Heat of displacement

The heat of displacement is the heat released when one mole of a metal is displaced from its salt solution by a more electropositive metal. Malay: haba penyesaran · Chinese: 置换热.

A displacement reaction is a redox reaction in which a metal higher in the electrochemical series pushes a metal lower down out of its salt solution. It is exothermic, so the heat of displacement is negative, and it is quoted per mole of the metal that is deposited.

Example. When zinc powder is added to copper(II) sulfate solution, zinc displaces copper: Zn + CuSO₄ → ZnSO₄ + Cu. The blue solution fades and the temperature rises as copper is deposited. To find the heat of displacement you measure the temperature rise, put the mass of the solution and the rise into Q = mcθ, then divide Q by the moles of copper displaced to get a value in kJ mol⁻¹.

Confusion to avoid. Do not confuse the heat of displacement with the heat of precipitation. Displacement is a redox change in which one metal replaces another and electrons are transferred; precipitation is the formation of an insoluble solid from ions with no change in oxidation number. They share the Q = mcθ method but describe different reactions.

The heat of displacement ties this chapter back to the reactivity series you learned earlier: the larger the gap between the two metals, the more heat is released, which is why SPM Chemistry may ask you to predict which pair gives the greater temperature rise. In the experiment the displacing metal is added as a powder so that its surface area is large and the reaction is quick and complete, and the salt solution is in excess so that all of the metal reacts. Remember that m in Q = mcθ is the mass of the salt solution, not the mass of the metal added, and the final step is to divide Q by the moles of the metal deposited to reach the value per mole.