Unit: 11: Solution Chemistry Solutions. Mixture A combination of 2 or more kinds of matter, each of which retains its own composition and properties To

Unit: 11: Solution Chemistry

Solutions

Mixture

A combination of 2 or more kinds of matter, each of which retains its own composition and properties

To distinguish the components of a mixture one would have to inspect the mixture molecule by molecule

Mixtures differ according to the size of the particles in the mixture

Mixtures:

Are classified as either suspensions, colloids or solutions based on size of particles

Solutions contains smallest particles and suspension contains the largest particles

Types of Solution

Solution – homogeneous mixture of two or more substances of ions or molecules. E.g. NaCl (aq)

Solvent = component which is the component in greater amount.

Solute = component which is present in the smaller amount.

Soluble: means it can be dissolved

Gaseous = gases are completely miscible in each other.

Liquid = gas, liquid or solid solute dissolved in solute.

Solid = mixture of two solids that are miscible in each other to form a single phase.

Colloid – appears to be a homogeneous mixture, but particles are much bigger, but not filterable. E.g. Fog, smoke, whipped cream, mayonnaise, etc.

Suspension: larger particle sizes, filterable. E.g. mud, freshly squeezed orange juice.

Solubility and the Solution Process

The solid dissolves rapidly at first but as the solution approaches saturation the net rate of dissolution decreases since the process is in dynamic equilibrium.

When the solution has reached equilibrium the amount of solute does not change with time;

At equilibrium: the rate of dissolution = rate of solution

Fig. 12.2 Solubility Equilibrium

3 Factors that influence the rate of dissolving

1. Increase the surface area of the solute

2. Agitate the solution (stir or shake)

3. Heat the solvent

Solubility is

• the maximum amount of solute that dissolves in a specific amount of solvent.

• expressed as grams of solute in 100 grams of solvent water.

g of solute

100 g water

Solubility

At 40C, the solubility of KBr is 80 g/100 g H2O.Identify the following solutions as either1) saturated or (2) unsaturated. Explain.

A. 60 g KBr added to 100 g of water at 40C.

B. 200 g KBr added to 200 g of water at 40C.

C. 25 g KBr added to 50 g of water at 40C.

Learning Check

A. 2 Amount of 60 g KBr/100 g water is less than the solubility of 80 g KBr/100 g water.

B. 1 In 100 g of water, 100 g KBr exceeds the solubility of 80 g KBr water at 40C.

C. 2 This is the same as 50 g KBr in 100 g of water, which is less than the solubility of 80 g

KBr/100 g water at 40C.

Solution

Solubility and the Solution Process

Saturated solution: maximum amount of solute is dissolved in solvent. Trying to dissolve more results in undissolved solute in container.

Solubility: Amount of solute that dissolves in a solvent to produce a saturated solution. (Solubility often expressed in g/100 mL.)

E.g. 0.30 g of I2 dissolved in 1000 g of H2O.

Unsaturated solution: less than max. amount of solute is dissolved in solvent.

E.g. 0.20 g of I2 dissolved in 1000 g of H2O.

Supersaturation = more solute in solution than normally allowed; we call this a supersaturated solution.

Factors Affecting Solubility

“like dissolves like” = substances with similar molecular structure are usually soluble in each other.

Gases = generally completely soluble in each other because of entropy

Molecules in gas phase are far apart from each other and not interacting strongly with each other in solution.

Mixing of Gas Molecules

Energy Changes and the Solution Process

Intermolecular forces are also important in determining the solubility of a substance. “like” intermolecular forces for solute and solvent will make

the solute soluble in the solvent.

Hsoln is sometimes negative and sometimes positive.

Solvent

Solvent – solvent

Solute

Solute – solute

+Solution

Solute – solvent

Solvent – solvent interactions: energy required to break weak bonds between solvent molecules.

Solute – solute interactions: energy required to break intermolecular bonds between the solute molecules.

Solute – solvent interactions: H is negative since bonds are formed between them.

Molecular Solutions

Molecular compounds with similar chemical structures and polarities tend to be miscible.(not soluble in each other)

Homologous alcohol series have polar and non-polar ends.

Ionic Solutions

Solubility affected by: Energy of attraction (due Ion-dipole force) affects the

solubility. Also called hydration energy, Lattice energy (energy holding the ions together in the

lattice. Related to the charge on ions; larger charge means higher lattice

energy. Inversely proportional to the size of the ion; large ions

mean smaller lattice energy.

Solubility increases with increasing ion size, due to decreasing lattice energy; Mg(OH)2(least soluble), Ca(OH)2, Sr(OH)2, Ba(OH)2(most soluble) (lattice energy changes dominant).

Energy of hydration increases with for smaller ions than bigger ones; thus ion size. MgSO4(most soluble),... BaSO4 (least soluble.) Hydration energy dominant.

Solubility: Temperature Dependence

All solubilities are temperature dependent; must report temperatures with solubilities.

Most solids are more soluble at higher temperatures. Exceptions exist. All gases are less soluble at higher temperatures.

Effect of Temperature on Solubility

Solubility• Depends on

temperature.• Of most solids

increases as temperature increases.

• Of gases decreases as temperature increases.

Temperature related to sign of Hsoln;

negative means less soluble at high temperatures positive means more soluble (Le Chatelier’s

principle). E.g. Predict the temperature dependence of the

solubility of Li2SO4, Na2SO4 and K2SO4 if their Hsoln are 29.8 kJ/mol, 2.4 kJ/mol and +23.8 kJ/mol, respectively.

A. Why do fish die in water that is too warm?

Learning Check

Because O2 gas is less soluble in warm water, fish cannot obtain the amount of O2 required for their survival.

Solution

Solubility: Pressure Dependence

Pressure has little effect on the solubility of a liquid or solid, but has dramatic effect on gas solubility in a liquid.

Gas solubilities in liquids always increases with increase pressure

Henry’s law S = kHP. Allows us to predict the solubility of a gas at any pressure.

E.g. At 25C P(O2 in air) = 0.21 atm. Its solubility in water is 3.2x104M. Determine its solubility when pressure of O2 = 1.00 atm.

Solubility and Pressure

Henry’s Law states • the solubility of a gas

in a liquid is directly related to the pressure of that gas above the liquid.

• at higher pressures, more gas molecules dissolve in the liquid.

• (increase pressure = increase solubility of gas)

Example Problem Henry’s Law

The solubility of methane, the major component of natural gas, in water at 20C and 1.00atm pressure is 0.026g/L. If the temperature remains constant, what will the solubility of this gas be at the following pressure: .6 atm

Solution: Given 1 L = 0.26 g K = equilibrium constant [CH4] = KP[CH4] .026g/L =K (1atm) K = .026g/atm solubility = (.026g atm)(.6 atm)= .0156g

Example Henry’s Law

To increase the solubility of a gas at constant temperature from 0.85g/ml (1 atm) to 5.1 g/ml, the pressure would have to be increased to what?

Solution: Given .85g = 1 ml [gas] = KP[gas] .85g/L =K (1atm) K = .85g/atm solubility: 5.1g/ml = (.85g/atm) (x) x= 5.96 atm

Example Problem Henry’s Law

The solubility of methane, the major component of natural gas, in water at 20C and 1.00atm pressure is 0.026g/L. If the temperature remains constant, what will the solubility of this gas be at the following pressure: 1.8 atm

Solution Henry’s Law

Solution: Given 1 L = 0.26 g K = equilibrium constant [CH4] = KP[CH4] .026g/L =K (1atm) K = .026g/atm solubility = (.026g atm)(1.8

atm)= .0468g

Why could a bottle of carbonated drink possibly burst (explode) when it is left out in the hot sun ?

A. The pressure in a bottle increases as the gas leaves solution as it becomes less soluble at high temperatures. As pressure increases, the bottle could burst.

Units of Concentration Physical properties of solutions

are often related to the concentration of the solute in the solution.Molarity

Mole fraction: The same quantity we have used in fractional abundances as well as with gases (Dalton’s law). A unitless number.Weight (mass) Percent (wt%) – similar to mole fraction except use mass of each.

solution of liter

solute of molesMolarity

B mol A molA of mol

X

210xsolution of mass

solute of mass%wt

Percent by mass

The number of grams of solute dissolved in 100g of solvent


solute of mass%wt

Percent by mass

A solution of sodium chloride is prepared by dissolving 5 g of slat in 550 g of water. What is the concentration of this solution given as percent by mass?


solute of mass%wt

210H2O 550g NaCl 5g

NaCl of 5g% xwt

Percent by mass example

Determine mass % of solution made from dissolving 30.0 g H2O2 with 70.0 g H2O.

30.0 g H2O2 . x100

30.0 g H2O2 +70.0 g H2O.

Answer: 30%


A 7.5% by mass aqueous solution of sodium chloride has a mass of 650 g. What mass of sodium chloride is contained in the solution? What mass of water is contained in the solution?

Solution:

650g solution x 7.5g NaCl/100g solution = 48.8g NaCl

650 g solution x 92.5gH2O/100g solution = 601 g H2O


A 60% by mass solution of H2SO4 in water is prepared using 225g of H2SO4. What mass of solution is produced? What mass of water is required?

Solution:

225g H2SO4 x 100g solution/60g H2SO4 = 375 g solution

225 g H2SO4 x 40gH2O/60g H2SO4 = 150 gH2O

Units of Concentration Other units: parts per million

(ppm) and parts per billion (ppb) for small concentrations.

solution of liter

solute of molesMolarity


solute of mass%wt


solute of massppm


solute of massppb

Units of Concentration: Molality

Molality(m): defined as the mol of solute per kg of solvent. Unlike Molarity this unit is temperature independent.

(kg) solvent of masssolute mol

(m)Molality

Molality example

A solution contains 17.1 g sucrose ( C12H22 O11) dissolved in 125G H2O . Find the molal concentration of the solution

Solution

Mass solute/mass solvent x 1/molar mass solute x 1000 g/1kg

17.1g C12H22 O11 /125g H2O x 1 mol C12H22 O11 / 342g C12H22 O11 x 1000g/1kg =

.4 molality

Molality example

What is the molality of a solution in which 3 moles of NaCl is dissolved in 1.5Kg of water?

Solution

3moles/1.5Kg = 2m


(m)Molality

Molality example

What is the molality of solution in which 25g of NaCl is dissolved in 2.0 Kg of water?

Solution

Convert grams NaCl to Moles Nacl

25g NaCl x 1 molNaCl/58g NaCl = .4274 moles NaCl

.4274NaCl / 2Kg = .2137m


(m)Molality

Molarity

The Molarity of a solution is the number of moles of solute per liter of solution.

solvent of

solute mol (m)Molarity liters

Molarity example Problem

What mass of K3PO4 is required to prepare 4.00 L of 1.5 M solution?

Solution:

1.5M = X/4L solve for x X= 6 moles

Convert moles to grams 6 moles K3PO4 x 212gK3PO4 /1 molK3PO4 = 1272 g

solvent of



What volume of .075M solution can be prepared using 90g of NH4Cl

Solution:

Convert grams NH4Cl to moles 90g x 1/153g(wt PT) = 1.698 mole NH4Cl

.75M = 16.98moesl/x solve for x X =2.26L

solvent of



What is the molarity of a solution that contains 210g of Al2(SO4)3 in 2.75 liters of solution

Solution:

Convert grams Al2(SO4)3 to moles 210g x 1mole/ 369g (wt PT) = .0569 moles Al2(SO4)3

.0569/2.7 L = .2069M solvent of


Vapor-Pressure Lowering of Solutions: Raoult’s Law

Raoult’s Law:Psoln = PsolvxXsolv

Non–volatile solute: vapor pressure decreases upon addition of solute.

Linear for dilute solutions Vapor pressure lowering : P = Po P = Po(1Xsolv)

Raoult’s law:

states that the partial vapor pressure of each component of an ideal mixture of liquids is equal to the vapor pressure of the pure component multiplied by its mole fraction in the mixture.

Thus the total vapor pressure of the ideal solution depends only on the vapor pressure of each chemical component (as a pure liquid) and the mole fraction of the component present in the solution

Is used to determine the vapor pressure of a solution when a solute has been added to it

Raoult's law is based on the assumption that intermolecular forces between unlike molecules are equal to those between similar molecules: the conditions of an ideal solution. This is analogous to the ideal gas law

Raoult's law: Example

25 grams of cyclohexane (Po = 80.5 torr, MM = 84.16g/mol) and 30 grams of ethanol (Po = 52.3 torr , MM = 92.14) are both volatile components present in a solution. What is the partial pressure of ethanol?

Raoult's law

Solution:

Moles cyclohexane: 25g x 1mol/84.16 = .297 moles

Moles ethanol: 30 g x 1mol/92.14 = .326 moles X ethanol: .326/(.326)+(.297) =.523

PxPo = (.523) (52.3 torr) =27.4 torr

Raoult's law: Example

A solution contains 15 g of mannitol C6H14O6, dissolved in 500g of water at 40C. The vapor pressure of water at 40C is 55.3 mm Hg. Calculate the vapor pressure of solution ( assume mannitol is nonvolatile)

Solution:

Molecular wt water = 18g Convert grams of water to moles 500g x 1mol/18g =27.78 mole water Mass mannitol = 182 g Convert grams mannitol to moles 15g x 1mol/182g = .0824 moles

Total moles = 27.78 + .0824 =27.86 Mole fraction water = 27.78 (water)/27.86 (total moles)

= .997 Solution vapor pressure = .997 x 55.3 mmHg = 55.13

mmHg

BP Elevation and FP Depression of Solutions

The magnitude of the change in FP and BP is directly proportional to the concentration of the solute (molality) – expressed in terms of the total number of particles in the solution.

BP Elevation The magnitude of the BP increase is given by the equation:

where Kb has units of °Ckg/mol or °C/m FP Depression: linear variation with composition and given by:

where the units for this constant are the same as for Kb

E.g. Determine freezing point depression when 5.00 g of sucrose is added to 100.0 g of H2O. FM(sucrose) = 342.3 g/mol. Kf = 1.86°C/m.

E.g. Determine the BP elevation for the sucrose solution in the previous example. Kb = 0.521 C/m.

mKT bb

mKT ff

A property that depends on the number of solute particles but is independent of their nature is called colligative property

Physical Behavior of Solutions: Colligative Properties

Compared with the pure solvent the solution’s: Vapor pressure is lower Boiling point is elevated Freezing point is lower Osmosis occurs from solvent to solution when

separated by a membrane. Colligative properties are directly proportional to

the molal conc. Of a molecular solvent.

Osmosis and Osmotic Pressure

Osmosis: the passage of solvent through a membrane from the less concentrated side to the more concentrated side.

Osmotic pressure: the amount of pressure necessary to stop Osmosis. Small molecules such as water can move through certain types of materials

(membranes). The tendency for this to occur is related to the molarity of the solution, is also a

function of the temperature and is measured with a device called a Thistle tube.

where M = is molarity of solute particlesMRT

E.g. Determine osmotic pressure of a solution containing 0.100 g of hemoglobin (molecular mass = 6.41x104 amu) in 0.0100 L at 1.00C.

E.g. Osmotic pressure of a solution containing 50.0 mg of a compound in 10.0 mL of water was 4.80 torr at 5.00C. Determine FM of the compound.

Reverse Osmosis

Application of a pressure to the solution (that is equal to or greater than the Osmotic pressure) and the solvent flows from the more concentrated side to the other one.

This process is used to obtain pure water from salt water.

Documents

Unit: 11: Solution Chemistry Solutions. Mixture A combination of 2 or more kinds of matter, each of which retains its own composition and properties To