1. Equilibrium Constants
201 Chemistry

2. Equilibrium: Crash Course Chemistry #28

3. Kc is a constant at a certain temperature
Equilibrium constant
Each equilibrium reaction has an equilibrium constant Kc.
aA + bB ↔ cC + dD
Kc is a constant at a certain temperature
Kc = products [C]c x [D]d
reactants [A]a x [B]b
[] = concentration
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4. Write the equilibrium expression from the following
a) 2NH3(g) ↔ N2(g) H2(g)
b) 2N2(g) H2(g) ↔ 4NH3(g)
c) CH3COOH(aq) + H2O(l) ↔ CH3COO (aq) + H3O +(aq)
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5. Equilibrium Constants
Indicates the extent to which the equilibrium favours products or reactants.
If Kc is very large (>>1 e.g. 1x 106) then at equilibrium there will be far more products than reactants.
H2(g) + Cl2(g) ↔ HCl(g) Kc = 4.0 x 1031 at 300 oC
This indicates that at equilibrium there is virtually no H2 and Cl2 present.
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6. Equilibrium Constants
 If Kc is very small (<<1 e.g. 1x 10-6) then there are far more reactants than products.
CH3COOH(aq) + H2O(l) ↔
CH3COO(aq) + H3O+(aq)
Kc = 1.8 x 10-5 at 25 oC
Very few of the ethanoic acid molecules split up into hydrogen and ethanoate ions.
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7. Calculations of Kc
A mixture of hydrogen and iodine was heated in a sealed flask at 491oC and, at equilibrium, the concentration of hydrogen was 2.5 x 10-2 mol L-1, iodine was 2.5 x 10-2 mol L-1, and hydrogen iodide was 1.71 x10-1 mol L-1. Calculate Kc. H2(g) + I2(g) ↔ 2HI(g)
Kc = [HI]2 = (1.71 x 10-1)2 = 46.8
[H2][I2] (2.5 x 10-2) x (2.5 x 10-2)