Chemical Equilibrium

Transcription

1 Chemical Equilibrium Not all reactions proceed to completion Chemical Equilibrium a state in which the ratio of concentrations of reactants and products is constant

2 Chemical Equilibrium: Some Rules In evaluating an equilibrium constant, always remember the following: The concentrations of solutes should be expressed in molarities. The concentration of gases should be expressed in atmospheres. The concentrations of pure solids, liquids or solvents are unity (1).

3 Chemical Equilibrium: Le Châtalier's Principle The position of a chemical equilibrium is independent of the route by which equilibrium is reached. Apply stress to a chemical system = disruption of equilibrium Stress : change in either temperature, pressure or total concentration of a reactant or product To relieve stress, the equilibrium shifts to offset the sudden change.

4 Equilibrium and Thermodynamics Enthalpy (ΔH) amount of heat absorbed or released Entropy (S) amount of order / disorder in the system Gibbs' Free Energy (ΔG) determines if the reaction is favored, not favored or in equilibrium based on ΔH and S.

5 In Depth: Le Châtalier's Principle The equilibrium constant K eq vs. the reaction quotient Q. Q= [C ' ]p [ D ' ] q [ A' ] m [ B ' ] n

6 Chemical Equilibrium: The Solubility Product Solubility Product a specific type of chemical equilibrium in which a solid salt dissolves into its ions in a solution Example: Hg 2 Cl 2 (s) Hg 2 +2 (aq) + 2Cl - (aq) K sp = [Hg 2 +2 ][Cl - ] 2 = 1.2 x 10-18

7 The Common Ion Effect Given the example in the previous slide, what would happen if we add sodium chloride into the solution? The Common Ion Effect a salt will be less soluble if one of its constituent ions is already present in the solution.

8 Application: Selective Precipitation Given the following: PbI 2 (s) Pb +2 (aq) + 2I - (aq) Hg 2 Cl 2 (s) Hg 2 +2 (aq) + 2Cl - (aq) Is it possible to separate the two metal ions in the solution? How?

9 Chemical Equilibria Acids and Bases Three (3) definitions of acids/bases Arrhenius Bronsted-Lowry Lewis

10 The Nature of H + and OH - A proton cannot exist by itself!

11 ph Power of H + Autoprotolysis of water What is the equilibrium constant for this reaction?

12 Acid/Base Strength Depends on how much H + is produced. Strong = complete dissociation Weak = partial dissociation Write down the equilibrium equation for the weak acid HA (A is an anion). Write down the equilibrium equation for the weak base B. Brain teaser: how is the equilibrium constant for autoprotolysis related to acid/base strength?

13 Chemical Equilibrium In Action Calculate for the [Ba +2 ] concentration from Ba(IO 3 ) 2 in a solution prepared by mixing 200 ml of M Ba(NO 3 ) 2 and 100 ml of M NaIO 3. K sp (Ba(IO 3 ) 2 ) = 1.57 x 10-9

14 Systematic Treatment of Equilibrium

15 Systematic Treatment of Equilibrium Most of the time, chemical equilibria are rarely straightforward: BaSO 4 (s) Ba +2 + SO 4-2 SO H 3 O + HSO H 2 O 2H 2 O H 3 O + + OH -

16 Systematic Treatment: Charge- Balance Equations Possible since the species in aqueous solutions are ions. Premise: all electrolyte solutions are neutral. no. mol/l (+) = no. mol/l (-) Remember: a charge-balance equation is based on the equality in molar charge concentrations. no. mol/l (+) = [A] x no. of (+) charge

17 Systematic Treatment: Charge- Balance Equations Example: Give the molar concentrations of positive/negative charge for the following ions: Na + Ba +2 OH - SO 4-2 CH 3 COO -

18 Systematic Treatment: Charge- Balance Equations Example: Given the system of equilibria in the previous slide, construct its charge-balance equation. BaSO 4 (s) Ba +2 + SO 4-2 SO H 3 O + HSO H 2 O 2H 2 O H 3 O + + OH -

19 Systematic Treatment: Mass- Balance Equations Restatement of the law of conservation of matter The quantity of all species in a solution containing a particular atom or group of atoms must equal the amount of that atom/group delivered to the solution.

20 Systematic Treatment: Mass- Balance Equations Example M acetic acid in water-- CH 3 COOH CH 3 COO - + H + c acetic acid = 0.05 M = [CH 3 COOH] + [CH 3 COO - ] Question: Why is [H + ] NOT included in defining the MB expression of c acetic acid?

21 Exercise. Given the following equilibrium reactions for 0.05 M sulfuric acid: H 2 SO 4 HSO H + HSO 4 - SO H + Give the MB expression for the acid's formal concentration c sulfuric acid.

22 Another Mass-balance Expression Example. La(IO 3 ) 3 partially dissolves in water, as shown in the equation-- La(IO 3 ) 3 La IO 3 - La +3 's relationship with iodate can be defined in the following MB expression: [IO 3- ] = 3 x [La +3 ] WHY?

23 MB Equations in a System of Equilibria Challenge! In the previous set of equilibria, write down all the possible MB expressions present in the system. BaSO 4 (s) Ba +2 + SO 4-2 SO H 3 O + HSO H 2 O 2H 2 O H 3 O + + OH -

24 Systematic Treatment of Equilibria: A 6-Step Approach (1) Write the pertinent reactions. (2) Write the charge-balance equation. (3) Write mass-balance equations. There may be more than one. (4) Write the equilibrium constant for each reaction. (5) # of equations # of unknowns. If not, find more equilibria or fix some concentrations at known values. (6) Solve for all unknowns, by hook or by crook.