chapter 10 - reaction rates and equilibrium

• Dynamic equilibrium


  • The rate of the forward reaction is equal to the rate of the reverse reaction
  • The concentrations of reactants and products do not change
• For a reaction to remain in equilibrium, the system must be closed
• Le Chatelier's principle


  When a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of that change
• At equilibrium, the forward and reverse reactions occur at the same rate
• Adding more reactants


  Equilibrium shifts to the right, more products are formed
• Removing reactants


  Equilibrium shifts to the left, more reactants are formed
• Equilibrium can be disrupted by changes in temperature, pressure or concentration
• More products formed


  Equilibrium shifts to the right
• More reactants formed


  Equilibrium shifts to the left
• you calculate the rate of reaction by doing change in concentration/time

  units = mol dm-3 s-1
• the rate of reaction is faster at the start of the reaction as each reactant is at its highest concentration
• the rate of reaction slows down as the reaction proceeds, because the reactants are being used up
• once one of the reactants has been completely used up, the rate of reaction is zero
• factors that can alter the rate of reaction

  • concentration
  • temperature
  • use of a catalyst
  • surface area of sold reactants
• collision theory 


  two reacting particles must collide for a reactant to occur
• a collision will be effective hen these 2 conditions are met

  • the particles collide with the correct orientation
  • the particles have sufficient energy to overcome the activation energy
• when the concentration of a reactant is increased, the rate of reaction increases as there are more particles in the same volume
• when the pressure of a gas is increased, the rate of reaction increases as there are more gas molecules in a smaller volume
• methods for following the progress of reaction

  • monitoring the removal of a reactant
  • following the formation of a product
• methods for following reaction progress (gases)

  • monitoring the volume of gas produced at regular time intervals
  • monitoring the loss of mass of reactants
• catalysts are not used up in a chemical reaction
• catalysts may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place
• at the end of the reaction the catalyst is regenerated
• a catalyst increases the rate of a chemical reaction by providing an alternative pathway or lower activation energy
• a homogeneous catalyst has the same physical state as the reactants

  the catalyst reacts with the reactants to form an intermediate

  the intermediate then breaks down to give the product and regenerates the catalyst
• heterogeneous catalysts have a different physical state from the reactants

  they are usually solids in contact with gaseous or solutions

  reactant molecules are absorbed (weakly bonded) onto the surface of the catalyst

  after reaction the product molecules leave the surface of the catalyst by desorption
• catalysts reduce thee energy needed for chemical reactions
• making products faster and using less energy because of catalysts can cut costs and increase profitability
• the boltzmann distribution describes the spread of molecular energies in gases

  
• features f the boltzmann distribution

  • no molecules have serio energy ‘ the curve stars at the origin
  • the area under the curve is equal to the total number of molecules
  • there is no maximum energy for a molecule ‘ the curve does not meet the x’axis at high energy. the curve old need to reach infinite energy to meet the x’axis
• boltzmann distribution and temperature

  as the temperature increases, the average energy fo the molecule also increases

  the peak fo the graph is lower on the y’axis and further along the x’axis

  
• boltzmann distribution and temperature

  • more molecules have an energy greater than or equal to the activation energy
  • therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction
  • collisions will also be more frequent as the molecules are moving faster, but the increases energy of the molecules is muc more important than the increased frequency of collisions
• the boltzmann distribution and catalysts

  a greater proportion of molecules now have an energy equal to, or greater than the lower activation energy, so there is an increase in the rate. of reaction

  
• using aqueous chromate ions and drichromate ions to show concentration shifts

  • adding dilute sulfuric acid, makes less of the H+ ions
  • adding sodium hydroxide, OH- ions react with H+ to make water, so makes less of water
• investigating changes to the position of equilibrium with temperature

  • forward and reverse directions have the same value for the enthrall change, but opposite signs
  • in increase in temperature shifts the equilibrium to the endothermic direction
  • a decrease in temperature shifts the equilibrium position in the exothermic direction
• using cobalt chloride to investigate temperature and equilibrium

  • in boiling water, solution turns a blue colour
  • in iced water solution turns a pink colour

  
• increase in temperature (forward exothermic reaction)- equilbrium shifts to left, more reactants made
• increase in temperature (forward endothermic)-position of equilibrium shifts to the right, more products are made
• increasing the pressure of the system will shift the position of the equilibrium to the side with the fewer molecules, reducing the pressure of the system
• a catalyst does not change the position of equilibrium, but speeds up the rate in which equilibirum is established