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Equilibria

This GCSE Chemistry quiz is all about equilibrium. The word equilibrium means something is in a state of balance. In chemistry, it refers to a situation in which the concentrations of the reactants and the products are constant. The plural is equilibria and this word refers to the study of concentrations in chemical reactions. Only reversible chemical reactions are subject to equilibria. In a non-reversible reaction, the products do not react with each other. In a reversible reaction, the products can react together to re-form the products.

There are quite a number of reversible reactions, for example, the manufacture of ammonia. During a reversible reaction, both the forwards reaction of reactants going to products AND the backwards reaction of products going to reactants are taking place at the same time. At equilibrium, the two chemical reactions are going at the same rate and so the concentrations of each substance present in the mixture remains the same.

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It is possible to disturb equilibria in reversible reactions by changing the conditions. If you understand rates of reaction, understanding equilibria should be reasonably straightforward for you. In industrial chemistry, getting the balance right is often a compromise between the speed of reaction and the yield, as you will have seen in your studies of the Haber Process.

Firstly, let's look at an increase in temperature. In reversible reactions, if one of the reactions is exothermic and the other endothermic, the yield of products will be affected. A temperature increase will favour the endothermic reaction since there will be more energy available to be absorbed from the surroundings. So if the forward reaction is endothermic, a higher percentage of the reactants will be converted into products, and vice-versa if it is the reverse reaction that is endothermic.

Secondly, we will consider what effect an increase in concentration has on equilibria. If the concentration of the reactants is increased, that will increase the rate of the forward reaction, so more product will be made and vice-versa. Where the reaction is between gases, the equivalent of an increase in concentration is an increase in pressure. If there are fewer gas molecules on the products side than on the reactants side, the higher pressure will favour the products. As always, with equilibria, the opposite is also true.

When discussing changes to equilibria, chemists usually talk about pushing the equilibrium to the left or to the right. An equilibrium that is pushed to the right actually indicates that the forward reaction is favoured and that you will get more product; one that is pushed to the left will have less product and more reactants.

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  1. At the point of equilibrium...
    At this point there is no overall change in the amount of products or reactants
  2. The Haber process is a reversible reaction. If the reaction were at equilibrium, hydrogen and nitrogen would be being produced as quickly as ammonia gas was being broken down. How is the forward reaction rate increased?
    The system attempts to achieve equilibrium by increasing the rate of the forward reaction, i.e. by making more ammonia to replace that which has been removed
  3. If a reversible reaction is exothermic in the forward direction, in the reverse direction it will be...
    Knowing which direction of the reversible reaction is exothermic can help you to predict what might happen when the temperature is changed
  4. What effect will adding a catalyst have to the equilibrium of a system?
    It speeds up both the forward and reverse reactions equally
  5. How do we represent a reversible reaction symbolically?
    This shows the two directions that the reaction will proceed in, both forwards and reverse. Sometimes the arrow is drawn with a larger arrow in one direction to show the predominant reaction direction
  6. A stable equilibrium can be achieved in a...
    A closed system is one in which no reactants or products can enter or leave
  7. What is a reversible reaction?
    A good example of a reversible reaction is the formation of ammonia from hydrogen and nitrogen
  8. At equilibrium, the amount of product(s) and reactant(s) is constant. Which of the following would increase the amount of product in a reaction?
    If the product is removed, there will be less of the reverse reaction occurring. The forward reaction will continue, forming more of the product to replace the quantity removed to re-establish the equilibrium. Adding more of the reactants would have the same effect
  9. At equilibrium, if the rate of the forward reaction increases, the rate of the reverse reaction...
    At equilibrium, the rate of the forward and reverse reaction are the same. To compensate for the increased rate of the forward reaction, the reverse reaction must also increase
  10. One example of a reversible reaction is the Haber process. Pick the correct symbol equation for the reaction between hydrogen and nitrogen to produce ammonia in the Haber process.
    You should have immediately dismissed the first two answers since they don't contain the double arrow that shows this is a reversible reaction. Ammonia is an important chemical in industry and is used to manufacture nitric acid and fertilisers

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