Paper 2 - AQA Chemistry

• Rate of a chemical reaction

  Can be found by measuring the amount of a reactant used or the amount of product formed over time
• Drawing and interpreting graphs showing the quantity of product formed or quantity of reactant used up against time


  1. Quantity of reactant used up vs time

  2. Rate of reaction = Amount of product formed / Time

  3. Rate of reaction = Amount of reactant used / Time
• Quantity of reactant or product

  Can be measured by the mass in grams or by a volume in cm3
• Units of rate of reaction

  g/s or cm3/s
• Quantity of reactants


  Can also be measured in terms of moles and for rate of reaction the unit can be measured in mol/s
• Using graphs to show reaction rates

  1. Quantity of product formed e.g. Volume of gas vs Time

  2. The slope/gradient of these graphs is a measure of reaction rate

  3. Where the slope is steepest, the reaction rate is fastest

  4. When the slope is horizontal the reaction rate is zero and the reaction has stopped
• Mean rate of a reaction


  Can be calculated from given information about the quantity of a reactant used or the quantity of a product formed and the time taken
• Calculating the rate using the gradient

  1. Draw a tangent to the curve on the graph

  2. Calculate the gradient of the tangent by drawing a tangent triangle

  3. Use the slope/gradient of the tangent as a measure of the rate of reaction at that time
• Common ways of measuring rate

  • Loss in mass of reactants
  • Volume of gas produced
  • Time for a solution to become opaque or coloured
• Measuring mass loss


  1. Can be done if a heavy gas like carbon dioxide is given off

  2. Typically used for marble chips (CaCO3) and acid reaction
• Measuring the volume of gas evolved over time

  1. Using a gas syringe

  2. Gas volume reading are taken at regular time intervals and a graph plotted

  3. The gradient of this graph will be equal to the reaction rate

  
• Measuring the disappearance of a cross through a mixture of acid and sodium thiosulfate


  1. The mixture becomes cloudy as solid sulfur is produced

  2. Time how long it takes for the cross on a piece of paper to disappear
• If investigating the effect of changing temperature on one of these reactions, heat the reactants up to the required temperature in a water bath
• If investigating effect of changing temperature then the control variables will be: volume and concentrations of the acid, volume and concentrations of the sodium thiosulfate, size of flask, size of cross on paper
• Method for investigating effect of changing concentration of sodium thiosulfate

  1. Measure 25cm3 volume of sodium thiosulfate

  2. Place sodium thiosulfate in conical flask

  3. Measure 25cm3 volume of hydrochloric acid

  4. Place conical flask on cross

  5. Add hydrochloric acid to conical flask

  6. Swirl

  7. Start stop clock

  8. Measure time for cross to become no longer visible

  9. Repeat and find mean

  10. Repeat for different concentrations of sodium thiosulfate
• Do not say amount here. Say volume or concentration.
• Collision theory


  • Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
  • The minimum amount of energy that particles must have to react is called the activation energy
• Factors which affect the rates of chemical reactions

  • The concentrations of reactants in solution
  • The pressure of reacting gases
  • The surface area of solid reactants
  • The temperature
  • The presence of catalysts
• Increasing concentration


  Increases the number of particles in the same volume and so increases the frequency of collisions and therefore increases the rate of reaction
• Increasing the surface area

  Increases the frequency of collisions and so increases the rate of reaction
• Increasing temperature


  • Increases the speed of the reacting particles so that they collide more frequently and more energetically
  • More of the collisions are successful as more particles have energy greater than the activation energy
  • This increases the rate of reaction
• Increasing the pressure of a gas

  Increases the number of gas molecules in the same volume and so increases the frequency of collisions and therefore increases the rate of reaction
• Surface area to volume ratio

  The smaller the piece the larger the surface area to volume ratio
• Catalysts


  • Increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy
  • Catalysts change the rate of chemical reactions but are not used up during the reaction
• Different reactions need different catalysts. Enzymes act as catalysts in biological systems.
• Reaction profile for a catalysed reaction

  1. Activation energy without catalyst

  2. Activation energy with catalyst
• Reversible reactions


  • The products of the reaction can react to produce the original reactants
  • Represented as A + B ⇌ C + D
• Equilibrium


  • Reached when the forward and reverse reactions occur at exactly the same rate
  • Occurs in apparatus which prevents the escape of reactants and products
• At equilibrium the amounts of reactants and products remain constant
• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction


  The same amount of energy is transferred in each case
• 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
• Changing concentration


  • If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again and the concentration of the reactant is reduced
  • If the concentration of a product is decreased, more reactants will react until equilibrium is reached again
• Changing temperature


  • If the temperature is increased the equilibrium will always move in the endothermic direction so that the increase in temperature is reduced
  • If the temperature is increased, the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases
• Changing pressure


  • In gaseous reactions, if pressure is increased the equilibrium will move towards the side of the reaction that has the least number of gaseous molecules so that the increase in pressure is reduced
  • An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules
  • A decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules
• If a reaction has the same number of gaseous molecules on both sides of the equation, then increasing pressure will have no effect on the position of equilibrium
• Increasing temperature and pressure will also increase the rate of reaction. Explain this by using the collision theory from earlier in this chapter.
• Catalysts do not affect the position of equilibrium as they speed up the forward and the backward reactions by the same amount.
• Boiling points and viscosity of hydrocarbons

  Increase as the molecules get bigger (because the intermolecular forces become larger as the molecules become bigger)
• Volatility (how easily a liquid vaporises) and Flammability of the fuels


  Decrease as the molecules get bigger
• Crude oil


  Finite resource found in rocks, remains of an ancient biomass consisting mainly of plankton that was buried in mud