4.6.2.1,4.6.2.2,4.6.2.3 - Reversible reactions and dynamic equilibrium

Reversible chemical changes

In a reversible reaction, the products can react to form the original reactants.

Write a double arrow, $\ce{<=>}$, to show that both directions are possible. In the general equation

the forward reaction changes A and B into C and D. The reverse reaction changes C and D into A and B. The arrows do not mean that both directions are equally fast at all times.

Conditions can change the direction in which a reversible change is observed. Blue hydrated copper sulfate becomes white anhydrous copper sulfate when heated, releasing water:

The waters shown before the arrow are part of the hydrated crystal. The forward dehydration releases water, as vapour under heating. Adding water to white anhydrous copper sulfate restores the blue hydrated form. This is a useful visible example of products remaking reactants.

Another example is ammonium chloride. On heating it can form ammonia and hydrogen chloride gases:

The gases can react together in a cooler region to form white solid ammonium chloride again. This explains why the white deposit can appear away from the heated sample. It is not evidence that every gas simply condensed into the original solid unchanged.

These are chemical changes because different chemical substances form. The double arrow identifies the possibility of reverse reaction, not a universal claim that every chemical change reverses easily. Observation of reversal requires suitable conditions and access to the necessary products. Demonstrations involving heating or irritating gases require suitable apparatus, eye protection and teacher-directed risk controls.

Opposite energy transfers in reverse reactions

If one direction of a reversible reaction is exothermic, the opposite direction is endothermic by the same amount.

The forward and reverse reactions exchange the same starting and finishing substances. If the forward change lowers the system's energy, the reverse change must raise it by the same amount. This follows from conservation of energy: completing a forward and reverse cycle returns the substances to their original state, so the cycle cannot create an extra supply of energy.

Heating blue hydrated copper sulfate to the white anhydrous form and water takes in energy: dehydration is endothermic. Adding water to the anhydrous solid releases energy as the hydrated form is restored: hydration is exothermic. The colour observation identifies the substances while the temperature or heating evidence helps establish transfer direction.

For a reversible change, suppose one fixed amount of reactants releases $40\,\mathrm{kJ}$ on forming products. Reversing exactly that change takes in $40\,\mathrm{kJ}$. The comparison requires the same chemical change and amount of substance; easier initiation does not change the overall energy difference.

The activation energies of the two directions need not be equal. On a common reaction profile, they are measured from different starting energy levels to the peak. The equal-and-opposite rule applies to the overall difference between reactants and products, not to those two barriers.