3.1/2 Intro to equilibrium, equilibrium law and reaction quotient

What is equilibrium?

Equilibrium is a physical property of reaction in which the rate of the forward reaction is equivalent to the rate of the reverse reaction

What conditions must be met for equilibrium to occur in a reaction

1. The system must be closed, i.e., no new material is being added, and no material is removed.

2. Temperature and pressure must remain constant within this closed system

What is the position of the equilibrium and how can it be determined

When a substance has reached equilibrium, the concentration of reactants and products remains constant, but this does not necessarily mean that the concentrations of reactants and products are equal.

• If there are more reactants than products, we say the equilibrium lies to the left

• if there are more products than reactants, we say the equilibrium lies to the right

If the forward reaction is endothermic, what must the reverse reaction be

Reversible reactions must be endothermic in one direction and exothermic in the other; therefore, in this scenario, the reverse reaction is exothermic.

What is equilibrium law

The equilibrium law is using a balanced chemical equation to create a mathematical expression that relates the concentrations together. For example, in this chemical equation, I will demonstrate how to find the equilibrium law.

• aA + bB ↔ cC + dD

Small letters are coefficients, while capital letters are molecules.

Equilibirum law for this reversible reaction:

{ [C]^c [D]^d} / { [A]^a [B]^b} = K

Concentration of molecules is represented by [capital letters]; these concentrations are then raised to the value of their exponents.

Products are in the numerator and reactants in the denominator.

What conditions must be met to create an equilibrium law expression for a reaction

1. The reaction must be balanced

2. solids and pure liquids are omitted

What does the k value tell us

• k > 1, there are more products than reactants

• k < 1, there are more reactants than products

We now know how to find the K value using the concentrations, how do we do the opposite?

To find the concentrations knowing the k value and key indicated information from the given question, you must compare the Initial, Change, and Equilibrium (ICE) values using an ICE block table.

See notes for more detail