Catalysis — Concepts, Formulas & Examples

A catalyst increases the rate of a reaction without being consumed. CBSE Class 12 covers catalysis in the surface chemistry chapter; NEET tests types and examples. Industrial chemistry depends on catalysis for efficiency.

Core Concepts

What catalysts do

Provide an alternative reaction pathway with lower activation energy. They do not change the equilibrium — just the rate of reaching it. They are recovered at the end of the reaction.

This is a crucial distinction. A catalyst speeds up both forward and reverse reactions equally. It does not shift equilibrium to produce more product — it just gets you to equilibrium faster. The $\Delta G$ of the reaction is unchanged; only the activation energy ( $E_a$ ) is lowered.

k = A \cdot e^{-E_a/RT}

A catalyst lowers $E_a$ , which increases $k$ (rate constant) exponentially. Even a small reduction in $E_a$ can increase the rate dramatically. For example, reducing $E_a$ by 10 kJ/mol roughly doubles the rate at room temperature.

Types of catalysis

Homogeneous — catalyst and reactants in the same phase (acid catalysis in esterification). Heterogeneous — different phases (Fe in Haber process, Pt in catalytic converter). Biological — enzymes.

Homogeneous catalysis examples:

Acid-catalysed esterification: $\text{RCOOH} + \text{R'OH} \xrightarrow{\text{H}^+} \text{RCOOR'} + \text{H}_2\text{O}$
Lead chamber process: SO2 oxidation catalysed by NO (all in gas phase)
Enzyme catalysis in solution

Heterogeneous catalysis — the adsorption model:

Weak chemical bonds form between reactant molecules and active sites on the catalyst surface. This is chemisorption — stronger than physical adsorption.

Adsorption weakens the bonds within reactant molecules (e.g., H-H bond in H2 weakens on Pt surface). The weakened molecules react more easily with each other.

The product molecules have less affinity for the surface and leave, freeing the active sites for fresh reactant molecules.

Industrial examples

Haber process (NH3 synthesis) — Fe catalyst. Contact process (H2SO4) — V2O5 catalyst. Ostwald process (HNO3) — Pt catalyst. Catalytic hydrogenation — Ni catalyst for margarine.

Process	Reaction	Catalyst	Conditions
Haber	$\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$	Fe with Mo promoter	450°C, 200 atm
Contact	$2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3$	V2O5	450°C, 1-2 atm
Ostwald	$4\text{NH}_3 + 5\text{O}_2 \to 4\text{NO} + 6\text{H}_2\text{O}$	Pt-Rh gauze	800°C
Hydrogenation	Oil + H2 → Fat (margarine)	Ni (finely divided)	200°C, high pressure
Catalytic converter	CO, NOx, hydrocarbons → CO2, N2, H2O	Pt, Pd, Rh	Exhaust temperature

Enzymes

Biological catalysts, highly specific. Substrate binds active site, reaction happens, product leaves. Active at specific pH and temperature. Examples — amylase, pepsin, trypsin.

Why enzymes are extraordinary catalysts:

They increase reaction rates by factors of $10^6$ to $10^{12}$ — far more than any industrial catalyst
They are highly specific (lock-and-key model or induced fit model)
They work at body temperature and neutral pH — mild conditions
They can be regulated (inhibitors, activators, allosteric regulation)

Enzyme kinetics — Michaelis-Menten:

v = \frac{V_{max}[S]}{K_m + [S]}

where $v$ = reaction rate, $V_{max}$ = maximum rate (all enzyme saturated), $[S]$ = substrate concentration, $K_m$ = Michaelis constant (substrate concentration at half $V_{max}$ ).

A low $K_m$ means the enzyme has high affinity for the substrate. At high $[S]$ , rate plateaus at $V_{max}$ because all active sites are occupied.

Poisoning and promoters

A poison reduces catalytic activity by binding the active site. A promoter increases activity without being a catalyst itself. Mo is a promoter in the Haber process.

Examples of catalyst poisoning:

CO poisons Pt in fuel cells (binds strongly to Pt surface, blocking H2 adsorption)
As2O3 poisons Pt in the Contact process
H2S poisons Fe in the Haber process

Catalyst poisoning is a favourite NEET question. The key concept: a poison occupies the active sites permanently, preventing reactant molecules from adsorbing. The catalyst’s activity drops even though the catalyst is chemically intact.

Shape-selective catalysis

Zeolites are microporous aluminosilicates with defined pore sizes. Only molecules that fit into the pores can reach the active sites and react. This selectivity is called shape-selective catalysis. ZSM-5 zeolite converts methanol to gasoline — a remarkable industrial application.

Worked Examples

Platinum adsorbs CO, NOx and unburnt hydrocarbons from car exhaust and catalyses their conversion to CO2, N2 and H2O. Rhodium and palladium are often used alongside.

Enzymes lower activation energy enormously, making reactions feasible at 37°C that would otherwise need high temperatures. This is how life operates at mild conditions.

Finely divided iron has a much larger surface area per gram. Since heterogeneous catalysis happens at the surface, more surface area means more active sites and faster reaction. This is why catalysts are often used as fine powders or deposited on high-surface-area supports.

A competitive inhibitor resembles the substrate and competes for the active site — it can be overcome by increasing substrate concentration ( $V_{max}$ unchanged, $K_m$ increases). A non-competitive inhibitor binds elsewhere on the enzyme and changes the active site’s shape �� it cannot be overcome by more substrate ( $V_{max}$ decreases, $K_m$ unchanged).

Common Mistakes

Saying a catalyst shifts equilibrium. It does not — only rates change.

Confusing promoter and catalyst. Promoter enhances a catalyst; it is not itself catalytic.

Writing that catalysts are consumed. They are regenerated at the end of the reaction.

Saying all catalysts are solids. Catalysts can be gases (NO in lead chamber process), liquids (H2SO4 in esterification), or dissolved ions (Co2+ in decomposition of KClO3).

Confusing homogeneous and heterogeneous catalysis. The defining criterion is phase: if catalyst and reactants are in the same phase, it is homogeneous; if in different phases, heterogeneous.

Exam Weightage and Revision

Catalysis carries 1-2 NEET questions per year (often as part of surface chemistry). CBSE Class 12 boards allocate 3-5 marks. Questions are mostly factual — match the catalyst to the process, identify the type, or explain why activity decreases.

Question Type	NEET Frequency	Example
Catalyst-process matching	Every year	Name the catalyst in the Haber process
Homogeneous vs heterogeneous	Most years	Classify these examples
Enzyme specificity	Every 2 years	What is the lock-and-key model?
Catalyst poisoning	Occasional	Why does CO poison Pt?
Shape-selective catalysis	Rare	What is ZSM-5?

Memorise five industrial processes with their catalysts. This single table covers most NEET questions on catalysis.

Practice Questions

Q1. What is the difference between a catalyst and a promoter? Give one example of each.

A catalyst provides an alternative pathway with lower activation energy and is regenerated at the end. Example: Fe in the Haber process. A promoter enhances the activity of a catalyst but cannot catalyse the reaction alone. Example: Mo (molybdenum) in the Haber process increases the activity of Fe catalyst.

Q2. Why does enzyme activity decrease at high temperatures?

Enzymes are proteins. Above the optimum temperature (typically 37-40°C for human enzymes), the protein denatures — its three-dimensional structure unfolds, destroying the shape of the active site. Without the correct active site shape, the substrate cannot bind and the enzyme loses activity. This is irreversible at very high temperatures.

Q3. In the Contact process, V2O5 is used instead of Pt. Why?

Platinum was the original catalyst but it is extremely expensive and easily poisoned by impurities (like As2O3) in the SO2 feed gas. V2O5 is much cheaper, more resistant to poisoning, and gives comparable conversion rates. The economic advantage is decisive for an industrial process that runs continuously.

Q4. What is shape-selective catalysis? Give an example.

Shape-selective catalysis occurs when only molecules of a certain size and shape can enter the catalyst’s pores and react at the active sites. Zeolites (like ZSM-5) have uniform pore sizes that act as molecular sieves. Example: ZSM-5 catalyses the conversion of methanol to gasoline — only molecules small enough to fit the pores are formed as products.

FAQs

Can a catalyst make an impossible reaction happen?

No. A catalyst cannot make a thermodynamically unfavourable reaction ( $\Delta G > 0$ ) proceed. It can only speed up reactions that are already thermodynamically possible but kinetically slow. The catalyst removes the kinetic barrier, not the thermodynamic one.

Why are platinum group metals such good catalysts?

Platinum, palladium, and rhodium have partially filled d-orbitals that can form temporary bonds with reactant molecules (chemisorption). The bonds are strong enough to hold and weaken the reactants but not so strong that products cannot leave. This “Goldilocks” binding strength is what makes them excellent catalysts.

How do catalytic converters clean car exhaust?

The converter contains Pt, Pd, and Rh on a honeycomb ceramic structure. Oxidation catalyst (Pt/Pd) converts CO → CO2 and unburnt hydrocarbons → CO2 + H2O. Reduction catalyst (Rh) converts NOx → N2 + O2. Three pollutants in, three harmless gases out.

Memorise one industrial process with its catalyst for each type. Four rows total.

Colloidal Properties and Catalysis

The connection between surface chemistry and catalysis is direct: heterogeneous catalysis happens at surfaces, and colloids have enormous surface area per unit mass.

Adsorption types:

Physisorption: Weak van der Waals forces. Low heat of adsorption (20-40 kJ/mol). Reversible. Multiple layers can form. Example: gas molecules adsorbing on charcoal.
Chemisorption: Chemical bond formation. High heat of adsorption (80-240 kJ/mol). Irreversible. Only one layer forms. Example: H2 on Pt surface.

Heterogeneous catalysis works by chemisorption: reactant molecules form chemical bonds with the catalyst surface, weakening their internal bonds and making them more reactive. The catalytic cycle is: adsorb → react → desorb.

Why finely divided catalysts work better: A 1 cm cube of iron has a surface area of 6 cm2. Breaking it into 1-micrometre particles increases the total surface area to about 60,000 cm2 — a 10,000-fold increase. More surface area means more active sites for chemisorption and faster reaction rates. This is why industrial catalysts are often nanoparticles deposited on porous supports.

Selectivity — The Secret Power of Catalysts

A good catalyst does not just speed up reactions — it speeds up specific reactions preferentially. This selectivity is what makes catalysts invaluable:

Hydrogenation: Lindlar’s catalyst (Pd on CaCO3, poisoned with Pb) reduces alkynes to cis-alkenes only — it stops at the double bond. Regular Pd/C would reduce all the way to the alkane.
Oxidation: PCC oxidises primary alcohols to aldehydes only. KMnO4 goes all the way to carboxylic acids. Same reaction type, different selectivity.
Enzyme specificity: Hexokinase phosphorylates glucose but not fructose (even though they differ by only one OH group position). This lock-and-key specificity is unmatched by any synthetic catalyst.

Catalysis in Green Chemistry

Catalysis is one of the twelve principles of green chemistry because:

Catalytic reactions need less energy (lower activation energy = lower temperature)
Better selectivity means fewer byproducts (less waste)
Catalysts are reused (not consumed like stoichiometric reagents)

Example: the traditional stoichiometric oxidation of an alcohol to a ketone uses CrO3 (toxic Cr waste). A catalytic version uses O2 with a Pd catalyst — the only byproduct is water.

Catalysis is why modern chemistry is feasible. Without catalysts, fertiliser production would not scale and cars would be far more polluting.