Week 11: Properties of solutions Flashcards
Terms used in solutions
Solute = the dissolved substance. Solvent = the liquid doing the dissolving. Soluble = can dissolve in the solvent. Dissolution = the process of dissolving a solute in a solvent.
Spontaneity
Solutions will form unless the solute-solute or solvent-solvent interactions are too strong relative to the solute-solvent interactions.
Processes in which the energy content of the system decreases tend to be spontaneous
Processes occurring at a constant temperature in which the randomness or dispersal in space of the system increases, tend to occur spontaneously
In most cases, the formation of a solution is favoured by the increase in randomness which accompanies mixing
Solution terms (solubility and saturated solution)
Solubility = the maximum amount of solute that can dissolve completely in a given amount of solvent at a given temperature and pressure.
Saturated solution = a solution that has more solute that can dissolve in the given amount of solvent.
Saturated solutions and solubility
When a solid is added to a solvent it begins to dissolve – solute molecules break away from the solid and are solvated by the solvent.
The concentration of solute particles increases; there is more chance of a solute particle colliding with the surface of the solid and it become re-attached to the solid again – crystallisation.
Two opposing processes therefore occur at the same time – dissolution and crystallisation.
When the rates of the two processes are equal, no further increase in the amount of solute in solution is possible – dynamic equilibrium.
Effect of temperature
Temperature affects the solubility of most substances, as shown by the data in the graph and table below.
For most solids dissolved in a liquid, an increasing temperature results in increase solubility.
The solubility of a gas in water usually decreases with increasing temperature.
Effect of pressure - gases in water
Small changes in pressure have little effect on the solubility of solids in liquids or liquids in liquids but have a marked effect on the solubility of gases in liquids.
The solubility of a gas in a liquid is directly proportional to the pressure of that gas above the solution.
Thus, the amount of a gas dissolved in solution will double if the pressure of that gas over the solution is double.
Henry’s Law
For a solution in which the gas does not react with the solvent, the relationship between the pressure of the gas and the solubility of the gas is given by Henry’s law.
Often used expression of Henry’s Law
c1 / P1 = c2 / P2
An equilibrium constant
Equilibrium can be expressed as;
a A + b B ⇌ c C + d D
where small letters are stoichiometric constants and the capital letters are substances.
At equilibrium, the concentration of products (A, B) and products (C, D) do not change. Thus, a constant called Kc (equilibrium constant) can be established. Kc is dependent on temperature, always specify temperature when reporting Kc. Using the stoichiometric equation above, the following equilibrium can be written. The square brackets refer to the concentration in mol/L so the equation very briefly refers to molar concentration of the starting material over molar concentration of the products.
what is Molality (Molal Concentration, mol/kg)
Molality is a measure of number of moles of solute present in 1 kg of solvent in contrast to the definition of molarity (mol/L).
In some cases, mass of solvent is more relevant for measuring solution as the volume of a solution can vary depending on its temperature and pressure. Volume can be changed at different temperatures whilst the mass is fixed.
What is the mass percent of sodium hydroxide in a solution that is made by dissolving 8.00 g NaOH in 50.0 g H2O?
The formula we’ll use here is:
Mass percent (m/m%) = (mass of solute / mass of solution) × 100%
The solute is NaOH, water is the solvent, and the solution is the combination of solute and solvent. Therefore, the mass of solution is 58.0 g.
Mass percent (m/m%) = (8.00 g / 58.0 g) × 100% = 13.8%
Colligative properties of solutions
Colligative properties are those properties that depend only on the number of particles in solution, not their identity. These include:
Vapour pressure lowering
Boiling point elevation
Freezing point depression
Osmotic pressure
For example, separate solutions of NaCl and NaBr, prepared by dissolving a mole of each salt in 1 kg water, would change the boiling point and the freezing point of each solution by the same amount? Important – only works for non-volatile solutes.
Raoult’s law
Raoult’s law states ‘the partial pressure exerted by solvent vapour above a solution, Psolution, equals the product of the mole fraction of the solvent times the vapour pressure of the pure solvent P°solvent.
Raoult’s law example
What is the vapour pressure of a solution of 10.0 g of candle wax in 40.0 g of tetrachloromethane, CCl4, at 23 °C? The molecular formula of candle wax is C22H46 and the vapour pressure of pure CCl4 at 23 °C is 1.32 x 104 Pa.
Estimating a freezing point using a colligative property
Estimate the freezing point of a solution made from 10.00 g of urea, CO(NH2)2 (M = 60.06 g mol-1), in 100.0 g of water. Kf water = 1.86 K mol-1 kg.
To find out the freezing point of solution, molality of the solution (mol/kg) needs to be calculated.
Moles of urea = (10 g) / (60.06 g/mol) = 0.1665 mol
Molality (m) = (0.1665 mol of urea) / (0.1 kg of water) = 1.665 mol/kg
∆Tf = Kf m = 1.86 K kg mol-1 1.665 mol kg-1 = 3.10 K = 3.10 °C
The estimated freezing point will be -3.10 °C
Osmosis
The concentration of the solvent in higher in the solution containing less solute so the rate with which the solvent passes from the less concentrated to the more concentrated solution is greater than the rate in the opposite direction. Thus there is a net movement of solvent molecules form the less concentrated solution to the more concentrated solution. This is called osmosis.
Osmotic pressure obeys a law similar to the ideal gas law (PV=nRT).
πV = nRT, where π is the osmotic pressure and V is the volume of solution.
π = (n/V)RT = MRT, where M is the molarity of the solution.
Since M is concentration, π = MRT can often expressed in
π = cRT
