Properties of Solutions Solution Formation and Solubility

Objectives When you complete this presentation, you will be able to define and distinguish between solute, solvent, and solution discuss the factors contributing to solvation define solubility and discuss the factors contributing to solubility determine the solubility of a salt given a graph or table of solubility vs. temperature

Solution Formation Solutions are homogeneous mixtures that may be solid (alloy), liquid (sugar water), or gas (air). Solutions are composed of - a solute (what is dissolving) a solvent (what it is dissolving in)

Solution Formation How fast a solute dissolves in a solvent depends on Agitation Temperature Surface area of the solute

Solution Formation Agitation is stirring. Agitation increases the rate at which a solute will dissolve. More stirring ⇒ faster solvation Agitation does not effect the amount of solute in solution.

Solution Formation As the temperature increases, the speeds of the solvent molecules increase. This means that the solvent particles hit the solute molecules faster and harder. Solutes dissolve faster in hotter solvents. Temperature may also affect the amount of solute dissolved.

Solution Formation Smaller solute particles have larger surface areas per amount of solute. This larger surface area means that there is more of the solute surface for the solvent to work on. Smaller particles dissolve faster than larger particles. Surface area does not effect the amount of solute dissolved in the solvent.

Solubility Solubility relates to the amount of solute that can be dissolved in a solvent. If you add 36.0 g of NaCl to 100 g of water at 25°C, all of the salt will dissolve. Add 1.0 more gram - only 0.2 g of that will dissolve. The last 0.8 g will not dissolve. The NaCl has reached its limit of solubility.

Solubility However, the situation is not so simple: At the limit of solubility, solid solute is dissolving into the solvent as fast as aqueous solute is precipitating onto the solid solute. This is an equilibrium situation: solute(s) ⇄ solute(aq) This is a saturated solution.

Solubility A saturated solution contains the maximum amount of solute for a given amount of solvent. The solubility of a solute is the maximum amount of the solute that dissolves in a solvent at a given temperature. Solubility is usually measured in grams of solute per 100 g of solvent. A solution where the amount of solute is less than the solubility is called an unsaturated solution.

Solubility There are a number of factors that affect the solubility of a solute in a given solvent. Temperature affects the solubility of solids, liquids, and gases in solvents. Pressure affects the solubility of gases in a solvent.

Solubility The temperature of a solvent affects the solubility of solids, liquids, and gases. In general, the solubility of solid solutes increases as the temperature of the solvent increases. In general, the solubility of gaseous solutes decreases as the temperature of the solvent increases. The solubility of liquid solutes in liquid solvents is generally unaffected by the temperature of the solvent.

Solubility We can use a graph of the solubility of a solute to determine the amount we can get into a given amount of solvent.

Solubility For example: This shows the solubility of nine different salts in water.

Solubility To determine the solubility of one of these salts: first, determine the temperature of the solvent, then, draw a line over to the solubility.

Solubility Example 1 Find the solubility of lead(II) nitrate at 30°C. The temperature of the solvent is 30°C.

Solubility Example 1 Find the solubility of lead(II) nitrate at 30°C. Draw the line over to solubility

Solubility Example 1 Find the solubility of lead(II) nitrate at 30°C. At 30°C, 66 g of lead(II) nitrate will dissolve in 100 g of water.

Solubility Example 2 Find the solubility of potassium dichromate at 75°C. The temperature of the solvent is 75°C.

Solubility Example 2 Find the solubility of potassium dichromate at 75°C. Draw the line over to solubility

Solubility Example 2 Find the solubility of potassium dichromate at 75°C. At 75°C, 52 g of potassium dichromate will dissolve in 100 g of water.

Solubility Sample Problems: Find the solubility of each salt in 100 g of solvent at the given temperature. NaNO3 at 30°C NaCl at 70°C Pb(NO3)2 at 35°C KClO3 at 65°C Ce2(SO4)3 at 15°C CaCl2 at 25°C 96 g 39 g 70 g 27 g 6 g 87 g

Solubility We can also determine the temperatures where a maximum solubility occurs: first, determine the solubility then, draw a line down to the temperature.

Solubility Example 3 Find the temperature where the solubility of calcium chloride is 70 g/100 g H2O The solubility of the solute is 70 g/100 g H2O.

Solubility Example 3 Find the temperature where the solubility of calcium chloride is 70 g/100 g H2O Draw the line down to temperature.

Solubility Example 3 Find the temperature where the solubility of calcium chloride is 70 g/100 g H2O Calcium chloride is soluble at 70 g/100 g H2O at 16°C.

Solubility Example 4 Find the temperature where the solubility of potassium chloride is 55 g/100 g H2O The solubility of the solute is 55 g/100 g H2O.

Solubility Example 4 Find the temperature where the solubility of potassium chloride is 55 g/100 g H2O Draw the line down to temperature.

Solubility Example 4 Find the temperature where the solubility of potassium chloride is 55 g/100 g H2O Potassium chloride is soluble at 55 g/100 g H2O at 93°C.

Solubility The solubility of gases in a liquid is also affected by pressure. As the partial pressure of a gas over a liquid increases, the amount of the gas in the liquid also increases. This is the effect seen in bottles of carbonated drinks. When the bottle is sealed, the partial pressure of CO2 is high to keep a high amount of CO2 in solution. When the bottle is opened to the air, the partial pressure of CO2 over the liquid falls and more CO2 comes out of solution.

Solubility The relationship between the solubility of a gas, S, and the partial pressure of the gas, P, over the solvent is direct. This is known as Henry’s Law S1 S2 P1 P2 =

Solubility Example 5 If the solubility of a gas in water is 0.77 g/L at 3.5 atm pressure, what is its solubility at 1.0 atm? Assume temperature is held constant. S1 = 0.77 g/L P1 = 3.5 atm S2 = ? g/L P2 = 1.0 atm (0.77 g/L)(1.0 atm) (3.5 atm) S1 S2 P1 P2 = S2 = S1 × P2 P1 0.22 g/L ➔ = =

Summary Solutions are homogeneous mixtures that may be solid (alloy), liquid (sugar water), or gas (air). Solutions are composed of a solute (what is dissolving) and a solvent (what it is dissolving in). How fast a solute dissolves in a solvent depends on agitation, temperature, and surface area. Solubility relates to the amount of solute that can be dissolved in a solvent.

Summary At the limit of solubility, solid solute is dissolving into the solvent as fast as aqueous solute is precipitating onto the solid solute, which is an equilibrium situation: solute(s) ⇄ solute(aq). A saturated solution contains the maximum amount of solute for a given amount of solvent. A solution where the amount of solute is less than the solubility is called an unsaturated solution. The solubility of a solid increases as the temperature increases, and the solubility of a gas decreases as the temperature increases.

Summary The solubility of gases in a liquid is also affected by pressure. As the partial pressure of a gas over a liquid increases, the amount of the gas in the liquid also increases.