OCR Unifying Concepts

What is the rate of reaction?
The change in concentration of a reactant per unit time. Therefore its units are moldm-3s-1.It is a measure of the rate of use of reactants and the rate of production of products.

What is the rate equation?

Use [A] and [B] as the reactatnts and m and n as the orders of reactions.

rate of reaction=k[A]m[B]n

Why can’t the;rate equation be predicted from the stoichiometric equation?

How is it calculated?

The stoichiometric equation cannot be used to find the rate equation because the units for k may be different for each reaction.;The rate equation must be worked out by experimentation.
How is the overall order of reaction calculated?

The overall order of reaction is the sum of the individual orders of the reactants.

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The order of reaction may be determined by which method?

Describe this method.

The order of reaction may be determined by the initial-rates method. The initial rate is measured for several experiments, using different concentrations of reactants. One concentration is changed whilst the others are fixed, so that a clear and systematic set of results is obtained.
The order of reaction may be determined from a single experiment. Explain how this is done.
Draw a concentration-time graph for this experiment. The shape of the graph, indicates the order of reaction (see fig12.7 pg 109 of Chemistry 2 for shapes). The gradient of a tangent drawn at a;point is the rate of reaction at that point.
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