Topic 5: Energy changes

• 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.
• Exothermic reactions


  Transfer energy from the reaction to the surroundings
• In exothermic reactions


  The temperature of the surroundings increases
• Exothermic reactions


  • Certain oxidation, combustion and neutralisation reactions
• Exothermic energy changes


  Negative, because energy has been transferred from the reaction
• For the energy profile for an exothermic reaction

  The reactants are higher than the products
• In exothermic reactions


  The products have less energy than the reactants because energy is released into the surroundings
• The difference between the energy of the reactants and products in an exothermic reaction

  The energy that has been released into the surroundings
• Exothermic reactions


  • Found in hand warmers and in self-heating cans like for food and drink
• Endothermic reactions


  Take in energy from the surroundings
• In endothermic reactions


  The temperature of the surroundings decreases
• Endothermic reactions


  • Thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate
• Endothermic reaction changes


  Positive, because energy has been gained from the surroundings
• For the energy profile for an exothermic reaction

  The products are higher than the reactants
• In endothermic reactions


  The products have more energy than the reactants because energy is taken in from the surroundings
• The difference between the energy of the reactants and products in an endothermic reaction

  The energy that has been taken in by the reaction
• On an energy profile for exothermic and endothermic reactions, the energy rises to a peak, which is the activation energy.
• Activation energy


  The minimum amount of energy that particles must have in order to react, because reactions can only occur when the reacting particles collide with each other with sufficient energy
• Activation energy


  The energy from the energy of the reactants to the peak energy of the reaction
• Energy is required to break chemical bonds


  Breaking bonds is an endothermic reaction
• Energy is released when making chemical bonds

  Making bonds is an exothermic reaction
• Every chemical bond has an energy value, which tells us the energy required to break that bond.
• Energy of reaction


  Sum of bonds broken - sum of bonds made
• Cell


  Composed of two different metals which are placed into an electrolyte. It produces electricity from a chemical reaction.
• Electrolyte


  A solution that can conduct electricity, like a solution of an ionic compound
• When two different metals in an electrolyte are connected with a voltmeter


  There will be a potential difference between them and electric current will flow through the wire
• A cell can only produce electricity for a certain amount of time. Eventually, the chemicals in the cell run out and the reaction stops.
• Cells can only produce electricity if two metals with different reactivities are used.
• The size of potential difference between the two metals in the cell


  Depends on the difference in reactivity between the two metals
• The electrolyte also affects the potential difference.
• Battery


  Consists of two or more cells in series to produce a greater voltage
• In some alkaline batteries, at some point, the reactants in the batteries run out and no more electricity is produced, and there is no way that these reactions can be reversed, so these are non-rechargeable batteries.
• Rechargeable batteries can be recharged because we can reverse the chemical reactions when we apply an electrical current.
• Fuel cell


  A fuel such as hydrogen is reacted with pure oxygen or air. Inside the fuel cell, a chemical reaction takes place, producing an electric current.
• For a hydrogen fuel cell, the only waste product is water, as in the fuel cell, only hydrogen and oxygen react.
• Half equation at the negative electrode of the hydrogen fuel cell

  2H2 → 4H+ + 4e-
• Half equation at the positive electrode of the hydrogen fuel cell


  O2 + 4H+ + 4e- → 2H2O
• Overall equation for the hydrogen fuel cell

  2H2 + O2 → 2H2O
• In a hydrogen fuel cell, hydrogen is being oxidised.
• Advantages of hydrogen fuel cells

  • They will produce electricity for as long as you provide hydrogen, but rechargeable batteries run out and need to be recharged
  • They do not get less efficient the longer they run, but rechargeable batteries can store less electricity the more charging cycles they go through and eventually need to be replaced
  • They can also be a source of drinkable water like on space-craft