chem 2

All chemical reactions that reach equilibrium present certain characteristics:

  1. The reaction must take place in a closed system (no reactant or product can enter or leave the system.

  2. The temperature must be constant.

  3. All reactants and all products are present, and they are in constant dynamic motion. This means that equilibrium is dynamic, not static.


ICE (initial change equilibrium) Table

initial concentration H2 = o.5 M

initial concentration I2 = o.5 M

T = 430oC

Keq = 54.3

H2 + I2 ←→ 2HI

[HI]2eq[H2]eq⋅[I2]eq=54.3\frac{\left\lbrack HI\right\rbrack^2eq}{\left\lbrack H2\right\rbrack eq\cdot\left\lbrack I2\right\rbrack eq}=54.3


            H2             I2            2HI

I        0.5              0.5             0

Δ\Delta     -x                -x             +2x

E     (0.5-x)        (0.5-x)          2x


Keq = (2x)2(0.5x−x)(0.5−x)=54.3\frac{\left(2x\right)^2}{\left(0.5x-x\right)\left(0.5-x\right)}=54.3

x1 = 0.39

x2 = 0.69 → reject because it is greater than the original concentration

→ H2eq = I2 = 0.5 - 0.393 = 0.107M

HIeq = 2(0.393) = 0.786M


To help decide whether or not the approximation is justified, divide the initial concentration by the value of Keq

CiKeq\frac{Ci}{Keq}

>500, the approximation is justified

100-500, it may be justified

<100, it is not justified, the equilibrium expression must be solved


Reaction quotient

To predict the direction in which the reaction must proceed to reach equilibrium, you substitute the concentrations of the reactants and products into an expression that is identical to the equilibrium expression. Because these concentrations may not be the concentrations that the equilibrium system would have, the expression is given a different name: the reaction quotient (Q)

Q=[HI]2[H2][I2]=80Q=\frac{\left\lbrack HI\right\rbrack^2}{\left\lbrack H2\right\rbrack\left\lbrack I2\right\rbrack}=80

Q = 25

If Q is greater than Keq, the numerator must be very large. The concentrations of the chemicals on the right side of the equation must be greater than their concentrations at equilibrium. In this situation, the system attains equilibrium by moving to the left.

If Q is less than Keq, the system attains equilibrium by moving to the right.


Le Chatelier Principle - the law of mobile equilibrium

It states that if a stress is placed on a reversible reaction at chemical equilibrium, the equilibrium will shift to relieve the stress, thereby restoring equilibrium.

It describes how a chemical equilibrium shifts in response to a stress or disturbance within an enclosed system.