C3.5 Production of ammonia (an example of a reversible reaction)

Defining a reversible reaction and dynamic equilibrium

Whilst most GCSE Chemistry has been taught with the viewpoint that reactants react together to make products, in reality, many chemical reactions can also occur in reverse, particularly those where reactants and products remain in the same state (be it liquid or gaseous).

A reversible reaction is indicated by using a particular symbol:    

If we assume the reaction A + B C + D     we describe,
the forward reaction as A+B → C + D
the reverse reaction as C + D → A + B
Slide09Furthermore, if the forward reaction is exothermic (generates heat), then the reverse reaction is endothermic (absorbs heat) by the same degree.

Similarly, if the forward reaction is endothermic, then the reverse reaction is exothermic by the same degree.

The adjacent figure shows how the formation of ammonia is an exothermic reaction.

 

Returning back to the example of A + B C + D, if A and B are placed in a suitable container, and sufficient activation energy provided, then A and B will react to generate C and D. The rate of the forward reaction is initially high, but as the concentration of reactants fall, so does the rate of the forward reaction. In contrast, as C and D are made, some C and D will react together to regenerate A and B. The rate of this reverse reaction is initially zero but gradually increases as more C and D are made.

Eventually, the rates of the forward and reverse reactions equal other, and the concentration of reactants and products in the reaction mixture remain unchanged: only at this point is the reaction is said to have achieved dynamic equilibrium.

Equilibria graph

Position of equilibrium and ‘yield’ of equilibrium reaction

A common misunderstanding is to assume that the concentrations of reactants and products are equal at dynamic equilibrium. In fact, the symbol,  , is probably better expressed as either  ⥂ or  ⥄ which indicates the reality of the reaction at dynamic equilibrium.

To describe the relative of amount of product compared to reactant in a reaction mixture at dynamic equilibrium we use the term position of equilibrium.

So, at dynamic equilibrium, if there is a higher molar concentration of product compared to reactant, then we describe this as the position of the equilibrium has moved to the right.
Conversely, if there is a higher molar concentration of reactant compared to product at dynamic equilibrium, then we describe this as the position of the equilibrium has moved to the left.

For example, in an aqueous solution of 1 mol dm−3 hydrochloric acid, which is a strong acid that has almost completely dissociated in water, the following quantities are likely to be present as the position of the equilibrium has moved to the right:
HCl (aq)              ⇌       H+ (aq)          +   Cl (aq)
0.01 mol dm−3             0.99 mol dm−3      0.99 mol dm−3

However, in an aqueous solution of 1 mol dm−3 ethanoic acid, which is a weak acid that has only partially dissociated in water, the following quantities are likely to be present as the position of the equilibrium has moved to the left:
CH3COOH (aq)    ⇌        H+ (aq)         + CH3COO(aq)
0.95 mol dm−3           0.05 mol dm−3     0.05 mol dm−3

The *yield of the reaction describes the relative amount of product compared to reactant in the reaction mixture at dynamic equilibrium. Hence, an equilibrium reaction where the position of the equilibrium is towards the right side indicates a high yield. Whereas, an equilibrium reaction where the position of the equilibrium is towards the left side indicates a low yield.

*it is not necessary to state yield ‘of product’, as the term ‘yield’ refers to the amount of product(s).


Law of Equilibria (Le Chatelier’s principle)

When a system at equilibrium is subjected to a small change (e.g. change in temperature, pressure or concentration), then the position of the equilibrium moves in the direction to oppose the change.

The production of ammonia, by the Haber process (N2 + 3H⇌  2NH3), represents an important industrial process that illustrates how knowledge about the Law of Equilibria is used to maximise yield.

Key rules:

  1. Increasing temperature will move the position of the equilibrium in the direction of the endothermic reaction.
  2. Decreasing temperature will move the position of the equilibrium in the direction of the exothermic reaction.
  3. For gaseous equilibria, increasing the pressure of the reaction mixture will move the position of the equilibrium in the direction of the reaction that produces fewer moles of gas.
  4. For gaseous equilibria, decreasing the pressure of the reaction mixture will move the position of the equilibrium in the direction of the reaction that produces more moles of gas.
  5. For gaseous equilibria, if both sides of the equation have equal moles of gas, then altering the pressure has no overall on the position of the equilibrium (and thus does not affect yield).

Pages: 1 2