Chemistry

Rate of Reaction: Formula, Examples, Calculation units & Factors

A rate of reaction is defined as how fast a chemical process happens. In chemistry, we use the term “rate of reaction” to describe the speed at which reactants are converted into products by words such as “slow,” “rapid,” or “instantaneous.” A slow reaction rate might be one that takes days, while a rapid rate could happen in seconds. There are many factors that affect the reaction rate, including temperature and concentration.

How can you determine the Rate of Reaction?

It can be determined by the following two methods:

  1. Physical Methods
  2. Chemical Methodsrate of reaction

Physical Methods

In these methods, we measure the change of some physical properties of reactants or products. The following are the methods generally used:

Physical MethodsObserved Physical Property
1.Refractometric methodChange in refractive index.
2.Spectroscopic MethodAbsorption of ultraviolet or infrared radiations.
3.Calorimetric methodChange in color intensity.
4.Conductivity methodChange in electrical conductivity.
5.pH methodChange in pH observed.
6.Polarimetric methodChange in the optical rotation of plane-polarized light.

Chemical Method

In this method, generally, samples are drawn from the reacting vessel at regular intervals of time. The reaction is stopped by chilling the sample by adding a suitable chemical. The number of reactants or products can be determined by titrating the samples with a proper chemical reagent.

CH3COOH + H2O ↔  CH3OH + CH3COOH

The titration is followed by measuring acetic acid by titrating the sample at regular intervals of time with standards alkaline (NaOH).

With the passage of time, more and more acetic acid is produced. Then, a graph is plotted between concentration and time and the rate of reaction is calculated by the graph.

6 Factors Affecting the Rate of Reaction

The rate of reaction is affected by the following factors:factors affecting rate of reaction

  1. The concentration of reactants.
  2. Nature of reactants.
  3. Temperature
  4. Presence of catalyst.
  5. The surface area of reactants.
  6. Radiation
  • The concentration of Reactants

the rate of reaction varies with the concentration of the reactants.

Rule

According to the law of mass action rate of reaction is directly proportional to the concentration of reactants.

Effect

The greater the concentration of the reactants, the higher the rate of reaction.

Reason

If the concentration of the re4actant increases, so, the number of colliding reactant molecules also increases, which increases the frequency of collision between them. Hence, the rate of reaction increases.

  • Effect of Temperature

It varies with temperature.

  • Effect of Exothermic Reaction

In an exothermic reaction, the rate of reaction decreases with the increase in the temperature.

  • Effect on Endothermic Reaction

In endothermic reactions, the rate of reaction increases with the increase in temperature.

Rule

It has been observed that for every 10K rise in temperature, the rate of the reaction becomes double.

Reason

  • If the temperature increases, the molecular velocity increase, so, the frequency of the collision increases, hence, the rate increases.
  • It has been observed that for every 10K rise in temperature, the number of effective collisions becomes double, hence, the rare is also doubled.

Milk sour more rapidly in summer than in winter

The rate of reaction is affected by temperature. By increasing temperature the kinetic energy of the molecule increases. Hence, the rate of reaction increases. The sour taste of milk is produced during a chemical reaction. In summer the temperature of the environment is greater as compared to winter that is why in summer spoiling of the milk proceeds much more rapidly than in winter.

Food kept in refrigerators can be stored for a longer

It is because the rate of decomposition of food various directly with temperature which is quite low in refrigerators. Hence, the decomposition of food becomes slow waste and so the food kept in the refrigerator can be stored for a longer time.

  • Effects of Surface Area of Reactants

The rate of reaction varies with the surface area of reactants.

Effect

The greater the surface area of reactants, the higher is the rate of reaction.

  • Effect of Catalyst

It is the substance that alters the rate of the reaction without being used in the chemical reaction.

  • Effect of Nature of Reactants

The rate of reaction varies with the nature of reactants.

Reason

The activation of energy for the effective collision of the different substances is different.

Effect

The rate of reaction having high activation energy is low.

  • Effect of Radiation

The sunlight contains ‘photons’ when these high energy photons strike the reactants molecules they provide the necessary activation energy to them for a reaction.

Activation Energy

It is the extra amount of energy that the reactant molecules have to absorb so it’s internal energy becomes equivalent to threshold energy.

Mathematical formula

Activation energy = Threshold energy – Average Internal Energy

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