topic 6 - the rate and extent of chemical change

• The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed
• Collision theory: for a chemical reaction to happen, reactant particles must collide with each other with enough energy and the correct orientation
• A collision that produces a reaction is called a successful collision
• The activation energy is the minimum amount of energy needed for a collision to be successful. It is different for different reactions
• mean rate of reaction = quantity of reactant used / time taken

  or

  mean rate of reaction = quantity of product formed / time taken
• The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time. The steeper the line, the greater the rate of reaction. The gradient of the line is equal to the rate of reaction

  
• The greater the frequency of successful collisions, the greater the rate of reaction
• Increasing the concentration/pressure of a reaction solution/gas increases the rate of reaction as frequency of collisions increases
• The rates of two or more reactions can be compared using a graph of mass or volume of product formed against time.

  
• Tangents are used on rate of reaction graphs to find the rate of reaction.

  rate of reaction = change in mass or volume of product / change in time
• For a given mass of solid, large lumps have a smaller surface area to volume ratio than small lumps or powders. If a large lump is divided or ground into a powder, its total volume stays the same but the area of exposed surface and the sa:vol ratio increases
• If the sa:vol ratio of a reacting solid is increased, more reactant particles are exposed at the surface meaning frequency of collisions increase and therefore the rate of reaction increases
• If the temperature of the reaction mixture is increased, the rate of reaction increases
• A catalyst is a substance that increases the rate of a reaction, does not alter the products of the reaction and is not changed or used up at the end of the reaction
• A catalyst provides an alternative reaction pathway with a lower activation energy
• In some chemical reactions, the products of the reaction can react together to produce the original reactants. These are reversible reactions:

  A + B ⇌ C + D
• If a reversible reaction is exothermic in one direction, it will be endothermic in the other. The same amount of energy is transferred in both the reactions
• When a reversible reaction happens in a closed container, it reaches a dynamic equilibrium. At equilibrium:

  • the forward and backward reactions are still happening
  • the forward and backward reactions have the same rate of reaction
  • the concentrations of all the reacting substances remain constant
• The position of equilibrium depends on the conditions of the experiment (e.g. pressure, concentration). If any of these factors are altered, then the position of equilibrium changes.
• The equilibrium position of a reversible reaction is a measure of the concentrations of the reacting substances at equilibrium
• In the Haber Process, nitrogen gas is reacted with hydrogen gas to make ammonia gas. The forward reaction is exothermic.

  N2(g) + 3H2(g) ⇌ 2NH3(g)
• In the Haber Process, the equilibrium position is:

  • to the left if the concentrations of N2 and H2 are greater than the concentration of NH3
  • to the right if the concentration of NH3 is greater than the concentrations of N2 and H2
• According to Le Chatelier’s principle, the equilibrium position can be changed by changing the reaction conditions through:

  • changing the pressure
  • changing the concentration
  • changing the temperature
• According to Le Chatelier: When a change is made to a system at equilibrium, the position of equilibrium moves to counteract the change that was made. For example, if the temperature is increased, the position of equilibrium moves in the endothermic direction to reduce the temperature
• If the pressure if increased in a reaction involving gases, the equilibrium position moves in the direction of the fewest molecules of gas to reduce the pressure
• There are fewer molecules on the right hand side of the equation for the Haber Process. If the pressure is increased, the equilibrium position moves to the right
• In a reversible reaction, if the reaction is exothermic in one direction it is endothermic in the other. If the temperature is increased the equilibrium moves in the direction of the endothermic process
• If the concentration of a reactant is increased, the equilibrium position moves in the direction away from this reactant, and so more of the products are produced. If one of the products is removed from a reaction, then the position of equilibrium moves to the right to make more of that product.