Rates of Reaction

Define rates
The rate of reaction is how fast a reaction is proceeding.

Finding the rate equation
The rate equation can be found using the rate-determining step.

A reaction is made up of a number of different steps, with one of the steps being the rate determining step. This means that this step is a slow step and therefore the rate is dependent on this step, and on this step only.

SN1

The slow step is the first step, and therefore the rate equation would be:

rates 1
r = k[halogenoalkane]

SN2

This is a 1 step reaction and therefore the rate equation will be related to all the reactants.

rates new
r = k[halogenoalkane](Nu-]

Some other examples

rates 3
r = k[A]

rates 4
r=k[A][X]

Order of reaction
The order of reaction shows how many molecules are colliding together in the rate determining step. For example, SN1 is a first order reaction since 1 molecule takes part in the rate determination state while SN2 is a second order reaction since 2 molecules collide together in the rate determining state. It can also be said that SN2 is first order with respect to both the halogenoalkane and the nucleophile since 1 molecule of each is used in the rate determining step.

Finding the order of reaction
The rate of reactions can be found using a series of experiments in which the concentration is changed and the rate of the reaction is calculated.

rates table

If the concentration of A doubles and the rate remains the same it is zero order.
If the concentration of A doubles and the rate doubles it is first order.
If the concentration of A doubles and the rate quadruple it is second order.

Finding the rate using experiments
A number of experiments can be done in order to analyse the rate of reaction. All of this measures either the concentration of the reactant or the concentration of the product at specific time intervals and then a graph is produced. The rate can be found by calculating the gradient.

Titration
A titration can be used to calculate the concentration of an acid, base a redox reagent. Once the sample from the reaction is taken this has to be quenched in order to stop the reaction.

Gas syringe
A number of reactions evolve gases. The volume of the gas can be recorded and used to find the rate of the reaction.

Calorimeter
A number of chemicals have colours, and the intensity change during a reaction can be measured in order to be used to find the rate of the reaction.

Pressure
Reactions involving gases can have a pressure change. If this is so this pressure change can be related to the concentration of the reactants and products and therefore it can be used to find the rate.

Conductivity
A number of reactions produce ions, and therefore there would be a change in the conductivity of the solution. This can be used to measure the rate of the reaction.

Collision theory
The collision theory states that the reactants have to collide, they have to collide at the right orientation and they have to collide with enough energy for the reaction to take place. Taking this into consideration a number of factors will affect the rate of reaction.

Surface area
The higher the surface area the bigger the area where a collision can take place and therefore more collisions will occur.

Temperature
The temperature increases the speed of the molecules so more collisions will take place. Also, the collision that takes place will have more energy so there will be an increase in the number of collisions with enough energy.

Catalyst
A catalyst can act in one of two different ways:
It can hold the molecules at the right orientation making the collisions easier.
It can reduce the activation energy and therefore there will be more collisions with the required energy for a reaction to take place.

Concentration
Increasing the concentration will increase the collisions.

Pressure
For gases increasing the pressure would increase the chances of collision.

Arrhenius equation

The Arrhenius equations related temperature to the rate constant and this shows that an increase in 10 oC will more or less double the rate constant.

rates 5
Iodination of a ketone

This is an example in which the rate equation includes a chemical that is not seen in the reaction. This is the H+ which is regenerated at the end of the reaction, while the I2 is not used in the rate determining step.

rates 6

 

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