4.3 how fast - rates

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Rate of reaction and activation energy

Reactions occur when reactant particles collide with a minimum amount of energy called the activation energy.

The activation energy for a reaction is the minimum amount of energy that

particles need to have if a collision between them is to

result in a reaction.

The rate of reaction is defined as the change in

concentration of reactants (or products) per unit time.


Factors affecting rate of reaction


Rates: true or false?


Measuring rates of reaction

To measure the rate of reaction, we need to be able to measure either how quickly the reactants are used up or how quickly the products are formed. The method used for measuring depends on the substances involved.

If a graph of concentration or volume of products or reactants is plotted against time, the gradient will be the rate of reaction.

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Measuring rates


Rates of reaction: testing


The rate equation

The rate equation is an equation that relates the concentrations of substances involved in a reaction to the rate of the reaction. For the reaction A + B C

the rate of reaction depends on the concentrations of A and B ([A] and [B]) and various constants in the following way:

Rate = k [A]m [B]n

k is the rate constant (units depend of values of m and n)

m is the order of reaction with respect to A

n is the order of reaction with respect to B.

m + n = overall order of the reaction


Determining the rate equation

The rate equation can be determined by completing a series of experiments varying the concentrations of each of the reactants.

A similar set of experiments can be carried out keeping [A] constant and varying [B] to determine how changing [B] affects the rate of reaction.

To determine how [A] affects the rate, several different experiments can be carried out in which [B] is kept constant and [A] is changed. The data can then be used to work out the relationship between rate and [A].

A + B C


Orders of reaction


Orders of reaction: summary


The effect of temperature on k

When temperature increases, rate of reaction increases. This is because the rate constant, k, increases with temperature.

556

575

629

666

700

781

4.45 × 10–5

1.37 × 10–4

2.52 × 10–3

1.41 × 10–2

6.43 × 10–2

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k (m

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temperature (K)rate constant, k

(mol dm3 s–1)


Rate equations: true or false?


Example rate calculations


Rate calculations


Rate determining step

Many reactions take place using more than one step.

In a reaction such as this one, it is highly unlikely that all three reactant particles will collide at the same time. It is more likely that this reaction will take place in a number of steps that involve other species, called intermediates.

BrO3– + 6H+ + 5Br

– 3Br2 + 3H2O

In a multi-step reaction, the slowest step is called the rate determining step.

Any step that occurs after the rate determining step will not affect the rate of reaction, and therefore will not appear in the rate equation.


More about the rate determining step


The rate equation and mechanisms


Rate determining step: questions


Glossary


What’s the keyword?


Multiple-choice quiz

4.3 how fast - rates

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