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Rate of reaction and activation energy. The rate of reaction is defined as the change in concentration of reactants (or products) per unit time.
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Rate of reaction and activation energy The rate of reaction is defined as the change in concentration of reactants (or products) per unit time. 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. Reactionsoccur when reactant particles collide with a minimum amount of energy called the activation energy.
Measuring rates of reaction concentration of reactants concentration of products time time 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.
The rate equation Rate = k[A]m[B]n m + n = overall order of the reaction 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: • 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.
Determining the rate equation A + B C The rate equationcan be determined by completing a series of experiments varying the concentrations of each of the reactants. 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 similar set of experiments can be carried out keeping [A] constant and varying [B] to determine how changing [B] affects the rate of reaction.
The effect of temperature on k When temperature increases, rate of reaction increases. This is because the rate constant, k, increases with temperature. rate constant, k (moldm3s–1) temperature (K) k (moldm3s–1) temp. (K) 556 4.45 × 10–5 575 1.37 × 10–4 629 2.52 × 10–3 666 1.41 × 10–2 700 6.43 × 10–2 781 1.34
Rate determining step BrO3– + 6H+ + 5Br– 3Br2 + 3H2O 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. In a multi-step reaction, the slowest step is called therate 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.
