Lecture 7 -- Chemical Equilibrium

1Chemical EquilibriumChapter 141

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223Textbook Chapters to Cover14.2 The Concept of Dynamic Equilibrium14.3 The Expression of the Equilibrium Constant14.4 The Equilibrium Constant K14.5 Calculating the Equilibrium Constant from Measured Quantities14.6 The Reaction Quotient: Predicting the Direction of Change14.7 Find Equilibrium Constants14.8 Le Chateliers Principle: How a System at Equilibrium Responds to Disturbances34

Dynamic equilibrium occurs when the forward rate and the reverse rate of the reaction are equal.

Concentrations (or partial pressures) no longer change over time.Dynamic Equilibrium5Equilibrium & Reaction Ratesgeneric equilibriumforward reaction:A B Rate = kf[A]reverse reaction:A B Rate = kr[B]At equilibrium, kf[A] = kr[B] Rearranging,

N.B. Keq changes with T, not with concentrationAt equilibrium, the concentrations of reactants and products no longer change, but they are not necessarily equal to one another.5

The Equilibrium Constant67The Equilibrium ConstantConcentration-based equilibrium constant, KcPressure-based equilibrium constant, KPRelation between Kc and KPSolids & liquids in equilibriumWe will not cover activities (you will see activities in more advanced chemistry courses.)8Conc-Based Equilib. ConstantFor generic reaction

with A, B, C, and D in the aqueous or gas phaseConcentration-based equilibrium constant, Kc is:

[ ] indicates conc at equilibrium in mol L-1Keq ratio always has products over reactants8

9Pressure-Based Equilib. ConstantKP used for gas-phase reactionsEquilibrium partial pressures (bar) instead of mol/L

e.g.2NO2(g) N2O4(g)9

10ExamplesFe3+(aq) + SCN(aq)Fe(SCN)2+(aq)

N2(g) + 3H2(g) 2NH3(g)

K >> 1 implies that products are favouredE.g. H2(g) + Br2(g) 2 HBr(g)K = 1.9 x 1019 at 25 CK Keq then rxn proceeds towards reactantsIf Q < Keq then rxn proceeds towards productsIf Q = Keq then rxn is already at equilibrium333434

3435Back to the exampleInitial values: PNO = 0.050 bar; PN2 = 2.00 bar; PO2 = 8.00 bar

Q < KP (4.8410-4) so rxn proceed towards productsSay, PNO,i = 0.100 bar, PN2 = 2.00 bar, PO2 = 8.00 barThen Q = 6.2510-4 rxn proceeds towards reactants35

362.008.000.050- x- x

ICE0.050 + 2x+ 2x8.00 - x2.00 -x

37SolutionUsing the quadratic formula

with a = 3.999516b = 0.20484c = -5.24410-3The roots are x = +0.0187 and -0.0700

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3738Quadratic equationKeq calculations will often require the use of the quadratic equation -- RELEARN IT!One of the roots is usually unphysicale.g. x = -0.070 in the previous example yields PNO = -0.090 bar

38Quadratic formula song:http://www.youtube.com/watch?v=IvXgFLV2gOk 3839Quadratic equationUsing the physical root, x = 0.0187, we find that:PNO = 0.050 + 2(0.0187) = 0.087 barPN2 = 2.00 - 0.0187 = 1.98 barPO2 = 8.00 - 0.0187 = 7.98 barCheck your answer: Plug the values found back into the KP expressionANSWER: 4.8310-4

393940Avoiding quadraticsExpression for KP (slide 36)

KP 0

Stress: Increase VolumeReactantsProductsNo Change

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51LC: Speed Round Part 3Will the stress cause the equilibrium to shift towards reactants or products, or produce no change? N2(g) + 3H2(g) 2NH3(g) H < 0

Stress: Increase pressure by adding heliumReactantsProductsNo Change51

52ExamplesWill the stress cause the equilibrium to shift towards reactants or products? 52H > 0Decrease temperatureReactantsH > 0Increase volume..Products

H < 0Increase pressure by adding helium.No change

53SummaryDynamic equilibrium is established when forward & reverse rates are equalKeq constructed from gas and solution-phase onlyQ constructed from non-equilib concentrationscompare with K to decide which way rxn proceedsEquilib calcs best handled with ICE tablere-learn quadratic equation!LeChateliers principle predicts shift in equilibriumP, V or T change53Suggested exercisesReview questions 2-20.

Problems by topic 21, 23, 25, 27, 29, 31, 33, 35, 39, 43, 45, 47, 51, 53, 57, 61, 63, 65, 67, 81, 87, 99, 101.

Note: answers to all odd-numbered problems are found in Appendix III.

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