PHYSICAL PROPERTIES OF SOLUTIONS

PHYSICAL PROPERTIES OF SOLUTIONS

Important characteristics pf solutions:

• They are homogenous mixtures, homogenous because its composition and properties are uniform and mixture, because it contains two or more substances in proportions that can be varied.
• Solutions may be gases, liquids or solids.
• Each substance in a solution is a component of the solution. Usually, the component with the highest concentration is termed the solvent (other components are termed solutes)

Solvent – a component that is present in the greatest quantity or that determines the state in which the solution exists.

Solute – substance dissolved in the solvent.

  

Types of Solution

Component 1	Component 2	State of Resulting Solution	Example
Gas	gas	gas	air
Gas	liquid	liquid	soda water
Gas	solid	solid	H2 in Pd
Liquid	liquid	liquid	ethanol in water
Solid	liquid	liquid	NaCl in H2O
Solid	solid	solid	Brass (Cu/Zn), Solder (Sn/Pb)

Most solutions we will deal with are those in a liquid state, where the solvent is H2O (i.e. aqueous solutions)

• The liquid states, and solid state, are known as condensed states.
• In condensed states, the attractive forces between molecules are strong enough (in comparison to the temperature-induce kinetic energy) to hold neighboring molecules together.

o   In solids, the neighbours are held rigid.

o   In liquids, the neighbour molecules can slide past each other.

Homogenous mixtures (solutions) can form only when the following attractive forces are approximately equivalent:

• Attraction between solvent and solute molecules
• Attraction of solute molecules for other solute molecules
• Attraction of solvent molecules for other solvent molecules

If the attractive forces of solute molecules for other solute molecules are greater than the attractive forces of solute molecules for water, then the solute will not dissolve

• For ionic solids, lattice energy describes the attractive forces between the solute molecules (i.e. ions)
• For ionic solids to dissolve in water, the water-solute attractive forces have to be strong enough to overcome the lattice energy.

The process known as solvation is where the solute-solvent interactions are strong enough to separate, surround and disperse a solute.

•          If the solvent is water, then solvation is referred to as hydration.
  
  

SOLUTION CONCENTRATION   

                                                    

Concentration – a measure of the quantity of solute in a given quantity of solvent

        Concentrated solution has a relatively large quantity of dissolved solute.

        Dilute solution contains only a small quantity of solute.

       Saturated solution contains the maximum amount of solute that the solvent can dissolve.

        Unsaturated solution contains less solute than it has the capacity to dissolve.

        Supersaturated solution contains more solute than is present in a saturated solution.

Saturated Solutions and Solubility

• When a salt (NaCl) crystal is initially place in a sample of water the solution is devoid of hydrated Na⁺ and Cl^- ions:
• As the water molecules surround, separate and disperse the Na⁺ and Cl^- ions, the solution becomes populated by the hydrated ions:
• The dispersed ions in the solution will collide with water molecules, the surfaces of the container and potentially other ions as well.
• If the original crystal has not completely dissolved, then dispersed ions can also collide with the remaining crystal. These collisions can result in the incorporation of the ions back into the NaCl crystal lattice.
• Thus, there are two opposing processes that can potentially occur:

1.      Dissolving of the crystal, resulting in hydration of the individual Na+ and Cl- ions, and

2.     Collision of dispersed ions resulting in an increase in the crystal mass (a process also known as crystallization)

Note that when a hydrated ion collides with a crystal and is incorporated into the crystal lattice, the waters that are hydrating the ion are released (i.e. the exact reverse of the hydration process)

These two opposing processes of dissolving and crystallization can be represented as follows:

• If the rate of dissolution is greater than the rate of crystallization, then the crystals of NaCl in the solvent will get smaller.
• If the rate of crystallization is greater than the rate of dissolution, then the crystals of NaCl in the solvent will get larger.
• If the rates of the two opposing processes are equal, then the size of NaCl crystals will remain unchanged and the system is said to be in a dynamic equilibrium.

A solution that is in dynamic equilibrium with undissolved solute is said to be saturated (i.e. no more solute will dissolve into the solvent under the current conditions)

• The concentration of solute present in the solution under conditions saturation is known as the solubility of that solute.
• For example, at 0^oC 35.7g of NaCl can be dissolved in a total volume of 100mL of H₂O. This is solubility of NaCl in H₂O at 0^oC.
• At higher temperatures, usually more solute can be dissolved, and the solubility is higher.

Under some conditions, it is possible to produce a supersaturated solution of a solute.

• Solute dissolved to saturation at a high temperature
• The is carefully and slowly cooled to a lower temperature (the idea is not to induce the formation of tiny crystals that can serve to nucleate crystal growth)
• At the lower temperature, the concentration of solute is higher than the equilibrium concentration at that temperature. The introduction of a “seed” crystal will stimulate rapid crystal formation

Factors Affecting Solubility

      Factors that can affect solubility:

•       Properties of solute
•       Properties of solvent
•      Temperature 
•       Pressure (gases)

Polar Solutes in Polar Solvents

Polar solutes tend to dissolve readily in polar solvents

•         Interactions between polar soltues are typically dipole- dipole (or hydrogen-bonds)
•      Interactions between molecules of a polar solvent are also dipole-dipole (or hydrogen-bonds)
•    Thus, the energies associated with disrupting solute-solute interactions and solvent-solvent interactions are approximately equivalent
•         Entropic forces can subsequently drive the dissolution process
•      Polar liquids tend to dissolve readily in polar solvents

Pairs of liquids that mix in any proportion are termed miscible. Liquids that do not mix are termed immiscible.

•        Ethanol contains a hydroxyl (OH) functional group that is similar in structure to water. Attractive forces between ethanol molecules include Hydrgen bonds, like the attractive forces between water molecules. Ethanol is miscible in H₂O
•         Octane (gasoline) molecules contain only C-H and C-C bonds and are essentially non-polar molecules. Attractive forces between octane molecules include primarily London dispersion forces. Octane is not miscible in H₂O
•     Ethanol is an alcohol (contains an OH functional group). Octanol is also an alcohol (it contains single OH functional group).  However, it contains a string of 8 carbon groups compared to the two in ethanol. The carbon groups cannot participate in Hydrogen bonding (only dispersion forces). The single OH group in octanol is not enough to provide solubility for octanol, and octanol is essentially immiscible in H₂O.

The observation that if similar attractive forces exists between solute-solute molecules and solvent-solvent molecules results in miscible solutions (i,e. the ability of the solvent to dissolve the solute ), has led to the following generalization:

“Like dissolves like”

(In other words, substances with similar intermolecular attractive forces tend to be soluble in one another)

Example: Which of the following would be a better solvent for molecular I₂(s):CCl₄ or H₂O?

·     I₂ is a non-polar molecule. Using the like-dissolves-like rule, I₂ will be more soluble in the non-polar solvent CCl₄ than the polar solvent H₂O.

Pressure Effects on gases Solubility

·       If we increase the pressure of a gas(at constant T), the physical interpretation is that more gas molecules are striking the surface of the container in a given amount of time(kinetic molecular Theory)

·   A gas is constant with a solution is “dissolved” when gas molecules strike the surface of the solution (and are surrounded and dispersed by solvent).

·    Thus, increasing the pressure (at constant T) results in more collisions of the gas molecules, per unit time, with the surface of solvent. This results in greater solubility.

Temperature effects on Gases and Solubility

·     Solvated gas molecules with enough kinetic energy can escape from the surface of a liquid (requires de-solvation of solvent molecules).

·     Kinetic energy increases with increasing temperature

·     Thus, increasing the temperature reduces the solubility of gas molecules in a solvent

Temperature effects on solid solutes and solubility

•         Insolubility of solid solutes is related to the inability of solvent molecules to overcome the attractive forces between solute molecules
•         Increases temperature results in increased in kinetic energy of the solvent molecules, as well as the solute molecules.
•         The increased kinetic energy of the solute molecules favors separation of solute molecules. Increased kinetic energy of the solvent molecules allows them to separate the solute molecules easier
•         Thus, increasing temperature increases the solubility of solid solutes

    Types of Concentration Units

Example:

            The dehydrated form of Epsom salt is magnesium sulfate. What is the percent MgSO4 by mass in a solution made from 16.0g MgSO4 and 100 mL of water at 25ºC The density of water at 25ºC is 0.997g/mL?

Practice Exercise

            An aqueous solution contains 167.0g CuSO4 in 820 mL solution. The density of the solution is 1.195g/mL. Calculate the percent CuSO4 by mass in the solution.

Example:

Practice Exercise

1.      An 11.3mL sample of CH₃OH (density = 0.793g/mL) is dissolved in enough water to produce 75.0mL of a solution with a density of 0.980g/mL. What is the solution concentration expressed as (a) mole fraction of water; (b) molarity og CH₃OH; (c) molality of CH₃OH.

Factor = number of replaceable H⁺ for acids

            = number of replaceable OH^- for bases

            = number of positive valence for salts

Practice Exercise

            Calculate the normality of 15.0% HNO3 by mass solution, with a density of 1.12g/mL.

Dilutions

        You dilute a solution whenever you add  solvent to a solution - adding a solvent results in a solution of lower concentration. you can calculate the concentration of a solution following a dilution by applying this equation:

M₁V₁ = M_fV_f

where M is molarity, V is volume, and the subscript i and f refer to the initial and final values.


RECORDED LECTURES