Reaching Equilibrium (Edexcel GCSE Chemistry)

Predicting the Conditions

Predicting the Ideal Conditions

• We have previously seen that ammonia is manufactured using the Haber Process

• The hydrogen and nitrogen react to form ammonia in the following reversible reaction:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)              ∆H = -92 kJ mol^-1

• The formation of ammonia is exothermic, so using Le Chatelier's Principle we would predict that

  • The reaction will produce a higher yield at low temperatures

  • Using a high pressure would increase the yield as there are fewer moles of gas on the right than the left of the equation

Reaching Equilibrium

• Equilibrium occurs when during the course of a reversible reaction, the rate of the forward reaction equals the rate of the reverse reaction

• This means that products are being formed in the forward reaction as fast as reactants are being formed in the reverse reaction

• It is reached at a faster rate when:

  • A higher pressure is used as there are more successful collisions

  • A higher temperature is used as the particles have greater kinetic energy

  • A higher concentration is used as there are more particles per given volume, hence there are more collisions

  • A catalyst is used as it speeds up the rate of reaction, allowing it to reach equilibrium faster

Choosing the Conditions

Economic Considerations

• Like all industries, companies that manufacture and sell chemical goods do so to make a profit

• Part of the industrial process is the economic decision on how and where to design and implement a manufacturing site

• The availability and cost of raw materials is a major consideration which must be studied well before any decisions are taken

• In the Haber Process the raw materials are readily available and inexpensive to purify:

  • Nitrogen - from the air

  • Hydrogen- from natural gas

• If the cost of extraction of raw materials is too high or they are unavailable then the process is no longer economically viable

• Many industrial processes require huge amounts of heat and pressure which is very expensive to maintain

• Production energy costs are also a factor to be considered carefully and alongside the raw materials issue

Temperature: 450ºC

• A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made

• If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made

• However at a lower temperature the rate of reaction is very slow

• So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly

Pressure: 200 atm

• A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made

• A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating fewer molecules (2 molecules of gaseous products) so a higher yield of products will be made

• However, high pressures can be dangerous and very expensive equipment is needed

• So 200 atm is a compromise pressure between a lower yield of products being made safely and economically

Choosing the conditions for the Haber Process

Catalyst

• The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached

• This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)

• As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.

  So a catalyst is used as it helps the reaction reach equilibrium quicker

• It allows for an acceptable yield to be achieved at a lower temperature by lowering the activation energy required

• Without it the process would have to be carried out at an even higher temperature, increasing costs and decreasing yield as the higher temperature decomposes more of the NH₃ molecules

Diagram showing the effect of catalyst on equilibrium position