Chemical kinetics is the study of rates of chemical reactions, the factors affecting rate of chemical reactions and the mechanism by which reactions occur.
The branch of chemistry concerned with the study of chemical reaction rates and the variables (factors) that affect them.
Different chemical reactions take different times to go into completion. eg:
Rusting of iron can take some days up to years.
Precipitation reactions take few seconds or minutes.
The rate of reaction is the speed at which a chemical reaction takes places / proceeds.
The rate (speed) of chemical reaction can be determined by:
Either measuring the amounts of product formed and time taken to form it.
Or measuring the amounts of reactants used up and the time taken to use it up.
Thus the rate of chemical reaction can be defined as:
the amount of products formed per unit time in a chemical reaction.
the amount of reactants used up per unit time in a chemical reaction.
Collision theory
Collision theory states that “in order for the reaction to occur reacting particles must collide with sufficient energy and must collide with the correct orientation”
Activation energy is the minimum energy that the colliding particles of the reactants must achieve in order to react.
It can also be defined as a minimum energy that the reacting particles must possess in order for a chemical reaction to occur.
Factors affecting rate of chemical reactions
Concentration of reactants
Temperature
Pressure of reactants
Surface area
Catalyst
Light
Concentration of the reactants
This factor affects reactions involving solutions only.
An increase in concentration of a reactant increases its number of reacting particles. This increases the chances of collision.
Generally, as the concentration of reactants increases the rate of reactions also increases and decrease in concentration decreases the rate of reaction. Thus the concentration is directly proportional to the rate of reaction.
Temperature
When there is an increase in temperature increases the rate of chemical reaction and decrease in temperature decreases the rate of chemical reaction.
This happens because as the temperature increases, the reacting particles gain kinetic energy. This causes the collision frequency between the reactants to increase. This increases the rate of reaction.
Thus temperature is directly proportional to the rate of chemical reaction.
Pressure of reactants
For the reactions involving gases, the rate of reaction is affected by the pressure of the gas. As the pressure increases, the rate of a chemical reaction increases.
Surface area
For the reaction involving solid reactant, reducing the size of the reactant increases the surface area.
Generally, the greater the surface area (smaller particle size) of reactants, the higher the rate of chemical reaction because greater number of particles collide more frequently with each other and the smaller the surface area, the lower the rate of reaction.
. Grinding or crushing solid lamps to powder form (many small particles).
Catalyst
Catalyst is a substance that alters the rate of chemical reaction but remains unchanged at the end of the reaction.
Properties of catalysts
They remain unchanged at the end of reaction.
They are specific.
They can undergo physical change in a reaction.
They are used in small quantities and give a good result.
Chemical reaction
Catalyst
Decomposition of hydrogen peroxide
Manganese (IV) oxide
Contact process
Vanadium (V) oxide
Haber process
Iron
Catalysts increase the rate of chemical reaction by lowering the action energy of reactants. It is also said to provide an alternative pathway for the reaction to occur.
Light
Some reactions depend on the presence of light. As light increases the rate of chemical reactions also increases. Example this reaction is the substitution reaction in the saturated hydrocarbons.
REVERSIBLE AND IRREVERSIBLE REACTIONS
Some chemical reactions can occur in only one direction that is from reactants to products. These are said to be irreversible reactions.
Irreversible reaction is the one which proceeds in one direction only to form a product.
Zn + 2HCl → ZnCl2
The arrow shows the direction of reaction.
A reversible reaction is the one that can proceed in both directions under suitable conditions.
NH4Cl(s)⇋ NH3(g) + HCl(g)
The arrows show that the reaction proceeds in both direction.
Chemical equilibrium
Chemical equilibrium is a reversible reaction where the rates of the forward reaction equal to the rate of reverse reaction.
Condition necessary for chemical equilibrium to take place:
Reaction must be reversible.
It should take place in a closed vessel (closed system). None of the reactants or products escapes from or enter into the reaction mixture.
Characteristics of equilibrium
It is dynamic. This means that the molecules/ions/atoms of reactants and products are continuously reacting. Reactants change to products and products continuously change to reactants.
The concentrations of reactants and products remain constant at equilibrium.
It requires a closed system.
The forward and reverse reactions occur at the same rate.
LE CHATELIER'S PRINCIPLE
"If a system in equilibrium is subjected to a change, the position of equilibrium will change in such a way as to try to eliminate the effects of the change."
Factors affecting equilibrium reactions
Concentration
When the concentration of one or more of the reactants is increased the position of equilibrium moves to the right to reduce the effect of the increase in concentration of reactant. More products are formed until the equilibrium is restored.
When the concentration of one or more of the products is increased, the position of equilibrium moves to the left to reduce the effect of the increase in concentration of product. More reactants are formed until the equilibrium is restored.
Temperature
When temperature in equilibrium reaction is increased, the reaction that absorbs heat from the surroundings (endothermic reaction) is favoured.
Lowering temperature favours the reaction that releases heat to the surroundings (exothermic reaction).
Example
What is the effect of increasing temperature in the following equation:
2NO(g) + O2(g) ⇋ 2NO2(g) H = -ve
Soln
In this equation the forward reaction is exothermic, therefore the reverse reaction is endothermic reaction. Increasing temperature therefore favours the reverse reaction, more reactants will be formed.
Pressure
When the pressure is increased, position of equilibrium shifts in the direction that produces the smaller number of molecules of a gas.
When the pressure is decreased, the position of equilibrium shifts in the direction that produces the larger number of molecules of gas.
Example
What is the effect of increasing pressure in this reaction:
N2O4(g) ⇋ 2NO2(g)
Soln
The equilibrium position equilibrium shifts towards reactants. More N2O4 forms.
Note
Change in pressure only affects reactions where reactants or products are gases.
If there are equal numbers of molecules of gas on reactant side and product side, the position of equilibrium is not affected by a change in pressure.
In a reaction involving gases, solids and liquids, it is only the molecules of gases that count when determining how pressure affects the position of equilibrium.
Catalyst does not affect the equilibrium reactions. What it does is just to speed up reactions (forward and backward/reverse reactions) to reach the state of equilibrium faster.
Endothermic and exothermic reactions
There is energy change when a chemical reaction takes place. The energy usually is in the form of heat; it can be absorbed from or released to the surroundings.
Basing on the change in heat energy there are two types of reactions:
Exothermic reaction
Endothermic reaction.
Exothermic reaction is the reaction that releases heat to the surroundings. It is denoted by H=-ve Endothermic reaction is the reaction that absorbs heat energy from the surroundings. It is denoted by H=+ve
Energy level diagram is a graph that shows the energy change in a chemical reaction.