Additional Science-Chemistry Chapter 5

GCSE AQA Additional Science-Chemistry Chapter 5

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Exothermic and Endothermic Reactions

When chemical reactions take place energy is transferred as bonds are broken and made. Reactions that transfer energy to the surroundings are called 'exothermic' reactions. The energy transferred often heats up the surroundings and so the temperature increases. Exothermic reactions include combustion such as burning fuels and metals, respiration and neutralization between acids & alkalis.

Endothermic reactions take in energy from the surroundings. Some cause a decrease in temperature and others require a supply of energy. Thermal decomposition reactions need a supply of heat to keep going. Photosynthesis is an important endothermic reaction that uses light energy.

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Exothermic Reactions

Heat is released when we add water to white anhydrous copper(II) sulfate (anhydrous means 'without water'). This reaction makes blue hydrated copper(II) sulfate crystals. The reaction gives out heat.

Respiration is a very special kind of burning. IT involves reacting sugar with oxygen inside the cells of every living thing. This makes the energy needed for all the reactions of life, and also makes water and carbon dioxide as waste products.

The temperature increase can be detected using a thermometer.

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Endothermic Reactions

Endothermic reactions are much less common than exothermic ones.

When we dissolve some ionic compounds like potassium chloride or ammonium nitrate in water the temperature of the solution drops.

Thermal decomposition reactions are also endothermic. E.g., the decomposition of calcium carbonate to form calcium oxide and carbon dioxide. This reaction only takes place if we keep heating the calcium carbonate strongly.

The most important endothermic reaction of all is photosynthesis. This is the reaction in which plants turn carbon dioxide and water into sugar and oxygen using energy from the Sun.

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Energy and Reversible Reactions

In reversible reactions, the forward and reverse reactions involve equal but opposite energy transfers. If it is exothermic in one direction then it must be endothermic in the other. The amount of energy released by the exothermic reaction exactly equals the amount taken in by the endothermic reaction.

Changing the temperature of a reversible reaction in a closed system (when nothing is added or taken away from the reaction mixture) changes the amounts of the reactants and products. If we increase the temperature, the amount of products from the endothermic reaction increases. If we decease the temperature, the amount of products the exothermic reaction increases. We can change the yield of the reaction by changing the temperature.

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Energy and Equilibrium

If a reaction is exothermic:

  • An increase in temperature decreases the yield of the reaction, so the amount of products formed is lower
  • A decrease in temperature increases the yield of the reaction, so the amount of products formed is larger

If a reaction is endothermic:

  • An increase in temperature increases the yield of the reaction, so the amount of products formed is larger
  • A decrease in temperature decreases the yield of the reaction, so the amount of products formed is lower
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Haber Process

Changes in pressure affect the yield of reversible reactions that have different numbers of molecules of gases in the reactants and products. An increase in pressure will increase the yield of a reaction that has fewer molecules of gases in the products than in the reactants.

If a reaction produces a larger volume of gases:

  • An increase in pressure decreases the yield of the reaction, so the amount of products formed is lower
  • A decrease in pressure increases the yield of the reaction, so the amount of products formed is larger

If a reaction produces a smaller volume of gases:

  • An increase in pressure increases the yield of the reaction, so the amount of products formed is larger
  • A decrease in pressure decreases the yield of the reaction, so the amount of products formed is lower
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Haber Process (cont.)

equation for the haber process (http://scienceaid.co.uk/chemistry/applied/images/habereq.png)

Four molecules of reactant gases produce two molecules of ammonia gas. So increasing the pressure will produce more ammonia. However, increasing the pressure increases the costs of the process and so a compromise of a reasonably high pressure is used (200-350 atmospheres pressure).

The reaction is exothermic, so lower temperatures give higher yields. However, the reaction is slower at lower temperatures because the rate decreases and the catalyst (iron) does not work as well, so a compromise temperature is used (400-450°C)

The conditions are chosen to produce ammonia as economically as possible. Industrial processes are being developed that use low temperatures and pressures to reduce energy use and waste.

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