C6 - Rate and extent of chemical change

• How can the rate of a chemical reaction be measured?

  1. Measuring the volume of gas given off by a reaction over time.
  2. Measuring the loss of mass of a reaction over time when a gas is produced.
  3. Measuring the amount of light that passes through a reaction mixture (as a precipitate forms) over time – this can be done by inspection or using a light sensor and data logger.
• What factors affect rate of reaction?

  • Concentration of the reactants in solution
  • the pressure of reacting gases
  • Temperature at which the reaction is carried out
  • Surface area of solid reactants
  • The use of a catalyst
• The rate of a chemical reaction can be changed by altering the temperature. If the temperature is increased:

  • the reactant particles move more quickly
  • they have more energy
  • the particles collide successfully more often
  • the rate of reaction increases
  • this is due to more particles having energy greater than or equal to the activation energy

  
• If the concentration or pressure is increased:

  • there are more reactant particles per unit volume (ie the reactant particles become more crowded)
  • there is a greater chance of the particles colliding, which leads to more successful collisions per second
  • the rate of reaction increases

  
• If the surface area of a reactant is increased:

  • more particles are exposed to the other reactant
  • there is a greater chance of particles colliding, which leads to more successful collisions per second
  • the rate of reaction increases

  
• A catalyst is a substance which changes the rate of reaction but is unchanged at the end of the reaction. A catalyst is specific to a particular reaction:

  • different catalysts catalyse different reactions
  • not all reactions have suitable catalysts

  
• Enzymes are biological catalysts and are produced by living organisms to speed up the biochemical reactions they need in order to survive.

  Without the use of catalysts, these biochemical processes would happen too slowly and the organism would not survive.

  Enzymes are often more effective than any non-biological alternatives, and so they are used commercially to speed up a number of different reactions e.g. brewing.

  
• What is activation energy?

  The amount of energy needed for a reaction to occur
• What is a reversible reaction?

  A reaction that can create the initial products and reverse and create the reactants
• Energy changes in reversible reactions

  If a reaction is exothermic in one direction, it will be endothermic in the other direction. The same amount of energy is transferred in both the forwards and reverse reaction.

  hydrated copper sulfate ⇌ anhydrous copper sulfate + water

  CuSO4.5H2O(s) ⇌ CuSO4(s) + 5H2O(l)

  The forward reaction is endothermic and the reverse reaction is exothermic.

  
• 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 equilibrium position of a reversible reaction is a measure of the concentrations of the reacting substances at equilibrium.
• Background to 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)

  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
• 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

  When a change is made to a system at equilibrium, the position of equilibrium moves to counteract the change that was made. 
• Changing the pressure of a reversible reaction

  If the pressure is 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:

  N2(g)+3H2(g)⇌2NH3(g)1+3=4 molecules2 molecules

  If the pressure is increased, the equilibrium position moves to the right.

  
• Changing the Temperature of a reversible reaction

  If the temperature is increased, the equilibrium position moves in the direction of the endothermic process.

  For example, in the Haber process:

  N2(g) + 3H2(g) ⇌ 2NH3(g) (forward reaction is exothermic)

  If the forward reaction is exothermic, the backward reaction must be endothermic. Therefore, if the temperature is increased, the equilibrium position moves in the endothermic direction (to the left) to reduce the temperature. This means that less ammonia (NH3) will be produced.
• Changing the concentration of a reversible reaction

  If the concentration of a reactant (on the left) is increased, the equilibrium position moves in the direction away from this reactant, and so more of the products are produced (on the right). If one of the products is removed from a reaction (on the right), then the position of equilibrium moves to the right to make more of that product.