The Effect of Catalysts

Kinetics

What is a catalyst?

A substance that increases the rate of a chemical reaction and is chemically unchanged at the end of the reaction.

How does a catalyst increase the rate of reaction?

It provides an alternative reaction pathway with a lower activation energy.

Does a catalyst lower the activation energy of the original reaction pathway?

No. It provides a different pathway that has a lower activation energy.

What is a reaction pathway?

The reaction, or series of reactions, that reactants undergo to change into products.

What happens to activation energy when a catalyst is used?

The activation energy is lower because an alternative reaction pathway is provided.

How does a catalyst affect the Maxwell–Boltzmann distribution curve?

It does not change the curve; it lowers the activation energy.

How is a catalyst shown on a Maxwell–Boltzmann distribution graph?

The activation energy line for the catalysed reaction is drawn to the left of the activation energy line for the uncatalysed reaction.

Why does lowering activation energy increase reaction rate?

A greater fraction of molecules have energy equal to or greater than the activation energy, so there are more successful collisions per second.

What does the area to the right of Eₐ represent on a Maxwell–Boltzmann distribution?

The fraction of molecules with sufficient energy to react.

What happens to the fraction of molecules with E ≥ Eₐ when a catalyst is added?

It increases because the activation energy is lower.

How should you explain the effect of a catalyst on reaction rate in an exam?

A catalyst provides an alternative pathway with a lower activation energy, so a greater proportion of particles have E ≥ Eₐ and there are more successful collisions per second.

What is a reaction profile diagram?

A diagram showing how enthalpy changes as a reaction progresses from reactants to products.

What are the axes of a reaction profile diagram?

Enthalpy, H, on the y-axis and progress of reaction on the x-axis.

How is activation energy shown on a reaction profile?

The energy difference between the reactants and the highest point of the reaction pathway.

How does a catalysed reaction profile differ from an uncatalysed one?

The catalysed pathway has a lower maximum because it has a lower activation energy.

Does a catalyst change the enthalpy change, ΔH, of a reaction?

No. The reactant and product energy levels remain the same, so ΔH is unchanged.

What does an exothermic reaction profile look like?

The products are at a lower enthalpy level than the reactants.

What does an endothermic reaction profile look like?

The products are at a higher enthalpy level than the reactants.

What may appear on the reaction profile of a catalysed multi-step reaction?

One or more intermediates and multiple activation-energy peaks.

What is an intermediate?

A species formed during a reaction that is subsequently used up in a later step.

What is a heterogeneous catalyst?

A catalyst that is in a different phase from the reactants.

What type of catalysts are commonly used in industry?

Heterogeneous catalysts.

How does a solid heterogeneous catalyst work with gaseous reactants?

Gas molecules adsorb onto the catalyst surface, react there, and the product molecules then desorb.

Why is the surface of a heterogeneous catalyst important?

It provides sites where reactant particles can adsorb and react.

What are two major economic advantages of catalysts in industry?

They increase reaction rate, allowing more product to be made in a given time, and allow reactions to occur at lower temperatures, reducing energy costs.

How can catalysts improve atom economy?

They can provide reaction pathways that produce fewer unwanted by-products and increase the proportion of reactants converted into the desired product.

How can catalysts reduce industrial waste?

They can reduce the formation of unwanted by-products.

How can catalysts make product separation easier?

They can reduce the number of unwanted products that need to be separated from the desired product.

Why can using fewer reaction steps be beneficial industrially?

It can reduce energy costs, increase overall yield, reduce product losses during separation and make the process quicker.

Why can percentage yield improve when fewer reaction steps are required?

Product can be lost at each reaction or separation step, so fewer steps can reduce overall losses.

How can catalysts make chemical processes more sustainable?

They can lower energy requirements, reduce waste and unwanted products, and improve atom economy.

What is an example of a heterogeneous catalyst used in the Haber process?

Solid iron.

Why does a catalyst not change the position of equilibrium?

It speeds up both the forward and reverse reactions, so equilibrium is reached faster without changing its position.