the rate of reaction

• The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product formed over time:
• mean rate of reaction = quantity of reactant uses / time taken
• mean rate of reaction = quantity of products formed / time taken
• The quantity of reactant or product can be measured by the mass in grams or by a volume in cm3 .
• The units of rate of reaction may be given as g/s or cm3 /s.
• factors affecting the rates of chemical reactions:

  • concentration of reactants
  • the pressure of reacting gases
  • the surface area of solid reactants
  • temperature
  • catalyst
• how does a concentration change affect the rate of reaction?

  increasing the frequency of collisions therefore increasing the rate of reaction
• how does pressure change affect the rate of reaction?

  increase the frequency of collisions therefore increasing the rate of pressure
• how does the surface area affect the rate of reaction?

  increase the amount of surface available to react with causing an increase of the frequency of collisions therefore the rate of reaction
• how does the temperature affect the rate of reaction?

  the reactant particles move more quickly as they have more energy so particles collide successfully more often and the rate of reaction increases. this is due to more particles having energy greater than or equal to the activation energy
• how does the catalyst affect the rate of reaction?

  lowers the activation energy
• 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.
• collision 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 surface area of solid reactants:

  • surface area increases due to the reactant being broken down into small pieces
  • increase the surface area to volume ratio
  • since there is a larger area of solid with the particles can react
  • more frequent collisions
  • increase rate of reaction
• reversible reactants: reactants that can be converted back into the original products when the reaction is complete
• The direction of reversible reactions can be changed by changing the conditions.
• ammonium chloride ---> (heat) ammonia + hydrogen chloride
• ammonia + hydrogen chloride --> (cold) ammonium chloride
• 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.
• endothermic reaction:

  hydrated copper sulfate (blue) --> anhydrous copper sulfate (white)+ water
• exothermic reaction:

  water + anhydrous copper sulfate (white) --> hydrated copper sulfate (blue)
• equilibrium: a state in a reaction where the rate of the forward reaction is equal to the rate of the reverse reaction
• equilibrium occurs when a reversible reaction occurs in an 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 relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.
• If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.
• The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.
• le chateliers principle: when a change is made to a system at equilibrium the position equilibrium moves to counteract the change that was made
• if the concentration of one of the reactants or products is changed the system is not in equilibrium
• the concentrations of all the substances will change until equilibrium is reached again
• the concentration of a reactant is increased more products will be formed until the equilibrium is reached again
• if the concentration of a product is decreased more reactants will react until equilibrium is reached again
• if the temperature of a system at equilibrium is increased:

  • amount of products at equilibrium increase for an endothermic reaction
  • amount of products at equilibrium decrease for an exothermic reaction
• if the temperature decreases:

  • products decrease for an endothermic reaction
  • products increase at the exothermic reaction
• increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules shown by the symbol equation
• a decrease in pressure causes the equilibrium positon to shift towards the side with a larger number of molecules as shown by the symbol equation