Question

1- a. If ΔHsolution is slightly endothermic, a solution might form. Explain why
b. If ΔHsolution is highly endothermic, a solution can never form. Explain why.
c. If ΔHsolution is exothermic, a solution will always form. Explain why.

Answer 1

A solution is a homogeneous mixture of two or more substances. The enthalpy change (ΔH) of a solution refers to the amount of heat released or absorbed during the dissolving process when pressure is constant. This ΔH can either be positive (endothermic) or negative (exothermic). When understanding the enthalpy of solution, we can think about a hypothetical three-step process-

Step 1: Breaking of solute molecules

At first, the solute molecules separate from each other. The enthalpy of this process is called ΔH₁. This is always an endothermic process. ΔH1>0.

Step 2: Breaking of solvent molecules

Like the first step, this reaction is always endothermic (ΔH₂>0) because energy is required to break the interaction between the solvent molecules.

Step 3: Combining the two

At the third step separated solute and the solvent molecules join together to form a solution. The enthalpy of combining these two substances to form the solution is ΔH₃ and is an exothermic reaction (releasing heat since interactions are formed) ΔH₃<0.

So ΔH (ΔH= ΔH₁+ ΔH₂+ ΔH₃) can either be endothermic, exothermic or zero), depending on how much heat is required or release in each step.

But If ΔH becomes highly positive (highly endothermic), change of Gibbs free energy (ΔG) becomes positive. For any reaction or change ΔG must be negative. If ΔG becomes positive somehow, the reaction will not proceed.

ΔG= ΔH-TΔS        T is temperature here.

ΔS (entropy change) is always positive as entropy always increases during the formation of the solution. Hence a solution can never form if ΔH is highly endothermic.