Unit 5 - Chpt 13 & 17 - Equilibrium and Thermochemistry Part II • Equilibrium basics • Equilibrium Expressions with pressures • Heterogeneous Equilibrium & Applications • Le Chatelier’s Principle • Thermo - Entropy and Free Energy • HW set1: Chpt 13 - pg. 629-637, #10-14, 21ac, 22ac, 23ac, 24ac, 28, Due Thurs. Jan 17 • HW set2: Chpt 13 #40, 43, 48, 52, 57, 63, 64 Due Wed Jan 23
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Unit 5 - Chpt 13 & 17 - Equilibrium and Thermochemistry Part II Equilibrium basics Equilibrium Expressions with pressures Heterogeneous Equilibrium & Applications.
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Unit 5 - Chpt 13 & 17 - Equilibrium and Thermochemistry Part II
• Equilibrium basics• Equilibrium Expressions with pressures• Heterogeneous Equilibrium & Applications• Le Chatelier’s Principle• Thermo - Entropy and Free Energy• HW set1: Chpt 13 - pg. 629-637, #10-14, 21ac,
22ac, 23ac, 24ac, 28, Due Thurs. Jan 17• HW set2: Chpt 13 #40, 43, 48, 52, 57, 63, 64 Due
Wed Jan 23
Chemical Equilibrium
• The state where the concentrations of all reactants and products remain constant with time.
• On the molecular level, there is enormous activity. Equilibrium is not static, but is a highly dynamic situation.
Macroscopically static Microscopically dynamic
Concentration with time
• Changes in Concentration
N2(g) + 3H2(g) 2NH3(g)
Concentrations reach levels where the rate of the forward reaction equals the rate of the reverse reaction.
Rates with time
• The Changes with Time in the Rates of Forward and Reverse Reactions
Concept check equilibrium
Consider an equilibrium mixture in a closed vessel reacting according to the equation:
H2O(g) + CO(g) H2(g) + CO2(g)
You add more H2O(g) to the flask. How does the concentration of each chemical compare to its original concentration after equilibrium is reestablished? Justify your answer.
jA + kB lC + mD
• A, B, C, and D = chemical species.• Square brackets = concentrations of species at equilibrium.• j, k, l, and m = coefficients in the balanced equation.• K = equilibrium constant (given without units).
j
l
k
m
[B][A]
[D] [C]K =
Consider the following reaction at equilibrium:
Equilibrium Expression Math
• Equilibrium expression for a reaction is the reciprocal of that for the reaction written in reverse.
• When balanced equation for a reaction is multiplied by a factor of n, the equilibrium expression for the new reaction is the original expression raised to the nth power; thus Knew = (Koriginal)n
.
• K values are usually written without units.
K constant - more stuff• K always has the same value at a given
temperature regardless of the amounts of reactants or products that are present initially.
• For a reaction, at a given temperature, there are many equilibrium positions but only one value for K. Equilibrium position is a set of equilibrium
• Δn = sum of the coefficients of the gaseous products minus the sum of the coefficients of the gaseous reactants.
• R = 0.08206 L·atm/mol·K• T = temperature (in kelvin)PV = nRT ; C in conc =n/V (molar volume) ; C = P/RTK expression plug in P/RT for each, derived pg 602-3
Calc K from Kp (last problem)
N2(g) + 3H2(g) 2NH3(g)
Using the value of Kp (3.9 × 104) from the previous example, calculate the value of K at 35°C.
Homogeneous Equilibria
• Homogeneous equilibria – involve the same phase:
N2(g) + 3H2(g) 2NH3(g)
HCN(aq) H+(aq) + CN-
(aq)
Heterogeneous equilibria
• Heterogeneous equilibria – involve more than one phase:
2KClO3(s) 2KCl(s) + 3O2(g)
2H2O(l) 2H2(g) + O2(g)
Heterogeneous equilibrium (cont)
• The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present. The concentrations of pure liquids and
solids are constant.
2KClO3(s) 2KCl(s) + 3O2(g)
Applications of Equilibrium• Extent of a reaction• A value of K much larger than 1 means that
at equilibrium the reaction system will consist of mostly products – the equilibrium lies to the right. Reaction goes essentially to completion.
• A very small value of K means that the system at equilibrium will consist of mostly reactants – the equilibrium position is far to the left. Reaction does not occur to any significant extent.
Reaction Quotient - Q• Apply the law of mass action using initial
concentrations instead of equilibrium concentrations in K expression.
• Q = K; The system is at equilibrium. No shift will occur.
• Q > K; The system shifts to the left. Consuming products and forming reactants,
until equilibrium is achieved.
• Q < K; The system shifts to the right. Consuming reactants and forming products, to