Question

Briefly Explain why solution formation can be endothermic but still favorable.

And also

Describe how energetically a solution forms.

Answer 1

Dissolution happens in three steps.

Solute-solute attractions must be broken (consumes energy, endotherm), solvent-solvent attractions must be broken (also endotherm), and finally solute-solvent attractions form. This results in a lower energy state and is exotherm.

Dissolution will be endotherm if it takes more energy to break the mentioned attractions, than is released in the last step. To find out we need chemical thermodynamics.

ΔG=ΔH−TΔS

ΔG

= Change in Gibb's free energy for a reaction. ΔG<0

= reaction is spontaneous. Otherwise it is NOT spontaneous (ie. will not occur on its own).

ΔH

= Change in enthalpy. ΔH<0 = reaction is exotherm. If larger than 0, the reaction is endotherm. Already now we can see that an endotherm reaction will make ΔG

for the reaction more positive, and thus the reaction is less likely to be spontaneous.

T

= temperature in K.

ΔS

= Change in entropy. ΔS>0 = reaction leads to a state of higher entropy. Entropy describes disorder in a system. Dissolution of a solute in a solvent will always lead to a state of higher disorder, since we go from having all the solute concentrated in a cluster (more ordered) to being spread evenly throughout the solution (more disordered). We can see from the equation that if the reaction leads to higher entropy, then higher temperatures increase likeliness of ΔG<0

, and the reaction will be spontaneous. On the other hand, if we're going to a more ordered state, lower temperatures increase likelihood for a spontaneous reaction.

we know that the dissolution reaction is endotherm. That means that ΔH>0 for this particular reaction. Yet we know that the reaction is spontaneous, This means ΔG<0. The only way to make that happen is for ΔS to be positive. So the reaction must go to a more disordered state. Since it's a dissolution, we know that it does. The solute goes from being concentrated in a cluster (ordered) to being spread throughout the solvent (disordered) AT HIGHER TEMPERATURE. It is quite easy to see, that for a dissolution reaction where ΔS>0

, you can increase the chance of the reaction being spontaneous by increasing temperature!

A solution tends to be more soluble in a solvent to which they're structurally and electrically similar. (like dissolves like)

A solution forms if solute -solvent interactions > solute solute interactions + solvent-solvent interactions.

Effectively a solution forms if the total energy required to break the interactions between pure solute and pure solvent is lesser than new interactions between energy released in solute and solvent. and so the overall proess is exothermic .