a) The Haber process is a reversible reaction. What does 'reversible reaction' mean?<br>b) The graphs show the amount of ammonia produced at different temperatures and pressures.
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i) What conditions favor the production of ammonia?<br>ii) In industry the reaction is carried out at $$450^\circ\text{C}$$ and 200 atmospheres pressure. Use the graph to explain why these conditions are used.<br>c) i) Some energy has to be put into the chemicals to start the reaction. Explain how this energy helps the reaction to start.<br>ii) Here is an energy level diagram for this reaction. Is this reaction exothermic or endothermic? Explain your answer in detail.
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(a) A reversible reaction is one that can go in both directions, so the products can react to form the reactants again, $$\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}$$; it never goes to completion but reaches an equilibrium.<br>(b)(i) The graphs show that the percentage of ammonia rises as the pressure increases and falls as the temperature increases, so high pressure and low temperature favour ammonia.<br>(ii) At low temperature the yield is high but the reaction is far too slow, so $$450^\circ\text{C}$$ (with an iron catalyst) is a compromise that gives a reasonable yield quickly. Higher pressures give more ammonia, but very high pressure needs expensive, thick-walled plant and is more dangerous, so 200 atmospheres is an economic compromise.<br>(c)(i) The input is the activation energy. It is used to break bonds in the reactant molecules (the strong $$\mathrm{N \equiv N}$$ and the $$\mathrm{H{-}H}$$ bonds) so that colliding molecules have enough energy to react; once new N-H bonds start forming, the energy they release keeps the reaction going.<br>(ii) It is exothermic. The products ($$\mathrm{2NH_3}$$) are at a lower energy level than the reactants ($$\mathrm{N_2 + 3H_2}$$), so the energy released in forming the new bonds is greater than the energy taken in to break the old ones, and the difference is given out as heat.