C3 Topic 4 - Gases, Reversible Reactions and Ammonia

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Calculating Volumes

Avogadros law - one mole of any gas occups 24dm^2 (24,000cm^3) at room temp + RTP

24dm^3 is the molar volume of gas

Volume of gas (dm3) = moles x 24               moles = mass / Mr

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Reversible Reactions

where the products of the reaction can react to produce the original reactants.

If this reaction takes place where no reactants/products can escape (closed system) then it reaches a state of equilibrium - the amounts will reach a permanent balance. It is a DYNAMIC equilibrium because reactions are taking place in both directions but there is no overall effect because the reactions are at the same rate so cancel each other out.

The position of the equilibrium (relative amounts of products+reactants) depends on temperature and pressure, which increase or decrease rate of reaction, but can result in less product (position). Raising reaches equilibrium faster as increase reaction rate.

All reactions are exothermic one way and endothermic the other. If you raise the temp, the endothermic one will be favoured to use up extra heat. If you lower the temp, the exothermic one will be favoured to give out more heat.

Many reactions have greater volume of products/reactants. Raise pressure: favours reaction that make less volume. Lower pressure: favours reaction that makes more volume.

Catalysts are better as speed up both reactions the same amount - equilibrium quicker but doesnt change equilibrium position.

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Haber Process

A reversible reaction between Nitrogen N2 and Hydrogen 3H2 = Ammonia 2NH3 and heat

Nitrogen extracted from air (78% nitrogen, 21% oxygen). Hydrogen obtained from natural gas. Because its reversible, not all the products will convert - dynamic equilibrium.

Industrial manufacture of ammonia uses high pressures (of 200 atmospheres) as it favours the forward reaction of ammonia. Highest pressure to give best % yield.

The forward reaction is exothermic so incr. temp moves the equilibrium the wrong way so the yield is slightly less. However, high temps increase rate of reaction. Compromise - 450C.

Ammonia is formed as gas but it cools in condensor, liquefies and is removed.

The iron catalyst used increase reaction rate but doesnt affect % yield.

Nirtogenous fertilisers, made from ammonia, increase plant growth. But if they enter rivers/lakes they can cause eutrophication - excessive growth of algae layer on surface. Plants living below th surface die as they can no longer photosynthesis - algae blocks light. Decomposers then feed on dead plants, use up all the oxygen in water, so fish die. - Farmers should be careful not to apply excess fertiliser.

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