Chapter 13 – Water as a Solvent The latent heat of fusion of water – the amount of energy needed to change a fixed

amount of water from solid to liquid phase at 0°C. The latent heat of vaporisation of water – the amount of energy needed to change a

fixed amount of water from a liquid to a gas at 100°C. The higher the latent heat of a substance, the greater the amount of energy it needs

to change phase. Water has much higher latent heat values than other substances of a similar size

owing to the stronger intermolecular forces within its solid and liquid states.

Specific heat capacity of a substance is the amount of energy needed to raise the temperature of one gram of the substance by one degree Celsius.

The higher the specific heat capacity the more energy the substance must absorb to raise its temperature.

Water retains heat 5 times more effectively than land, but may take 5 times longer than land to heat up.

The density of water varies across the three states. When water freezes it expands (unlike other substances). Ice has a lower density than water, allowing ice to float.

A substance will conduct electricity only if it contains charged particles that are free to move. Water is covalently bonded and has neutrally charged molecules (not to be confused with polarity!), so it should not conduct electricity.

Pure water does conduct electricity slightly. Water reacts slightly with itself to form a small number of ions. The self-ionisation of water

o H2O(l) + H20(l) H3O+(aq) + OH- (aq) However – only a few ions are formed so pure water was a conductivity of virtually

zero. Water gains its conductivity from other substances that dissolve in it.

Effects of hydrogen bonding on the properties of water:o High melting and boiling pointso Expansion of water upon freezingo Ability of water to dissolve other substances

Behaviour of substances in water:o They may remain undissolvedo They may dissociateo They may form ionso They may form hydrogen bonds with water

A saturated solution is one that contains the maximum amount of solute for the volume of solution at a particular temperature.

An unsaturated solution has less than the maximum amount of solute in the solvent.

A supersaturated solution can be created by slowly cooling a saturated solution. This creates a solution that contains more solute than it should at the given temperature. A small addition to the solution will cause the excess solute to crystallise.

Hydrated ions are surrounded by water molecules. Salts with solubility greater than 0.1mol L-1 are classed as soluble. Solubility curves are a graph of temperature vs. solubility of a substance. Rules for solubility are listed on page 297.

o In summary all salts containing Na+, K+, NH4+ and NO3

- are soluble

Polar gases will dissolve in water easily. Increase in temperature generally decreases the solubility of gases in liquids.

The Concentration of Substances

Concentration is defined in terms of the amount of solute per volume of solvent. A concentrated solution may be diluted by the addition of more solvent. Concentration can be expressed in many ways.

C=mV (Concentration equals mass over volume) is the general rule.

When very small quantities of solute are dissolved the concentration can be measured in parts per million (ppm)

ppm=massof solute∈micrograms (μg)massof solution∈grams

Percentage by mass (%w/w)o Expresses the concentration of the solute as a percentage of the mass of

solution.o So 98% sulphuric acid means there are 98grams per 100 grams of solution.

concentration∈%w /w= massof solute∈gramsmassof so lution∈grams

x100%

Percentage mass/volume (%w/v)o Mass of solute per 100mL of solution

concentration%w /v= massof solute∈gramsvolume of solution∈mL

x100%

Percentage by volume (%v/v)o Volume of solute in volume of solution

concentration%v /v= volume of solute∈mLvolumeof solution∈mL

x 100%

Improving water quality Distillation

o Separates mixtures with components with different boiling temperatures.

o Can use a fractionating column to separate components with similar boiling temperatures.

Reverse osmosiso Osmosis is the diffusion of a solvent through a membrane towards an area of

higher concentration of dissolved substances. o To purify water the water solvent needs to move away from the dissolved

substances – hence reverse osmosis.o Requires energyo Forces pressurised water through a series of membranes. The membranes

have tiny pores that allow the smaller water molecules to pass but block contaminants.

o Often use a series of membranes with decreasing pore size to prevent clogging.

Questions:1. The specific heat of ethanol is 2.4 J g-1 °C-1 and of water is 4.2 J g-1 °C-1.a) If 100g of ethanol and 100g of water at the same temperature and under the

same conditions are each given the same amount of heat energy the temperature of:A) Both liquids will increase by the same amountB) Ethanol will be higherC) Water will be higherD) Both liquids will decrease by the same amount

b) The amount of heat energy required to heat 250g of ethanol from 15°C to 22°C isA) 0.60kJB) 1.75kJC) 4.2kJD) 13.2kJ

2. A solution of ammonia contains 6.3mg of ammonia dissolved in 100mL of solution. What is the concentration of the solution in g L-1?A) 6.3x10-4

B) 6.3x10-2

C) 6.3D) 63

3. A saturated solution is formed when 0.3g of a salt is dissolves in 15mL of water. The solubility of the salt in g/100g of water isA) 0.045B) 2.0C) 4.5D) 30

4. When the temperature of solutions containing dissolved salts or dissolved gases increases, the solubility of:A) Both gases and salts increasesB) Gases increases and salts decreasesC) Gases decreases and salts increasesD) Both gases and salts decreases