chem 1

Equilibrium

Reversible reactions

When a reaction results in an almost complete conversion of reactants to products, chemists say that the reaction goes to equilibrium.

The reactions appear to stop because they are reversible: a reversible reaction is a chemical reaction that can occur in both the forward and the reverse directions.

N2(g) + 3H2(g) → 2NH3(g) delta gibbs free energy = -33.1 kJ

Forward reaction N2(g) +3H2(g) → 2NH3(g)

Reverse reaction 2NH3(g) → N2(g) + 3H2(g)    or    N2(g) + 3H2(g) ← 2NH3(g)

The decrease in the concentration of the reactants causes the rate of the forward reaction to slow. As soon as NH3 is present, the reverse reaction can occur, slowly at first, but at an increasing rate as the concentration of ammonia increases.

As the reaction proceeds, the rate of the forward reaction continues to decrease and the rate of the reverse reaction continues to increase until the two rates are equal.

The system has reached a state of balance or equilibrium. The word equilibrium means that opposing processes are in balance. At that point, ammonia produced at the same rate it is decomposed, so the concentrations of N2, H2, and NH3 remain constant.

Chemical equilibrium is a state in which the forward and reverse reactions balance each other because they take place at equal rates.

Rate(forward react.) = Rate(reverse react.)

At equilibrium, the concentrations of reactants and products are constant. The amounts of concentrations of reactsnts and products are seldom equal.

Law of chemical equilibrium states that at a given temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value.

The general equation for a reaction at equilibrium: aA + bB ←→ cC + dD

If the law is applied to this reaction: $Keq=\frac{\left\lbrack C\right\rbrack^{c}\cdot\left\lbrack D\right\rbrack^{d}}{\left\lbrack A\right\rbrack^{a}\cdot\left\lbrack B\right\rbrack^{b}}$ , where Keq (constant only at a specific temperature) is an equilibrium constant (numerical value of the ratio of product concentration to reactant concentration, with each concentration raised to the power equal ro its coefficient in the balanced equation.

A numerically large Keq means that the equilibrium mixture contains more reactants than products.

Keq > 1: products are favoured at equilibrium. Forward reaction is favoured.

Keq < 1: reactants are favoured at equilibrium. Reverse reacton is favoured.

Keq (also Kc) does not depend on the initial concentration.

Homogeneous equilibria.

Gaseous hydrogen iodine is produced by the equilibrium reaction of H2 with I2.

H2(g) + I2(g) ←→ 2HI(g)

$Keq=\frac{\left\lbrack HI\right\rbrack^2}{\left\lbrack H2\right\rbrack\cdot\left\lbrack I2\right\rbrack}$ —> doesn’t include liquids or solids

This reaction is a homogeneous equilibrium, which means that all the reactants and products are in the same physical state. All participants are gases.

Heterogeneous equilibria

When the reactants and products are present in more than one physical state, the equilibrium is called a heterogeneous equilibrium. Liquid ethanol is a pure substance. No matter the amount, the concentration remains constant.

Equilibrium constants.

For a given reaction of a given temperature, Keq will always be the same regardless of the initial concentrations of reactants and products.