Download Chapter 18

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts
no text concepts found
Transcript
Chapter 18
Chemical Equilibrium
http://www.learner.org/resources/series61.html?pop
=yes&pid=806# - Molecules in Action Starting at
15:30-23:52
(FYI: Co(H2O)6+2 + 4Cl- -> CoCl4 + 6 H2O)
Reversible Reactions
• Reversible Reaction
– Reaction that can proceed in either direction
–↔
• Chemical Equilibrium
– Rate of forward rxn = rate of reverse direction
• The Equilibrium Constant (K)
– Compares concentrations of products to
reactants at equilibrium
Reversible Reactions
• For the reaction:
aA + bB  cC + dD
K = [C]c[D]d
[A]a[B]b
Interpretation  K = [products]coefficients
[reactants]coefficients
Reversible Reactions
• Example:
H2 + I2 ↔ 2HI
Given data:
Experiment
[H2]
[I2]
[HI]
1
0.0004953
0.0004953
0.003655
2
0.001141
0.001141
0.008410
3
0.003560
0.001250
0.01559
4
0.002252
0.002336
0.01685
Calculations of K:
K
Reversible Reactions
• Example:
– A mixture of N2, O2, & NO at equilibrium has [N2] =
6.4x10-3M, [O2] = 1.7x10-3M, and [NO] = 1.1x10-5M.
Find K for the reaction:
N2(g) + O2(g) ↔ 2NO(g)
Reversible Reactions
• FYI – SOLIDS AND LIQUIDS HAVE
CONCENTRATIONS SO LARGE THEY
ARE ESSENTIALLY NOT GOING TO
CHANGE (a intensive property), SO YOU
CAN almost always IGNORE THEM IN
THE K EXPRESSION!!
Shifting Equilibrium
• Remember LeChatlier????
• Changes in pressure
– Affects systems with GASES involved
– Move mixture to a smaller container…
• Creates increased pressure
• Rxn will shift to side with FEWER mole of gas to
help alleviate that pressure
• Example
– N2 (g) + 3H2 (g) ↔ 2NH3 (g)
– In a smaller container – will shift RIGHT, K stays the
same!!
Shifting Equilibrium
• Changes in concentration
– Add more of a substance, shifts to use it up
(shifts away from an added substance), K
stays the same
• Changes in temperature
– A reversible rxn is ENDO in one direction and
EXO in the other
– An increase in temp causes a shift so the
ENDO rxn occurs more
– K does change!!
Shifting Equilibrium
• Common-ion effect
– Adding a substance with an ion which is also
in the rxn shifts equilibrium
– Example
• CH3COOH + H2O ↔ H3O+ + CH3COO-1
• Adding NaCH3COO gives CH3COO-1, so causes a
shift LEFT
Calculations involving shifts in
equilibrium
• Reaction Quotient
– Same form as K, but can be used at any point in a
rxn, NOT just at equilibrium
– Q = [C]c[D]d
[A]a[B]b
• If Q = K, then
– The system is at equilibrium
• If Q > K, then
– Need more reactants (less product), so shift LEFT
• If Q < K, then
– Need more product, so shift RIGHT
Calculations involving shifts in
equilibrium
• Example:
K for the rxn N2 (g) + 3H2 (g) ↔ 2NH3 (g) is 2.37x10-3. At a
given point in the rxn, the concentrations are: [N2] = 0.683M, [H2] =
8.80M & [NH3] = 3.65M. Calculate the value of Q and determine the
direction of the rxn.
Equilibria of Acids, Bases, and
Salts
• For weak acids: An acid losing its
hydrogen ion has an equilibrium constant,
Ka
• Example:
CH3COOH + H2O ↔ H3O+ + CH3COO-1
Ka = [H3O+][CH3COO-1]
[CH3COOH]
** H2O is not in expression, because it is a liquid so it
has a concentration which essentially does not change **
Equilibria of Acids, Bases, and
Salts
• Buffers
– Has both the acid and its conjugate base in
the solution (or base and it conjugate acid).
– Example
• CH3COOH & CH3COO-1
• NH3 & NH4+1
Equilibria of Acids, Bases, and
Salts
• Hydrolysis (adding water)
– Anion hydrolysis
• Weak acid anions (F-1, CH3COO-1) can react with
H2O to remove a proton
• F-1 + H2O ↔ HF + OH-1
– A BASIC SOLUTION IS FORMED
– Cation hydrolysis
• Weak base cations (NH4+1) can react with H2O to
add a proton
• NH4+1 + H2O ↔ H3O+1 + NH3
– AN ACIDIC SOLUTION IS FORMED
Solubility Equilibrium
• Solubility Product = Ksp
• Used for slightly soluble or insoluble
substance dissolving
• Example
– AgCl (s) ↔ Ag+1 (aq) + Cl-1 (aq)
{AgCl is insoluble, but
still a LITTLE will dissolve!!}
– K = [Ag+1][Cl-1]
• AgCl not included because it is a solid. So
concentration does NOT change.
Precipitation Calculations
• KBaSO4 = 1.1x10-10. If [Ba+2] = 5.0x10-3M &
[SO4-2] = 2.5x10-3M, will a precipitate
form?
BaSO4 (s) ↔ Ba+2 (aq) + SO4-2 (aq)
Related documents