Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109	General Chemistry	Summer 2002
Lecture Notes::15 July	© R. Paselk 2002
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Solutions

Solution Concentrations-a Review & Some New Stuff.

Solutions: a solution occurs when one chemical is completely dissolved or dispersed in another. We most commonly think of solutions as being liquid, but solid solutions also occur, such as the various metal alloys like steel, brass and bronze.

In a solution the substance present in highest concentration is considered to be the solvent, while components in lesser amounts are considered to be solutes. If you dissolve a sugar cube in water you get a sugar solution, where water is the solvent, and sugar is the solute.

Example:

• What is the solvent in 80 proof rum: 80 proof = 40% alcohol in water, so water is the solvent.
• What is the solvent in 151 proof rum: 151 proof = 75.5% alcohol in water, so alcohol is the solvent.

Concentration Measures

(Check out the problems in the Labbook Appendix!)

Concentration Terms:

• Percent Concentration
  • Mass percent
  • Volume percent
• Molarity: The most commonly used concentration term in chemistry = moles of solute dissolved in 1 L of solution.
  • Two types of situation arise giving two kinds of problems:
  • Making molar solutions.

Example: Make up a 1.00000 L solution of 0.25 M NaCl (note that water is the "default" solvent).

Example: What is the concentration of a solution made by dissolving 10.00 g of KI in enough water to make
1.00000 L?

• Dilution problems (see 12 June).
• Molality: = moles of solute dissolved in 1 kg of solvent.
• Mole fraction: = moles of solute dissolved in total moles of solution = n_a / S n

Example: What is the mole fraction of a solution of 10.0 moles of glycerol dissolved in 15.0 moles of water?

(10 mol) / (10 mol + 15 mol) = 10/25 = 0.400

Solubility

• All gases are completely soluble in each other.
• Liquid solutions
  • "Like dissolves like."
  • Solvation - the process of surrounding a solute with solvent molecules).
    • Hydration (solvation with water).
  • Saturation - the maximum amount of solute which can be in solution in equilibria with its pure state.
    • Supersaturation - dissolving solute in excess of saturation - unstable to the addition of solute (carbonated beverages, honey, etc.)
  • Temperature effects:
    • Gases decrease in solubility with increasing temperature. [example of oxygen solubility]
    • Many solids increase in solubility with increasing temperature, but not always true.
  • Pressure effects:
    • Gases increase in solubility with increasing pressure (Henry's Law: P = kC, where k is a constant specific to the solution). [example of carbonated beverages - p 523 in Zumdahl).
      • Note that Henry's Law only holds for gases that are non-reactive with the solvent - e.g. works for oxygen and nitrogen in water, does not work for HCl and water (goes to H⁺ + Cl^-)
    • Solubility of solids generally uneffected by pressure, since both liquids and solids essentially incompressible.

Colligative properties

Colligative properties (properties which depend only on the number or concentration, not on the type, of particles). [Exchange across surfaces model]

Be able to solve problems for:

• Vapor pressure lowering (Raoult's Law): P = XP°, where P° = the vapor pressure of the pure substance and X = its "mole fraction", that is the number of moles of substance divided by the total number of moles of all substances in the solution (moles solute/(moles solute + moles solvent)) In other words the vapor pressure of a substance in solution is proportional to the molecular percentage of that substance in the solution.

Example: What is the vapor pressure of water in 80 proof alcohol (X_H2O = 0.79) at 25° C (vapor pressure = 23.76 mmHg).

P = XP°

P = 0.79 (23.76 mmHg) = 18.77 mmHg = 19 mmHg

• Boiling point elevation: DT_b = k_bm, where m = molality = moles solute/kg solvent, and k_b is a constant specific to the solvent.
• Freezing point depression: DT_f = -k_fm, where m = molality = moles solute/kg solvent, and k_f is a constant specific to the solvent.
• Osmotic pressure (p): pV = nRT; or, dividing both sides by V, p = MRT, where M = molarity.

Example: What are the osmotic pressures of 1.00 M sugar and 1 M aluminum chloride solutions at 25°C?

p_sugar = MRT = (1 mol/L)(0.0821 L*atm/mol*K)(298 K) = 24.5 atm

p_AlCl3 = MRT = (1 mol/L)(4 mol ion/mol)(0.0821 L*atm/mol*K)(298 K) = 97.9 atm

Colloids: defined by particle size = 1.0 nm< colloid < 100 nm (particles in solution are 0.1 - 1.0 nm in diameter, whereas particles > 100 nm dispersed in a fluid are considered to be in suspension.) Colloids generally do not settle out.

• Biomacromolecules are often colloidal (proteins, DNA & RNA)
  • particles in inks are sometime colloidal.

Chemical Kinetics

Study of rates and mechanisms of reactions. Experimentally, look at rates of reactions, use this information to guess mechanisms

• Rate is a measure of how fast the reaction goes. Can measure how fast a reactant is used up, or how fast a product appears. Use the stoichiometry of the reaction to relate a particular rate to the overall reaction rate.
• A mechanism is a detailed description of the steps leading from reactants to products.

Concentration Dependence of Reaction Rates: Concentrations are assumed to be in Molarity unless otherwise specified.

Consider the reaction:

• Let's assume that if [A] is doubled while all other concentrations are unchanged, that the rate doubles. We can then say that the rate is proportional to [A]; r µ [A]
• Let's now assume that if [B] is doubled the rate quadruples, while 3 x [B] gives 9 x rate (again, all other concentrations are unchanged). We can say that the rate is proportional to the square of [B]; r µ [B]².
• Finally let's say that if [C] is changed, no effect is seen on the rate; r µ [C]⁰ or r = constant.
• We can now combine these expressions to give

This expression is referred to as a Rate Law with the sum of various exponents referred to as the order of the reaction. The overall order of this reaction is thus 3rd order - it is first order in A, second order in B, and zero order in C.

Syllabus / Schedule

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© R A Paselk

Last modified 15 July 2002