energy changes

• the interaction of particles often involves transfers of energy due to the breaking and formation of bonds
• an endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases
• endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate; some sports injury packs are based on endothermic reactions
• an exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases
• exothermic reactions include combustion, many oxidation reactions and neutralisation reactions; everyday uses of exothermic reactions include self-heating cans and hand warmers
• energy is conserved in a chemical reaction, so this means the amount of energy in the universe (or system) 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 of energy transferred
• if a thermometer detects a temperature increase the reaction is exothermic and if a thermometer detects a temperature decrease the reaction is endothermic
• chemical reactions can only occur when the reacting particles collide with each other and with sufficient energy
• the minimum amount of energy that the particles must have to react is called the activation energy
• reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction
• in an exothermic reaction, the energy put in to break the reactant bonds is less than the energy transferred to the surroundings in the formation of new bonds, so this means the energy of the products is lower than the energy of the reactants
• in an endothermic reaction, the energy transferred to the reactants to break the reactant bonds is greater than the energy transferred to surroundings in the formation of new bonds, so products have more energy than the reactants
• in a chemical reaction the bonds between atoms in the reactants must break apart and the atoms must rejoin in a different way to produce new products
• during a chemical reaction:

  • energy must be supplied to break bonds in the reactants
  • energy is released when bonds in the product are formed
• the energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies
• the amount of energy required to make or break a particular bond is called the bond energy
• the overall (enthalpy) change of a reaction = total energy needed to break bonds in the reactants - total energy released when bonds in the products are formed
• in an exothermic reaction the energy released from forming new bonds is greater than the energy needed to break existing bonds
• in an endothermic reaction the energy needed to break existing bonds is greater than the energy released form forming new bonds
• particles gain energy in breaking bonds as enough energy is needed to overcome the bonds, so breaking bonds is endothermic
• cells contain chemicals which react to produce electricity
• the voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte
• a simple cell can be made by connecting two different metals in contact with an electrolyte
• in a simple cell:

  • the more reactive metal forms ions more easily, readily releasing electrons
  • the electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes so the electrons flow around the circuit
• batteries consist of two or more cells connected together in series to provide a greater voltage
• in non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up
• 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 and this reverses the chemical reactions taking place allowing the cycle to be repeated
• fuel cells are supplied by an external source of fuel (such as hydrogen) and oxygen or air
• in fuel cells, the fuel is oxidised electrochemically within the fuel cell to produce potential difference
• the overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water
• hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries
• in fuel cells, the polarity on the electrodes is the other way around: the anode is the negative electrode and the cathode is the positive electrode
• hydrogen fuel cell advantages:

  • will produce electricity as long as you provide hydrogen, while rechargeable batteries will run out and need to be recharged
  • HFC do not get less efficient the longer they run, while rechargeable batteries store less electricity the more charging cycles they go through and will eventually need replacement
  • HFC do not produce any pollution and the water they produce can be a source of drinking water
  • no power is lost in transmission as there are no moving parts unlike an internal combustion engine
• hydrogen fuel cell disadvantages:

  • run on hydrogen which is a flammable, potentially explosive gas so it is difficult to store safely while no dangerous fuels are required with rechargeable batteries
  • HFC produce a relatively low potential difference so several are needed together, while rechargeable batteries produce a much greater potential difference
  • HFC are affected by low temperatures, becoming less efficient