C6 The Rate and Extent of Chemical Change

• mean rate of reaction

  mean rate of reaction = quantity of reactant used/time taken
• mean rate of reaction

  mean rate of reaction = quantity of product formed/time taken
• Factors which affects the rates of chemical reactions:

  • the concentrations of reactants in solution
  • the pressure of reacting gases
  • the surface area of solid reactants
  • the temperature
  • the presence of catalysts.
• Collision theory

  • Collision theory explains how various factors affect rates of reactions.
  • According to this theory, chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
• Activation energy

  The minimum amount of energy that particles must have to react
• • Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.
  • Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction.
• Catalysts

  • Catalysts change the rate of chemical reactions but are not used up during the reaction.
  • Different reactions need different catalysts.
  • Enzymes act as catalysts in biological systems.
  • Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy.
• Reversible Reactions

  In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions and are represented:

  A +B ⇌C + D

  The direction of reversible reactions can be changed by changing the conditions.
• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred in each case. 
• Equilibrium

  When a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.
• The effect of changing concentration

  • If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.
  • If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.
  • If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.
• The effect of temperature on changes on equilibrium

  If the temperature of a system at equilibrium is increased:

  • the relative amount of products at equilibrium increases for an endothermic reaction
  • the relative amount of products at equilibrium decreases for an exothermic reaction.

  If the temperature of a system at equilibrium is decreased:

  • the relative amount of products at equilibrium decreases for an endothermic reaction
  • the relative amount of products at equilibrium increases for an exothermic reaction.
• The effect of pressure changes on equilibrium

  For gaseous reactions at equilibrium:

  • an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction
  • a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.
• Le chatelier's principle

  If any system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change
• If the temperature of a system at equilibrium is increased:

  The equilibrium will shift towards the direction in which the reaction is endothermic
• If the temperature of a system at equilibrium is decreased:

  The equilibrium will shift towards the direction in which the reaction is exothermic