Chemistry 2e · Kinetics

Catalysis

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On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Catalysis uses a substance, a , to increase a reaction’s rate by providing a different pathway with lower . More collisions can then lead to reaction at the same temperature. The catalyst participates in elementary steps but is regenerated overall, rather than permanently consumed.

Catalysis changes kinetics, not the starting or ending energy levels. It can make equilibrium arrive sooner, but it does not change the equilibrium constant, reaction enthalpy, or equilibrium composition at a fixed temperature.

Why this matters

Catalysts make chemical manufacture faster, more selective, and often less energy-intensive. Catalytic converters limit harmful vehicle emissions, enzymes coordinate cell chemistry, and industrial catalysts help transform nitrogen and hydrogen into ammonia. A catalyst can also favor one product when multiple products are possible.

For kinetics questions, identify the altered elementary steps and the new activation-energy barrier. This connects catalysis to collision theory, mechanisms, and energy diagrams.

The college version

Core Concepts

Catalysts and activation energy

The activation energy E_a is the barrier reactant particles must overcome on a pathway to products. Lowering E_a lets more particles have effective collisions and increases the rate constant. A catalyst does not make every collision successful; it supplies a more accessible route.

On a reaction-coordinate diagram, catalyzed and uncatalyzed routes have the same reactant and product heights, hence the same overall energy change. The catalyzed route has a lower highest peak and may have several smaller peaks from multiple mechanism steps.

Homogeneous, heterogeneous, and enzymatic catalysis

In , catalyst and reactants share a phase, often solution. The catalyst may form an intermediate and be released later. In acid catalysis, temporary proton transfer can make a bond-breaking or bond-forming step easier.

In , they are in different phases, commonly a solid with gases or liquids. Reactants adsorb at surface active sites, where bonds can weaken and molecules are held in useful orientations; products desorb. Finely divided solids expose more active sites.

Enzymes are usually protein catalysts. Their active sites bind selected substrates, position them, and stabilize the transition state. activity depends on temperature, pH, and substrate concentration; extreme conditions can disrupt protein structure.

Catalysts do not rewrite equilibrium

For a reversible reaction, a catalyst lowers barriers for both directions. Dynamic equilibrium is reached faster, but neither equilibrium state becomes more stable. It changes speed of approach, not the thermodynamically determined equilibrium ratio.

How It Works / Step-by-Step Process

Consider alkene hydrogenation using nickel, platinum, or palladium. The net reaction adds H₂ across a carbon-carbon double bond to form an alkane.

Alkene and H₂ molecules contact the metal surface and adsorb at available active sites.
Interaction with the metal weakens the H-H bond, allowing hydrogen atoms to be held separately on the surface.
The adsorbed alkene is positioned near the hydrogen atoms, which transfer by lower-energy surface steps.
The alkane product desorbs from the metal, leaving the active sites available for more reactant molecules.

The metal is absent from the net equation because it is regenerated. Impurities can block active sites; this catalyst poisoning lowers activity.

Common Confusions

Common Confusion	Correct Understanding
A catalyst is consumed because it appears in a mechanism.	It can be consumed in an early step and regenerated in a later step; cancel it when obtaining the net reaction.
A catalyst makes a reaction more product-favored.	It changes rate, not ΔG^°, K, or the equilibrium composition at fixed temperature.
Catalysis means adding energy to reactants.	A catalyst supplies an alternative mechanism with a lower activation barrier.
A solid catalyst works indefinitely.	Sites can be blocked, poisoned, or saturated.
Enzymes work faster at any higher temperature.	Temperature can increase rate only until structural disruption or denaturation reduces enzyme activity.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of a chemical reaction as climbing over a hill. A catalyst builds a lower route over the same hill, so more particles can get across each second. It does not move the starting or ending places, and it is ready to help again afterward.

Worked example

Worked example 1: how much faster is the catalyzed pathway?

Use the Arrhenius equation k = Ae^-E_a/RT to compare the rate constants of catalyzed and uncatalyzed routes at the same temperature. Take the uncatalyzed activation energy as E_a,u = 80 kJ/mol, the catalyzed value as E_a,c = 50 kJ/mol, R = 8.314 J mol^-1K^-1, T = 298 K, and assume the frequency factor A is the same for both pathways. The ratio of rate constants is

$k c k u = Ae -E a,c /RT Ae -E a,u /RT = e (E a,u - E a,c)/RT$

Substitute the energies in joules so the units cancel with RT:

$k c k u = e (80, 000 - 50, 000)/((8.314)(298)) = e 30, 000/2477 = e 12.11 \approx 1.8 \times 105$

At 298 K the catalyzed rate constant is roughly 180,000 times larger. This is why a modest lowering of the barrier — not added energy — can turn an impractically slow reaction into a practical one.

Worked example 2: the energy diagram stays anchored

Draw the reaction-coordinate diagram twice, once per pathway. In both drawings, the reactant and product energy levels are identical; only the highest peak differs (80 kJ/mol uncatalyzed, 50 kJ/mol catalyzed). The reaction does not release less heat: ΔH is unchanged. In a reversible reaction, the reverse barrier is also lowered, so equilibrium arrives earlier without moving.

Key takeaways

A catalyst increases reaction rate by offering a pathway with lower activation energy.
It is regenerated overall, even though it may be used in individual mechanism steps.
Catalyzed and uncatalyzed reactions have the same reactants, products, and overall ΔH.
A catalyst speeds both forward and reverse reactions and does not change K at a fixed temperature.
Heterogeneous catalysts work at active surface sites; more accessible surface can mean a faster reaction.
Enzyme activity can fall when temperature or pH disrupts an active site.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

What feature of a reaction-coordinate diagram changes when a catalyst is added?
Show answer
The catalyzed pathway has a lower highest activation-energy barrier; reactant and product energies remain the same.
Why does a catalyst not change the equilibrium constant at fixed temperature?
Show answer
It speeds both forward and reverse processes by lowering their barriers, without changing the thermodynamic energy difference between reactants and products.
What happens at an in heterogeneous catalysis?
Show answer
Reactants adsorb, are oriented or activated at the surface, react through surface steps, and products desorb.
In the alkene hydrogenation walkthrough, why is the metal absent from the net equation?
Show answer
It is regenerated after helping the elementary steps, so it cancels from the overall reaction.
How can catalyst poisoning slow a reaction?
Show answer
Poison molecules occupy or alter active sites, reducing the number of places where reactants can adsorb and react.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

catalyst: Substance that increases reaction rate and is regenerated by the overall reaction.
activation energy: Energy barrier that must be overcome along a particular reaction pathway.
homogeneous catalysis: Catalysis in which catalyst and reactants occupy the same phase.
heterogeneous catalysis: Catalysis in which catalyst and reactants are in different phases, commonly a solid catalyst with gaseous or liquid reactants.
active site: Specific surface location or enzyme region where reactants bind and reaction is facilitated.
adsorption: Attachment of particles to a surface; distinct from absorption into the bulk of a material.
enzyme: Biological catalyst, typically a protein, that binds substrates at an active site.

Sources & references

openstax.org — Chemistry 2e

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