Chem

• Solution

  Homogeneous mixture where the solvent dissolves the solute, and the solvent is the component present in greatest amount, determining the phase of the solution
• Concentrated solution

  Contains a lot of solute
• Dilute solution

  Contains little solute
• Enthalpy of solution (ΔHsoln)

  • Determines whether a solution can form
  • Depends on intermolecular forces in solution
• Types of solutions based on ΔHsoln

  • Ideal solution (ΔHsoln = 0, ΔVsoln = 0)
  • Non-ideal solution where solute-solvent interactions dominate (ΔHsoln < 0, forms exothermically)
  • Non-ideal solution where solute-solute and solvent-solvent interactions dominate (ΔHsoln > 0, forms endothermically or can't form)
• Rule of thumb for predicting solubility

  "Like dissolves like" (not always true, but a good guide)
• Dissolving ionic compounds in H2O

  1. Ion-dipole forces disrupt the solid

  2. Hydrated ions in solution
• Saturated solution

  Dynamic equilibrium between continuous dissolving and crystallizing, with constant concentration of dissolved solid and quantity of undissolved solid
• Solubility of ionic compounds in H2O

  Increases with temperature
• Solubility of gases in H2O

  Decreases with increasing temperature
• Solubility of gases in all solvents

  Increases with increasing pressure
• Henry's Law

  Solubility is proportional to pressure, Cgas = kPgas
• Molarity

  Moles of solute / volume of solution
• Percent composition

  (Part/Whole) x 100 with appropriate units
• ppm, ppb, ppt

  (Part/Whole) x 10^6, 10^9, 10^12 respectively
• Molality

  Moles of solute / mass (kg) of solvent
• Mole fraction

  Moles of component / total moles present
• Conversions between concentration units

  1. Start with a fixed amount of solution or solvent

  2. Use appropriate equations and definitions
• Colligative properties

  Depend on quantity (concentration) of solute particles, not on their identity
• Raoult's Law

  Psolution = XsolventP°solvent (adding a solute lowers vapor pressure compared to pure solvent)
• Freezing point depression

  ΔTf = mKf (freezing point is lower than pure solvent)
• Boiling point elevation

  ΔTb = mKb (boiling point is higher than pure solvent)
• Osmosis

  Solvent passes from dilute to concentrated solution separated by semi-permeable membrane, until osmotic pressure π is reached
• Osmotic pressure (π)

  π = MRT or πV = nRT
• Isotonic solutions

  Solutions with the same osmotic pressure
• Electrolytes

  Produce ions in solution, so # particles > # formula units dissolved
• Modifying colligative property equations for electrolytes

  1. Use Van Hoff factor (i)

  2. ΔTf = imKf, ΔTb = imKb, π = iMRT
• Van Hoff factor (i)

  • Number of ions produced in solution per formula unit of solute (at infinite dilution)
  • Reduced by ion pair formation in more concentrated solutions
• Colloids

  Mixtures with particle size between solutions and suspensions, won't settle out, can be distinguished by Tyndall effect
• Reaction rate

  Change in concentration per unit time (can refer to products or reactants)
• Factors affecting reaction rate

  • Concentration
  • Temperature
  • Surface area (particle size)
  • Catalysts/inhibitors
• Reaction rates are always > 0 (but remember: change in concentration of a substance being consumed is < 0)
• [A]

  Concentration of A
• Calculating reaction rate

  1. For a reaction forming compound "A", rate = Δ[A]/Δt

  2. Units of reaction rate are always concentration/time (usually mol L^–1 s ^–1 or M ^s –1)

  3. Also need to consider stoichiometry of the reaction (balanced equation): for the reaction aA --> bB, reaction rate = – 1/a x Δ[A]/Δt = 1/b x Δ[B]/Δt
• Instantaneous rate

  The rate at one point in time
• Average rate

  All the instantaneous rates averaged over a period of time (usually whole reaction)
• Initial rate

  Instantaneous rate at start of reaction
• Initial rate is usually the highest rate: rates are usually higher at higher reactant concentrations
• Rate law

  A relationship between reaction rate and reactant concentrations
• General rate law

  1. For the general reaction aA + bB + ... --> products, the rate law is Rate = k[A]^m[B]^n ...

  2. Values of m, n,... must be determined experimentally, not from the overall balanced equation

  3. m, n,... are the order of the reaction with respect to each reactant A, B, etc.

  4. The sum m + n + ... is the overall order of reaction