4.6 CHEMISTRY

• For a chemical reaction to take place reacting particles collide with each other and the reacting particles collide with sufficient energy
• the factors affecting a chemical reaction are: temperature, concentration, surface area, catalysts and pressure
• a catalyst is a substance that speeds up the rate of reactions by providing an alternate reaction pathway with a lower activation energy.
• collision theory states that a reaction can only take place when the particles of reactants come together, NOTE: the reacting particles don't have to bump into each other but also need to collide with enough energy to allow the reaction to take place.
• when increasing temperature

  • temperature increases the rate of reaction
  • a higher temperature increases the speed/average kinetic energy of particles.
  • this increases the frequency of successful or increases the energy of collisions
• Increasing surface area

  • increases the rate of reaction
  • increases the surface exposed to the reactant
  • increases the frequency of collisions
• Adding a catalyst

  • increases the rate of reaction
  • provides an alternate reaction pathway
  • the alternate reaction pathway has a lower activation energy
  • increases the frequency of successful collisions
• something is reproducible when the same results are obtained by different people using the same method
• something is repeatable when the same results are obtained from the same experiment with the same equipment and the same conditions and the same person
• Equilibrium is the stage in a reversible reaction where the rate of the forward reaction is equal to the rate of the backward reaction
• In an equilibrium the concentration of product and reactant stays constant
• Factors that affect equilibrium

  • Temperature
  • Pressure
  • Concentration
  • A catalyst
• Le Chateliers principle: If a system is at equilibrium, a change in the conditions will cause the equilibrium position to shift to counteract the change
• Changing temperature

  • a + b =c
  • forward reaction is exothermic
  • Therefore increasing the temperature would mean equilibrium would shift to the left hand side as the forward reaction is exothermic and equilibrium would favour an endothermic reaction
  • To try and decrease the temperature
  • a+b = c
  • forward reaction is exothermic
  • Therefore decreasing the temperature would mean equilibrium would shift to the right hand side as the forward reaction is exothermic
  • To try and increase the temperature
  • Therefore the yield of C would increase
• Changing Concentration

  • a+b=c
  • if the concentration of a was increased, equilibrium would shift to the right hand side
  • To try and reduce the concentration of a
  • a+b=c
  • if the concentration of a was decreased, equilibrium would shift to the left hand side to try and increase the concentration of a
• changing pressure

  • 2a = b
  • if we increase the pressure, equilibrium would shift to the right hand side
  • because an increase in pressure favours the side with fewer gaseous molecules
  • The yield of b would increase
• a closed system is when no matter can enter or exit the system
• the equation for the haber process is Hydrogen + nitrogen = Ammonia

  NOTE equals sign means reversible reaction sign

  3H2 + N2 = 2NH3
• Ammonia is made from Nitrogen from the air and hydrogen from methane
• The conditions used for the haber process are:

  1. 450 degrees C (temperature)
  2. 200 atm (pressure)
  3. catalyst (iron)

  NOTE THIS IS FOR INDUSTRIAL PROCESSES
• Issues and solutions with the haber process

  The forward reaction in the haber process is exothermic therefore increasing the temperature too much will mean equilibrium will move the wrong way, away from ammonia and towards nitrogen and hydrogen, but if the temperature is too low it can affect the rate of reaction
• Issues and solutions 2

  Increasing the levels of pressure means there are less gaseous molecules on the products side (ammonia), therefore more yield of ammonia is produced which is a good thing.

  However using high pressures may require strong reaction vessels which is extremely expensive and if it isn't controlled there is a risk of an explosion.
• Ammonia is very useful as it is used in fertiliser so it is very important in the production of cleaning products