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14 Equilibrium Essay

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Chapter 14. CHEMICAL EQUILIBRIUM
14.1 THE CONCEPT OF EQUILIBRIUM AND THE EQUILIBRIUM CONSTANT Many chemical reactions do not go to completion but instead attain a state of chemical equilibrium.
Chemical equilibrium: A state in which the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products remain constant. ⇒ Equilibrium is a dynamic process – the conversions of reactants to products and products to reactants are still going on, although there is no net change in the number of reactant and product molecules.

For the reaction: N2O4(g)

2NO2(g)

N2O4

Forward rate

concentration

Rate

NO2

Reverse rate
time

time

The Equilibrium Constant
For a reaction:

aA + bB

equilibrium constant:

cC + dD
Kc =

[C ]c [ D] d
[ A] a [ B]b

The equilibrium constant, Kc, is the ratio of the equilibrium concentrations of products over the equilibrium concentrations of reactants each raised to the power of their stoichiometric coefficients.

Example. Write the equilibrium constant, Kc, for N2O4(g)

2NO2(g)

Law of mass action - The value of the equilibrium constant expression, Kc, is constant for a given reaction at equilibrium and at a constant temperature. ⇒ The equilibrium concentrations of reactants and products may vary, but the value for Kc remains the same.

Other Characteristics of Kc
1) Equilibrium can be approached from either direction.
2) Kc does not depend on the initial concentrations of reactants and products. 3) Kc does depend on temperature.

Magnitude of Kc
⇒ If the Kc value is large (Kc >> 1), the equilibrium lies to the right and the reaction mixture contains mostly products.
⇒ If the Kc value is small (Kc
14.2 WRITING EQUILIBRIUM CONSTANT EXPRESSIONS

Calculating Equilibrium Constants, Kc
Kc values are listed without units ⇒ don't include units when calculating Kc. If equilibrium concentrations are known, simply substitute the concentrations into the equilibrium constant expression:

Example. For the reaction, CO + 3H2
CH4 + H2O, calculate Kc from the following equilibrium
concentrations: [CO] = 0.0613 M; [H2] = 0.1839...

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Keywords

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