This document discusses aqueous chemistry and chemical equilibrium. It introduces key concepts like the equilibrium constant K, reaction quotient Q, and Le Châtelier’s principle. K is a ratio that quantifies concentrations at equilibrium. Q is similar but used to predict the direction of reactions not yet at equilibrium. Le Châtelier's principle states that if a system at equilibrium experiences a change, it will shift to counteract the change.

1. Aqueous Chemistry

3. I. Introduction
II. The Equilibrium Constant (K)
III. Values of Equilibrium Constants
IV. The Reaction Quotient (Q)
V. Equilibrium Problems
VI. Le Châtelier’s Principle

4.  Equilibrium will be the focus for the next
several chapters.
 Most reactions are reversible, meaning
they can proceed in both forward and
reverse directions.
 This means that as products build up, they
will react and reform reactants.
 At equilibrium, the forward and backward
reaction rates are equal.

6.  Equilibrium does not mean that
concentrations are all equal!!
 However, we can quantify concentrations at
equilibrium.
 Every equilibrium has its own equilibrium
constant.

7.  equilibrium constant: the ratio at equilibrium
of the [ ]’s of products raised to their
stoichiometric coefficients divided by the [ ]’s
of reactants raised to their stoichiometric
coefficients.
 The relationship between a balanced equation
and equilibrium constant expression is the
law of mass action.

8.  For a general equilibrium aA + bB  cC +
dD, the equilibrium expression is:

9.  Write the equilibrium constant
expression for the reaction:
2H2(g) + O2(g)  2H2O(g).

10.  Large values of K mean that the equilibrium
favors products, i.e. there are high [ ]’s of
products and low [ ]’s of reactants at
equilibrium.
 Small values of K mean that the equilibrium
favors reactants, i.e. there are low [ ]’s of
products and high [ ]’s of reactants at
equilibrium.

11.  If an equilibrium contains pure solids or
pure liquids, they are not included in the
equilibrium constant expression.

12.  Note that units are
not included when
calculating K’s.
 Thus, equilibrium
constants are
unitless.

13.  What happens when we mix reactants
together and wait?
 Can we predict what will happen when we
have a mixture of reactants and products?
 The reaction quotient, Qc or Qp, is used to
predict in which direction an equilibrium
will move.

14.  You already know the formula because
it’s the same as for Kc or Kp!!
 The difference is, we don’t know if the
reaction is at equilibrium, thus, we
cannot set the ratio equal to K!
 For the reaction aA + bB  cC + dD:

15.  The value of Q relative to K tells you whether
the reaction will form more products or more
reactants to reach equilibrium.
 Q < K means reaction forms products.
 Q > K means reaction form reactants.
 Q = K means reaction is at equilibrium.

16.  Consider the reaction N2O4(g)  2NO2(g)
with Kc = 5.85 x 10-3. If a reaction mixture
contains [NO2] = 0.0255 M and [N2O4] =
0.0331 M, which way will the reaction
proceed?

17. By Jenna Kennedy
Dec. 15 2009
Chem. 12 A

18.  Born on Oct. 8 1850
 Attended Collège Rollin, and the
École polytechniques in 1869.
 Between the years 1884-1914 he
published about 30 papers on either
the principle of chemical equilibrium
or the variation of the solubility of
salts in an ideal solution.
 He taught chemistry at the Collège
de France, from 1898 – 1907.
 In 1907 he was made a member of
the Royal Academy of Science, and
the Académie des sciences.

19.  This principle is used to describe the effect a
change in conditions can have on a chemical
equilibrium.
 “If a chemical system at equilibrium experiences a
change in concentration, temperature, volume, or
partial pressure, then the equilibrium shifts to
counteract the imposed change.” (Le Chatelier)
 This is a basic summary of Le Chatelier’s Principle.

20.  An increase in the concentration of the
reactants, means a shift to the right.
(consumes more materials)
 An increase in the concentration of the
products, means a shift to the left.
(makes more products)
 If there is a decrease then the shift is to the
opposite side.
 ex. CO + 2 H2 ⇌ CH3OH

21.  An increase in temperature would mean a
shift to the opposite side of the equation.
(heat was added to the reactants, then the
products side would increase)
 ex. N2 + 3 H2 ⇌ 2 NH3 + 92kJ

22.  An increase to the pressure, or a decrease
in the volume would result in a shift
towards the side with the least moles of
gas.
 A decrease in the pressure, or an increase
in the volume results in a shift towards the
side with the most moles of gas.
 ex. N2 + 3 H2 ⇌ 2 NH3

23.  What is the effect of removing C (s) from the
system:
◊C (s) + H2O (g) H2 (g) + CO (g)
 A decrease on the reactants side, means a
shift to the left.

24.  Clark, Jim. “Le Chatelier’s Principle.” 2002. Online. Dec. 10,
2009.
http://www.chemguide.co.uk/physical/equilibria/lechatelier
.html
 “Henry Louis Le Chatelier.” Wikipedia. 2009. Online. Dec. 10,
2009.
http://en.wikipedia.org/wiki/Henry_Louis_Le_Chatelier
 “Le Chatelier.” Wikipedia. 2009. Online. Dec. 10, 2009.
http://en.wikipedia.org/wiki/Le_Chatelier's_principle
 Therrien, E. Chemistry 12. Halifax, NS: Auburn High, 2009.