Topic 7

• Rate of reaction


  Amount of reactant used or product formed / Time (s)
• Units for rate of reaction

  • g/s
  • cm3/s
  • mol/s
• Measuring rate of reaction when gas is given off


  1. Measure time and collect gas in an upside down measuring cylinder in a trough of water or in a gas syringe to measure the volume of gas produced

  2. Measure time and the change in mass
• Measuring rate of reaction 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
• Using the precipitation method to investigate rate of a reaction is not very accurate
• Reason precipitation method is inaccurate


  It's subjective so people are likely to disagree over the exact point at which the cross is no longer visible
• Measuring rate of reaction using a digital balance

  When a gas is produced as this will cause mass to decrease. The experiment can be carried out on a digital balance and the rate of reaction can be calculated by recording the mass at regular time intervals
• Activation energy


  The minimum amount of energy required for a reaction to occur between two reacting particles
• For a reaction to occur, particles must collide at the correct orientation with sufficient energy
• What can happen when rate of reaction increases

  • More frequent collisions
  • Energy of collisions increases
• Ways to increase rate of reaction

  • Increase the temperature
  • Increase the concentration
  • Increase surface area to volume ratio
  • Add a catalyst
  • Increase the pressure (for gases)
• How temperature affects rate of reaction

  Increasing temperature increases the rate of reaction. This is because the reactants have more energy so more particles have energy above the activation energy meaning more collisions will be successful. Collisions also occur more frequently because the particles have more kinetic energy
• How surface area affects rate of reaction

  Increasing the surface area of reactants increases the rate of reaction. This is because a greater surface area means there are more exposed particles so more frequent successful collisions
• How a catalyst affects rate of reaction

  A catalyst increases the rate of reaction. This is because it provides an alternate reaction pathway with a lower activation energy. More particles will have sufficient energy to overcome the activation energy and react so more successful collisions occur in the same time
• How concentration affects rate of reaction

  Increasing concentration increases the rate of reaction. This is because there are more reacting particles in the same volume so there are more frequent successful collisions
• How pressure affects rate of gaseous reaction

  Increasing the pressure of a gaseous reaction increases the rate of reaction. This is because there are more reacting particles in the same volume of gas (or the same number of particles in a smaller volume) so more frequent successful collisions occur
• Measuring rate of reaction for marble chips reacting with hydrochloric acid

  Since gaseous carbon dioxide is released, the rate can be measured by using a digital balance to measure the change in mass over a period of time
• Ways to increase rate of reaction for marble chips and hydrochloric acid

  • Increase the surface area of the marble chips by turning them into a powder
  • Increase concentration of acid
  • Increase temperature
• Finding rate of reaction from a graph

  Plot: X axis: time, Y axis: amount of reactant used or product formed. Draw a tangent to a point on the graph and find the gradient of this line to find the rate of reaction at that time
• Steep gradient on gas vs time graph

  Fast rate of reaction
• A graph showing time and the amount of gas given off during a reaction has an initially steep curve
• Gradient of a graph

  Rate of reaction at that time
• A graph showing time and the amount of gas given off during an experiment has a steep gradient
• Steep gradient


  Fast rate of reaction
• Describing the shape of a graph showing time and amount of gas given off during a reaction

  1. Initially the curve is very steep as the rate of reaction is relatively fast at the start

  2. The curve becomes less steep as the reactants get used up because there are fewer successful collisions occurring

  3. At the end, the graph is a flat line because all the reactants have been turned into products
• Catalyst


  A substance which speeds up the rate of a reaction without being chemically changed at the end
• How a catalyst speeds up a reaction

  It provides an alternate reaction pathway with a lower activation energy
• Enzymes


  Biological catalysts that increase the rate of reactions in living cells
• Endothermic


  A reaction that takes in heat energy from the surroundings
• Exothermic


  A reaction that gives out energy to the surroundings
• Neutralisation reactions are exothermic
• Displacement reactions can be either exothermic or endothermic
• The dissolution of a salt in water can be either exothermic or endothermic
• Precipitation reactions are exothermic
• Measuring the temperature change of a neutralisation reaction

  1. Measure initial temperature of the solutions

  2. Mix both reactants in a polystyrene cup

  3. Record the highest temperature reached

  4. Calculate the temperature change
• Minimising heat loss when measuring temperature change of a reaction

  1. Use polystyrene cup

  2. Place reaction cup in a beaker full with cotton wool for extra insulation

  3. Lid on the reaction cup
• Bond energies in chemical reactions

  • Energy is needed to break bonds and is released when bonds are made
  • Exothermic: Energy released from breaking bonds is greater than the energy used to make bonds
  • Endothermic: Energy released in forming new bond is greater than the energy used to break old bonds

  
• Exothermic reactions


  • Combustion
  • Neutralisation
• Endothermic reactions


  • Thermal decomposition
  • Photosynthesis
• Calculating energy change of a reaction from bond energies (higher)

  Energy change (kJ mol-1) = Total energy of bonds broken - total energy of bonds made