Solutions - chapter 16Section 16.1 - properties of solutions

What is a solution?

A homogeneous mixture that may be solid, liquid or gases

The solvent properties determine if the solute will dissolve

How can you speed up the rate of solution formation?

Solution properties continued

● Agitation○ Stirring sugar makes it dissolve faster in tea○ This is because the sugar is interacting more with the surface of the sugar○ Only affects speed at which solute dissolves

● Temperature○ Water molecules move faster at higher temperatures ○ This leads to more collisions, faster rate of dissolution

● Particle size of the solute○ The smaller the particle the larger the surface area○ The larger the surface area = more collisions with water molecules

Why can you only dissolve so much salt in 100ml of water?

● The Solubility of a substance is the amount of solute that can be dissolved in a given quantity of solvent at a specified temperature and pressure

● Often expressed as grams of solute per 100 g of solvent

● Or, with gases g/L of solution● What happens if you no more

solution dissolves?

Saturated vs Unsaturated solutions● A Saturated solution contains the maximum amount of solution for any given

amount of solvent at constant temperature and pressure conditions● For salt, this is 36.2g of salt in 100 ml of water at 25 C ● At this point an equilibrium exists between the solution and solute ● An Unsaturated solution contains less solute than a saturated solution

Miscible vs Immiscible ● One of two things happens when you

mix two liquids together● They either mix perfectly (dissolve in

each other in all proportions): and are miscible

● They are insoluble in each other and do not mix - Immiscible

●

Can you increase the solubility of a compound?

Yes, temperature affects the solubility of solid and liquid solutes, and temperature and pressure affect the solubility of gaseous solutes

If you exceed the solubility of a substance, a Supersaturated solution is created

Supersaturated solutions

What about gases? ● As temperature increases gas solubility decreases

○ Potential for harm from companies dumping hot water into ecosystems

● Pressure has a large effect on solubility of gases - Henry’s law● Solubility and pressure are directly proportional● This idea is used to make carbonated beverages

Concentrations of Solutions

Section 16.2

What is the difference between these cups of tea?Assuming they all contain the same amount of milk

What is meant by Molarity? ● Concentration is a measure of the amount of solute dissolved in a certain

amount of solvent● Molarity is the number of moles of solute dissolve in a litre of solvent● We use molarity as a measurement of concentration

○ Small amount of solute = Dilute Solution○ Large amount of solute = Concentrated Solution

● These are qualitative descriptions - whether something is concentrated or dilute depends what you are comparing them against

How do you make a 0.5 M solution?

How do you calculate Molarity? This equation can also be rearranged to calculate the moles of solute of you know the volume and number of moles

If you have a 1M solution, is it easy to make a 0.5M solution? ● Yes! ● You just need to dilute it● Diluting a solution reduces the number of moles of solute per unit volume. It

does not change the number of moles of solute in the solution

Are there other measures of Concentration? ● Why is rubbing alcohol referred to as 91% isopropyl alcohol by volume?● This 91% tells us that 91% of the solution is isopropyl alcohol, or that 91m of

isopropyl alcohol was diluted with enough water to make 100 ml of solution● This only works where the solution and the solvent are both liquids, and you

can express the concentration of the solute as a percent of the solution by volume

● If a solute is a solid, the concentration of the solid can be expressed as percent by mass

Colligative properties of Solutions

Section 16.3

Differences between solutions and pure solvent

● Physical properties of solutions can be different than pure solvents● A Colligative property is a property of solutions that depends only on the

number of solute particles,but not on the identity of the particles● There are 3 colligative properties that we will discuss

○ Vapor pressure lowering○ Freezing point depression○ Boiling point elevation

Vapor pressure lowering

● Vapor Pressure - The pressure exerted by a vapor above it’s liquid in a closed system

● Solutions with a solute always have a lower vapor pressure than a pure solvent

● Why?

The effect of vapor pressure lowering depends on the type of solute - why?

Vapor pressure lowering explained● When water molecules solvate a solute, they slow down, meaning they have

less kinetic energy and fewer molecules will have enough energy to escape the liquid and become a vapor

○ This results in a lower vapor pressure

● The decrease in vapor pressure is proportional to the number of solute particles in solution

● As molecular solutes do not dissociate, they produce the fewest solute particles and have the smallest effect on vapor pressure

● Ionic compounds have a great effect● CaCl2 will lower vapor pressure more than NaCl - why?

Freezing point depression● When a substance freezes, particles are arranged in an orderly pattern● Solutes make it harder for particles to be arranged in the correct order, and

more kinetic energy must be removed from the system● As such, the freezing point of a solution is lower than the freezing point of a

the pure solvent○ This difference is the freezing point depression

● The size of the freezing point depression is proportional to the number of solute particles in solution

● This why in cold places salt is spread on roads

Boiling Point elevation● The boiling point is the temperature where the vapor pressure of the liquid is

equal to atmospheric pressure○ Adding a nonvolatile solute to a solvent makes this harder, and increases the boiling point

● Boiling point elevation: the difference in temperature between between the boiling point of a solution and the pure solvent

● Again, the size of the boiling point elevation is dependant on the number of particles in solution - more solute particles = higher boiling point elevation

Writing assignmentExplain how antifreeze in a car’s cooling system makes use of both boiling point elevation and freezing point depression for the benefit of the driver.

Calculations involving colligative properties

Section 16.4

Why do people add salt when cooking pasta?

MolalityColligative properties depend on the number of solute particles dissovled in a given amount of solvent

Molality (m) is the number of moles of solute dissolved in 1 kg of solvent. It is also referred to as molal concentration

Molality vs Molarity● Molality depends on the mass of the

solvent● Molarity depends on the volume of

the solvent● Why the need for two similar terms?● If water is the solvent it does not

matter, as 1kg = 1L ● But, for other solvents it is not so

easy● Molality is also unaffected by

temperature changes, whereas molarity is - why?

Mole fractionsA mole fraction is another way to express the concentration of solutions

It is the ratio of moles of that solute to the total moles of the solvent and solute

You can also find the mole fraction of the solvent

Mole fractions do not have units

How do you calculate the freezing point depression of boiling point elevation?● Changes in freezing or boiling point are usually very small, but can be

calculated if you know the molality and have some reference data about the solvent

● This reference data is constant value for a solvent○ For freezing point depression it is Kf - the change in freezing point for a 1 molal solution of a

molecular solute ■ Units are °C/m

○ For Boiling point elevation it is kb - the change in boiling point for a 1 molal solution of a molecular solute

■ Units are °C/m

The formulas