Chemical Equilibrium: Chemical Reactions

Topics: Chemical equilibrium, Chemical reaction, Equilibrium constant Pages: 3 (671 words) Published: October 4, 2013
Chapter 13 Chemical Equilibrium page 1

How far does a chemical reaction proceed?
Many reactions, however, do not go to completion. For example decomposition of N2O4(colorless) to NO2(dark brown):

N2O4(g) 2NO2(g)

As time passes the concentration of N2O4 decreases and concentration of NO2 increases until both concentration level off at constant amount. Why do the reaction appear to stop after concentrations reach their constant amount? The results show the concentrations reach constant amount, not because the reactions stop, but because the rate of the forward reaction is equal the rate of reverse reaction. We have Chemical Equilibrium in this situation. Chemical equilibrium is a dynamic process. We use two arrow to show chemical equilibrium:

N2O4 2 NO2

Equilibrium expression and eqilibrium Constant Kc: aA + bB cC + dD Based on Law of Mass Action, equilibrium expression is:

[C]c [D]d
Kc = [ ] = concentration of chemical species at equilibrium (molarity) [A]a[B]b
The equilibrium constant (Kc) for a reaction at a particular temperature always has a same value. It is a number that tells us the ratio of concentration of the products to reactants at equilibrium at a particular temperature. Example: [NH3]2 1. N2(g) + 3H2(g) 2NH3 K1 = [N2][H2]3 2.When we reverse the reaction : [N2][H2]3 2NH3 N2 + 3H2 K2 =1 / K1 K2= [NH3]2 3. When the balanced equation for a...
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