Explain the action of a catalyst in lowering activation energy, describe enzymes as biological catalysts, and interpret rate graphs of product formed against time

What this dot point is asking

SEAB wants you to explain how a catalyst speeds up a reaction by lowering the activation energy without being used up, recognise enzymes as biological catalysts, and interpret rate graphs that plot the amount of product against time. This dot point completes the rates topic and connects it to the energy profile from the energetics dot point.

The answer

What a catalyst does

A catalyst is a substance that speeds up a reaction but is not used up in the reaction, so it can be recovered unchanged at the end and used again. A small amount of catalyst can speed up a large amount of reaction. Different reactions need different catalysts; for example, manganese(IV) oxide catalyses the decomposition of hydrogen peroxide, and iron catalyses the manufacture of ammonia.

How a catalyst works

A catalyst provides an alternative reaction pathway with a lower activation energy. The activation energy is the minimum energy a collision must have to react. By lowering it, the catalyst means a greater proportion of collisions now have enough energy to be successful, so more collisions react per second and the rate increases.

On an energy profile, the catalysed pathway has a lower hump (lower activation energy) than the uncatalysed one. Importantly, the catalyst does not change whether the reaction is exothermic or endothermic, and it does not change the overall energy released or absorbed; the reactants and products sit at the same energies. It only lowers the barrier between them.

Enzymes as biological catalysts

Enzymes are catalysts made by living things (they are proteins) that speed up the reactions in cells. Like other catalysts, they lower the activation energy and are not used up. They are highly specific (each enzyme catalyses a particular reaction) and work best within a narrow range of temperature and pH; outside that range they stop working effectively. Enzymes are used in food production, washing powders and brewing.

Reading rate graphs

A common way to follow a reaction is to plot the amount of product (such as the volume of gas given off) against time. The graph has a characteristic shape:

• It rises steeply at the start, where the reaction is fastest (reactant concentration is highest, so collisions are most frequent).
• The curve becomes less steep as reactants are used up and the rate falls.
• It levels off (becomes horizontal) when the reaction is complete (a reactant has run out), and no more product forms.

The gradient (steepness) of the curve at any point shows the rate at that moment: a steeper gradient means a faster rate. A reaction with a catalyst, or run at higher temperature or concentration, rises more steeply and levels off sooner, but reaches the same final amount if the amounts of reactant are unchanged.

Examples in context

Example 1. Catalytic converters in cars. A car's catalytic converter uses metals such as platinum to speed up the conversion of harmful exhaust gases into less harmful ones, without the metal being used up. The catalyst lowers the activation energy of these reactions so they happen quickly in the hot exhaust, an important environmental application.

Example 2. Enzymes in washing powders. Biological washing powders contain enzymes that catalyse the breakdown of food and stain molecules at low temperatures, saving energy compared with hot washes. Because the enzymes are sensitive to temperature, these powders are designed to work in warm rather than very hot water.

Try this

Q1. State two features of a catalyst. [1 mark]

• Cue. It speeds up the reaction and is not used up (can be recovered unchanged).

Q2. Explain how a catalyst increases the rate of a reaction. [2 marks]

• Cue. It provides an alternative pathway with a lower activation energy, so a greater proportion of collisions have enough energy to react.

Q3. On a graph of gas volume against time, state what the flat (horizontal) part of the curve shows. [1 mark]

• Cue. The reaction has finished (a reactant has run out), so no more gas is being produced.