Chemistry 1027A/B Study Guide - Final Guide: Reaction Mechanism, Half-Life, Nernst Equation

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13 Apr 2012

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Chemistry 1200B Final 1
Chapter 1: Chemical Equilibrium
1.1: The Equilibrium Constant
Equilibrium: amounts remain constant, overall process reaches equilibrium (dynamic, not static
For any equilibrium, there is a mathematical relationship between the pressures or
concentrations of the reactants and products K, the equilibrium constant
Equilibrium constants are temperature-dependent
If all the components of an equilibrium system are in the same phase, the system is
homogeneous (if they are in different phases, it is heterogeneous)
The concentration of any solid is considered to be constant, and is therefore omitted from the
equilibrium constant expression
How k is affected by:
o Reversing a reaction: k is inverted (1/k)
o Multiplying or dividing: k is put to the power of whatever you multiply by
o Combining equilibria: add together to give overall equation
The magnitude of the equilibrium constant is an indication of the relative amount of product or
reactant present at equilibrium
o If k is greater than 1, the equilibrium lies towards the right (more products than
o If k is less than one, it lies to the left (more reactants than products)
If the amounts of products and reactants can be determined, a quantity known as the reaction
quotient, Q, can be calculated this number indicates whether or not the system is at
equilibrium, and the direction in which the reaction must proceed to attain equilibrium
o Q expression is the same as K expression, except that the concentrations used to
calculate Q are not necessarily the equilibrium concentrations used as comparison
o Q can be found at some point, and is compared to K
o A system is at equilibrium only when Q=K
o If Q is less than K, the reaction must proceed to the right to obtain equilibrium. If Q is
greater than K, then the reaction must proceed to the left to obtain equilibrium
Once a system has reached equilibrium, the concentrations or pressures of the reactants and
products remain constant (system hasn’t stopped reacting, but forward and reverse rates are
o Changing the system by changing the temperature, pressure, adding reactants will
disturb the equilibrium
Le Chatelier’s principle: If a chemical system at equilibrium is disturbed by a change in
temperature, pressure, or the concentration of a participant in the equilibrium, the equilibrium
will shift in such a way as to minimize the disturbance
One way to disturb a gaseous equilibrium is to change the total pressure of the system
o Can be achieved by changing the volume of a system
o If an inert gas is added, the result will be an increase in the total pressure, but there will
not be any changes to the partial pressures of the reactant and products (so no change
in position of equilibrium)
If the temperature of the system is changed, the value of K will change
o In an exothermic reaction (heat is lost to the surroundings): increasing the temperature
has the same effect as increasing the amount of one of the products (shift of equilibrium
to the left, decreasing K)
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Chemistry 1200B Final 2
o Adding heat to an endothermic reaction will shift to the right, increasing K
Gibbs Free Energy change can be used to determine whether a reaction proceeds spontaneously
or not
o G = spontaneous: equilibrium lies to the right and k is large
o +G = nonspontaneous: equilibrium lies to the left, k is small
Delta G = Standard Delta G + RTlnQ
Van’t Hoff equation: used to determine K values at given temperatures (given)
1.2: Solubility of Ionic Compounds
Solubility: the amount of that substance that will dissolve in a certain volume of a solvent
o Soluble: 10g or more can dissolve in a litre of solvent
o Slightly soluble: 0.1g-10g
o Insoluble: less than 0.1g can dissolve
Soluble: nitrates, alkali metal, NH4, halides (Cl, Br, I), sulfates
Insoluble: sulfides, carbonates, phosphates, hydroxides
Solvent: liquid or a gas that dissolves another solid, liquid, or gas (solutes dissolve in a solvent)
Saturated: no more additional solute can dissolve in the solution (at equilibrium)
Salts that are very soluble have a very large Ksp value (usually go to completion)
Solubility product constant is an equilibrium constant, and is the same for a given solid at a
given temperature vs. solubility, which can vary at a given temperature due to other conditions
The reaction quotient:
o If Q<Ksp, no solid
o If Q>Ksp, solid will formed precipitation, solution is supersaturated
Common Ion Effect: the solution may already contain an ion in common with the dissolving salt
can significantly reduce the solubility of the solid relative to that in pure water
o Common ion shifts equilibrium
1.3: Weak Acids and Bases
Acids: increase hydrogen ion concentration
Bases: increase hydroxide ion concentration
o Strong acids and bases completely ionize in solution, whereas weak acids and bases do
not ionize completely
o Weak acid and base solutions proceed until an equilibrium is achieved (Ka and Kb
Arrhenius theory of acids and bases:
o An acid produces H3O+ in water
o A base produces OH- in water
Bronsted-Lowry theory
o An acid is a proton donor
o A base is a proton acceptor
Lewis Acids and Bases
o A Lewis acid can accept a pair of electrons from another atom, forming a coordinate
covalent bond
o A Lewis base donates a pair of electrons
o The species formed is called an acid-base adduct
Ammonia and amines behave as Bronsted-Lowry bases
o The nitrogen atom of NH3 carries a nonbonding pair of electrons, which attracts the
partially positive hydrogen of water hydrogen is transferred to the amine
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Chemistry 1200B Final 3
o The amine donates the nonbonding pair of electrons to form a new N-H bond, resulting
in NH4
Metal cations act as Lewis acids: form complex ions when they react with Lewis bases such as
water, NH3 or OH sometimes called coordination complexes
o Oxides of non-metals also behave as Lewis acids when they react with hydroxide
Weak acid equilibrium: Ka = H+A- / HA
Weak base equilibrium: Kb = BH+OH-/B Ka and Kb values are much less than one
% ionization = x/c times 100%
o If the % ionization is less than 5%, the approximation that x is much smaller than c is
used (making c-x around c)
The more dilute the solution of a weak acid or base, the greater the percent ionization
In the equation of the ionization of a weak acid in solution:
o HA + H2O H3O + A-
o HA is the weak acid, and A- is the conjugate base of the weak acid
o The conjugate base has one less hydrogen, more negatively charged, will behave like a
weak base when in solution
The equilibrium constant for the ionization of water is Kw = 1x10-14
o Ka x Kb = Kw
The stronger the weak acid is, the weaker its conjugate base will be
Strong acids have extremely weak conjugate bases their bases are so weak, that they are
spectator ions and do not affect pH at all (like Cl as the spectator ion for HCl
A salt is an ionic solid containing cations and anions when a salt dissolves in water, its cation
and anion separate from each other
o If the cation and anion can affect the pH, then if Ka> Kb, the salt is acidic
The relative strengths of weak acids are primarily determined by the relative thermodynamic
stabilities of the conjugate bases
What affects the stability of conjugate bases?
o Resonance: increases the stability of the ion (cumulative effects also)
o Electronegativity of the atom bearing the negative charge: more stable = more
o Inductive effect: distance effect, further stabilizes by pulling electrons away
Polyprotic: they contain more than one ionisable hydrogen atom these acids ionize in stages,
each with a different Ka value
1.4: Buffer Solutions
Buffer: both the weak acid and its conjugate base are added to the same solution maintains
pH. Requirements:
o Must contain a weak acid that will react with any OH
o Contain a weak base that will react with any H3O
o The acid and the base in the buffer must not react with each other (weak acid or base
and its own conjugate, in equilibrium)
Acid buffer: weak acid and its conjugate base start with acid, and add a salt that contains the
conjugate base, or react the weak acid with a lesser amount of strong base
Base buffer: weak base and its conjugate acid
The outstanding feature of a buffer is its ability to resist changes in pH
There are two reasons why mole amounts work:
o The parent and conjugate are always in the same volume
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