energy changes

• Endothermic reaction


  Reaction that takes in energy from the surroundings so the temperature of the surroundings decreases
• Endothermic reactions


  • Thermal decompositions
  • Reaction of citric acid and sodium hydrogencarbonate
• Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.
• If a reaction transfers energy to the surroundings

  The product molecules must have less energy than the reactants, by the amount transferred
• Exothermic reaction


  Reaction that transfers energy to the surroundings so the temperature of the surroundings increases
• Exothermic reactions


  • Combustion
  • Many oxidation reactions
  • Neutralisation
• Everyday uses of exothermic reactions

  • Self-heating cans
  • Hand warmers
• Some sports injury packs are based on

  • Endothermic reactions
• Activation energy


  Minimum amount of energy that particles must have to react
• Required practical: measuring energy change

  1. Put polystyrene cup in a beaker for insulation and support

  2. Measure required volumes of solution with measuring cylinder/pipette

  3. Clamp thermometer into place making sure the thermometer bulb is immersed in liquid

  4. Measure the initial temperatures of the solution or both solutions if 2 are used

  5. Transfer reagents to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance

  6. Stir mixture

  7. Measure final temperature
• Required practical: measuring energy change: neutralisation

  1. Put polystyrene cup in a beaker for insulation and support

  2. Measure required volume of hydrochloric acid with measuring cylinder/pipette

  3. Clamp thermometer into place making sure the thermometer bulb is immersed in liquid

  4. Measure the initial temperature of the acid

  5. Add 5cm3 of sodium hydroxide

  6. Stir mixture and measure temperature

  7. Repeat steps 5 and 6 until 40cm3 of NaOH added
• During a chemical reaction

  • Energy must be supplied to break bonds in the reactants
  • Energy is released when bonds in the products are formed
• Energy change of reaction

  Difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed
• Exothermic reaction


  Energy released from forming new bonds is greater than the energy needed to break existing bonds
• Endothermic reaction


  Energy needed to break existing bonds is greater than the energy released from forming new bonds
• Bond energies


  Energy needed to break bonds and the energy released when bonds are formed
• Calculating energy change of reaction

  Energy change of reaction = sum of bonds energies broken - sum of bonds energies made
• Bond energies


  • H-H: 436 kJ/mol
  • O-H: 464 kJ/mol
  • O=O: 498 kJ/mol
• Exothermic reaction


  Products are lower in energy than the reactants
• Endothermic reaction


  Products are higher in energy than the reactants
• Non-rechargeable cells and batteries

  Chemical reactions stop when one of the reactants has been used up
• Fuel cells


  Supplied by an external source of fuel (eg hydrogen) and oxygen or air, fuel is oxidised electrochemically within the fuel cell to produce a potential difference
• Cells


  Contain chemicals which react to produce electricity, voltage produced is dependent on type of electrode and electrolyte
• Batteries


  Consist of two or more cells connected together in series to provide a greater voltage
• Rechargeable cells and batteries

  Chemical reactions are reversed when an external electrical current is supplied
• Simple cell


  Made by connecting two different metals in contact with an electrolyte, the bigger the difference in reactivity between the two metals the larger the voltage
• Half equations for the electrode reactions in the hydrogen fuel cell

  1. At the negative electrode, hydrogen gas reacts with hydroxide ions to produce water, producing electrons

  2. At the positive electrode, oxygen gas reacts with water to produce hydroxide ions, using up electrons
• The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water
• Advantages of Hydrogen fuel Cells

  • No pollutants produced -only water
  • Do not need recharging
  • Hydrogen can be renewable if made by electrolysis using renewable energy
• Disadvantages of Hydrogen fuel Cells

  • Hydrogen is highly flammable/explosive
  • Hydrogen is difficult to store/takes up a lot of space
  • Hydrogen not renewable if produced using fossil fuels
  • Not many hydrogen filling stations