Equilibrium Constant and Reaction Quotient 6 of 16 Constants | Periodic Table At

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Equilibrium Constant and Reaction Quotient

6 of 16

Constants | Periodic Table

At equilibrium, the concentrations of reactants and products can be predicted using the equilibrium constant, Kc, which is a mathematical expression based on the chemical equation. For example, in the reaction

aA+bBcC+dD

where a, b, c, and d are the stoichiometric coefficients, the equilibrium constant is

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

where [A], [B], [C], and [D] are the equilibrium concentrations. If the reaction is not at equilibrium, the quantity can still be calculated, but it is called the reaction quotient, Qc, instead of the equilibrium constant, Kc.

Qc=[C]tc[D]td[A]ta[B]tb

where each concentration is measured at some arbitrary time t.

Part A

A mixture initially contains A, B, and C in the following concentrations: [A] = 0.700 M , [B] = 1.10 M , and [C] = 0.600 M . The following reaction occurs and equilibrium is established:

A+2BC

At equilibrium, [A] = 0.500 M and [C] = 0.800 M . Calculate the value of the equilibrium constant, Kc.

Express your answer numerically.

Explanation / Answer

Since the reaction equation is A + 2B ---------> C

We can write equilibrium constant Kc = [C]1 / [A]1 [B]2

First we have to find concentration of B ( [B] ),

A + 2B ----------------------> C

initial conc. 0.7 1.1 0.6

equilibrium con. 0.7 - x 1.1-y x+y

From above,

0.7 - x = 0.5 => x = 0.2

x + y = 0.8 => y = 0.6

hence equilibrium concentration of b is [B] = 1.1 - 0.6 = 0.5

Hence,

Kc = (0.8) / ((0..5) * (0.5)2 )

Kc = 6.4 <---------------Answer

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