Reversibility of Chemical Reactions
When we read the chemical equation for a reaction we often describe the process as the reaction of reactants to form products. This is often written and interpreted in such a way that it appears this process is fixed and that the reverse can not occur. Many of the chemical reactions that occur are in fact, reversible. This means that the chemical reaction can proceed in the reverse direction and the products can react to produce reactants. Further to this, reactions that do not appear reversible, may in fact be, but require large amounts of energy to do so.
In a chemical equation:
⇌ or ↔ indicates that a reaction is reversible
→ indicates that a reaction is irreversible
The chemicals that make up the reactants and the products are together described as the chemical system. All other factors that are not involved chemically in the reaction are described as the surroundings or the environment. These factors may include temperature and pressure.
The environment or surroundings can determine many things, such as the speed at which a particular reaction occurs, how far that reaction proceeds and if the reaction is reversible, which direction the reaction will proceed.
Not all reactions are reversible, but those that are can proceed in both directions at the same time. This means that reactants are forming products and at the same time, products are forming reactants. These are referred to as the forward and reverse reactions based on the way the chemical equation is written. The overall rates at which these occur determine the overall direction that the reaction is moving. When the forward and reverse reactions are occurring at the same rate the reaction is said to be at equilibrium and the reaction appears to have stopped. This is an important distinction – the reaction is still happening, but the forward and reverse reactions are occurring at the same rate.
- Some reactions are reversible
- Forward and reverse reactions occur at the same time
- Equilibrium occurs when the forward and reverse reactions occur at the same rate
Analysing the Reversibility of Chemical Reactions
Below are some common examples of reactions that are used to demonstrate the reversibility of chemical reactions. The Cobalt and Iron reactions are particularly useful because of the obvious and contrasting colours that are displayed to illustrate the reaction proceeding in different directions.
Cobalt(II) chloride hydrated and dehydrated
The two different coloured Co(II) ions, Co(H2O)62+(aq)(pink) and CoCl42-(aq)(blue), exist together in equilibrium in solution in the presence of chloride ions:
Co(H2O)62+(aq) + 4Cl–(aq) ⇌ CoCl42-(aq) + 6H2O(l) ∆H = positive (endo)
This reaction can be simplified:
Co2+(aq) + 4Cl–(aq) ⇌ CoCl42-(aq) ∆H = positive (endo)
This equilibrium can be disturbed by changing the temperature or by changing the chloride ion concentration. The addition of NaCl or HCl will increase the Cl– concentration and the addition of AgNO3 will remove Cl– by precipitating AgCl). These changes can be easily reversed and observed.
- Addition of NaCl and HCl will increase the Cl– concentration and shift the system to the right and turn blue
- Addition of AgNO3 will remove Cl– and will shift the system to the left and turn pink.
- An increase in temperature will shift the system to the right and turn blue
- A decrease in temperature will shift the system to the left and turn pink
Iron(III) nitrate and potassium thiocyanate
The two different coloured ions, Fe3+(aq) (yellow) and Fe(SCN)2+ (aq)(red), exist together in equilibrium according to the following equation:
Fe3+(aq) + SCN– (aq) ⇌ Fe(SCN)2+ (aq) ∆H = negative (exo)
This equilibrium can be disturbed by changing the SCN– ion concentration or by changing the temperature. These changes can be easily reversed and observed.
- Addition of SCN– will shift the system to the right and turn red
- An increase in temperature will shift the system to the left and turn yellow
- A decrease in temperature will shift the system to the right and turn red
The combustion of magnesium releases large amounts of energy and requires a similarly large amount of energy to be input to reverse the reaction. As previously noted, this is what makes some reactions to be considered irreversible.
2Mg (s) + O2 (g) → 2MgO (s)
Burning steel wool
The combustion of steel wool also requires a large amount of energy to be reversed and is also considered an irreversible reaction.
4Fe (s) + 3O2 (g) → 2Fe2O3 (s)
An equilibrium was established with a cobalt chloride solution. Predict the impact on the equilibrium and any observations that would be made if some hydrochloric acid was added:
The addition of hydrochloric acid would increase the Cl– concentration. This will result in the equilibrium shifting to the right to favour the products. The solution would be observed to become blue in colour
An equilibrium was established with Iron(III) nitrate and potassium thiocyanate. Predict the impact on the equilibrium and any observations that would be made if some potassium thiocyanate was added:
The addition of potassium thiocyanate would increase the SCN– concentration. This will result in the equilibrium shifting to the right to favour the products. The solution would be observed to become red in colour