Unit 4 Equilibrium – 7.1 Intro
Consider:
At equilibrium, the rates of the forward and reverse reactions are equal. How the equilibrium is
established is not relevant. Once equilibrium is established, it is impossible to tell whether we
started with separate solutions of Ca2+(aq) and SO42–(aq), or with excess CaSO4(s) added to pure
water.
Consider:
Equilibrium occurs in a closed system. Explanation: competition between collisions of reactants
to form products and collision of products to form reactants.
In industry: How drive the reaction forward? (to make lime and CO2) Answer: _____________
Compare this idea to OSMOSIS in biology. (Osmosis is the movement of water down a
concentration gradient across a semi-permeable membrane).
Another example:
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Another reaction:
ex. problem (pg. 433)
ICE chart:
Perform algebra…
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7.2 Equilibrium Constant, K
Equilibrium Law:
p. 441 shows how the law is related to Reaction Kinetics constants
Example:
Therefore, always ignore liquids
or solids in equilibria equations.
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7.3 Le Châtelier’s Principle:
When a chemical system at equilibrium is disturbed by a change in a
property, the system adjusts in a way that “opposes” the change.
(or “resists” – “Hell no, we won’t go!)
ex.
As blood circulates to the lungs, the high concentration of dissolved oxygen shifts the
equilibrium to the right and the hemoglobin becomes oxygenated. As the blood circulates
throughout the body, cellular respiration consumes oxygen. This removal of oxygen shifts the
equilibrium to the left, and more oxygen is released.
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from pg. 456:
7.5 Quantitative Changes in Equilibrium Systems
Recall:
Reaction quotient is Q.
We can think of Q as being similar to K, with the difference being that K is calculated using
concentrations at equilibrium, where as Q may or may not be at equilibrium. The same
mathematical equation is used for calculating both K and Q. The result of such a trial
calculation must be one of three possible situations.
• Q is equal to K, and the system is at equilibrium.
• Q is greater than K, and the system must shift left (toward reactants) to reach equilibrium,
because the product-to-reactant ratio is too high.
• Q is less than K, and the system must shift right (toward products) to reach equilibrium
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sample problems pg. 464-480 …. lots of algebra
a) perfect squares – pg. 467
ex.
b) imperfect squares – pg. 472
- use approximation to simplify the algebra
- must check if acceptable by the “100 rule”:
c) K is too large to ignore – may need to solve
quadratic equation
7.6 Solubility Product Constant, Ksp
Ksp = the value obtained from the equilibrium law applied to a saturated solution
Note: everything is “soluble” to an extent:
sample problems: pg. 485-6
Table 4 on pg. 487 (same as 11U text) solubility cut off is > or < 0.1 M
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Will a precipitate form?
sample problem pg. 488
7.7 Energy & Equilibrium: The Laws of Thermodynamics
(note: part of this section is crazy. It is scary for some, much like the atomic theory section was)
First Law of Thermodynamics
The total amount of energy in the universe is constant. Energy can be neither created
nor destroyed, but can be transferred from one object or place to another, or
transformed from one form to another.
Enthalpy and Entropy Changes Together Determine Spontaneity
Entropy, S, is a measure of disorder or randomness.
In general, a system will experience an increase in entropy (ΔS > 0) if
• the volume of a gaseous system increases,
• the temperature of a system increases, or
• the physical state of a system changes from solid to liquid or gas, or liquid to gas
(Sgas > Sliquid > Ssolid)
Free energy (or Gibbs free energy) energy that is available to do useful work (see
aquariums pg. 499)
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In general, a change at constant temperature and pressure will occur spontaneously if
it is accompanied by a decrease in Gibbs free energy, G. Changes are spontaneous if
the change in G, ΔG, is negative, ΔG < 0 (Gfinal is less than Ginitial ).
read together pg.500-501
Use entropy to explain….
a) Why a pregnant woman feels tired much of the time.
b) Why snowflakes form pretty patterns (or ice on a window) compared to liquid water which
just sits there like a puddle of water.
Second Law of Thermodynamics
All changes either directly or indirectly increase the entropy of the universe.
Mathematically, ΔSuniverse > 0
The entropy of the universe is equal to the sum of the entropy of the system and the entropy of
the surroundings. A change in the entropy of the universe may occur as a result of a change in
the entropy of the system or of the surroundings, or of both.
ΔSuniverse = ΔSsystem - ΔSsurroundings
This means that a system’s entropy, ΔSsystem, can decrease (the system becomes more ordered),
so long as there is a larger increase in the entropy of the surroundings, ΔSsurroundings, so that the
overall entropy change, ΔSuniverse, is positive.
Predicting Spontaneity
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ΔG < 0 change is spontaneous
ΔG > 0 change is nonspontaneous
(pg. 504)
Third Law of Thermodynamics
The entropy of a pefrectly ordered pure cyrstalline substance is zero at absolute zero.
Mathematically, S = 0 at T = 0 K
Now, for something very useful…molar calculations…
and
Use balanced equation & find the values of ΔH°f and S° from the chart of thermodynamic
data in Appendix C6 to help find ΔG°. (sample pg. 506)
Equilibrium and ΔG
• if ΔG < 0, the reaction is spontaneous in the forward direction;
• if ΔG > 0, the reaction is nonspontaneous in the forward direction but is spontaneous in the
reverse direction;
• if ΔG = 0, there is no “preferred” direction. The system is at equilibrium.
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Temperature and Equilibrium
Since the value of ΔG is equal to zero for any system at equilibrium…
We can predict the temperature of a phase change. (sample: pg. 510)
Chpt. 8: Acid-Base Equilibrium
Brønsted-Lowry Theory
a Brønsted–Lowry acid is a proton donor, and
a Brønsted–Lowry base is a proton acceptor.
amphoteric (amphiprotic) - _______________________________________________
Reversible Acid–Base Reactions
The Autoionization of Water
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ex. pg. 535… much simpler & shorter than it looks
Hydrogen Ion Concentration and pH
See sample problems in the section (p 545, 547)
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8.2 Weak Acids & Bases
percent ionization – nice but not as common and tabulated as “Ka” values
Ionization Constants for Weak Acids
ICE type sample problem: pg. 554
There is a similarly derived ionization constant for bases:
The Relationship Between Ka and Kb is shown on pg. 559-560. Summary:
Therefore, if you’re given one, you’re expected to be able to find the other. Appendix C9 on pg.
803 has Ka and Kb values. Sample problem: pg. 563 -578
8.3 Acid-Base Properties of Salt Solutions
Salts That Form Neutral Solutions
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Salts That Form Acidic Solutions
A cation such as the ammonium ion, NH4(aq), acts as a Brønsted–Lowry acid.
Salts That Form Basic Solutions
Salts That Act as Acids and Bases
sec. 8.4 (Acid-Base Titration), sec. 8.5 (Buffers)
Read the sections and answer the following questions:
1. What is the purpose of performing a titration?
2. Why is a primary standard used and what are 2 examples of primary standards?
3. Draw a typical titration curve. Explain the shape of the curve.
4. Compare the meaning of buffer in chemistry to that in two other aspects of life such as a) computer memory,
b) the use of masking tape when painting a room.
5. Why are buffers important in biological systems (i.e. the human body)?