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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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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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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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 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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