MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

RATES OF REACTION

THE COLLISON THEORY The collision theory explains rates of reaction in terms of colliding particles. Essentially it says that before two particles can react they have to undergo a

collision with each other. However, if the rate of a reaction is compared to the number of collisions which

take place it is found that only certain collisions produce a reaction. The collision theory also states that there must be enough energy in the collision

to overcome the activation energy (i.e. break bonds in the reactant molecules) and also states that the molecules must collide in the correct orientation

Reactions can only occur when collisions take place between particles having sufficient energy. The energy is usually needed to break the relevant bonds in one or either of the reactant molecules. This minimum energy is called the Activation Energy

The Activation Energy is defined as the minimum energy which particles need to collide to start a reaction

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

FACTORS WHICH AFFECT RATES OF REACTION

The factors which determine the rate of a chemical reaction are the following.

1. Concentration; the higher the concentration, the more there are in a given volume and so the more often the particles will collide in a set time.

2. Pressure in a gas; for a gas increase in pressure increases the number of particles in a given volume and so this is the same as concentration.

3. Temperature; the higher the temperature, the faster the particles move and so the greater the number of collisions in a set time. Also the energy of the particles increases, so more effective collisions take place.

4. Surface area for solid and liquid/gas; the reaction will occur where the two types of particle meet, at the surface, so the greater the surface area, the greater the number of collisions in a set time.

5. A catalyst provides an alternate reaction route of lower activation energy allowing more collisions in a set time to produce a reaction.

MAXWELL BOLTZMANN DISTRIBUTIONThe Maxwell-Boltzmann energy distribution shows the spread of energies that molecules of a gas or liquid have at a particular temperature

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

INCREASING TEMPERATUREAs the temperature increases the distribution shifts towards having more molecules with higher energies

The total area under the curve should remain constant because the total number of particles is constant

At higher temperatures the molecules have a wider range of energies than at lower temperatures.

At higher temps both the Emp and mean energy shift to high energy values although the number of molecules with those energies decrease

MEASURING THE RATES OF REACTIONThe rate of reaction is defined as the change in concentration of a substance in unit time Its usual unit is mol dm-3s -1

When a graph of concentration of reactant is plotted vs time, the gradient of the curve is the rate of reaction. The initial rate is the rate at the start of the reaction where it is fastest Reaction rates can be calculated from graphs of concentration of reactants or products

In the experiment between sodium thiosulphate and hydrochloric acid we usually measure reaction rate as 1/time where the time is the time taken for a cross placed underneath the reaction mixture to disappear due to the cloudiness of the Sulphur .

Na2S2O3 + 2HCl 2NaCl + SO2 + S + H2O

This is an approximation for rate of reaction as it does not include concentration. We can use this because we can assume the amount of Sulphur produced is fixed and constant.

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

EFFECT OF INCREASING CONCENTRATION AND INCREASING PRESSUREAt higher concentrations (and pressures) there are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions.

Note: If a question mentions a doubling of concentration/rate then make sure you mention double the number of particles per unit volume and double the frequency of effective collisions

If concentration increases, the shape of the energy distribution curves do not change (i.e. the peak is at the same energy) so the Emp and mean energy do not change They curves will be higher, and the area under the curves will be greater because there are more particles

More molecules have energy > EA (although not a greater proportion)

COMPARING RATE CURVES

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

EFFECT OF INCREASING TEMPERATURE At higher temperatures the energy of the particles increases. They collide more

frequently and more often with energy greater than the activation energy. More collisions result in a reaction

As the temperature increases, the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases

EFFECT OF INCREASING SURFACE AREAIncreasing surface area will cause collisions to occur more frequently between the reactant particles and this increases the rate of the reaction.

EFFECT OF CATALYSTSDefinition: Catalysts increase reaction rates without getting used up.

Explanation: They do this by providing an alternative route or mechanism with a lower activation energy

Comparison of the activation energies for an uncatalysed reaction and for the same reaction with a catalyst present

If the activation energy is lower, more particles will have energy > EA, so there will be a higher frequency of effective collisions. The reaction will be faster

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

HOMOGENOUS AND HETEROGENOUS CATALYSTSCatalysts are divided into homogeneous (in the same phase as the reactants, e.g. in solution for liquid-phase reaction; or all in gas phase) and heterogeneous (different phase – usually a solid catalyst for gas-phase or liquid-phase reactants) catalysts.

Heterogeneous catalysts such as surface catalysts involve steps such as; diffusion to surface, adsorption on surface, reaction at surface, deadsorption from surface, diffusion from surface. If two reactant molecules collide they may react if they have enough energy.  If they are brought together on the surface of a catalyst the activation energy may be lower so at a given temperature the reaction will be faster. 

Homogeneous catalysts can form intermediates which contain the catalyst but then decompose to form products. 

e.g. The reaction; A  B + C  high activation energy

Catalysed reaction A + catalyst A-catalyst low activation energy Intermediate

A-catalyst B + C + catalyst low activation energy

Note - The catalyst may change oxidation state during the reaction.

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL

MISS CHOHANS NOTES ON rates of reaction UNIT 2 EDEXCEL