Notes

Catalysts and Enzymes

Catalyse your revision on catalysts and enzymes

1. No pressure, no sweat, for we have catalysts

Some reactions are sluggish. You will need to crank up the temperature and pressure to get them going. This inadvertently consume a lot of energy, which is awful for the environment. Thankfully, there is an alternative that does not break a sweat: adding catalysts.

2. Catalysts provide an easier way out by lowering activation energy

Catalysts speed up a reaction by changing how it proceeds. They intervene to provide an easier way to convert reactants into products. In terms of energy, they provide an alternative reaction pathway with a lower activation energy. We can draw this in an energy profile diagram. whereby the “hump” that represents activation energy, Ea is lower.

A lower activation energy means a lower energy requirement for reactants to undergo effective collisions. Therefore, a greater proportion of reactants will have enough energy to overcome the activation energy. This increases the frequency of effective collisions and hence the speed of reaction.

A catalyst is a substance that increases the speed of reaction by providing an alternative pathway with a lower activation energy.

3. Long live catalysts!

So how exactly do catalysts provide an alternative pathway? We will answer this question by looking at a specific catalyst: palladium. It speeds up the reaction between carbon monoxide and oxygen to form carbon dioxide.

Firstly, palladium speeds up the reaction by adsorbing oxygen molecules on its surface. This strains the covalent bond between the two oxygen atoms, making it easier to break. Secondly, palladium also adsorbs carbon monoxide. This brings the two reactants close together, making it easier for them to undergo effective collision to form carbon dioxide.

Crucially, the product leaves the surface of the catalyst. This regenerates the catalyst to its original state, allowing it to catalyse the formation of another carbon dioxide molecule. That’s why only a small amount of catalyst is needed to process a large amount of reactants.

A catalyst is needed in small amount as it is regenerated and not used up in a reaction.

4. Different catalysts for different occasions

CatalystReaction
IronHaber process to make ammonia
Platinum
Rhodium
Catalytic converters to remove pollutants
Aluminium oxide
Silicon dioxide
Cracking of large hydrocarbons into smaller ones
NickelHydrogenation of alkene into alkane
Phosphoric acidHydration of ethene into ethanol

Besides palladium, there are many other types of catalyst for different reactions. While all these catalysts play the same general role of increasing reaction speed, they work in different ways.

5. Biological catalysts found in cells have a special name: enzymes

Catalysts are extremely useful to speed up industrial processes. So humans make catalysts. However, catalysts make humans too. Without biological catalysts called enzymes, vital reactions like photosynthesis and respiration would be too slow to support life.

Dot-and-cross diagram of RuBisCO, which is a lot larger than carbon dioxide

Most enzymes are made up of proteins. Proteins are large biological molecules consisting innumerable non-metallic elements like carbon, nitrogen, hydrogen, oxygen and sometimes sulfur.

An example is RuBisCO, a plant enzyme with a funky name cooler than that of Elon Musk’s son. It speeds up the capture of carbon dioxide in the first step of photosynthesis. However, as a large molecule of protein, it is more than ten thousand times larger than carbon dioxide.

Enzymes are biological catalysts that speed up chemical reactions in living cells.

6. Out of shape, out of steam: denatured enzymes can no longer speed up a reaction

While the dot-and-cross diagram of RuBisCO looks messy and random, the atoms are arranged in a highly specific manner to give it a highly specific shape. This shape is extremely crucial to allow the right reactants to bind and the reaction to speed up.

Unfortunately, when temperature increases or pH changes, some bonds are broken. The enzyme loses its shape and becomes denatured. The denatured enzyme can no longer speed up the reaction.