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Aa + BB: Chapter 13 - Chemical Equilibrium

1. Chemical equilibrium is the state where the concentrations of reactants and products remain constant because the forward and reverse reaction rates are equal. The equilibrium constant K relates the concentrations of products and reactants. 2. Equilibrium involving gases uses the equilibrium constant Kp, which relates partial pressures. Kp and Kc are directly related by temperature and moles of gas. 3. Le Chatelier's principle states that if a stress is imposed on a system at equilibrium, it will shift to reduce the effect of that stress, such as changes in concentration, pressure, temperature, or addition of other substances.

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57 views3 pages

Aa + BB: Chapter 13 - Chemical Equilibrium

1. Chemical equilibrium is the state where the concentrations of reactants and products remain constant because the forward and reverse reaction rates are equal. The equilibrium constant K relates the concentrations of products and reactants. 2. Equilibrium involving gases uses the equilibrium constant Kp, which relates partial pressures. Kp and Kc are directly related by temperature and moles of gas. 3. Le Chatelier's principle states that if a stress is imposed on a system at equilibrium, it will shift to reduce the effect of that stress, such as changes in concentration, pressure, temperature, or addition of other substances.

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Alex Ting
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© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Oleg Andreyev, Alex Ting

Chapter 13 – Chemical Equilibrium

 Definition: The state where the concentrations of all reactants and products remain constant
because the forward and reverse reaction rates are EQUAL.

aA + bB   pP + nN Kc = [P]p[N]n / [A]a[B]b

Solids and Liquids are excluded in formula

If using the reverse reaction of original:


Kc = 1 / K’C

o Small equilibrium constants mean that the reaction favors the reactions. While large equilibrium
constants mean the products are favored

 Equilibrium involving Gas – Kp

Kc = Kp[RT]- Δn Δn = (Sum of Coefficients of Products) – (Sum of Coefficients of Reactants)

Kp = Kc[RT] Δn

 CO2(g) + H2(g) ⇄ CO(g) + H20 (g) Kp = PCOPH20 / PCO2PH2

Ex. CaCO3(s) ⇄ CaO(s) + CO2(g)

Kc = [CO2] Kp = PCO2

 Dropping of X
o If K < 0.01, then must calculate % Error. E% = Part/Total x 100%
o If % Error is less than 5%. Then OK to Drop X

 Reaction Quotient
o Q: Calculated the same as K but [Initial] are used instead of [Final]
 Q = K : Reaction is at EQUILIBRIUM
 Q > K : Too much product; Reaction shifts LEFT.
 Q < K : Too much reactant; Reaction shifts RIGHT
Oleg Andreyev, Alex Ting

Chapter 13

 LeChatelier’s Principle

If a stress is imposed on a system at EQUILIBRIUM, then it will shift to REDUCE the effect.

Possible stresses: 1. Addition/Subraction of reactant or product


2. Change in Pressure
Inc: Shift to less #mol of gas /// Dec: Shift to more #mol
3. Change in Volume
Inc: Shift to more mol /// Dec: Shift to less mol
4. Change in Temperature
Inc: Shift to Endothermic /// Dec: Shift to Exothermic
5. Addition of Common Ion
6. Addition of Inert Gas – NO CHANGE
7. Secondary Reaction
8. Catalyst addition

Examples:
1. In which of the following systems would the number of moles of the substances present at equilibrium NOT be
shifted by a change in the volume of the system at constant temperature?

(A) CO(g) + NO(g) <===> CO2(g) + 1/2 N2(g)


(B) N2(g) + 3 H2(g) <===> 2 NH3(g)
(C) N2(g) + 2 O2(g) <===> 2 NO2(g)
(D) N2O4(g) <===> 2 NO2(g)
(E) NO(g) + O3(g) <===> NO2(g) + O2(g)
Ans: 1. E
2. CaCO 3(s) ⇄ Ca2+(aq)+ CO32-(aq) Ksp = 2.8 x 10-9
CaSO4(s) ⇄ Ca2+(aq)+ SO42-(aq) Ksp = 9.1 x 10-6

The values for the solubility products for the two reactions above are given at 25 oC.

a. What is the concentration of CO32- ions in a saturated 1.00L solution of CaCO 3.

b. Excess CaSO4(s) is placed in the solution in (a). Assume that the volume of the solution does not change.
I. What is the concentration of the SO42- ion?

II. What is the concentration of the CO32- ion?


Oleg Andreyev, Alex Ting

3. N2(g) + 3H2(g) = 2NH3(g) ∆H = -92.4kj

When the reaction above took place at a temperature of 570K, the following equilibrium concentrations were
measured:

[NH3] = 0.20 mol/L [N2] = 0.50 mol/L [H2] = 0.20 mol/L

a. Write the expression for Kc and calculate its value.

b. What is the value of Kp for the reaction?

c. Describe how the concentration of H 2 will be affected by each of the following changes to the system at
equilibrium:
I. The temperature is increased.

II. The volume of the reaction chamber is increased.

III. N2 gas is added to the reaction chamber.

IV. Helium gas is added to the reaction chamber.

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