Unit 5 - Energy Changes

Exothermic and Endothermic Reactions

Chemicals store a certain amount of energy and different chemicals store different amounts. If the products of a reaction store more energy than the original reactants then they must have taken in the difference in energy between the surroundings. But if they store less. then the excess energy was transfereed to the surroundings. The overall level energy doesn't change.

An Exothermic reaction is one which transfers energy to the surroundings usually by heating. This is shown in a rise in temperature. Examples include:

  • combustion
  • neutralisation reactions
  • many oxidation reactions

An Endothermic reaction is one which takes in energy from the surroundings. This is shown in a fall in temperature and these are much less common than exothermic reactions. Examples includes:

  • the reaction between citric acid and sodium hydrogencarbonate
  • thermal decomposition
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Energy Transfer Practical

You can measure the amount of energy released by a chemical reaction in solution by taking the reagents, mixing them in a polystrene 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 by putting the cup into a beaker filled with cotton wool to give more insulation and putting a lid on the cup to reduce the energy lost to evapouration. 
  • This method works for neutralisation reactions or reactions between metals and acids or carbonates and acids.
  • You can also use this method to investigate what effect different variables have on the amount of energy transferred e.g. the mass or concentration of the reactants used.
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Reaction Profiles

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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 exisiting bonds (endothermic) and energy is released when new bonds are formed (exothermic).

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 binds is greater tahn the energy released by forming them.

Bond Energy Calculations:

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Cells and Batteries

The voltage of a cell depends on many factors:

  • Different metals will react differently with the same elctrolyte - this is what causes the charge difference, or the voltage, of the cell. So the type of electrodes used will affect the volatge of the cell.
  • The bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell.
  • You can predict what the voltage of a cell might be from the information about the voltages of other cells.
  • The electrolyte used in a cell will also affect the size of the voltage since different ions in solution will react differently with the metal electrodes used.

A battery is formed by connecting two or more cells together in series. The volatges of all the cells in a battery are combined so there is a bigger voltage overall.          

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Cells and Batteries

An electrochemical cell is a basic system made up of two different electrodes in contact with an electrolytes:

  • The two electrodes must be able to conduct electricity and so are usually made up of metals.
  • The electrolyte is a liquid that contains ins 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 then connected by a wire, the charge is able to flow and and electricity is produced.
  • A voltmeter can also be connected to the circuit to measure the voltage of the cell.

In non-rechargeable batteries the reactants get used up:

  • In some cells, the chemical reactions that happen at the electrodes are irreversible.
  • Eventually, the reacting particles (ions in electrolyte and metal ions on electrode) are used up and turn into products of the reaction.
  • This means the reaction can't happen and so no electricity is produced.
  • The products can't be turned back into reactants, so the cell can't be recharged.
  • Non-rechargable batterires e.g. alkaline contain cells which use irreversible reactions. Once one of the reactants is used up, they don't produce any more charge and you have to replace them. Rechargeable cell reactions are reversed by connecting it to an external current.
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Fuel Cells

A fuel cell is an electrical cell that's supplied with fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently.

When the fuel enters the cell it becomes oxidised and sets up a potential difference within the cell. There are a few different types of fuel cells, using various fuels and electrolytes.

One important examples is the hydrogren-oxygen fuel cell. This fuel cell combines hydrogen and oxygen to produce nice clean water and release energy.

Hydrogen-Oxygen Fuel Cells involve a redox reaction:

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Advantages and Disadvantages of Fuel Cells

Coventional fuels for vehicles e.g. petrol have a finite supply and they're very polluting. So vehicles that use electrical energy are becoming more and more popular. Batteries are one way of getting cleaner energy but hydrogen-oxygen fuel cells might be even better.

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