Rate of reactions IGCSE ChemistrySome reactions are very fast and some are very slow. But some chemical reaction takes place a moderate speed. Explosives and precipitation reaction are examples of quick reactions. Rusting or iron and fossil formation are examples of very slow reactions. Reaction of Zinc with dilute hydrochloric acid is an example of medium fast reaction. Neutralisation reaction and combustion are very fast reactions.In chemistry, speed of a reaction is called rate of reaction. Rate of a reaction is measured in terms of amount of reactants disappear during a reaction in unit time or the amount of products formed in unit time. For a solid rate of reaction is g/sec (unit time)For solution rate of reaction is mole/secFor a gas rate of reaction is cm3/secDuring a chemical reaction, amount of reactants decreases with time while the amount of products increases with the time. Finally no reactant will be left and the reaction comes to an end.The changes of reactant amounts and product amount are shown graphically as follows.

From the graph you can find the concentration of reactant or product at any time.Rate of a reaction is measured by:Rate is = Fast reaction takes less time and slow reaction takes more time. So rate of reaction is inversely proportional to time.Hydrochloric acid and zinc reacts to for zinc chloride and hydrogen gas.HCl + Zn ZnCl2 + H2Hydrogen gas is collected in a gas syringe and the volume is noted at 30 sec intervals of time. Volume is then plotted against time.

The rate of reaction in the above experiment can be measured by:How fast Zinc reacts away (disappears) in unit timeHow fast Hydrochloric acid reacts away in unit timeHow fast Zinc chloride is formed in unit time andHow fast Hydrogen gas is formed in unit time.

The experiment was conducted separately with acid at different temperature and zinc powdered. The results are plotted on a graph paper.

Rate of reaction decreases with time. Why?Collision of particlesFor a chemical reaction to takes place, the reactant particles should collide. Reaction takes place only if the particles with enough energy collide. The minimum amount of energy particles require for a reaction to happen is called activation energy. With time more and more reactant particles change in to products, frequency of collision will decrease. So reaction rate will decrease with time and finally stops.

There are many factors which affect rate of reaction:1. Concentration.This factor is mainly for dissolved chemicals (solution). Concentration is measured as moles/dm-3 . When the reactant concentration is high (chemicals dissolved in water), frequency of successful collision will be higher and hence larger rate of reaction.Hydrochloric acid and zinc reacts to for zinc chloride and hydrogen gas.HCl + Zn ZnCl2 + H2Production of Hydrogen is faster if we use more concentrated acid. Hydrogen gas is collected at different intervals of time and plotted the values of volume against time.For a gas, higher pressure will lower the volume and so particles in smaller space. Higher pressure will increase the reaction rates when gases are used.2CO(g) + O2(g) 2CO2(g) Higher pressure makes the carbon monoxide molecules closer to Oxygen molecules. So collision rate is high and the rate of reaction will be also high.If a gas is involved, high pressure makes reaction faster. When the pressure is high, the volume becomes smaller. It means the particles come closer in smaller space and chances of more effective collision and reaction rate is high

2. Temperature.Increase in temperature increases kinetic energy of the particles. When the kinetic energy is higher, they move faster with enough activation energy (more reactant particles will have required energy). So more frequent successful collision takes place and rate of reaction will increase.So increase in temperature will help:1. For the increase in the movement of particles and hence more frequent collision.2. More molecules having required activation energy so that more successful collisions.

3. Surface area.Reactions take place on surfaces. So if the reactant solids are powdered, more surface area will be exposed and the reaction rate will increase.

4. CatalystCatalyst is a substance which speeds up a reaction and is not used up in a reaction. Catalyst will not involve in the chemical reaction. It lowers the activation energy by providing surface area for the reactants and speeds up the reaction. So at the end of a reaction, catalysts stay the same. Catalyst will not help to make more products.

Enzymes are biological catalysts which speed up biochemical reactions. Iron is the catalyst used in the manufacture of ammonia (Haber Process).Vanadium(V)oxide is used in the manufacture of sulphuric acid (Contact Process).Nickel catalyst used to make margarine from vegetable oils.Manganese(IV)oxide is the catalyst used in the preparation of Oxygen gas from hydrogen peroxide.

Photochemical reactions are those reactions catalysed by light. For examples, reaction of methane with chlorine is catalysed by light.

In photography, silver halides are present on the photographic film. The reflected light from the object falls on the photographic plate and the Silver ions are changed into Silver atoms.

Ag+(aq) + e- Ag(s) (Silver ions are reduced to silver atoms)

Formation of silver atoms helps to form the negative.

Photosynthesis is a photochemical reaction where carbon dioxide and water are reacted in the leaves of plants to produce glucose and oxygen gas. This reaction is catalysed by light energy.Carbon dioxide + water glucose + Oxygen6CO2 + 6H2O C6H12O6 + 6O2