Chemical Equilibrium and the Equilibrium Constant�

Marc Chagall

Aleko and Zemphira by Moonlight

1942

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The Concept of Equilibrium

Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.

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At equilibrium:

rate_forward = rate_reverse

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This is called a dynamic equilibrium because the reaction is still occurring even though the overall concentrations of reactants and products are not changing.

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The Concept of Equilibrium

Example:

As colorless N₂O₄(g) warms it begins to decompose to brown NO₂(g):

N₂O₄(g) → 2NO₂(g)

• A mixture of N₂O₄ (initially present) and NO₂ (initially formed) appears light brown.

→

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The Concept of Equilibrium

Example:

• When enough NO₂ is formed, it can react to form N₂O₄:

2NO₂(g) → N₂O₄(g).

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←

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The Concept of Equilibrium

• At equilibrium, the rate that N₂O₄ reacts to form NO₂ is the same as the rate that NO₂ reacts to re-form N₂O₄:

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N₂O₄(g) ⇌ 2 NO₂(g)

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• The double arrow implies the process is dynamic (continues to react).

⇌

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Example of Equilibrium

As a system proceeds, both the forward and reverse reactions are occurring.

At equilibrium, the forward and reverse reactions are proceeding at the same rate (it’s dynamic).

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Example of Equilibrium

Once equilibrium is achieved, the amount of each reactant and product remains constant.

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Example of Equilibrium

For a system at equilibrium, both the forward and reverse reactions are occurring; as a result, we write its equation with a double arrow.

N₂O₄ (g) ⇋ NO₂ (g)

Forward reaction:

N₂O_{4 (g)} → 2 NO_{2 (g)}

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Rate law:

Rate_f = k_f [N₂O₄]

Reverse reaction:

2 NO_{2 (g)} → N₂O_{4 (g)}

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Rate law:

Rate_r = k_r [NO₂]²

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Example of Equilibrium

Remember, at equilibrium

Rate_f = Rate_r

k_f [N₂O₄] = k_r [NO₂]²

We can rearrange this as:

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K_eq is known as the equilibrium

constant

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The Equilibrium Constant

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The Equilibrium Constant

Important points about K_eq:

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• K_eq is temperature dependent. (Temp matters!)
• K_eq for concentrations in molarity is written as K_c (K_c for molarity)
• Only (aq) and (g) substances appear in the K_c equilibrium expression. Solids or liquids are not included since their concentrations are constant! (Only include (aq) and (g) substances!)
• K_eq for pressures is written as K_P (K_P for pressures)
• Only gases appear in the K_P equilibrium expression!
• Units are NOT included in values of K_eq (No unit for K!)

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What Does the Value of K_eq Mean?

If K_eq >> 1, the reaction is product-favored; product predominates at equilibrium.

Products

Reactants

K_eq >> 1

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What Does the Value of K_eq Mean?

If K_eq << 1, the reaction is reactant-favored; reactant predominates at equilibrium.

Products

Reactants

K_eq << 1

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What Does the Value of K_eq Mean?

For each of the following equilibrium constants, predict if the reaction is reactant-favored or product-favored.

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1. K = 3.5x10^-6
2. K = 1900
3. K = 2x10⁴
4. K = 0.00045

reactant favored

product favored

product favored

reactant favored

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K_eq Variations

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K_eq Variations

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K_eq Variations

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K_eq Variations

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K_P

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Try this

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Writing Equilibrium Constant Expressions

• The concentrations of the reacting species in the aqueous phase are expressed in M. In the gaseous phase, the concentrations can be expressed in M for K_C or in atm for K_P.
• The concentrations of pure solids and liquids do not appear in the equilibrium constant expressions.
• The equilibrium constant is a dimensionless quantity (no units).
• In quoting a value for the equilibrium constant, you must specify the balanced equation and the temperature.
• If a reaction can be expressed as a sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions.

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