Learning Objective

In this lesson we will learn how the rate of a chemical reaction is affected by the following factors: temperature, concentration, pressure, surface area, stirring and catalysts.

Learning Outcomes

By the end of this lesson you will be able to:

  • Explain how temperature affects the rate of a chemical reaction.

  • Explain how concentration of solutions affects the rate of a chemical reaction.

  • Explain how pressure of gases affects the rate of a chemical reaction.

  • Explain how surface area of solids and liquids affects the rate of a chemical reaction.

  • Explain how stirring affects the rate of chemical reactions involving heterogeneous mixtures.

  • Explain how catalysts affect the rate of a chemical reaction.

 

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Introduction

  • Based on collision theory, the rate of a chemical reaction depends on:
    1. The frequency of collisions between reactant particles.
    2. The energy of collisions between reactant particles.
    3. The orientation of collisions between reactant particles.
    Consequently, anything that influences any of these three factors will affect reaction rate.

 

Temperature

Effect

  • An increase in temperature will lead to an increase in reaction rate.

 
temperature reaction rate

An increase in temperature leads to an increase in reaction rate.

 

Reason

  • An increase in temperature will give reactant particles more energy, and as a result they will move faster.
    An increase in the energy of reactant particles means that a greater proportion of collisions will involve sufficient energy to break bonds.
    This will consequently increase the rate of reaction.
  • An increase in the movement of reactant particles will result in them colliding more frequently.
    This will further increase the rate of reaction.
  • Examples:
    Magnesium reacts more vigorously with water when heated over a Bunsen burner.
    The industrial production of nitric acid is performed at high temperature to increase the rate of reaction and therefore the efficiency of production.
    The cold temperatures of fridges and freezers reduces the rate of food spoilage.

 

Concentration and Pressure

Effect

  • An increase in concentration of solutions will lead to an increase in reaction rate.

 
concentration reaction rate

An increase in concentration leads to an increase in reaction rate.

 

  • An increase in pressure of gases will lead to an increase in reaction rate.

 
pressure reaction rate

An increase in pressure leads to an increase in reaction rate.

 

Reason

  • An increase in concentration of solutions or pressure of gases means there are more reactant particles in a given space.
    More particles in a given space will result in more frequent collisions.
    This will consequently increase the rate of reaction.
  • Examples:
    Acids that have a higher concentration react more vigorously with other substances.
    Gases stored under high pressure are more explosive when ignited.
    The industrial production of ammonia is performed at high pressure to increase the rate of reaction and therefore the efficiency of production.

 

Surface Area

Effect

  • An increase in the surface area of solids or liquids will lead to an increase in reaction rate.

 
surface area reaction rate

An increase in surface area leads to an increase in reaction rate.

 

Reason

  • An increase in surface area of solids or liquids exposes more reactant particles to collisions, resulting in a greater frequency of collisions.
    This will consequently increase the rate of reaction.
  • Examples:
    A steel bar will not burn when placed in a flame but steel wool will burn readily.
    Dust in flour mills and coal mines can be a serious safety issue as it is highly flammable, whereas grain and coal do not ignite readily.
    Fuel injectors create a fine mist from liquid fuels, enabling efficient combustion in car engines.

 

Stirring

Effect

  • Stirring or agitating heterogeneous reaction mixtures will lead to an increase in reaction rate.

 
stirring reaction rate

Stirring reaction mixtures leads to an increase in reaction rate.

 

Reason

  • Stirring a reaction mixture with two or more phases exposes more reactant particles to collisions, resulting in a greater frequency of collisions.
    This will consequently increase the rate of reaction.
  • Examples:
    Many industrial food and chemical processes, such as fermentation reactions, are carried out in large batch reactors that use stirrers to continually agitate the reaction mixture.

 

Catalysts

Effect

  • The presence of catalysts will lead to an increase in reaction rate.

 
catalyst reaction rate

The presence of a catalyst leads to an increase in reaction rate.

 

Reason

  • Catalysts are substances that lower the energy required to break bonds (activation energy), by orienting reactant molecules in a way that makes bonds easier to break.
    Lower activation energy means a greater frequency of successful collisions between reactant particles.
    This will consequently increase the rate of reaction.
  • Some reactions do not proceed at all without a catalyst.
  • Catalysts increase the rate of a chemical reaction, but are not themselves changed during the reaction.
  • A catalysed reaction is written as:

 
catalysed chemical reaction

 

    This shows that the reaction takes place in the presence of a catalyst, and that the catalyst is neither a reactant nor a product.
  • Examples:
    Hydrogen peroxide breaks down slowly if left on the shelf, but in the presence of manganese dioxide catalyst, it breaks down rapidly.
    A platinum catalyst enables the rapid reaction between hydrogen and oxygen in fuel cells.
    Most chemical reactions that take place in living things require specialised biological catalysts called enzymes, otherwise they would not proceed.

 

Summary

  • The rate of a chemical reaction is largely determined by the chemical properties of the reactants, but is also affected by factors that influence either of the following:
    1. The frequency of particle collisions.
    2. The proportion of particle collisions that are successful.
  • An increase in temperature will lead to an increase in reaction rate.
    Reactant particles have more energy, resulting in a greater proportion of collisions with sufficient energy to break bonds.
    Reactant particles move around more, resulting in a greater frequency of collisions.
  • An increase in the concentration of solutions or pressure of gases will lead to an increase in reaction rate.
    More reactant particles are in a given space, resulting in a greater frequency of collisions.
  • An increase in the surface area of solids or liquids will lead to an increase in reaction rate.
    More reactant particles are exposed to collisions, resulting in a greater frequency of collisions.
  • Stirring of heterogeneous mixtures will lead to an increase in reaction rate.
    More reactant particles are exposed to collisions, resulting in a greater frequency of collisions.
  • The presence of catalysts will lead to an increase in reaction rate.
    The energy required to break bonds is lowered, resulting in a greater proportion of collisions with sufficient energy to break bonds.

 
thermite chemical reaction rate

(Image: CaesiumFluoride, Wikimedia Commons)

 

(Header image: NASA, Wikimedia Commons)

 

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