C6 - The Rate and Extent of Chemical Change

• Collison theory

  For particles to react, they have to collide with sufficient energy in order to surpass ACTIVATION ENERGY
• Activation Energy

  The minimum amount of energy required for a chemical reaction to occur
• Increasing Concentration

  There's more particles, therefore more collisions and enough energy to react
• Increasing Temperature

  There's more kinetic energy, therefore more collisions and enough energy to react
• Increase Surface Area

  More area, therefore more likely to collide and enough energy to react
• Add a catalyst

  Lowers activation energy
• Increasing Pressure

  If pressure is increased, more frequent collisions and enough energy to react
• How can we find the rate of the reaction?

  • Measure the quantity of reactants used OVER TIME
  • Measure the amount of products formed OVER TIME
• Rate of Reaction Formula (Reactants)

  • Quantity of reactants used / time taken
• Rate of Reaction Formula (Products)

  • Amount of product formed / time taken
• Units of rate of reaction

  • g/s
  • cm^3 /s
• How can I calculate the mean rate of reaction from a graph?

  • Either the quantity of a reactant OR product / time taken
• How can I measure the rate of reaction from a graph?

  • Draw a tangent and calculate the gradient of the tangent to the curve by doing -> change in y / change in x
• Factors which affect the rates of chemical reactions

  • Concentration
  • Pressure
  • Surface Area
  • Temperature
  • Catalysts
• Why is a powder reactant used over one large piece?

  • Powder form has the highest surface area, increasing the rate of the reaction
• Catalysts

  • Catalysts increase the rate of reaction, by providing a different pathway for the reaction that has a lower activation energy - catalysts are not used up during the reaction
  • Enzymes act as catalysts in biological system
• Which graph shows the presence of a catalyst and WHY?

  • Graph 1 because the activation energy is smaller, we can tell this because the distance between the reactants to the peak of the curve is smaller

  
• Should you include a catalyst as one of the reactants?

  • No - Catalysts do NOT get used up in the reaction, but increases the rate of the reaction
• Reversable Reactions

  • When the products of the reaction can react to produce the original reactants.
• If a reversable reaction is exothermic in one direction...

  • It is endothermic in the opposite direction (vice versa)
• Equilibrium

  • When a reversable reaction occurs in a closed system, preventing the escape of reactants and products, equilibrium is reached when the forward and backwards reaction occurs at the same rate
• Le Chatelier's Principle

  • If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.
• What are the 3 factors which affect the position of equilibrium?

  • Temperature
  • Concentration
  • Pressure
• What happens if the concentration of the reactant is increased?

  • More products will be formed until equilibrium is reached again
• What happens if the concentration of the products is decreased?

  • More reactants will react until equilibrium is reached again
• Pressure in a reversible reaction

  • An increase in pressure causes the position of equilibrium 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.
• 2NO2(g) ⇋ N2O4(g) (-24 kJ mol-1)

  In the above reaction, increasing the temperature will:

  • According to Le Chatelier's principle if you increase the temperature, the position of equilibrium will shift to reduce the temperature. Because the forward reaction is exothermic and the backward reaction is endothermic, more reactants will react to take in heat energy to balance out the temperature.