Solutions: Type and Energy

Solutions: What are they made up of? All solutions are mixtures, but not all mixtures are solutions. What type of mixture is a solution? (Homogeneous or heterogeneous) and why? Solutions are homogeneous mixtures because they are made up of two or more substances that dissolve each other causing one phase (or layer) Solutions are made up of solutes (what is being dissolved, typically the minority component), and solvents (what is doing the dissolving, typically the majority component).

Types of Solutions Can solutions be made of solids, and/or gases? YES! Examples would be air (a mixture of gases) and brass (a mixture of solids). In aqueous solutions, water is the solvent, and a solid, liquid or gas is the solute. Remember, as a simple check to see if one substance will dissolve another, “likes dissolve likes”. This means, most of the time, polar dissolves polar, and non-polar dissolves non-polar, but the two won’t form a solution. Solubility – the amount of the substance that will dissolve in a given amount of solvent. – Example: the solubility of NaCl in water at 25 o C is 36 g NaCl per 100 g H 2 O.

Entropy Gases mix, but not to lower potential energy. Gases mix to increase entropy. Entropy – a measure of energy randomization or energy dispersal in a system. The pervasive, or universal, tendency for energy to spread out, or disperse, is the reason that two ideal gases mix (such as neon and argon). Here is another example of energy dispersal. What would happen if continually heated a substance with metal tongs and the tongs remained in the burner flame for too long? The heat, thermal energy, would disperse up the tongs, and you would burn your hand. Remember heat travels from hot to cold.

The Effect of Intermolecular Forces A solution always forms if the solvent-solute interaction are greater than or comparable to the solvent-solvent and the solute- solute interactions. Remember, if two liquids dissolve in each other, such as hexane and pentane, they are said to be miscible. Here’s the tricky part, even if the solvent-solute interactions are weak compared to the solvent-solvent and the solute-solute interactions, a solution MAY or MAY NOT form. This all depends on intermolecular forces. – Example – Water and hexane do not mix because the energy disparity, or inequality, is too large, and water can not form hydrogen bonds to hexane. However, water can mix with N 2, CO 2 and O 2 by inducing a dipole in these molecules. Here, the energy disparity is small.

Let’s Try a Practice Problem! Determine whether each compound is soluble in hexane. a.) water (H 2 O) b.) propane (CH 3 CH 2 CH 3 ) c.) ammonia (NH 3 ) d.) hydrogen chloride (HCl) a.) No b.) Yes c.) No d.) No If a molecule has both a polar and a nonpolar end, the longer the nonpolar end becomes, the more fat soluble it becomes and the less water soluble.

Energetics of Solution Formation In determining whether a solution, when formed, will emit or absorb heat, three steps must be considered: – Step 1: the separating of solute particles. This is always endothermic. – Step 2: the separating of solvent particles from each other. This is also always endothermic. – Step 3: the mixing of solute and solvent particles, joining together by IMFs. This is always exothermic. Enthalpy of solution formation can be determined using Hess’s Law. Here, the enthalpy change is the sum of the changes in enthalpy in the three steps above. – If the sum of exo. is approximately equal to endo., the heat of solution is about zero. Example: NaCl(aq). Here there is an increase of entropy that result in a solution, but the energy change of the solution remains nearly constant. – If the sum of the endo. < exo., the solution emits heats (is exothermic), when formed. Here, energy decreases, and entropy increases. Example: CaCl 2 (aq) – In the sum of endo.>exo., the solution absorbs hear (is endothermic), when formed. As long as the heat of solution is not too large, entropy will increase and a solution will be formed. Example: NaHCO 3 (aq) ΔH soln = ΔH solute + ΔH solvent + H mix

Aqueous Solutions and Heat of Hydration In ionic aqueous solutions, ΔH solvent and ΔH mix, can be combined into one term called the heat of hydration, ΔH hydration. ΔH hydration :the enthalpy change that occurs when 1 mol of the gaseous solute ions is dissolved in water. The heat of hydration is always largely negative due to the much stronger ion-dipole attraction between solvent and solute, than the hydrogen bonds between water molecules. ΔH soln = ΔH solute + ΔH hydration

Let’s Try a Practice Problem! The enthalpy of solution for cesium fluoride is kJ/mol. What can you conclude about the relative magnitudes of ΔH solute and the ΔH hydration ? The ΔH hydration must have been greater than the ΔH solute.

pgs #’s 29, 31, 35 & 37 Read pgs