chemistry topic 5

EXOTHERMIC AND ENDOTHERMIC REACTIONS

  • energy cannot be created or destroyed only transferred
  • an exothermic reaction transfers energy to the surroundings usually by heating, theres a rise in temp, eg combustion and neutralisation reactions, hand warmers 
  • an endothermic reaction transfers energy to the surroundings by taking energy in from the surroundings shown by a fall in temperature eg thermal decomposition and sport injury packs 
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MORE EXOTHERMIC AND ENDOTHERMIC REACTIONS

  • you can measure the amount of energy released by a chemical reaction (in solutions) by taking the temperature of reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction
  • the biggest problem with energy measurements is the amount of energy lost to the surroundings, you can reduce this a little bit by putting the polystyrene cup in a beaker of cotton wool to give more insulation and putting a lid on the cup to reduce energy lost by evapouration
  • reaction profiles- exothermic- reactants are higher than products
  • reaction profiles- endothermic- reactants are lower than products
  • activation energy or Ea is the amount of energy required to start a reaction
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BOND ENERGIES

  • during a chemical reaction old bonds are broken and new bonds are formed 
  • energy must be supplied to break the bonds, so it is endothermic
  • energy is released when new bonds are formed, so it is exothermic
  • in exothermic reactions the energy released by forming bonds is greater that the energy used to break them
  • in endothermic reactions the energy used to break the bonds is greater than the energy released by forming them 
  • overallenergy change= energy (bonds broken) - energy (bonds formed) 
  • if the result is negative it is an exothermic reaction
  • if the result is positive it is an endothermic reaction
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CELLS AND BATTERIES

  • chemical reactions in a cell produce electricity
  • an electrochemical cell is a basic system made up of two different electrodes in contact with an electrolyte, the two electrodes must be able to conduct electricity and so are usually metals
  • the electrolyte is a liquid that contains ions which react with the electrodes
  • the chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes, if the electrodes are connected via a wire the charge is aable to flow and electricity is produced
  • the type of electrolyte affects the voltage of the cell
  • in non rechargable batteries the reactant get used up, once any one of the reactants are used up the reaction can happen and so no electricity is produced
  • non rechargable batteries eg alkaline batteries contain cells which use irreversible reactions meaning once they stop working you have to replace them 
  • in rechargable batteries the reaction can be reversed by connecting it to an external electric current
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FUEL CELLS

  • fuel cells use fuel and oxygen to produce electrical energy 
  • a fuel cells is an electrical cell thats supplied with fuel and oxygen or air and uses energy from the reaction between them to produce electrical energy efficiently
  • when the fuel enters the cell it becomes oxidised meaning theres pd in the cell
  • an example of this is the hydrogen-oxygen fuel cell which produces water and releases energy
  • the hydrogen goes into the anode compartment, hydrogen loses electrons to produce H+ ions (oxidation)
  • the oxygen goes into the cathode compartment, oxygen gains electrons and reacts with the H+ ions from the electrolyte to form water (reduction)
  • the electrons flow through an external circuit from the anode to the cathode this is the electric circuit
  • the overall reaction is hydrogen + oxygen which makes water
  • petrol is finite , fuel cell vehicles dont produce any pollutants, fuel cells take a long time to be charged 
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