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Chemical Equilibrium: Reactions, Equilibrium Constants, and Shifting Equilibria, Lecture notes of Chemistry

Hofstra University Chemistry

A comprehensive overview of chemical equilibrium, focusing on reversible reactions, reaction quotients, equilibrium constants, and shifting equilibria. It includes examples of gaseous reactions, changes in pressure and temperature, and calculations for various reactions. The document also covers the le châtelier's principle and its implications for chemical systems at equilibrium.

Typology: Lecture notes

2023/2024

Uploaded on 03/19/2024

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EQUILIBRIUM

➢Chemical Equilibrium: point where forward and reverse rates of reaction are

equal

➢Concentrations of reactants and products remain constant

➢Usually studied for reversible reactions

○Ex. Acetic acid + Water Acetate + Hydronium (weak acid)

⇌

○Ex. Ammonia + Water Ammonium + Hydroxide⇌

○Ex. Phase transitions

➢Reaction Quotients: amounts of products and reactants can be quantitatively

compared using the reaction quotient Qc

○General Form: For the reversible reaction mA + nB ⇌xC + yD

■𝑄𝑐=[𝐶]𝑥[𝐷]𝑦

[𝐴]𝑚[𝐵]𝑛

■Quantities in brackets are concentrations for everything but

solids and pure liquids

○At equilibrium, Qcis always the same, the value itself is the

equilibrium constant: Kc

○A large Kcimplied equilibrium occurs when most of the reactants

have been converted to products

○A small Kc(< 1) implies equilibrium occurs when only a small

amount of reactants have been converted to products (higher reactant)

○Kc> Qcshift towards products

○Kc< Qcshift towards reactants

➢Equilibrium Constants (Gaseous Reactions)

○If all reactants and products in a reaction are gases, partial pressures

can be used instead of concentrations in calculating reaction quotients

and equilibrium constants

○𝐾𝑝=𝑄𝑝=(𝑃𝐶)𝑥(𝑃𝐷)𝑦

(𝑃𝐴)𝑛(𝑃𝐵)𝑚

○, where = ∑coefficients products - ∑coefficients

𝐾𝑝=𝐾𝑐(𝑅𝑇)∆𝑛 ∆𝑛

reactants

➢Shifting Equilibria

○Le Châtelier’s Principle: When a chemical system at equilibrium is

disturbed, it returns to equilibrium by counteracting the disturbance

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EQUILIBRIUM

➢ Chemical Equilibrium: point where forward and reverse rates of reaction are equal ➢ Concentrations of reactants and products remain constant ➢ Usually studied for reversible reactions ○ Ex. Acetic acid + Water ⇌Acetate + Hydronium (weak acid) ○ Ex. Ammonia + Water ⇌Ammonium + Hydroxide ○ Ex. Phase transitions ➢ Reaction Quotients: amounts of products and reactants can be quantitatively compared using the reaction quotient Qc ○ General Form: For the reversible reaction mA + nB ⇌ xC + yD ■ 𝑄 𝑐

[𝐶] 𝑥 [𝐷] 𝑦 [𝐴] 𝑚 [𝐵] 𝑛 ■ Quantities in brackets are concentrations for everything but solids and pure liquids ○ At equilibrium, Qc is always the same, the value itself is the equilibrium constant: Kc ○ A large Kc implied equilibrium occurs when most of the reactants have been converted to products ○ A small Kc (< 1) implies equilibrium occurs when only a small amount of reactants have been converted to products (higher reactant) ○ Kc > Qc shift towards products ○ Kc < Qc shift towards reactants ➢ Equilibrium Constants (Gaseous Reactions) ○ If all reactants and products in a reaction are gases, partial pressures can be used instead of concentrations in calculating reaction quotients and equilibrium constants ○ 𝐾 𝑝

𝑝

(𝑃𝐶) 𝑥 (𝑃𝐷) 𝑦 (𝑃𝐴) 𝑛 (𝑃𝐵) 𝑚 ○ 𝐾 , where = ∑coefficients products - ∑coefficients 𝑝

𝑐

∆𝑛 ∆𝑛 reactants ➢ Shifting Equilibria ○ Le Châtelier’s Principle : When a chemical system at equilibrium is disturbed, it returns to equilibrium by counteracting the disturbance

■ Disturbance could be change in temperature, pressure, concentrations ■ Adding catalyst will not change equilibrium ○ If the concentration of one or more of the products or reactants in a reaction at equilibrium is changed, the concentrations of all other species will change to restore equilibrium ➢ Change in Pressure: ○ Changes in pressure will only affect reactions in which the total number of gas molecules changes ○ Increases in pressure will cause equilibrium to shift in the direction with less total gas molecules ➢ Change in Temperature: ○ Changing the temperature of a reaction does not just shift Q from K, it changes the value of K ■ Increase in temperature = increase in rate constant ■ 𝐾 = 𝑘𝑓𝑜𝑟𝑤𝑎𝑟𝑑 𝑘𝑟𝑒𝑣𝑒𝑟𝑠𝑒 ○ Determining the effect of a temperature change on a reaction’s equilibrium requires looking at the reaction’s ∆H ○ If a reaction is exothermic (-∆H), increasing the temperature will favor the reverse reaction ○ If a reaction is endothermic (+∆H), increasing the temperature will favor the forward reaction ➢ Equilibrium Calculations: ○ ICE = I nitial C hange E quilibrium A B C Initial Conc. (M) Change (M) Equilibrium Conc. (M)

➢ Determine the equilibrium constant for the reaction I 2 (aq) + I-(aq) ⇌ I 3 - (aq) given that a solution with initial concentrations [I 2 ] = [I

] = 1.000 x 10 - M has an [I 2 ] equilibrium concentration of 6.61 x 10-4^ M. I 2 I-^ I 3 - Initial Conc. (M) 1.000 x 10-3^ 1.000 x 10-3^0 Change (M) -X -X +X Equilibrium Conc. (M) 1.000 x 10-3^ - X 1.000 x 10-3^ - X X 𝐾 𝑐

[𝐼 3 − ] [𝐼 2 ][𝐼 − ]

𝑋 (1.000× − −𝑋) 2

000 × 10 − − 𝑋 = 6. 61 × 10 − 𝑋 = 3. 39 × 10 − 𝐾 𝑐

3.39× − (6.61× − )

2 = 7. 75 × 10

2 ➢ Determine the equilibrium concentrations for all species in the reaction PCl 5 (g) ⇌ PCl 3 (g) + Cl 2 (g) for a system with an initial PCl5 5 concentration of 1.00 M given that the equilibrium constant for the reaction is 0.0211. PCl 5 PCl 3 Cl 2 Initial Conc. (M) 1.00 0 0 Change (M) -X +X +X Equilibrium Conc. (M) 1 - X X X 𝐾 𝑐

[𝑃𝐶𝑙 3 ][𝐶𝑙 2 ] [𝑃𝐶𝑙 5 ]

0211 = 𝑋 2 1−𝑋

2 𝑋 2

1. 0211𝑋 − 0. 0211 = 0 𝑋 = −𝑏 ± 𝑏 2 −4𝑎𝑐 2𝑎

−0.0211 ± (0.0211) 2 −4(1)(−0.0211) 2(1) 𝑋 = 0. 135, -0. [PCl 5 ] = 0.865 M / [PCl 3 ] = 0.135 M / [Cl 2 ] = 0.135 M

Chemical Equilibrium: Reactions, Equilibrium Constants, and Shifting Equilibria, Lecture notes of Chemistry

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EQUILIBRIUM

2 = 7. 75 × 10