16 HL Chemical Kinetics

This leads to a very common rate expression: Rate ∝ [D] or Rate = k[D] This rate expression means that if the concentration of D is doubled, then the rate doubles Equally, if the concentration of D halves, then the rate halves

Rate of reaction = k [A]^m [B]^n

Rate equations can only be determined experimentally and cannot be found from the stoichiometric equations

In the above rate equation: [A] and [B] are the concentrations of the reactants m and n are orders with respect to each reactant involved in the reaction

Products and catalysts may feature in rate equations Intermediates do not feature in rate equations

The order of a reactant shows how the concentration of a chemical, typically a reactant, affects the rate of reaction

It is the power to which the concentration of that reactant is raised in the rate equation

The order can a positive, negative or fractional value Orders that are a fraction suggest that the reaction involves multiple steps

When the order of reaction with respect to a chemical is 0 Changing the concentration of the chemical has no effect on the rate of the reaction Therefore, it is not included in the rate equation

When the order of reaction with respect to a chemical is 1 The concentration of the chemical is directly proportional to the rate of reaction, e.g. doubling the concentration of the chemical doubles the rate of reaction The chemical is included in the rate equation

When the order of reaction with respect to a chemical is 2 The rate is directly proportional to the square of the concentration of that chemical, e.g. doubling the concentration of the chemical increases the rate of reaction by a factor of four The chemical is included in the rate equation (appearing as a squared term)

The overall order of reaction is the sum of the powers of the reactants in a rate equation

• Identify two experiments where the concentration of one reactant changes, but the concentrations of all other reactants are constant • Calculate what happens to the concentration • Calculate what happens to the rate of reaction • Deduce the order of reaction with respect to that chemical • Repeat this for all of the reactants, one at a time, until you have determined the order with respect to all reactants

Once the order with respect to all of the reactants is known the rate equation can be constructed Zero order reactants are not included in the rate equation First order reactants are included in the rate equation - they do not require a power Second order reactants are included in the rate equation - they are raised to the power of 2

The rate constant, (k), of a reaction can be calculated using the initial rates and the rate equation

k = rate/concentration of reactants

When you are asked to calculate the rate constant, k, for a reaction you must also be able to deduce the units This is done by replacing the values in the rearranged rate equation with the units of that value The units can then be combined or cancelled as required

As the rate of reaction increases the rate constant will increase

Therefore, increasing the temperature also increases the value of the rate constant, k, assuming that the concentration of the reactants remains unchanged An exponential relationship between the rate of reaction and temperature is observed when seen on a graph (exponential curve)

The graph shows that the rate of reaction roughly doubles with an increase of 10 oC This general relationship does not apply to all reactions Also, it is not necessarily every 10 oC, the rate may double every 9 °C or 11 °C The number of degrees needed to double the rate also changes gradually as temperature increases

In a zero-order reaction, the concentration of the reactant is inversely proportional to the time This means that the reactant concentration decreases as time increases The graph is a straight line going down

The gradient of the line is the rate of reaction Calculating the gradient at different points on the graph, will give a constant value for the rate of reaction

When the order with respect to a reactant is 0, a change in the concentration of the reactant has no effect on the rate of the reaction Therefore: Rate = k

This equation means that the gradient of the graph is the rate of reaction as well as the rate constant, k