CHEMICAL REACTIONS (RATE OF CHEMICAL REACTION)

Desmond Akachukwu
1 Apr, 2022
When substances react together, they form products which may be solids, liquids or gases. Chemical reactions are usually represented by an equation. The species at the left are called the reactants while those at the right hand are called the products. 

Rate of chemical reaction

The rates of chemical reaction vary greatly depending on the type of chemical reaction. Some
reactions are fast, some are slow and others occur at irregular speeds.

  The rate of a chemical reaction is the amount (in moles) of the reactant used up or products formed at a given time.


The rate of chemical reaction can also be defined as the number of moles of reactant converted or products formed per unit time.

  Rate of reaction = change in concentration of reactant or product⁄time taken  

To determine the rate of a chemical reaction, we measure the rate at which the mass or the concentration of a reactant is decreasing or a product is increasing. In case this cannot be done, we measure a property which changes proportionately with the mass of a reactant or product. Generally, we determine the rate of chemical reaction by measuring the following:
  1. Decrease in the mass of the reaction system e.g. due to the escape of a gaseous product.
  2. Volume of gaseous products
  3. Amount of precipitate formed
  4. Time taken to arrive at an easily seen stage
  5. Changes in intensity of colour
  6. Changes in pH
  7. Changes in total gas pressure.

Rate curve:

Rate curve is the graph which shows the rate of reaction is known as rate curve. The curve passes through the origin because there is no loss in mass right at the start of the reaction. The gradient of the curve is steep at first because the rate of reaction is fast. The gradient however becomes less steep as the rate of the reaction slows down. Finally, the curve becomes horizontal indicating the end point of the reaction. From the rate curve we can
  1.  the average rate of the reaction and 
  2.  the rate at a particular instant during the reaction. 
A large gradient indicates a faster reaction rate while a small gradient indicates a slower one.


Collision theory

The collision theory states that for a chemical reaction to occur, the reacting particles must collide with one another. Meanwhile, not every collision between reacting particles is effective. Only small fraction of the collision results in chemical reaction. For chemical reaction to occur therefore, the reacting particles must possess a certain minimum amount of energy called Activation energy. The activation energy is equal to the energy barrier that must be overcome before chemical reaction can take place. Activation energy can be defined as the minimum amount of energy possessed by the reactant particles for a chemical reaction to occur. Every reaction has its own activation energy. If it is low, the reaction will occur spontaneously at room temperature but where the activation energy is high, the reaction will not occur until more energy is supplied usually in form of heat.

Activated complex: 

this is the intermediate, high energy particles formed by reactant particles which have attained the activation energy. Because of their high energy content, they are unstable and readily decompose to give the products or the reactants of the reaction.
Reaction profile: this is the graphical representation of the energy change during a chemical reaction.

Factors affecting rate of a chemical reaction

The rate of chemical reactions depend on the following factors
  1. Nature of reactants
  2. Concentration and pressure (for gases) of reactants
  3. Temperature of the reaction mixture
  4. Surface area of the reactants
  5. Presence of light
  6. Presence of catalyst

Nature of reactants: 

every substance possesses its own unique chemical nature. The rate of a chemical reaction is determined by the nature of the reactants because different substances have different energy contents. For example, when iron is placed in dilute hydrochloric acid, there is a slow evolution of hydrogen, if a piece of zinc is used, the reaction is rapid but with a piece of gold there is no evidence of reaction.

Concentration of reactants: 

an increase in the concentration of the reactants will result in a corresponding increase in effective collisions of the reactants and hence, in the rate of chemical reaction, while a decrease in the concentration of the reactants will have the opposite effect.
Pressure affects the concentration of gaseous reactants, hence the rate of the chemical reaction.
For example, a mixture of hydrogen and chlorine gases will react twice as fast if the partial pressure of hydrogen or chlorine is doubled. The concentration of solid and liquid reactants is unaffected by change in pressure.

Temperature: 

the rate of almost all chemical reactions increases with increased temperature. It is found that a temperature rise of 100C doubles the rate of reaction. When the temperature of a system is increased, heat energy is supplied to the reacting particles. The increase in energy results in an increase in the number of particles with activation energy or more. Thus, collision becomes more frequent and the reaction proceeds faster.
Surface area of reactants: when the surface area of a solid is increased by breaking it to smaller pieces or by grinding it to powder, there will be greater contact between the reacting species and hence, increased rate of reaction. The greater the surface area of the reactants the faster the rate of the reaction. For example, the reaction between dilute hydrochloric acid and powdered marble is faster than that of marble chip with dilute hydrochloric acid. The efficiency of solid catalyst is usually increased by increasing the surface area.

Light: 

some reactions known as photochemical reactions can be influence by the presence of light. For example, the reaction between hydrogen and chlorine takes place explosively in the presence of bright light; moderately in day light but almost negligible in the absence of light.

Other examples of photochemical reactions are:

  1. Conversion of silver halide to grey metallic silver
  2. Photosynthesis in plantsReaction between methane and chlorine
  3. Decomposition of hydrogen peroxide
  4. The reactant particles in these reactions absorb light energy and react rapidly in a series of chain reactions.

Catalyst: 

A catalyst is a substance that alters the rate of a chemical reaction but remains chemically unchanged at the end of the reaction. A catalyst provides an alternative route for the reaction. Thus, a positive catalyst provides an alternative route with lower activation energy while negative catalyst provides alternative route with higher activation energy.

Characteristics of catalysts

  1. It alters the rate of a chemical reaction
  2. Its chemical nature remains unchanged after the reaction though its physical appearance may change
  3. A catalyst is specific in action
  4. It cannot start a reaction
  5. It has no influence on the type of product formed
  6. It has no effect on the equilibrium position of a reversible reaction
  7. A catalyst will affect the rate of a reaction even though it is present in a very small amount.
  8. The effect of a solid catalyst is improved by increasing the surface area.
Source: class activities
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