Energy changes

• Energy changes


  During a chemical reaction, energy transfers occur
• Energy transfers


  • to the surroundings - exothermic
  • from the surroundings - endothermic
• Energy transfer can cause a temperature change
• Energy is always conserved in chemical reactions
• There is the same amount of energy in the Universe at the start of a chemical reaction as at the end of the chemical reaction
• Surroundings


  Everything that isn't the reaction
• If the thermometer records an increase in temperature, the reaction in the test tube is exothermic
• If the thermometer records a decrease in temperature, the reaction in the test tube is endothermic
• Reaction profile


  Shows whether a reaction is exothermic or endothermic
• Activation energy


  The minimum amount of energy that particles must have to react when they collide
• Reaction types


  • exothermic
  • endothermic
• In an exothermic reaction, energy is transferred to the surroundings and the temperature of the surroundings increases
• In an endothermic reaction, energy is transferred from the surroundings and the temperature of the surroundings decreases
• Bonds


  Atoms are held together by strong chemical bonds. In a reaction, these bonds are broken and new ones are made between different atoms
• Breaking bonds


  Requires energy, so is endothermic
• Making bonds


  Releases energy, so is exothermic
• If a lot of energy is released when making bonds and only a little energy is required to break them, then overall energy is released and the reaction as a whole is exothermic
• If a little energy is released when making bonds and a lot is required to break them, then overall energy is taken in and the reaction as a whole is endothermic
• Bond calculations


  1. Work out how much energy is required to break all the bonds in the reactants

  2. Work out how much energy is released when making all the bonds in the products
• Positive number


  Means an endothermic reaction
• Negative number


  Means an exothermic reaction
• The combustion of methane in oxygen is an exothermic reaction
• The overall energy transferred is -817 kJ/mol
• Rate of reaction


  How quickly the reactants turn into the products
• Measuring rate of reaction


  1. How quickly a reactant is used up

  2. How quickly a product is produced

  3. For reactions involving a gas, by measuring the change in mass or volume of gas
• Mean rate between two points in time


  Change in quantity / Change in time
• The unit for rate is cm^3/s when measuring volume of gas produced
• The unit for rate is g/s when measuring change in mass
• Steep gradient


  Means a high rate of reaction - the reaction happens quickly
• Shallow gradient


  Means a low rate of reaction - the reaction happens slowly
• Mean rate at specific time


  Calculated from the gradient of a tangent to the graph
• Collision theory


  For a reaction to occur, the reactant particles need to collide with enough energy to react
• Factors that increase rate of reaction

  • Increasing the frequency of collisions
  • Increasing the energy of the particles when they collide
• Conditions that increase rate of reaction

  • Increasing the temperature
  • Increasing the concentration of solutions
  • Increasing the pressure of gases
  • Increasing the surface area of solids
• Catalyst


  A substance that can be added to increase the rate of a reaction, without being consumed in the reaction
• A catalyst provides a different reaction pathway that has a lower activation energy, so more particles will collide with enough energy to react