C6-The Rate and Extent of Chemical Change

• The rate of reaction is how quickly the reactants turn into products
• To calculate the rate of reaction you can measure

  • how quickly a reactant is used up e.g. quantity of reactant used / time taken
  • how quickly a product is formed e.g. quantity of product formed / time taken
• A steep gradient means a high rate of reaction, so the reaction will happen quickly.
• A shallow gradient means a low rate of reaction, so the reaction will happen slowly
• Collision theory states that chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
• Activation energy is the minimum amount of energy that particles must have to react
• Increasing the concentration of solutions :

  • Increases the rate of reaction
  • This is because there are more reacting particles per unit of volume
  • This means the successful collisions are more frequent
• Increasing the surface area of solids :

  • Increases the rate of reaction
  • This is because the solid has a larger SA:V
  • This means the amount of particles at the surface increases
  • This leads to more frequent successful collisions
• Increasing the temperature :

  • Increases the rate of reaction
  • This is because the particles have more energy
  • This leads to the particles moving faster due to them having more kinetic energy
  • This increases the number of collisions
  • Each collision has more energy meaning more particles can overcome the activation energy barrier
  • This results in more successful collisions
• Increasing the pressure :

  • Increases the rate of reaction
  • This is because there is less space between particles
  • This means collisions will happen more frequently
• Catalysts increase the rate of reaction
• Catalysts provide a different reaction pathway that has a lower activation energy
• Practical : Investigating Rates of reaction, Opaque reaction

  • PLace a white tile with a black X painted on under a glass beaker]
  • Pour 25 cm$^3$ of sodium thiosulfate into the beaker
  • To this add 25 cm$^3$of a 0.1 g/cm$^3$ solution of hydrochloric acid
  • Start a timer
  • Watch the black X from above the beaker
  • Stop the timer when the cross is no longer visible through the reaction mixture
  • Record the time on the timer
  • Repeat steps increasing the concentration of hydrochloric acid
• A reversible reaction is when a chemical reaction is able to move in both the forward and backward direction
• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction
• Equilibrium is reached when the forward and reverse reactions occur at exactly the same rate
• Le Chatelier's Principle is if a chemical system at equilibrium experiences a change the equilibrium will shift to minimise that change
• Temperature and Le Chatelier's Principle :

  • If the temperature of the equilibrium is increased, the position of the equilibrium shifts in the endothermic direction
  • If the temperature of the equilibrium is decreased, the position of the equilibrium shifts in the exothermic direction
• Pressure and Le Chatelier's Principle :

  • If the pressure of the equilibrium is decreased, the position of the equilibrium shifts to the side with more molecules
  • If the pressure of the equilibrium is increased, the position of the equilibrium shifts to the side with less molecules
• Concentration and Le Chatelier's Principle :

  • If the concentration of reactants is increased, the position of the equilibrium shifts to the products
  • If the concentration of products is increased, the position of the equilibrium shifts to the reactants