LSS_Formation of Solutions Lesson

Formation of Solutions

The formation of a solution involves changes in energy. The formation of a solution may be an exothermic or endothermic process, depending on the relative strengths of intermolecular/interparticle interactions before and after the dissolution process.   The net energy involved in the formation of a solution is called heat of solution. This is also called molar enthalpy of solution and has the symbol, or ΔHsoln. (You will learn more about enthalpy in the module on thermochemistry). There is a general 3-step process that we can use to visualize the energy involved in solution formation.

Formation of a Solution Solvent Step 1 ΔΗ, Solute Step 2 ΔΗΣ Step 3 ΔΗ, Solution

In step 1, the solvent particles spread apart.

In step 2, the solute particles spread apart.

In step 3, the solute and solvent particles come together and mix.

  • Steps 1 and 2 require energy to separate the particles. This is endothermic and will take energy from the surroundings.
  • Step 3 releases energy. This is exothermic and will release energy to the surrounding.
  • The net result of these steps is the heat of solution , or ΔHsoln. Heat of solution can be either endothermic or exothermic depending on the specific values of steps 1, 2, and 3.

Calculating Heat of Solution

Heat of solution is often calculated using a two step process. This is possible because enthalpy is a process that is a state function. This means that the path we take to calculate its value is not important as long as the beginning and end are the same. In other words, a state function  is any function that is dependent only on the present  state of a system, not how the state was reached. Imagine climbing Stone Mountain. There are many paths that you could take to climb the mountain. You could take the path that is marked for walkers. You could stray off of that path and zigzag your way up the mountain. You could travel in a spiral up the mountain. As long as your ending location was the same in all three examples, the elevation that you reached is the same. Elevation is a state function. The distance you travel differs with each path. The time it takes to reach the top differs with each path. The amount of calories that you burn differs with each path. These are not state functions.

Look at the enthalpy diagram below to see what this means in the process of calculating a heat of solution.

It is important to note that an enthalpy diagram does not actually demonstrate what happens in a laboratory when a solution is formed. Instead, imagine that the solid is vaporized. This is shown in step 1. The energy required to separate the particles of the solid and vaporize them is known as the lattice energy.  

Enthalpy Diagram Potential Energy Step 1 Lattice Energy Step 2 Solvation Energy Overall Formation of Solution is EndothermicIn step 2, we imagine that the gas particles mix with the solvent and are solvated. (Because gas particles were formed in the first step, it is not required that the solvent particles separate before they mix.) The solute and solvent particles attract each other, so this step is exothermic, or releases energy. The amount of energy released is called the solvation energy. If the solvent is water, it is referred to as the  hydration energy.

The net energy change, the difference between the energy required in step 1 and the energy released in step 2, is the heat of solution. This is the same value as the direct method of forming the solution in the lab, as is shown by the purple arrow. The reason for calculating heat of solution using this indirect method (combining lattice energy and solvation energy) is that it allows us to use existing experimental data to calculate the heat of solution.   Remember that this is only possible because the enthalpy of solution is a state function.

If the energy required in step 1 exceeds the energy released in step 2, the solution forms with a + ΔHsoln, endothermic. If the energy required in step 1 is less than the energy released in step 2, the solution forms with a - Δ Hsoln, or exothermic.

Example: What is the ▲ Hsoin of the formation of an aqueous solution of NaBr? Is this an endothermic or exothermic process? Compound NaBr Lattice Energy (kJ mol-1) +728 Hydration Energy (kJ mol-1) -741 Solution: AHsoin = +728 kJ/mol + -741 kJ/mol -13 kJ/mol Because the AHsoln is negative, this is an exothermic process.

Remember to work on the module practice problems as you complete each section of content. 

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