Catalyst Corner

A third kind of change

You already know that a reaction needs collisions, and that a collision only works if it carries at least the activation energy. You also know that concentration, pressure, surface area and temperature all speed a reaction up by acting on the particles: how many there are in a space, how much of the solid is exposed, how fast they are moving. A catalyst does something none of those do. It leaves the particles exactly as they were and lowers the bar. It offers the reaction an alternative pathway with a lower activation energy, so a greater proportion of the collisions that were already happening now have enough energy to work. That is the whole idea, and every property you have to know follows from it. It is not used up, because it is not a reactant. It does not appear in the equation, because nothing about it is consumed or produced. And it is fussy about which reactions it will help, because a pathway that suits one reaction need not suit another.

Words for catalysts

Five terms. The first is the definition an examiner wants back almost word for word.

Property to consequence

Why it is not in the equation

A reaction is speeded up by a catalyst. Why does the catalyst not appear in the balanced chemical equation for that reaction?

• Because it is neither consumed nor produced, so there is nothing for the equation to record
• Because only a very small amount of it is needed
• Because a catalyst is not a chemical substance
• Because it only affects energy, and equations do not show energy

The catalysed curve

In the previous module, the uncatalysed reaction had produced 40 cubic centimetres of gas by 30 seconds and reached 48 cubic centimetres only by 60 seconds. This graph shows the SAME reaction with a catalyst added, in cubic centimetres against seconds. Four points are plotted. Plot the missing point at 20 seconds, where 45 cubic centimetres have been collected.

Same amount, sooner

Comparing the catalysed curve you have just completed with the uncatalysed one, what is the same and what is different?

• The final volume of gas is the same; the catalysed reaction simply reaches it sooner
• The catalysed reaction produces more gas in total
• The catalysed reaction produces less gas because the catalyst uses some up
• The catalysed reaction never finishes, because the catalyst keeps working

What it changes and what it does not

Almost every mark on this topic is won or lost on this division, so learn it as two lists rather than as a paragraph.

Which two are true of catalysts

Select the TWO statements that are correct.

• The same mass of catalyst is present at the end of the reaction as at the start
• A catalyst that works for one reaction may do nothing for a different one
• A catalyst increases the total amount of product that can be made
• A catalyst raises the energy of the reacting particles so more collisions succeed

The catalyst explanation

A catalyst increases the rate of a reaction without being _____ up. It works by providing an alternative _____ with a lower activation _____. Because less energy is needed, a greater proportion of collisions are _____, so the rate increases. The particles themselves are _____, and the final amount of product is exactly the same.

The error to avoid

One sentence loses more marks on this topic than everything else combined, and it sounds perfectly reasonable: a catalyst gives the particles more energy. It does not. Nothing about the particles changes at all. What changes is how much energy a collision needs in order to work, because the reaction is now taking a different route with a lower activation energy. Think of it as lowering a bar rather than lifting the athletes. It matters because the wrong version quietly contradicts everything else you have to say: if a catalyst gave particles energy, it would have to be supplying that energy from somewhere, and it would be changed by doing so, which is exactly what a catalyst is defined as not being. Get the direction right and the rest of the topic holds together.

Enzymes, worked

The specification asks you to know that enzymes are biological catalysts, and they make specificity concrete. Living things run at temperatures that are, chemically speaking, quite low. Many of the reactions a body depends on would be far too slow at those temperatures if they had to take their ordinary route. Raising the temperature is not an option for an organism, so the alternative is to lower the activation energy instead, which is what an enzyme does: it is a catalyst produced by a living thing to speed up a particular reaction. Notice the word particular. An organism does not make one general catalyst; it makes many, each suited to its own reaction, which is specificity seen from the inside. Here is a model answer to why enzymes matter, in the terms this course wants: enzymes are biological catalysts that increase the rate of reactions in living organisms by providing an alternative pathway of lower activation energy, so that reactions can happen fast enough at body temperature without the organism having to be hotter. Everything beyond that, including how an enzyme fits its reaction and what happens when it is heated, belongs to your Biology course rather than this topic.

The statement that is wrong

Four sentences from student answers about catalysts. Select the ONE that is incorrect.

• The catalyst provides an alternative pathway with a lower activation energy.
• The mass of catalyst recovered at the end equals the mass added at the start.
• The catalyst transfers energy to the reactant particles so that they collide harder.
• The catalyst does not appear in the balanced equation for the reaction.

From catalyst to rate

Put the steps of the explanation in the order that answers an explain question about a catalyst.

• A catalyst is added to the reaction mixture
• It provides an alternative pathway for the reaction
• That pathway has a lower activation energy
• A greater proportion of collisions now have enough energy to succeed
• The rate increases, though the final amount of product is unchanged

Choosing a catalyst

A company runs a reaction that is too slow to be worth doing. Take the decisions in order.

• A catalyst that works well for a different reaction is already in the building. Will it help?
• A suitable catalyst is found. How much product will the reaction now make?
• After the reaction, what has happened to the catalyst?
• Someone asks why the company does not simply heat the reaction instead. What is the chemical difference?

Explain the catalyst

A reaction produces a gas. When a catalyst is added, the same volume of gas is collected in half the time. Explain what the catalyst is doing and why the final volume is unchanged.

• Define a catalyst in the terms the specification uses
• Explain how it increases the rate, reaching the alternative pathway and the activation energy
• Explain what happens to the proportion of successful collisions, and why
• Explain why the final volume of gas is the same in both experiments
• Explain why the catalyst does not appear in the balanced equation
• State one thing a catalyst does NOT do, and correct the common wrong version of it