C5 Energy Changes

• 4.5.1.1 - Energy transfers

  • In a chemical reaction energy is conserved and the amount of energy in the universe at the end of the chemical reaction is the same as before it took place
  • If a reaction transfers energy to the surroundings the product molecules have less energy than the reactants
  • If the products of a reaction store more energy than the reactants they take in the difference in energy between the products and reactants from the surrounds
• Exothermic Reactions


  • Heat is given out because they transfer energy to the ussrounds usually by heat. This is shown by a rise in temperature
  • Examples are:
  • Burning fuels (combustion) give out lots of energy
  • Neutralisation reactions (acid + alkali)
  • Oxidation reactions like adding sodium to water
  • Hand warmers
• Endothermic Reactions


  • Heat is taken in as energy is taken in from the surroundings. This is shown by a fall in temperature
  • Less common than exothermic reactions
  • Examples include:
  • reaction between citric acid and sodium hydrogen carbonate
  • thermal decomposition (CaCO3 + heat > CO2 + CaO)
  • sport injury packs
• RPA 4 - Temperature Changes

  1. Measure 25cm^3 of 0.25mol/dm3 of hydrochloric acid and pour it into a polystyrene cup
  2. Put polystyrene cup in a beaker to prevent it falling
  3. Uses thermometer to measure initial temperature of acid and record it
  4. Measure 5cm^3 of sodium hydroxide solution and pour it into polystyrene cup
  5. Fit plastic lid on the cup and place thermometer through hole in the lid
  6. Use thermometer to gently stir solution and record the temp every 30 seconds of mixing
  7. Temperature should increase as its an exothermic reaction
• Part 2 Temperature Changes

  1. Wash out the polystyrene cup and repeat the experiment but each time you increase the volume of sodium hydroxide by 5cm3 until you reach 40cm3
  2. Once reached repeat experiment from the beginning so you have two sets of maximum temperatures reached at each volume so you can find a mean
  3. Plot a graph the results should show a straight diagnol line then it goes horizontally flat
• 4.5.1.2 - Reaction Profiles

  • Reaction profiles are diagrams that show the relative enrgies of the reactants and products in a reaction and how the energy changes over course of reaction
  • 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.
• Exothermic energy profiles

  • For exothermic reactions the products should be lower than the reactants
  • The initial rise in temperature/energy is the activation energy (Ea)

  
• Endothermic energy profiles


  • The greater the activation energy the more energy needed to start a reaction
  • The products are higher than the reactants
  • The difference in height represents the overall energy change during the reaction

  
• 4.5.1.3 - Bond energies

  • During a chemical reaction energy must be supplied to break existing bonds so bond breaking is an endothermic process
  • Energy is released when new bonds are formed so bond formation is an exothermic reaction
  • You can use known bond energies to calculate the overall energy change for a reaction
  • Overall energy change = sum of energies needed to break bonds in the reactants - energy released when new bonds are formed in products
• • In exothermic reactions the energy released by forming bonds is greater than the energy used to break them
  • In endothermic reactions the energy used to break bonds is greater than the energy released by forming them
  • Negative energy change = exothermic
  • Positive energy change = endothermic
• Using the bond energies given below calculate the energy change for the reaction between H2 and Cl2 forming HCl. H-H + Cl-Cl > H-CL H-Cl

  • Bond energies needed: H-H = +436kj/mol. Cl-Cl = +242kj/mol. H-Cl = +431kj/mol
  • Find energy required to break original bonds:
  • (1 X H-H) + (1 X Cl-Cl) = 436 + 242 = 678kj/mol
  • Find energy released by forming new bonds
  • 2 X H-Cl = 2 X 431kj/mol = 863kj/mol
  • Find overall energy change
  • 678 - 862 = -184kj/mol (exothermic reaction)
• 4.5.2.1 Cells


  • Cells contain chemicals which react to produce electricity
  • The voltage produced by a cell is dependent on the type of electrode and electrolyte
  • The two electrodes in the electrochemical cell must be able to conduct electricity so they are metal
  • Chemical reactions between the electrodes and the electrolyte set up a charge difference between the elctrodes
  • If the electrodes are connected by a wire the charge is able to flow and electricity is produced
• 4.5.2.1 - Batteries

  • Batteries are formed by two or more cells being connected together in series to provide a greater voltage.
  • In non-rechargeable cells the reactants get used up and the reaction stops e.g. Alkaline batteries are non-rechargeable.
  • Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.
• 4.5.2.2 - Fuel Cells

  • Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air.
  • When the fuel enters the cell it becomes oxidised and starts a potential difference within the cell
  • The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water.
  • Hydrogen fuel cells offer a potential eco friendly alternative to rechargeable cells and batteries.
• Hydrogen - oxygen fuel cells

  • Can be used in vehicles as a more eco friendly alternative as they produce less pollutants
  • Can be dangerous as hydrogen is explosive when mixed with hair and since hydrogen is a gas it takes up more space than a rechargeable battery
  • Involve a redox reaction
• Redox reaction of hydrogen - oxygen fuel cells

  • The electrolyte is often an acid (phosphoric acid)
  • The electrodes are often porous carbon with a catalyst
  • Hydrogen goes in the anode compartment, oxygen to the cathode
  • At the anode hydrogen loses electrons to produce H+ ions. = H2 > 2H+ + 2e-
  • At the cathode oxygen gains electrons from the cathode and reacts with the H+ ions from the electrolyte to make water: O2 + 4H+ + 4e- > 2H2O
  • Overall reaction: 2H2 + O2 > 2H2O