Chapter 14
Ions in Aqueous Solutions and Colligative Properties


  1. Compounds in Aqueous Solutions
    Solids compounds can be ionic or molecular
    Differ in the basic particles - ions v atoms
    Behavior is different when dissolved in water

    1. Dissociation

      applies to ionic compounds
      dissociation - definition

      NaCl(s) ---> Na+(aq) + Cl-(aq)

      1 mol ---> 1 mol + 1 mol = 2 mol of ions

      N.B. charge of each ion times coefficient = cancellation of charges

      CaCl2(s) ---> Ca+2(aq) + 2Cl-(aq)

      1 mol ---> 1 mol + 2 mol = 3 mol of ions

      N.B. charge of each ion times coefficient = cancellation of charges

      Number of ions produced from each formula.

      100% dissociation implies 1 mole of compound breaks down totally into its ions in solution. No part of the compound remains - only ions in solution.

      Sample Problem 14-1 page 426

      Write the equation for the dissociation of aluminum sulfate, Al2(SO4)3. How many moles of each ion are produced when you start with one mole of aluminum sulfate? What is the total number of ions?

      Al2(SO4)3 (s) ---> 2 Al+3(aq) + 3 SO4-2(aq)

      1 mol ---> 2 mol + 3 mol = 5 mol of ions
      N.B. charge of each ion times coefficient = cancellation of charges

      Homework: Chapter 141 Dissociation

      Precipitation Reactions

      No compound is completely insoluble - some small amount dissolves.

      Table 14-1 page 427

      Can’t write dissociation reactions for insoluble compounds.

      Can use table 14-1 to predict solubility of products when two solutions are mixed -- double replacement reactions we studied earlier in the year.

      If one of the compounds formed is insoluble we get a precipitate formed.

      Precipitate - definition

      Forms because the attraction between the ions of the precipitate is stronger than the attraction of the water molecules for each ion.

      (NH4)2S(aq) + Cd(NO3)2(aq) ---> 2 NH4NO3(aq) + CdS(s)

      First write the products of the double replacement reaction -- two solutions reacting -- then use table 14-1 to predict if the products individually will be soluble.
      1. Net Ionic Equations

        Used instead of formula equations for reactions that happen in solutions.

        Net ionic equation - definition

        Steps:
        a) take the balanced chemical equation and covert it into an ionic equation;
        b) soluble compounds are shown as ions, insoluble compounds are not shown as ions;
        c) cancel those ions that appear on both sides of the equation, exactly;
        d) rewrite the equation showing what is left -- usually two or more ions on the left and a compound on the right

        Example using equation above.

        Spectator Ions - definition

        Sample Problem 14-2 page 430

        Zinc nitrate solution and ammonium sulfide solution are mixed. Identify the precipitate after you write the formula equation, the ionic equation, and the net ionic equation and identify the spectator ions.

        a) write chemical equation

        Zn(NO3)2(aq) + (NH4)2S(aq) ---> ZnS(s) + NH4NO3(aq)

        b) balance the equation

        Zn(NO3)2(aq) + (NH4)2S(aq) ---> ZnS(s) + 2 NH4NO3(aq)

        c) write the ionic equation
        Zn+2(aq) + 2 NO3 -(aq) + 2 NH4+(aq) + S2-(aq) ---> ZnS(s) + 2 NH4+(aq) + 2 NO31-(aq)

        d) check to see that all charges cancel

        1(+2) + 2(-1) + 2(+1) + 1(-2) = 0 for the reactants

        zero + 2(+1) + 2(-1) = 0 for the products

        Homework Chapter 14.2 Net Ionic Equation


    2. Ionization

      The term applies to molecular compounds that form ions in solution.

      Ionization - definition

      ionization v dissociation

      formation v separation

      the extent of ionization depends on a) strength of the bonds within the molecule of the solute; b) strength of the attraction between the solute and solvent molecules

      if a above is greater than b above, the solution process will not occur; if a above is weaker than b above,the solution process and the formation of ions will occur

      HCl ---> H+(aq) + Cl1-(aq)

      1. The Hydronium Ion

        H+ does not exist alone; it attracts other ions or molecules -- it is a bare proton.

        a better way to represent when HCl is added to water is:

        H2O(l) + HCl(g) ---> H3O+(aq) + Cl1-(aq)

        figure 14-4 page 431

        H3O+ is known as the hydronium ion -- a hydrogen ion attached to a water molecule or a hydrated hydrogen ion

        Homework: Chapter 14.3, Ionization
    3. Strong and Weak Electrolyte

      If a substance in solution produces ions and conduct electricity it is known as an electrolyte -- ionic compounds that are soluble or molecular compounds that ionize.

      hydrogen halides -- all gases, all except HF conduct electricity well

      the strength of an electrolyte is directly related to the fact that whatever dissolves in water exists completely as hydrated ions in solution -- a strong electrolyte

      more ions means better electrical conductivity

      Figure 14-5 page 432

      1. Strong Electrolytes

        strong electrolyte - definition

        HCl, HBr, HI are 100% ionized and are strong electrolytes; all form acids in water solutions.

        The acids above, several other acids, and all ionic compounds are strong electrolytes.

        AgCl is considered insoluble: 0.000 089g AgCl/100 g water and yet the small amount that dissolves is complete dissociated into hydrated ions, therefore, it is considered a strong electrolyte.


      2. Weak Electrolytes

        In HF, there are dissolved ions and some molecules that are not ionized. The solution is called hydrofluoric acid.

        The bond between H and F is stronger than the bond between H and other halogens, therefore, hydrogen fluoride solution has fewer ions than the same amount of the other hydrogen halogen compounds in water.

        This can be represented as

        HF(aq) + H2O(l) === H3O+(aq) + F1-(aq)

        where the concentration of unionized HF is high compared to the concentration of the hydronium and fluoride ions.

        weak electrolyte - definition

        nonelectrolyte - definition

        HC2H3O2(aq) + H2O(l) ---> C2H3O21-(aq) + H3O+(aq)

        strong and weak v concentrated and dilute

  2. Colligative Properties of Solutions

    Whenever we add a solute to a solvent, the resulting solution has different properties than the pure solvent would have.

    The properties of the solution depend on the number of solute particles.

    Colligative Properties - definition

    1. Vapor-Pressure Lowering

      Boiling and freezing points depend on the vapor pressure of the liquid.

      A solution has a
      a) lower vapor pressure than the pure solvent and so
      b) the boiling point of the solution is higher than the boiling point of the pure solvent and the
      c) freezing point of the solution is lower than the freezing point of the pure solvent.

      This assumes a nonvolatile solute is added to a pure solvent.

      Nonvolatile substance - definition

      Figure 14-6 page 436

      Figure 14-7 page 437

      Boiling and freezing points depend on vapor pressure.

      The nonvolatile solute prevents the solvent from creating vapor pressure as readily and so lowers the vapor pressure and as a result raises the boiling point and lowers the freezing point.

      The more solute you have the lower the vapor pressure of the solvent will be and the effect on the bp and fp will be even greater.

      A 1m aqueous solution of glucose, a nonelectrolyte, lowers the vapor pressure eo water 5.5 x 10-4 atm at 250C.

      A 1 m aqueous solution of sucrose, a nonelectrolyte, lowers the vapor pressure eo water 5.5 x 10-4 atm at 250C.

      The lowering of the vapor pressure depends on the number of solute particles (concentration of the nonelectrolyte solute) which makes the lowering of vapor pressure a colligative property.

      Figure 14-6 page 436

      After the solute is added the solution remains liquid over a larger temperature range because of the bp increasing and the fp decreasing.

    2. Freezing Point Depression
      For water a 1m solution of a nonelectrolyte lowered the freezing point 1.860C i.e. the freezing point of the solution is
      -1.860C.

      A 2m solution of a nonelectrolyte in water lowers the freezing point by 2(-1.86) or -3.720C.

      For water the molal freezing point constant, Kf, is -1.860C/m

      Molal freezing point constant - definition

      Each solvent has its own unique molal freezing point constant.

      Table 14-2, page 438

      Freezing point depression has the symbol delta Tf

      Freezing point depression - definition

      Formula: delta Tf = Kf m

      Sample Problem 14-3 page 439

      a) What is the freezing point depression of water in asolution of 17.1 g of sucrose, C12H22O11, and 200. g       of water?
      b) What is the actual freezing point of the solution?

      sample prob 14-3


      Homework: Website, Chapter 14.4, Freezing Point Depression
    3. Boiling Point Elevation

      boiling point - definition

      adding a solute to a solvent lowers the vapor pressure and thus raises the boiling point. The solution boils at a temperature higher than the pure solvent.

      molal boiling point constant (Kb) - definition

      for water it is 0.510C/m

      the constant is unique to each solvent
    4. page 438

      boiling point elevation (delta tb) - definition

      delta tb = Kb m

      Homework: Chapter 14.5, Boiling Point Elevation
    5. Osmotic Pressure

      Figure 14-8 page 442

      Semipermeable membranes - definition

      Osmosis - definition

      Osmotic pressure - definition

      Cells and cell membranes

      Homework: Chapter 14.6, Osmotic Pressure
    6. Electrolytes and Colligative Properties

      The colligative properties depend on the number of solute particles added to a pure solvent. We measure that number by using molality (m).

      An electrolyte breaks down into more particles in solution than a nonelectrolyte.

      e.g. C12H22O11(s) ---> C12H22O11(aq)

      one molecule of the solid when added to water yields one molecule of the hydrated substance.

      CaCl2 ---> Ca2+(aq) + 2 Cl1-(aq)
      1. Calculated Values for Electrolyte Solutions

        one mole of CaCl2 produces 3 moles of ions ( one mole of calcium ions and two moles of chloride ions).

        Figure 14-9 page 443

        Thus, we would expect a solution of calcium chloride to lower the vapor pressure, raise the boiling point, and lower the freezing point three times more than the same concentration of sucrose solution.

        We need to take this into account when calculating the colligative properties of solutions.
      Actual Values for Electrolyte Solutions

      The values mentioned above for solutions of electrolytes are expected values. This is not always the case.

      Table 14-3 page 445

      Attractive forces need to be considered. These forces become more important as the concentration of the solution increases because of the increase in the number of solute particles and the closer these hydrated ions are.

      They also become less important as the concentration of the solution decreases (becomes more dilute) since the number of solute particles decreases and the hydrated solute particles are farther apart.

      Also, the higher the charge on the individual ions, the less the colligative properties are affected -- Debye-Huckel theory.

      Compare MgSO4 and KCl in table 14-3 page 445.

      Homework: Chapter 14.7, Electrolytes and Colligative Properties

      end of notes


      The boiling point is the temperature at which the equilibrium vapor pressure of a liquid is equal to the prevailing atmospheric pressure. back

      A nonelectrolyte is a substance whose water solution does not conduct electricity. back


      A precipitate       is an insoluble solid that results from mixing two solutions. back


      Dissociation is the separation of ions that occurs when an ionic compound dissolves.back

      Net ionic equation includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution. back

      Spectator ions are ions that do not take part in a chemical reaction and are found in solution both before and after the reaction. back

      Ionization is the process by which ions are formed from solute molecules by the action of the solvent. back

      The hydronium ion is a hydrated proton. back

      A strong electrolyte is any compound o which all or almost all of the dissolved compound exists as ions in an aqueous solution. back

      A weak electrolyte is a compound of which a relatively small amount of the dissolved compound exists as ions in an aqueous solution. back

      Colligative properties are those properties that depend on the concentration of solute particles but not on their identify. back

      A nonvolatile substance is a substance that has little tendency to become a gas under existing conditions. back

      Molal freezing point constant (Kf) is the freezing point depression of the solvent ina 1 molal solution of a nonvolatile nonelectrolyte solute. back

      The freezing point depression (delta tf) is the difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution. back

      The molal boiling point constant (Kb) is the boiling point elevation of the solvent in a 1 molal solution of a nonvolatile, nonelectrolyte solute. back

      The boiling point elevation, (delta tb) is the difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent. back

      Semipermeable membranes allow the movement of some particles while blocking the movement of others. back

      Osmosis is the movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration. back

      Osmotic pressure is the external pressure that must be applied to stop osmosis. back