rate of reactions

• Collision theory

  For a chemical reaction to take place, the particles must collide with enough energy to react (this is called a successful collision)
• Activation energy

  The minimum energy the particles need to react
• Rate of reaction

  Depends on the frequency of successful collisions
• Reactions are fastest at the beginning, slow down and eventually stop
• Reactions slow down over time because there are less reactant particles and so the successful collisions between reactant particles are less frequent
• The reaction stops when there are no more particles of one of the reactants
• Effect of concentration of solutions on rate of reaction

  The higher the concentration of reactants in solution, the faster the rate of reaction
• This is because particles are closer together and so successful collisions are more frequent
• If the concentration is doubled, successful collisions are twice as frequent. Therefore, the rate is proportional to the concentration
• Effect of pressure of gases on rate of reaction

  The higher the pressure of reactant gases, the faster the rate of reaction
• If the pressure is doubled, successful collisions are twice as frequent. Therefore, the rate is proportional to the pressure
• Effect of surface area of solids on rate of reaction

  The more pieces a solid is broken up into, the greater the surface area
• The greater the surface area to volume ratio of solid reactants, the faster the rate of reaction
• This is because there are more reactant particles on the surface, and so successful collisions are more frequent
• If the surface area is doubled, successful collisions are twice as frequent. Therefore, the rate is proportional to the surface area
• Effect of temperature on rate of reaction

  The higher the temperature, the faster the rate of reaction
• This is because the particles have more energy and so more of the collisions are successful, and the particles move faster and so collisions are more frequent
• If the temperature is doubled, the rate of reaction is much more than doubled as the increasing temperature increases both the success and frequency of collisions. Therefore, rate of reaction is not proportional to temperature
• Catalyst


  A substance that increases the rate of a reaction but is not used up
• Catalysts do not appear in the overall chemical equation for a reaction as they are not used up
• Catalysts work by providing a different reaction pathway that has a lower activation energy
• Reversible reactions


  Some reactions are reversible, meaning that once the products have been made they can turn back into the reactants
• If the forward reaction is exothermic, the reverse reaction is endothermic. If the forward reaction is endothermic, the reverse reaction is exothermic
• Dynamic equilibrium


  In a closed system, where no chemicals can get in or out, a reversible reaction can reach a state of dynamic equilibrium, where the forward and reverse reactions are taking place simultaneously but at exactly the same rate
• A good analogy of a system in a state of dynamic equilibrium is somebody walking up an escalator that is moving down
• Le Châtelier's Principle

  If a change is made to the conditions of a system at equilibrium, then the position of the equilibrium moves to oppose that change in conditions
• Factors that affect the position of an equilibrium

  • Temperature
  • Pressure
  • Concentration
• If the temperature is increased, the equilibrium position moves in the direction of the endothermic reaction to reduce the temperature
• If the pressure is increased, the equilibrium position moves towards the side with fewer gas molecules to reduce the pressure
• If the concentration of a substance is increased, the position of the equilibrium moves to reduce it by doing more of the reaction that uses it up
• If the temperature is decreased, the equilibrium position moves in the direction of the exothermic reaction to increase the temperature
• If the pressure is decreased, the equilibrium position moves towards the side with more gas molecules to increase the pressure
• If the concentration of a substance is decreased, the position of the equilibrium moves to increase it by doing more of the reaction that makes it
• Effects of changing factors on the yield of hydrogen

  • Increase temperature (equilibrium position moves right in endothermic direction to increase hydrogen formed)
  • Increase pressure (equilibrium position moves left to side with less gas molecules, decreasing hydrogen formed)
  • Increase concentration of steam (equilibrium position moves right to remove added steam, increasing hydrogen formed)