Chapter 18

Chemical Equilibrium

Example 1

Solid + Energy ---> Liquid

Example 2

Example 3

Fe3O4(s) + H2(g) ---> Fe(s) + H2O(g)


Ratefwd = Raterev

Mass-Action

mA + nB ---> pC + qD
Where m, n, p, and q represent the coefficients from the balanced equation

Mass-Action Expression for the equation is:
[C]p x [D]q
[A]m x [B]n

(the brackets represent the concentration of the substance in moles per cubic decimeter - mol/dm3 - raised to the power of the coefficient from the balanced equation)

Mass-Action Expression = [Products]
[Reactants]



Equilibrium Constant (Keq)



The Effects of Stresses on Systems at Equilibrium
Le Chatelier's Principle


A + B ---> C + D
<---

3 basic types of stresses that can disturb equilibrium 1. Effect of a change in concentration
A + B ---> C + D
<---

Predict the shift if:
A + 2B ---> 3C + D
<---
2. Changing Temperature, Pressure, and adding a Catalyst
C + CO2 + heat ---> 2 CO2
        <---
(endothermic reaction) shifts to the right, increasing the concentration of the reactant heat
  • when you remove heat from the system, you have a decrease in temperature, which favors the exothermic reaction because that is the reaction that releases the heat
H2 + Cl2 ---> 2 HCl + heat
(exothermic) shifts to the left
B. Pressure
General rules:

N2(g) + 3 H2(g) ---> 2 NH3(g)


N2(g) + 3 H2(g) ---> 2 NH3(g)





Ammonia--4th most important chemical produced commercially in the United States
3 H2(g) + N2(g) ---> 2 NH3(g) + 22.08 kcal