Chemical equilibria

What is chemical equilibrium?

Chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products.

Is chemical equilibrium static?

No, chemical equilibrium is dynamic. Reactions still occur, but the concentrations of reactants and products do not change.

How can equilibrium be illustrated using cities and cars?

If city A has 1,000 cars and city B has 2,000 cars, and 10 cars travel each way per hour, the total number of cars in each city stays constant, showing equilibrium.

What does a concentration vs. time graph show for a reversible reaction?

The concentration of reactants decreases while the concentration of products increases until both become constant, indicating dynamic equilibrium.

What does it mean when the concentration curve becomes horizontal?

It means the concentrations are no longer changing, and the system has reached dynamic equilibrium.

How does the rate of the forward reaction change over time?

It decreases as the concentration of the reactant decreases until it becomes constant at equilibrium.

How does the rate of the reverse reaction change over time?

It increases as the concentration of the product increases until it equals the forward reaction rate at equilibrium.

What is the formula for the equilibrium constant in terms of rate constants?

k = k_forward / k_reverse = [products] / [reactants]

What are the two types of equilibrium constants?

Kc (concentration) and Kp (partial pressure).

How do you write an equilibrium expression using the law of mass action?

Kc = [C]^c [D]^d / [A]^a [B]^b, where exponents are the stoichiometric coefficients.

How do you write Kp for a gas reaction?

Kp = (P_products)^coefficients / (P_reactants)^coefficients

How do you calculate Kc for N2 + 3Cl2 ⇌ 2NCl3 if [N2]=0.15, [Cl2]=0.25, [NCl3]=0.50 M?

Kc = [NCl3]^2 / ([N2] [Cl2]^3) = (0.5^2) / (0.15 * 0.25^3) ≈ 106.6

How do you calculate Kp for SO2 + 1/2 O2 ⇌ SO3 with partial pressures 0.10, 0.30, 0.45 atm?

Kp = P_SO3^2 / (P_SO2^2 P_O2) = (0.45^2) / (0.10^2 0.30) ≈ 67.5

How do you convert between Kc and Kp?

Kp = Kc (R T)^Δn, where Δn = sum of coefficients of products − sum of coefficients of reactants.

How does doubling the coefficients of a reaction affect K?

The new equilibrium constant K' = K^2

How does halving the coefficients affect K?

The new K' = K^(1/2)

How does reversing a reaction affect K?

The new K' = 1 / K

How do you calculate equilibrium concentrations using an ICE table?

Subtract or add changes based on stoichiometry from initial concentrations, then solve for unknowns using given equilibrium values.

Example: If 20 moles of NOCl are in 4 L, and [Cl2] at equilibrium is 2 M, what is [NO] at equilibrium?

Use ICE table: [NO] = 2 * x = 4 M (x = 2), [NOCl] = 5 − 2x = 1 M

Example: For CO + 1/2 O2 ⇌ CO2 with [O2]=0.10, [CO2]=0.75, Kc=1000, find [CO]?

Kc = [CO2]^2 / ([CO]^2 [O2]); solve for [CO] → x ≈ 0.0375 M

Example: NH3 ⇌ N2 + 3H2, with P_NH3=0.85 atm initially, P_H2=0.45 atm at equilibrium. Find P_NH3 at equilibrium.

Use ICE table: x = 0.15, P_NH3 = 0.85 − 2x = 0.55 atm