Lecture 14: Equilibrium and Kc Concepts

Reversible Reactions

  • Definition: Reversible reactions can proceed in both forward and backward directions.

  • Example:

    • Hemoglobin (Hb) binds to oxygen (O2) reversibly, forming Hb.(O2)4.

    • Reaction:

Chemical Equilibrium

  • Definition: Chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction.

  • You are getting to the point where the rate of reactions are the same , not the concentrations.

  • The concentrations of reactants and products do not need to be equal.

Equilibrium Constants (Kc)

  • General Formula:

    • The lowercase letters is showing the number of moles: this becomes a power in the expression.

    • The uppercase letters represents are concentrations.

    • Products over reactants.

    • Always double check the equation is balanced before forming your expression.

  • Example

  • Key Characteristics:

    • Expresses the relationship between the amounts of products and reactants (Ratio of how many products versus how many reactants)

    • Kc has no units

    • Kc is constant at a given temperature

    • Pure solids and liquids (only don’t include aqueous solutions where it is not the bulk solvent) are not included in the Kc expression.

Interpretation of Kc Values

  • Large Kc (Kc > 100): Indicates that products are favored at equilibrium; the reaction proceeds towards completion.

  • Small Kc (Kc < 0.01): Indicates that reactants are favored; little to no products are formed.

  • Kc between 0.01 and 100: Significant amounts of both products and reactants, a mixture.

  • (If Kc =1 then products=reactants).

Factors Affecting Kc

  • Kc is influenced by temperature changes; however, altering concentrations or pressure in a dynamic equilibrium does not change Kc.

Calculating Kc from Concentrations

  1. Balance the equation

  2. Form Kc expression

  3. Calculate Kc by plugging in the concentrations numbers

  • Example Calculations: