GCSE OCR Gateway Chemistry C5.2 f-i Catalysis and catalysts

GCSE OCR Gateway Chemistry C5.2 f-i

C5.2 Controlling reactions

Summary

Changing the physical conditions of a chemical reaction can change its rate and yield.

Common misconceptions

Learners often misinterpret rate graphs and think that catalysts take part in reactions and run out/get used up.

Underlying knowledge and understanding

C5.2f To be able to describe the characteristics of catalysts and their effect on rates of reaction

C5.2g To be able to identify catalysts in reactions

C5.2h To be able to explain catalytic action in terms of activation energy using reaction profiles

C5.2i To recall that enzymes act as catalysts in biological systems

Examples to include:

Catalysis of hydrogen peroxide with various black powders including MnO2 manganese (IV) oxide

Catalysis of the reaction of zinc with sulfuric acid using copper powder

Catalysts and catalysis

Catalysts are substances that change the rate of a chemical reaction but remain unchanged at the end of the reaction.  The catalyst can be used repeatedly time and time again as it is never consumed/used up.

Examples of catalysts are many and varied.

One common example is to compare the effect of iron (III) oxide, manganese (IV) oxide and copper (II) oxide on the decomposition of hydrogen peroxide (H₂O₂).

2H₂O₂    ⟶    2H₂O     +    O₂

The experiment itself is simple enough.  Just add tenth molar amounts of each solid powder to the same volume of 20vol hydrogen peroxide and see which powder causes the most vigorous fizzing of oxygen gas.

This way you’ll see that the manganese (IV) oxide is the best catalyst.  But is it used up in the reaction?

Simple way to check is to weigh the amount of oxide you put in and when the reaction is finished, filter off the oxide dry it in the oven and then re-weigh.  You should find that the weight of oxide has hardly fallen.  It is the same after as it was before i.e. it has not been consumed even though it has changed the rate of the reaction. 

How does a catalyst like manganese (IV) oxide work?

A catalyst provides an alternative route for the reaction to take place.  In the case of manganese (IV) oxide the decomposition of the hydrogen peroxide takes place on the oxide surface. When it does so it takes a lot less energy to start the reaction the activation energy.

We can see this if we compare reaction profiles for the same reaction with and without a catalyst. 

With a catalyst, reacting particles need a much lower activation energy so as to engage in a successful collision. 

The new route with the catalyst is faster because it carries a lower activation energy.

The difference is as is you travelled from home to London by motorway or side roads.

Some specific examples of catalysis:

1.    Decomposition of hydrogen peroxide

2H₂O₂    ⟶    2H₂O     +    O₂

Catalyst: Manganese (IV) oxide

2.    Reaction of zinc and sulphuric acid

Zn    +   H₂SO₄     ⟶       ZnSO₄      +      H₂

Catalyst: Copper sulphate solution  CuSO₄

3.    The catalytic converter

2CO   +   2NO      ⟶       2CO₂     +    N₂

Catalyst : Silver or platinum metal

4.    Tooth filling

Ultra violet light sets off a catalytic polymer reaction to harden the tooth filling material. 

5.    Enzyme action

Enzymes carry a specific active site where only certain specific molecules will fit and react together.  Enzymes are proteins (but not all proteins are enzymes) and fit specific organic reactions in nature.  Enzymes being organic usually work best at around 35^oC but are rendered ineffective at high or low temperatures.  This is called denaturing.