The rate of a chemical reaction is a very important topic under Chemical Kinetics which is the study of the rates of reactions. It provides information on how long it takes a reaction to occur; how fast a reaction goes; how the rate can be measured; and the pathways through which the reactants are converted into products(mechanisms). Chemical reactions go at different rates:
- Some reactions are s fast and instantaneous that their rates cannot be measured. These are reactions involving ions, e.g precipitation reactions. For instance, when a solution of silver salt is mixed with a solution of a chloride, a white precipitate of AgCl is produced instantaneously.
- Some reactions proceed at slow rates that their progress can be monitored e.g catalytic decomposition of hydrogen peroxide at room temperature
- Some reactions will not commence unless heat is applied, or the reactants are ignited.
Rates Of A Chemical Reaction
The rate of a chemical reaction is defined generally as the change in the quantity of reactant or product per unit time. It can also be defined with respect to the quantity being measured in the reaction. For instance:
- For a reaction in solution, the rate is the change in the concentration of reactant or product per unit time at a given temperature., i.e Rate = Change in concentration/Time taken
- For a reaction in which a solid is a reactant or producer, the rate is the change in mass of a substance per unit time, i.e rate=change in mass/time taken
- For a reaction in which a gas is a reactant or producer, the rate is the change in volume of gas per unit time at a given temperature i.e Rate = Change in volume/time taken
Determination Of Reaction Rates
The rate of a chemical reaction can be determined by measuring any of the following properties:
- Increase in the quantity of a product with time
- Loss of mass of a reactant
- The concentration of acid or base
- Change in pressure of the system
- Colour change
- Mass of precipitate
- Ph measurement
The Collision Theory
The collision theory of Arrhenius suggests that two or more particles of reactants must collide before the products can be formed. The collision theory is based on the principle of kinetic theory that: particles of matter are in continuous random motion colliding with one another. Hence the rate of reaction is directly proportional to the frequency of collision between the particles.
Factors Affecting The Rates Of Reactions
There are seven factors that affect the reaction rates and we would explain them below:
- Nature Of The Reactants: The particles of reactants are usually in the form of atoms, molecules, and ions.
- Concentration Of Reactants: The concentration of a solution is a measure of the number of reactant particles in the solution. When a solution is concentrated, there are more reactant particles per unit volume than when it is dilute.
- Temperature Effect: For most reactions, rate increases as temperature increases.There is an increase in reaction rate due to the number of reactants particles possessing the activation energy at a higher temperature.
- Surface Area Of Reactants: The surface area represents the state of subdivision of a solid reactant in a reaction, It is a measure of the area of the particles exposed, or available for physical or chemical attack.
- Prescence Of Catalyst: A catalyst is a substance that alters reaction rate without being used up during the reaction. In the presence of a catalyst, more particles possess kinetic energy greater than the activation energy. It leads to an increase in effective collisions, and hence, an increase in reaction rate.
- Effects Of Pressure:A change in pressure affects chemical systems in which one or more of the reactants or products are gases. An increase in pressure on a given mass of volume(Boyle’s law) and hence, an increase in concentration.
- Effects Of Light Or Radiation: Some reactions occur only in the presence of sunlight; such reactions will not occur in the dark.