibrium constant.

k = A e^(−Ea/RT) ; K depends on temperature

1. Both a catalyst and a temperature rise increase k and hence the rate, so (c) is wrong.
2. Only a catalyst lowers Ea; heating leaves the barrier unchanged, so (b) is wrong.
3. The equilibrium constant is temperature-dependent but catalyst-independent.
4. So raising the temperature changes K while a catalyst does not.

Ea(f) catalysed = 60 kJ mol⁻¹, ΔH = −30 kJ mol⁻¹

Catalysed Ea(backward)

ΔH is unchanged by the catalyst, so it still links the two barriers.

ΔH = Ea(f) − Ea(b) ⟹ Ea(b) = Ea(f) − ΔH

1. Ea(b) = 60 − (−30) = 90 kJ mol⁻¹.
2. Check the uncatalysed case: Ea(b) was 100 − (−30) = 130 kJ mol⁻¹.
3. So the backward barrier fell from 130 to 90 — a drop of 40 kJ mol⁻¹.
4. The forward barrier fell from 100 to 60 — also 40 kJ mol⁻¹. Both fell equally, exactly as required for ΔH to stay fixed.

Four pairs of quantities

The pair unaffected by a catalyst

Thermodynamic quantities are untouched; kinetic ones are not.

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1. Ea, k and rate are all kinetic quantities and all change under catalysis.
2. ΔH and K are thermodynamic quantities and are both unchanged.
3. Option (d) is wrong because it pairs Ea, which does change, with ΔH, which does not.
4. So only option (a) contains two quantities that are both unaffected.

Effect of a catalyst on k

The mechanism

The Arrhenius equation links a lower barrier directly to a larger rate constant.

k = A e^(−Ea/RT)

1. The catalyst lowers Ea.
2. A smaller Ea makes the exponent −Ea/RT less negative, so the exponential factor is larger.
3. With A unchanged, a larger exponential factor means a larger k.
4. The catalyst does not change A, temperature or concentration, ruling out the other options.

Step 1: H₂O₂ + I⁻ → H₂O + IO⁻ ; Step 2: H₂O₂ + IO⁻ → H₂O + I⁻ + O₂

Identity of the catalyst

A catalyst is consumed first and regenerated later; an intermediate is the reverse.

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1. I⁻ is present at the start, is used up in step 1 and reappears in step 2.
2. That pattern — consumed then regenerated — defines a catalyst.
3. IO⁻ is formed then consumed, which makes it the intermediate.
4. H₂O and O₂ are products.

Ea falls from 80 to 50 kJ mol⁻¹ at T = 300 K

Factor by which k increases

The ratio of rate constants depends exponentially on the difference in barriers.

k(cat)/k = e^((Ea − Ea(cat))/RT)

1. ΔEa = 80 − 50 = 30 kJ mol⁻¹ = 30000 J mol⁻¹.
2. Work in base 10: log(ratio) = 30000/(19.15 × 300) = 30000/5745 = 5.22.
3. Ratio = 10^5.22 ≈ 1.7 × 10⁵, of the order of 2 × 10⁵.
4. A modest-looking 30 kJ reduction produces a rate increase of about a hundred thousand times — which is why catalysis matters industrially.

A catalyst recovered unchanged

The reason

The catalyst does participate — it is simply returned at the end.

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1. The catalyst forms temporary bonds with the reactants, so it certainly takes part.
2. The resulting intermediate complex is transitory.
3. When it decomposes it releases both the products and the catalyst.
4. So the catalyst is chemically involved yet emerges unchanged — this is why option (b) is a common but incorrect answer.

Four candidate quantities

What a catalyst changes

Only the kinetic quantity in the list is affected.

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1. Equilibrium concentrations, ΔG and product enthalpy are all thermodynamic and unchanged.
2. By accelerating both directions, the catalyst allows equilibrium to be attained sooner.
3. So the time required to reach equilibrium is reduced.
4. NCERT phrases this as helping in attaining the equilibrium faster.

A catalysed reversible reaction

Effect on the backward activation energy

ΔH is fixed, so the two barriers must fall together.

ΔH = Ea(f) − Ea(b), and ΔH is unchanged

1. The catalyst creates a single new peak that both directions cross.
2. ΔH equals the difference between the two barriers and cannot change.
3. For the difference to remain fixed while the peak drops, both barriers must fall by exactly the same amount.
4. This is the mathematical reason behind NCERT's phrase 'to the same extent'.

Comparison of enzymes and MnO₂

The shared property

All catalysts share the same defining behaviour regardless of their chemical nature.

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1. Every catalyst, biological or inorganic, provides an alternative lower-energy pathway.
2. Every catalyst emerges without permanent chemical change.
3. Neither type alters ΔH or the equilibrium position, so (b) and (c) are wrong.
4. Neither is consumed stoichiometrically, which is why small amounts suffice.

ΔG = +25 kJ mol⁻¹, catalyst added

Effect on spontaneity

A catalyst is powerless against thermodynamics.

ΔG unchanged by a catalyst

1. A positive ΔG means the reaction is non-spontaneous in the forward direction.
2. A catalyst does not alter ΔG at all.
3. So the reaction remains non-spontaneous however good the catalyst.
4. NCERT states that a catalyst does not catalyse non-spontaneous reactions.

Catalyst classification (gap content)

The distinguishing feature

The classification is by phase, not by effectiveness.

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1. A homogeneous catalyst is in the same phase as the reactants, such as I⁻ in aqueous H₂O₂.
2. A heterogeneous catalyst is in a different phase, such as solid platinum with gaseous ammonia.
3. Both types lower Ea and neither changes ΔH.
4. Effectiveness varies case by case and is not part of the definition.

Overall role of a catalyst

The best summary

This single sentence resolves nearly every catalyst question.

Kinetics: Ea, k, rate — all change. Thermodynamics: ΔH, ΔG, K — all fixed.

1. A catalyst lowers Ea, raises k and increases the rate — all kinetic effects.
2. It leaves ΔH, ΔG, K and the equilibrium position untouched — all thermodynamic quantities.
3. So it changes kinetics but never thermodynamics.
4. Holding this one sentence lets you answer most catalyst questions without further reasoning.