Rates of reaction-catalyst

Higher and foundation tiers

Rates of reaction and catalysts

Catalysts are amazing! Catalysts are substances that speed up a chemical reaction without being used up. Catalysts are usually transition metals or transition metal compounds. As an example of a catalyst at work consider the following decomposition reaction of hydrogen peroxide:

hydrogen peroxide(l) water(l) + oxygen(g)
2H2O2(l) 2H2O(g) + O2(g)

a hard working lab tech! Hydrogen peroxide is a corrosive colourless liquid. Its main uses are as bleach in the paper and textile industries and also as a disinfectant; although it can also be used as an oxidising agent and also as a propellant e.g. in submarine torpedoes. According to the equation above hydrogen peroxide decomposes to form water and oxygen gas. However under normal conditions in the science lab hydrogen peroxide is stable and you will be waiting a long long time to collect any oxygen gas or water from this reaction!

Now imagine for a minute that the school science lab technician has to prepare a class set of test-tubes full of oxygen gas for say a year 8 science lesson. How would they do it? Could the lab technician use the decomposition reaction of hydrogen peroxide to prepare the test-tubes of oxygen gas? Well in theory yes since hydrogen peroxide decomposes to release oxygen gas but not if the decomposition reaction is too slow; I am sure the year 8 class won't wait weeks or even years to get their test-tubes full of oxygen gas. Luckily this is where catalysts come into play. The lab technician set-up the apparatus below to collect the test tubes of oxygen gas.

apparatus to show the decomposition of hydrogen peroxide into oxygen gas and water using a manganese dioxide catalyst.

You may notice that in the conical flask is a hydrogen peroxide solution and one spatulas of a solid compound called manganese dioxide has been added. Manganese dioxide is a catalyst for the decomposition of hydrogen peroxide. The hydrogen peroxide will decompose violently and rapidly in the presence of the manganese dioxide catalyst to form oxygen gas and water, so the lab tech will be able to collect a class set of test tubes all filled with oxygen gas in a few minutes.

Once the lab technician has filled a class set full of test-tubes for use by students it would be possible to simply filter out the manganese dioxide catalyst since catalysts do NOT get used up in the reaction. It is also worth mentioning that since the catalyst is not used up in the reaction only a small amount of it is needed.

Hydrogen peroxide in living cells

Hydrogen peroxide is produced in the human body as a by-product of cell activity. Since it is a corrosive substance it would quickly kill the body cells if its concentration rose to high levels. In the body there are catalysts called enzymes; one of these biological catalysts or enzymes is called catalase. Catalase will rapidly decompose hydrogen peroxide into oxygen and water to quickly remove it from the body cells. Catalase is not only present in the human body but also in the cells of plants and bacteria. If a small potato cube or a piece of apple is placed into a boiling tube containing hydrogen peroxide solution it will immediately start fizzing due to the action of the catalase enzyme present in the apple or potato. If a glowing splint is pushed into the test tube it will re-light proving that the gas produced is indeed oxygen. This is outlined below: plant and bacteria cells will decompose hydrogen peroxide into oxygen gas and water.  These cells contain the enzyme catalase.

More about catalysts and enzymes

Many industrial and commercial process use transition metal catalysts; for example:

These transition metal catalysts are often very expensive but the up-front cost of the catalyst can be returned in the long run since they allow many industrial processes to operate at much lower temperatures than would be possible without them; this will obviously save money and energy. The use of transition metal catalysts also speeds up the reaction taking place and so allows a larger amount of product to be made in a given time; this product can then be sold for profit.


Enzymes are biological molecules that are used as catalysts in living organisms; they consist of large protein molecules. Enzymes are essential for many of the processes to sustain life such as respiration and digestion. Enzymes in the human body are adapted to carry out a unique role; the table below lists a few examples of the many enzymes found in the human body and a brief explanation of the enzyme's role:

Enzyme Where is it produced? What it does.
protease stomach, pancreas and small intestine Breaks down proteins into amino acids
lipase pancreas and small intestine Breaks down lipids (fats and oils) into fatty acids and glycerol.
carbohydrase (amylase) salivary glands in the mouth and also in the pancreas and small intestine Breaks down large carbohydrate molecules into smaller simpler sugar molecules. Amylase is an example of a carbohydrase enzyme that breaks down starch to form the simple sugar glucose.

Enzymes are very large protein molecules which are made up of long chains of amino acids which are folded to give a very specific shape. Each enzyme has its own specific shape and this specific shape is designed to allow only certain molecules (substrates) to fit into certain active sites within the enzyme's structure. These active sites are the business end of the enzyme and this is where the enzyme will perform its function or job. Some enzymes will break up large molecules into smaller one while others do the opposite and help assemble larger molecules from smaller one.

The image below shows a substrate molecule slotting into the active site in an enzyme. The enzyme and substrate fit together much like a lock and key; in fact the diagram below is often referred to as a lock and key diagram which is a good description since only one key will open a lock and only one substrate molecule will fit into the active site in the enzyme. Once the substrate slots into the active site they bind together to form an enzyme-substrate complex and reaction then takes place very rapidly and the product is then released from the enzymes' active site. The active site on the enzyme is completely unchanged and as soon as the product is removed another substrate molecule will slot into the active site and the whole process will repeat again.

lock and key diagram showing how a substrate fits into the enzymes' active site

Using enzymes at home

Biological dishwasher tablets and washing powders all contain enzymes.

Biological washing powders and detergents contain artificial enzymes which perform a similar job to those found in the human digestive system. This makes sense since the stains we are likely to get on our clothes may include food, sweat, grass and even blood if we are having a bad day. After lunch or dinner you may put your dirty dishes in a dishwasher and add a tablet which also contains enzymes to digest the food which remains on the plates and cutlery.
These washing powders and dishwasher tablets will contain enzymes that include proteases, lipases and carbohydrases to break up or digest the stains on our dirty clothes and the food remnants left on the dirty dishes in the dishwasher.

How do catalysts work?

Before particles can react with each other they need to collide with enough energy to break the bonds in the reactants; this is called the activation energy. Catalysts are often described as surface active agents; this means is that reactions involving them take place on the surface of the metal catalyst or the active sites in an enzyme. The reactants adsorb onto the surface of the catalyst and are altered or changed in some way or perhaps aligned in a fashion that allows them to react in a way that requires less activation energy from the reaction without the use of a catalyst.

The catalyst basically gets from reactants to products via a route that requires less activation energy. More particles in the reactants are likely to have this new lower activation energy and therefore can react successfully; so the rate of reaction will increase.

The energy profile diagram below is for an exothermic reaction. It shows how a catalyst speeds up a reaction by lowering the activation energy.

energy profile diagram for a catalysed reaction and uncatalysed reaction.  The catalysed reaction has a lower activation energy.

Key points

Practice questions

Check your understanding - Questions on rates and catalysts

Check your understanding - Additional questions on rates and catalysts

Check your understanding - Quick quiz on rates and catalysts.