Chemical Equilibria Notes

Chemical Equilibria

Chemical reactions can be reversible, meaning reactants form products and products revert to reactants. This creates a forward and reverse reaction. Dynamic equilibrium is reached when the rates of the forward and reverse reactions are equal, resulting in no apparent change in the system, despite ongoing chemical activity.

Stoichiometry vs. Equilibria

Stoichiometry is used to determine limiting reagents and predict product amounts in unidirectional reactions. Equilibria involve reversible reactions, making it more complex to calculate equilibrium concentrations.

ICE Box

To calculate concentrations at equilibrium, an ICE box is used. ICE stands for Initial, Change, and Equilibrium.

Chapter 1: ICE Box Example

Starting with 1 mole of PCl5, and at equilibrium, 0.135 moles of PCl3 are measured.

1. Initial (I): Initial amounts of reactants and products.

   • Reactant: 1 mole

   • Products: 0 moles each

2. Change (C): Change in amounts as the system reaches equilibrium, denoted as x.

   • Reactant: -x (depleted)

   • Products: +x (formed)

   • If stoichiometric coefficients are different, adjust the change accordingly (e.g., 2x or 3x).

3. Equilibrium (E): Sum of initial and change.

   • Reactant: 1 - x

   • Products: x

Given [PCl_3] = 0.135 at equilibrium, then x = 0.135. Substitute x to find other concentrations.

Equilibrium Constant (Kc)

Every equilibrium has an equilibrium constant, K_c, defined as:

K_c = \frac{{\text{[Products]}^{\text{stoichiometric coefficients}}}}{{\text{[Reactants]}^{\text{stoichiometric coefficients}}}}

This indicates whether products or reactants are favored:

• K_c >> 1: Products are favored (numerator is larger).

• K_c << 1: Reactants are favored (denominator is larger).

Only gases and aqueous species are included in the K_c expression. Solids and pure liquids are excluded.

Example: Carbon Equilibrium

For a reaction involving solid carbon, carbon is not included in the K_c expression because concentration doesn't apply to solids.

Reaction Quotient (Q)

The reaction quotient, Q, predicts the direction a mixture will shift to reach equilibrium. It is calculated by plugging non-equilibrium values into the K_c expression.

• K_c > Q: The reaction will proceed towards products to reach equilibrium.

• K_c < Q: The reaction will proceed towards reactants to reach equilibrium.

• K_c = Q: The system is at equilibrium.

Chapter 2: Calculating Equilibrium Concentrations

Calculating equilibrium concentrations in terms of molarity (moles per liter) using an ICE box. Remember that molarity is the number of moles of a substance per liter of solution.

Chapter 3: Setting up the ICE Box

Set up an ICE box with initial concentrations, changes based on stoichiometry, and equilibrium concentrations. For example, consider the reaction:

2A \rightleftharpoons B + C

• Initial: Calculate the initial concentration of reactant A (molarity).

• Change: For every two moles of A, one mole of B and one mole of C are produced. So, the change is -2x for A and +x for B and C.

• Equilibrium: Sum of initial and change.

Plug the equilibrium concentrations into the K_c expression and solve for x. If the equation simplifies to a perfect square, take the square root to solve for x. Otherwise, the quadratic equation may be needed.

Solve for x and use it to calculate all equilibrium concentrations.