Chemical kinetics

Particles must collide for  reaction to take place.

Not all collisions lead to a reaction.

Particles must possess at least a minimum amount of energy (activation energy) if the collision is to lead to a successful reaction

Particles must approach each other in a certain way and be in a certain spacial orientation in order to collide correctly. This is called the STERIC EFFECT.

1. Concentration

2. Pressure

3. Surface area

4. Catalyst

5. Temperature

The rate of a chemical reaction can be measures by monitoring either the rate of formation of a given product, or by measuring how quickly one of the initial reactants is used up.

A + 2B ---------> C

E.g. in the above equation - the rate of formation of C or the rate of the disappearance of A or B could be measured.

In the reaction:

A + B --------> C + D

If the concentrations of one of the reactants or products were measured, they would change as follows:

Rate decreases over time because reactant concentration decreases over time.

The rate of an equation at different intervals during an equation can be found by drawing tangents to the curve.

Usually as a reaction proceeds, the gradient decreases, indicating that the reaction rate is also decreasing as it gets less steep.

Initial rate is shown by  line drawn adjacent to the curve from the point of (0,0), then then finding the gradient of this line.

As concentration is usually measured in moldm-3 and time is measured in s-1, rate is usually measured in moldm-3s-1

The order of a reaction for a particular reactant tells youwhat magnitude of effect increaing the concentratiohn of that reactant has on the reaction rate.

If for a particular reactant e.g. X:

The order is zero, this means that doubling the concentration of X has no effect on the rate of the reaction. [A]^0

The order is one, this means that doubling the concentration of X has a propotional effect on the rate of the reaction, so the rate will double too. [A]^1

The order is two, this means that doubling the concentration of X has an exponential effect on the rate of the equation, so the rate will quadruple. [A]^2

The overall order of an equation is found by adding up all of the separate order sof each component in the rate equation.

The rate constant (k) is a coefficient of proportionality relating the rate of a chemical reaction at a given temperature to the concentration of reactant (in a unimolecular reaction) or to the product of the concentrations of reactants.

k is the rate constant- the bigger it is, the faster the reaction. The rate constant is always the same for a certain reaction at a particular temperature. If the temperatuire increases however, k increases too.

Units for the rate constant are not fixed, they need to be worked out from the rate equation.

Example. Write rate equation for reaction between A and B where A is 1st order and B is 2nd order.

Rate= k[A][B]2 therefore the overall order is 3 (1+2)

Then, to calculate the unit of k:

1. Rearrange rate equation to give k as subject　
k= Rate / [A][B]2

2. Insert units and cancel
k = mol dm-3s-1 / mol dm-3(moldm-3)2

3. Simply fraction
k = s-1 / mol2dm-6

Therefore, k = mol-2dm6s-1

The rate of a chemical reaction is related to the
concentration of reactants by a rate equation of the form:

Rate = k[A]m [B]n

where m and n are the orders of reaction with respect to
reactants A and B and k is the rate constant.

The orders m and n are restricted to the values 0, 1, and 2.

The rate constant k varies with temperature as shown by
the equation:
k = Ae–Ea/RT

A is a constant, known as the Arrhenius constant,
Ea is the activation energy
T is the temperature in Kelvin
e is a mathematical constant (2.71828)
R is the gas constant (8.31 JK-1 mol -1)