Nuffield Advanced Chemistry - For my individual investigation I am looking at the decomposition of hydrogen peroxide into water and oxygen using an enzyme - catalase. I will investigate the reaction with varying temperatures and substrate/enzyme concentration. I will record the reaction by measuring the volume of oxygen produced using an inverted burette. Having done this experiment, I have reached a conclusion that the rate equation for this reaction at 25 C is rate = 0.02 [H<sub>2</sub>0<sub>2</sub>] [catalase] but my friends have told me that the rate constant (0.02) is too small. Is this true? If so, where could I have gone wrong? To calculate the rate constant I did a graph of initial rate of reaction against conc. of substance and then one for conc. of enzyme. I then calculate the gradient from these graphs. I know this sounds very confusing, but thank for your help.

Rates of reaction - kinetics (A2)

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For my individual investigation I am looking at the decomposition of hydrogen peroxide into water and oxygen using an enzyme - catalase. I will investigate the reaction with varying temperatures and substrate/enzyme concentration. I will record the reaction by measuring the volume of oxygen produced using an inverted burette.

Having done this experiment, I have reached a conclusion that the rate equation for this reaction at 25 C is

rate = 0.02 [H₂0₂] [catalase]

but my friends have told me that the rate constant (0.02) is too small. Is this true? If so, where could I have gone wrong?

To calculate the rate constant I did a graph of initial rate of reaction against conc. of substance and then one for conc. of enzyme. I then calculate the gradient from these graphs.

I know this sounds very confusing, but thank for your help.

You should be getting an answer in the region of 36 kJ mol^-1 for the catalysed reaction (as against about 79 for the uncatalysed reaction). The problem you are experiencing in the calculation centres around the concentration values you are using. All concentrations must be in mol dm^-3, of course but how can you do that for the enzyme?

There is a standard way of coping with this situation. It involves the use of the Michaelis constant. There are two sources of information to look at. One is the Nuffield Book of Data, p119 which gives the formula to be used for the rate equation and hence the rate constant. The other is one of the websites you will get if you put 'Michaelis constant' into a Google search. This should tell you how to calculate the Michaelis constant for your circumstances and also the conditions under which the method is valid.

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updated: 17 April 2007

Rates of reaction - kinetics (A2)

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