November 22nd 2012
Abid Khan
Chemical Equilibrium
Reversible Reaction:
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Many chemical reactions are reversible.
A reversible reaction is able to proceed in both forward and reverse directions.
i) Denoted using a double headed arrow.
N2O4(g) ↔ 2NO2 (g)
The forward and reverse reactions are constantly going but there is a point when there is no net
charge in the amounts of reactions and products.
When a chemical system reaches a point where there is no net observable change in the
reactants and products, the system is said to be at EQUILIBIRUM.
The forward and reverse reactions do not stop, but the rate of the forward reaction equals the
rate of the reverse reaction.
Since the 2 processes are still occurring chemical equilibrium is considered DYNAMIC.
At equilibrium, the rates of the 2 processes are the same but amount of reactants and products
are not necessarily equal.
In order for equilibrium to be established, the system must be closed (energy can go in and out
but matter cannot go in and out).
At a given temperature, the point at which a system reaches equilibrium is constant.
Equilibrium is reached in both chemical and physical changes.
Ex.1: Chemical System
2NH3(g) ↔ 3H2(g) + N2(g)
Ex.2: Phase Equilibirum
Br2(l) ↔Br2(g)
Ex.3: Solubility Equilibrium
𝑀𝑔𝐶𝑙2 (g) ↔ 𝑀𝑔2+ (aq) + 2𝐶𝑙 −
Representing Equilibirum Graphically
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You can graph a concentration vs time graph for a reversible function to determine the point at
which equilibrium is reached.
Since there is no net charge in the concentrations of reactants and products at equilibrium, the
slopes of all lines on the graph will be zero.
Qualitative Look at equilibrium:
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For a given temperature, the amounts of reactants and products present at equilibrium are
represented by the equilibrium constant (𝑲𝒆𝒒 ).
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𝐾𝑒𝑞 is determined by comparing the concentrations of the products to the concentrations of the
reactants at the equilibrium in the equilibrium expression.
The equilibrium expression is written as the concentration of the products raised to their molar
coefficients divided by the concentration of reactants raised to their molar coefficients.
Solids and liquids are considered 1 in the equilibrium expression, because they do not have a
concentration value.
Gas and aqueous are only ones that have a value in the equilibrium expression.
The lower case letters are the molar coefficients and upper case are the formulas:
Equilibrium Expression:
aA(g) + bB(i) + cC(g) ↔ dD(aq) + eE(s) + fF(g)
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𝐾𝑒𝑞=
𝑃𝑟𝑜𝑑𝑢𝑐𝑡
𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡
𝐾𝑒𝑞=
[𝐷]𝑑 [𝐹] 𝑓
[𝐴𝑎 ][𝐶 𝑐 ]
Examples:
1.
2𝐻2 𝑆(𝑔) + 𝐶𝐻4 (𝑔) ↔
𝐾𝑒𝑞=
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4𝐻2 (𝑔) + 𝐶𝑆2 (𝑔)
[𝐻2 ]4 [𝐶𝑆2 ]1
[𝐶𝐻4 ]1 [𝐻2 𝑆]2
𝑆8 (𝑔) + 8𝑂2 (𝑠) ↔ 8𝑂2
[𝑂2 ]8
𝐾𝑒𝑞=
[𝑆8 ]1
Homework:
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Equilibrium worksheet Questions #1-3
Remainder:
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Daiper lab due THIS MONDAY 26th NOVEMBER