Name: ________________________ EQUILIBRIUM CONSTANT, Keq or K The Law of Chemical Equilibrium:

(Guldberg & Waage, 1864)

States that: At equilibrium, there is a constant ratio between the concentration of the products and the concentration of reactants in any change. Example:

Chemical System: N2O4(g)  2 NO2(g)

Table 1: Different initial and equilibrium concentration values for the above system

Initial [ N2O4(g) ] At 100oC 0.1000 0.0000 0.0500 0.0750

Initial [ NO2(g) ] At 100oC 0.0000 0.1000 0.0500 0.0250

Equil. [ N2O4(g) ] At 100oC 0.0491 0.0185 0.0332 0.0411

Equil. [ NO2(g) ] At 100oC 0.1018 0.0627 0.0837 0.0930

Ratio: [ NO2(g) ]2 [ N2O4(g) ]

The Equilibrium Constant: A mathematical expression that represents the ___________ of product concentration to reactant concentration, with each concentration raised to the power of the coefficient. For any reaction:

aA

+ bB



cC

The equilibrium constant expression:

+

dD

Keq or K = [ C ]c [ D ]d [ A ]a [ B ]b

where A, B, C and D represent chemical formulas and a, b, c and d, their respective coefficients. NOTE: 1.

Pure liquids and solids are ________ included in the expression (their concentrations are already constants), thus only gases and solutions are included.

2.

Each chemical system has it’s own K value.

3.

K values change with a change in temperature, therefore K values are usually specified at a particular temperature. (Since forward and reverse reactions have different activation energies).

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Name: ________________________ 4.

K value for a system will depend on how the equation for the reaction is written.

Eg:

N2O4(g)  2 NO2(g) K = [NO2(g) ]2 [ N2O4(g) ]

vs.

2 NO2(g)  N2O4(g) K=

[ N2O4(g) ] [NO2(g) ]2

5.

Only equilibrium __________________ are used to calculate K.

6.

Units are not usually associated with K values.

7.

The value of K can indicate the position of an equilibrium or whether the formation of reactants or products are favored (in higher concentration):  If K > 1, the products are favored  If K < 1, reactants are favored  If K = 1, neither reactants nor products are favored

8.

Percent reaction is also an indicator of whether reactants or products are favored:  < 50% , reactants are favored  > 50% , products are favored  = 50 % , neither reactants nor products are favored

Sample Exercises: 1.

Write the expression for the equilibrium constant for each of the following chemical systems: a) The decomposition of trinitrogen hexaoxide into nitrogen dioxide. Both compounds are in the gaseous phase.

b)

The single replacement reaction between zinc metal and hydrochloric acid.

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Name: ________________________ 2.

Calculate K for the decomposition of trinitrogen hexaoxide into nitrogen dioxide at 25oC if the equilibrium concentrations of trinitrogen hexaoxide and nitrogen dioxide are 0.0045 mol/L and 0.030 mol/L, respectively.

3.

PCl5(g) in the amount of 0.0722 mol and at 250C was added to a 2.00 L container. The temperature was raised to 2500C and the PCl5 decomposed into PCl3(g) and Cl2(g). At equilibrium, the concentration of Cl2 was 0.0257 mol/L. Find K for the system at 2500C.

4.

The initial concentrations of hydrogen gas and iodine gas in a 2.00 L reaction vessel at 2120C were 0.357 mol/L. The equilibrium concentration of the hydrogen iodide produced was measured to be 0.570 mol/L. Calculate the equilibrium concentrations of hydrogen gas and iodine gas and the Keq.

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Name: ________________________ 5.

A 0.167 mol sample of nitrogen monoxide was placed in a 1.00 L container and decomposed into its elements at 5000C. If the percent reaction was 35%, find the equilibrium constant K.

6.

A 0.575 mol sample of PCl5(g) is placed in a 1.00 L reaction vessel and equilibrium is established at 1560C. If the percent reaction is 25%, calculate the equilibrium concentration of all species in the vessel.

7.

Oxygen and sulfur dioxide combine at 250oC to produce sulfur trioxide. The K value for the equilibrium is 10.0. Find the equilibrium concentration of sulfur trioxide if the equilibrium concentrations of the reactants are both 0.250 mol/L.

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