This unit describes how molecules collide and also gives an indepth of the kinetic theory

The collision theory of reaction

It states that particles (atoms or molecules) must collide with one another in order for the collision to give a product (reaction) colliding particles:

Colliding particles must:

  • Be of one type
  • Have energy equal to or above activation energy.
  • Have correct orientation.

Activation energy

Is the minimum amount of energy moving particles must have in order for the collision to result into chemical reactions.


Effective collision: will result into a chemical reaction.

Any factor which increase the numbers of effective collision increase the rate of reactions.

Factors affecting rates of chemical reactions

  1. Temperature
  2. Pressure
  3. Concentration of reaction particles


Increase in presence of gaseous reactants.

  1. Particle size (surface area of solid reactants)
  2. Presence of a catalyst.

The collision theory explains the factors affecting the rate of a chemical reaction.

  1. Temperature:

Rise in temp increases the speed of moving particles and theory energy of the particles. This increases the number of effective collide per second-rate of reactions. Increase in rise of temperature.

A 100C temp rise approximately doubles against time.



The steeper the curve, the higher the rate of chemical reactions


The lower curve (steeper curve) corresponds to a faster rate of reactions.

  1. Concentration of reactants

It is the number of particles per unit volume. An increase in the concentration of reactants in solution (or pressure of gaseous reactants) makes particles more crowed. This increases the number of collision per unit time hence rate of reaction increases.


Increase is pressure only affects gaseous reactants.

2. Surface area of solid reactants (particles size)

If one of the reactants is a solid, increase in its surface area increases the number of collision per second and rate of reaction increase. How do powered and finely divvied solid react more quickly to reaction.

3. A substance which changes rates of reaction but remains unchanged at the end is a catalyst.

Properties of catalyst

  • Can change physically but their chemical nature and quantity does not change.
  • Require in small quantities because can be used over and over again.
  • Catalysts do not initiate the reaction. It merely affects its rate. It changes the speed of reaction.
  • Catalysts are specific. In their reaction i.e. a catalyst catalyses one reaction or one type of reaction. Efficiency of reaction can be affected by traces of other substances. This is referred to as a promoter. One which reduces its efficiency is an inhibitor.
  • A catalyst changes the rate but has no effect on position of equilibrium .It has the same effect on forward and backward reaction. (it affects the forward and backward reaction to the same extent).


  1. Homogenous catalyst: One where catalyst and reaction are in same time e.g. decompositions of potassium chlorate to give oxygen using manganese (IV) oxide.
  2. Heterogeneous: is where catalyst and reactants are in different state e.g. decomposition of hydrogen peroxide using manganese (IV) oxide.
  1. Positive catalyst, where the catalyst increases reactants speed.
  2. Negative catalyst where the catalyst slows down fast reactions e.g. nuclear reactants.


It affects only photo chemical reactions i.e. mixture of chlorine and hydrogen does not occur in darkness or normal light but explodes in sunshine.


Light is required for photosynthesis


Light darkens silver salts on photographic paper by forming tiny particles of black                        silver.


Methods of measuring rates of chemical reactions

Method I

By measuring the time taken up for the reaction to come to completion

E.g. Reaction between metal and acid or acid and carbonate


Method II

Measuring the rate at which one of the products accumulates with time e.g. metal and an acid


In the reactions between magnesium metal and dilute hydrochloric acid, the rate at which hydrogen gas accumulates with time can be measured e.g. 50cm3of 0.2 m HCL is placed in a conical flask with side connection fitted with 100cm3 gas syringe as shown in the figure below.

rubber bang

About 4g of magnesium turnings are added to the acid at the same time a stop clock is started. The stopper is quickly replaced to avoid escape of hydrogen gas. The volume of hydrogen is recorded at regular time interval for example after 30 seconds.


A graph of volume of hydrogen against time is plotted.


  1. Describe the shape of the graph

The curve starts from the origin, initially it rises expansionary, it continues to rise but at a decreasing rate, finally the curves levels off.

Explain the shape of the graph.

The curve starts from the origin in because there is 0cm3 of the gas before the reaction starts, initially; there is a steep rise in curve because rate of reaction is high when rate has just started since there is high concentration of the reactants of the beginning.

The curve continues to rise but at a decreasing rate due to decrease in concentration of reactants when reaction nears completion. Finally the curve travelled off when one of the reactants or both get exhausted and arte of reaction is 0.


Method III:

By measuring the rate at which mass of reaction s decreases with time e.g. in the reaction which involves formation of gases which escape, Rate which mass of reaction decreases for example Dilute hydrochloric acid and magnesium.


Put 100cm3 of IM HCL acid into a conical flask and put a loose plug of cotton wool in the neck of the flask. Place the conical flask and direct reading clock pan balance and hot its mass (a gms). Remove the cotton wool from the neck of the flask and add 10 gms of medium size marble chips. Quickly replace the cotton wool to prevent acid spray from damaging the metallic balance.


Calcium carbonate reacts with acid to give carbon dioxide gas which escapes causing a decrease in mass of flask and its contents.


The mass of the flask is recorded and its contents. Note the mass of flask and its contents at regular time intervals for example after every 30 seconds.

mass flask


An experiment to investigate the effect of particles size on the rate of a chemical reaction

The above experiment is repeated using same mass of marble chips of smaller size keeping all other factors constant and then using same mass calcium carbonate power.

mass of flask

The steeper the curve the faster the rate of chemical reaction. Fast reactions take a short time. 

Experiment to investigate effect of concentration on the rate of chemical reaction

Consider reaction between dilute hydrochloric acid and solution this sulphate ,NaS2032

Theory of the experiment

2 colourless solutions of sodium this sulphate and dilute hydrochloric acid react to produce a yellow precipitate of sulphur.

The intensity of the precipitate represents the extent of the reaction.

The intensity of the precipitate is measured by carrying out the reaction in a glass beaker and placing on a white tile and putting a cross.(x).

The time taken for the disappearance of the cross when the contents are viewed from above will give the measure of the time taken for a certain fraction of the reactions to occur.


Measure 50cm3 of sodium this sulphate solution (containing H0gm/cm3 of sodium this sulphate) and transfer it into a glass beaker. Add 5cm3 of 2m HCL and simultaneously starts a stop clock.

Shake the beaker carefully and place it back on white paper with across marked on it observe the cross by looking down through the solution from above the beaker and stop the clock the moment the cross is no longer visible.

Vary the conc of sodium this sulphate solution by taking 40, 30, 20 and 10cm3 of the original this sulphate solution and making the total volume 50cm3 by adding 10, 20, 30, 40, cm3 of water respectively .This also insures that the depth of solution in the beaker is the same in each reactions so that the cross is displaced to the same extent in each experiment


Effect of surface area on the rate of chemical reaction

This can be done using calcium carbonate (marble chips) and dilute hydrochloric acid.

A known mass of large chips is reacted with excess hydrochloric acid. The mass of reaction mixture is recorded mixture is recorded at regular time intervals until reactions comes to completion.

This is repeated using the same mass of marble chips of small size and then with powdered calcium carbonate .Keeping other factors constant.




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