Chemistry – Chemical reactions - Reversible reactions and equilibrium | e-Consult

A reversible reaction for the formation of hydrated compounds, such as copper(II) sulfate (CuSO₄) or cobalt(II) chloride (CoCl₂), involves the loss of water molecules from the crystalline lattice to form anhydrous (water-free) compounds. This process is typically endothermic, meaning it requires energy (heat) to proceed.

Increasing the temperature will shift the equilibrium towards the formation of the anhydrous compound. This is because the system will absorb heat, and the forward reaction (loss of water) is endothermic. The equilibrium position will move to favour the products (anhydrous compound) to absorb the excess heat. This can be represented using Le Chatelier's principle: Adding heat to a system at equilibrium will shift the equilibrium in the direction that absorbs heat.

Decreasing the temperature will shift the equilibrium towards the hydrated compound. This is because the system will release heat, and the reverse reaction (absorption of water) is exothermic. The equilibrium position will move to favour the reactants (hydrated compound) to release the excess heat.

Therefore, by manipulating the temperature, we can influence the extent to which a hydrated compound dehydrates to form its anhydrous counterpart. The enthalpy change (ΔH) for the dehydration reaction is typically positive (endothermic), and this drives the equilibrium in the direction of higher temperature.