Solutions and Colloids

How Solutions Form

Solutions are homogeneous mixtures of two or more substances that have an enthalpy change (heat absorbed or released) associated with their formation.

A solution is a homogeneous mixture of two or more substances. The substance that dissolves a material to form a solution is called the solvent. The dissolved material in a solution is a solute. Thus the solvent dissolves the solute, and together they form a solution. A solution is perhaps most commonly thought of as the liquid that results when a solid solute dissolves into a liquid solvent, as in the case of sugar dissolved in water. In fact solids, liquids, and gases can participate as a solution. A mixture of two or more gases, such as air, is a solution. Gas can also be dissolved in a liquid or in a solid. Carbonated beverages contain dissolved CO2 gas, and H2 gas can be dissolved in solid palladium (Pd). Any mixture of two liquids, such as ethanol and water, is also a solution. Two or more solids can also be mixed to form a new solid, as in the case of many alloys. An alloy is a mixture of two or more metallic elements.

In a solution, the solute particles are evenly distributed throughout the mixture with the solvent, and a solution is therefore homogeneous by definition. When a solution forms, three things must happen. First, the solute-solute molecules move apart from one another as the solute dissolves. The solvent-solvent molecules must also move apart from one another to make room for the dissolved solute molecules. Because the like molecules are attracted to one another, both of these processes are endothermic. Lastly, as the molecules mix, the solute and solvent molecules are drawn to one another by attractive intermolecular forces, an exothermic process. Energy is absorbed by an endothermic reaction and released by an exothermic reaction. Enthalpy (H) is the internal energy of a system plus the work needed to displace the environment to produce the components of the system. The enthalpy change (ΔH{\Delta H}) of endothermic processes is always positive, and the enthalpy change of exothermic processes is always negative. The sum of the ΔH{\Delta H} associated with the three steps in the formation of a solution, called the enthalpy of solution (ΔHsoln{\Delta H}_{\rm{soln}}), is the net enthalpy change of the process.

This three-step process can be summarized as:

1. Solute molecules spread apart: ΔH1>0 \Delta H_1>0.

2. Solvent molecules spread apart: ΔH2>0 \Delta H_2>0.

3. Solvent and solute molecules move together: ΔH3<0 \Delta H_3<0.

And ΔHsoln=ΔH1+ΔH2+ΔH3 \Delta H_{\rm{soln}}=\Delta H_1+\Delta H_2+\Delta H_3.

Enthalpy of Solution

Pure components start at a certain enthalpy (energy of the particles plus work needed to produce components of the system). The solvent and solute molecules use energy to spread out, so the spread-out molecules exist at a higher enthalpy than at the start. When the molecules mix together, they release energy because of the attractive forces between them. If the energy released, ΔH3\Delta H_3, is less than the sum of ΔH1\Delta H_1 and ΔH2\Delta H_2, then ΔHsoln>0 \Delta H_{\rm{soln}}>0, and the process is endothermic. If ΔH3>ΔH1+ΔH2 \Delta H_3>\Delta H_1+\Delta H_2, the process is exothermic.
When a solute molecule is completely surrounded by solvent molecules, it is said to be solvated. A solute is hydrated if it is solvated by water molecules. Hydration enthalpy is the energy released when one mole of solute becomes completely hydrated. This energy explains why water is able to dissolve ionic compounds, which have strong attractive forces between the positive and negative ions. The hydration enthalpy for ionic compounds is approximately equal to the energy needed to separate the ionic compound into its ion components.