Rates of Reaction and Energy Changes

• Rate of reaction = Amount of reactant used or product formed / time (s)


  Units for rate of reaction:

  g/s

  cm^3/s

  mol/s
• How can you measure the rate of reaction when a gas is given off?

  Measure time and collect gas in an upside measuring cylinder in a trough of water or in a gas syringe to measure the volume of gas produced, measure time and change in mass.
• How can you measure the rate of reactions when a precipitate is formed?

  Put a black cross below a beaker containing one reactant. Time how long it takes for the cross to disappear after the second reactant is added.
• Why is using the precipitation method to investigate rate of a reaction not very accurate?

  Subjective as to when the cross is no longer visible.
• How can you measure rate of reaction using a digital balance?

  When a gas is produced mass will decrease. The rate of reaction can be calculated by recording the mass at regular time intervals.
• What does ’activation energy’ mean?

  The minimum amount of energy required for a reaction to occur between two reacting particles.
• What must happen for a reaction to occur?

  Particles must collide at the correct orientation with sufficient energy to react.
• What two things could happen when the rate of reaction increases?

  More frequent collisions, energy of collisions increases
• How can the rate of a reaction be increased?

  Increase the temperature, concentration, surface area : volume ratio, pressure (for gases) or add a catalyst.
• How does a catalyst affect the rate of reaction?

  Provides alternative reaction pathways with lower activation energy.
• How does pressure affect the rate of a gaseous reaction?

  More reacting particles in the same volume of gas.
• What is used to produce ethanol from glucose?

  Yeast
• Exothermic - Takes in heat, substance gets hotter, beaker gets colder.


  Endothermic - A reaction that gives out heat, colder substance, hotter beaker
• Neutralisation reactions are exothermic.


  Displacement reactions can be either.
• Salt dissolving in water can be either.


  Precipitation reactions are exothermic.
• How could you measure the temperature change of neutralisation reaction?

  Measure initial temperatures, mix both in polystyrene cup, record highest temperature reached
• How can heat loss be minimised?

  Use a polystyrene cup (doesn’t absorb heat), place reaction cup in a beaker filled with cotton wool for extra insulation, lid on the reaction cup
• Combustion is exothermic.


  Neutralisation is exothermic.
• Thermal decomposition is endothermic.


  Photosynthesis is endothermic.
• Energy change (kJ mol^-1) = total energy of bonds broken - total energy of binds made


  If the energy change is negative it is exothermic.
• What is a reaction profile?

  A graphical representation of the energy changes that occur during a chemical reaction and show the and the activation energy of the reaction.