Finding Equilibrium Constant (Keq)

A reaction takes place in a 3.0 L container in which nitrogen gas is added to hydrogen gas to produce ammonia vapor. At equilibrium there are 0.60 mole of nitrogen, 0.30 mole of hydrogen, and 0.80 mole of ammonia. Calculate the equilibrium constant for this reaction.

Find Concentration at Equilibrium

A reaction occurs where sulfur dioxide gas reacts with oxygen gas to form gaseous sulfur trioxide. At a particular temperature the equilibrium constant (Keq) for this reaction is 0.457. Find the equilibrium concentration of oxygen gas knowing that sulfur dioxide and sulfur trioxide have equilibrium concentrations of 1.4M and 0.95 M respectively.


Finding Initial Direction of Equilibrium Reaction

To determine the initial direction of an equilibrium reaction you must know the starting (initial) concentrations of each of the reactants and products. This will allow you to determine if the reaction proceeds to the products or reactants.

This is done by comparing the equilibrium constant (K) to the reaction quotient (Q). The reaction quotient is determined just the way the equilibrium constant is determined except using initial concentrations (pressures) instead of those at equilibrium. Then compare the value of the equilibrium constant to that of the reaction quotient.

Remember that the value of the equilibrium constant show the relative ratio of products to reactants at equilibrium. The reaction quotient also shows the ratio of products to reactants but to start with not at equilibrium.

So for the reaction 2 HI(g) à I2(g) + H2(g) if K = 0.500 and Q = 0.100

Then the ratio of products to reactants is greater at equilibrium than to start with, so it is necessary to produce more products by the time equilibrium is reached. As a result, the reaction will shift to the right towards I2(g) and H2(g). So initially the concentration of reactants will decrease and the concentration of products will increase.

A reaction occurs in which hydrogen gas reacts with iodine gas to form hydrogen iodide gas. At a temperature of 60°C the Keq is 0.80. Initially, the chemicals have the following molarities:

[H2] i = 1.0M [I2] i = 0.80M [HI]i = 0.60M.

Which way will the reaction proceed based upon these initial concentrations?


Finding Equilibrium Concentrations with known Initial Concentrations

·  Need to be able to determine the relative concentration change from start to equilibrium

·  This is done by setting up an I.C.E. chart.

This is a table that lists the starting concentrations, relative change in concentrations and equilibrium concentrations, such as: A(aq) + 2B(g) à 3C(aq)

Chemicals / Reactant A / Reactant B / Product C
Initial concentration / 3.0 / 3.0 / 0
Change in conc. / -x / -2x / +3x
Equilibrium conc. / 3.0 - x / 3.0 – 2x / 0 + 3x

Once the table is set up then you can write the equilibrium expression for the reaction and then solve for x and put this back into equilibrium conc. equation and find the equilibrium concentrations.

Example:

N204(g) 2NO2(g) Kc=0.25

Starting with [N204] = 0.40M and [NO2] = 0.60M what are their equilibrium concentrations?


Gaseous carbon monoxide reacts with water vapor to form carbon dioxide gas and hydrogen gas. To start with carbon monoxide and water have a respective molarity of 1.0M and 2.0M.

a)  At equilibrium what are the concentrations of water, carbon dioxide, and hydrogen if carbon monoxide equilibrium concentration is 0.50M?

b)  What is the equilibrium constant?


A reaction occurs by combining water vapor with dichlorine monoxide to form hypochlorous acid. The hypochlorous acid is formed when 1.0g of water is mixed 2.0g of Cl2O to make a solution volume of 1.0mL. This reaction has an equilibrium constant of 0.090. What are the equilibrium concentrations of the reactants and the products?