The Collision Theory assumes that there must be collision between reactant particles for a chemical reaction to occur. Although there are many such collisions, only a small fraction of them result in a reaction; these are called effective collisions.There are several factors that affects the rate of reaction in a chemical system, however, we first examine the role of activation energy in chemical reactions
In a reaction system, particles move about at different speeds as they process different amounts of energy. For a chemical reaction to occur, the colliding reactants particles must possess a certain minimum amount of energy, called activation energy.
In all chemical reactions, existing bonds in the reactant particles have to be broken first before new bonds can be formed to produce product particles. The breaking of bonds requires energy and is possible only if the reactants particles collide with sufficient energy to overcome this energy barrier. If the energy of the colliding reactant particles is less than the activation energy, they merely rebound from each other and no reaction occurs. Reaction occurs if the energy of the colliding reactant particles is equal to or more than the activation energy.
Every reaction has its own energy of activation. A reaction with low activation energy will take place spontaneously at room temperature. A reaction with a high activation energy will only take place if energy is supplied, usually in the form of heat, light or electrical energy. The addition of a catalyst lowers the activation energy of a reaction.
Let us consider the synthesis of water (steam) from gaseous hydrogen and oxygen. Both hydrogen and oxygen gases exist as stable diatomic molecules. During the reaction;
- The covalent bonds in the hydrogen and oxygen molecules are broken; then
- New bonds are formed between the hydrogen and oxygen atoms to give molecules of water
The breaking of the covalent bonds requires energy. An initial energy input is needed to activate the reactant molecules. This energy is the activation energy of the reaction. In this case, the energy is relatively high and a mixture of hydrogen and oxygen left undisturbed do not react under ordinary conditions. To overcome the energy barrier, energy must be supplied to the reaction system. This is usually provided by an electric spark. The net reaction of the formation of water is exothermic. Therefore, once the reaction begins, the heat energy evolved is enough so active the rest of the reactant molecules.
From the concept of the energy of activation, we see that
- Both exothermic and endothermic reactions generally require and initial input of energy to overcome the activation energy barrier
- Exothermic reactions, once started, may evolve enough energy to activate more reactant particles so that the reaction can proceed without any further external energy supply
- Endothermic reactions have to be continually supplied with an external source of energy for activating the reactant particles as well as to form the products.
Factors Affecting Rates of Reaction
From the Collision Theory, we see that the rate of a reaction would depend on the frequency of effective collisions between reactant particles. The factors which influence
- The energy content of the particles,
- The frequency of collision of the particles, and
- The activation energy of the reaction
will also affect the rate of chemical reactions. Some of these important factors are as follows.
- Nature of reactants
- Concentration/pressure (for gases) of reactants
- Surface area of reactants
- Temperature of reaction mixture
- Presence of light
- Presence of a catalyst
Note: When doing experiments to study the effect of one of the factors on rate of reaction, the other factors that also affect the rate of reaction must be kept constant. Only then can we assume that the changes in measurement that occur are due, to the factor under study.
Effect of the Nature of Reactants
When a piece of iron is placed in dilute hydrochloric acid, there is a slow evolution of hydrogen gas; with a piece of zinc, hydrogen is evolved rapidly, and with a piece of gold, there is no evidence of a reaction. Thus, the rate of a chemical reaction is determined by the chemical nature of the reactants as different substances have different energy contents.
Effect of Concentration of Reactants
Reactants particles will collide more often if they are crowed in a small space, i.e., frequency of collision is dependent upon concentration. An increase (or decrease) in the concentration of the reactants will result in a corresponding increase (or decrease) in effective collisions of the reactants and hence in the reaction rate.
Pressure affects the concentration of gaseous reactants. For example, a mixture of hydrogen and chlorine gases will react twice as fast if the partial pressure of hydrogen or chlorine is increased from 0.5 to 1.0 atm. The concentration of solid and liquid reactants is unaffected by changes in pressure.
The following reactants are used to study the effect of concentration of reactants on the rate of reaction.
- The reaction between hydrochloric acid and magnesium.
Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
Hydrogen gas is given off. The reaction rate is obtained by measuring the duration of effervescence. The concentration of the acid is varied, while the mass of the metal is kept constant.
- The reaction between a dilute acid and a trioxocarbonate (IV).
- The reaction between acidified potassium iodide and hydrogen perioxide.
Iodine is liberated in this reaction. Starch is added to react with the iodine to form a dark-blue coloured complex. We measure the reaction rate by the time taken for a blue colour of a given intensity to develop.
2I–(aq) + 2H+(aq) + H2O2 2H2O(l) + I2(g)
In the experiments on how the concentration of a reactant affects the reaction rate, we note that the time taken for the reactions to complete is inversely proportional to the rate of reaction.
We get a straight line passing through the origin. This shows that for this reaction, the rate is directly proportional to the concentration of potassium iodide, i.e. the reactant. For most reactions, the rate of reaction increases as the concentration of a reactant increases though the relationship may not be linear.
Note: In general, reaction rate decreases with time. This is because as reaction goes on the reactants are being used up, so their concentrations keep on decreasing.
Effect of Surface Area of Contact
When a dilute acid is added to marble chips and to powdered marble, we notice a marked difference in the rate of reaction. The acid reacts more vigorously and faster with the powdered marble than with the marble chips. The powdered marble offers a greater area of contact with the acid than the marble chips so that more marble particles can react with the acid in a given time. This is a very important factor especially when one of the reactants is a solid, because only the particles on the surface of the solid are in contact and hence able to react with the other reacting particles.
To bring about greater contact between the reacting particles, the exposed surface area of the solid reactant must be increased by subdividing or breaking the solid into smaller pieces. The greater the surface area of the reactant, the higher is the rate of reaction. For example, zinc dust burns more quickly in oxygen or chlorine than zinc granules. Even lead burns in oxygen when it is in dust form.
In a reaction where the reactants are gases, liquids or solids dissolved in solution, the thoroughness of mixing is important to ensure maximum contact between the reactant particles. For example, liquids of different densities react slowly when they are immiscible, unless continually stirred. Similarly, in industrial processes, such as the solvay process, elaborate devices are used to ensure thorough mixing of the gases with the liquids.
The efficiency of solid catalysts in increasing reaction rates depends on their surface areas. Catalysts in industrial processes are often prepared in a very finely divided form or are spread out on an inert support so that the largest possible surface area is exposed to the reacting particles. That is why we use platinum black (finely divided platinum powder appears black) and platinized asbestos (platinum deposited on asbestos support) but not a platinum block as the active catalyst.
The rate of reaction is faster, especially at the beginning, when powdered marble is used than when marble chips are used.
Effect of Temperature
We know that reactions are faster at higher temperatures and slower at low temperatures. Many reversible reactions, if left at ordinary temperatures, would take years or even centuries to come to equilibrium. In general, the reaction rate doubles for each rise of 10oC. The reverse effect of cooling a reaction in order to slow it down is true. For example, the refrigerator has low temperatures to slow down the chemical reactions that spoil food.
Increasing the temperature of a system can lead to an increase in reaction rate in two ways. When the temperature is raised, energy in the form of heat is supplied to the reactant particles, so that
- The number of particles with energies equal to or greater than the activation energy increases;
- The average speed of all the reactant particles increases due to the greater kinetic energy, leading to a higher frequency of collision.
As a result, the number of effective collisions increases and the reaction proceeds at a faster rate.
Effect of Light
Some reactions are influenced by light. The reaction between hydrogen and chlorine, for example, is negligible in the absence of light, moderate in daylight and explosive in bright sunlight. Such reactions are known as photochemical reactions.
Other examples include
- The decomposition of hydrogen peroxide,
- The reaction between methane and chlorine
- Photosynthesis in plants, and
- The conversion of silver halides to grey metallic silver (used in photography).
In these reactions, the reactant molecules become activated on absorbing light energy, and react rapidly together in a series of chain reactions.
Effect of Catalyst
The rate of reaction is also affected by the presence of a catalyst. A catalyst is a substance which alters the rate of a reaction but itself does not undergo any permanent change at the end of the reaction. In the preparation of oxygen by heating potassium trioxochlorate(V). in the presence of a small amount of the catalyst, manganese(IV)oxide, only moderate heat is needed to decompose potassium trioxochlorate (V). in the absence of the catalyst, potassium trioxochlorate(V) must be heated to a much higher temperature and for a longer time in order to obtain similar results.
A positive catalyst usually acts by lowering the energy barrier of a chemical reaction. Thus, in the presence of a catalyst, more reactant particles are able to react when they collide.