Chemical Equilibrium Chapter 18

Chemical Equilibrium Happens to any reversible reaction in a closed system Happens to any reversible reaction in a closed system –Exists when the rate of the forward reaction and the rate of the reverse reaction are equal.  The rate of a reaction is the change in concentration per unit time (usually Molarity per sec, M/sec)

–Occurs because molecules NEVER stop colliding and reacting. As product molecules form in any chemical process, they still have the opportunity to reform reactant molecules if they collide with sufficient energy. –At Equilibrium the concentrations of the reactants and products are constant – NOT NECESSARILY EQUAL!!!! Reaction is now at equilibrium beyond this point. Concentrations are not changing!

Fundamental Characteristics of a system at equilibrium Macroscopic properties (such as color change) do not change. Macroscopic properties (such as color change) do not change. Microscopic processes (collisions between molecules) continue. Microscopic processes (collisions between molecules) continue. –The balance of an equilibrium system can be affected by changes in temperature, concentration and pressure.

–There is a special numerical relationship between the concentration of the REACTANTS and the concentration of the PRODUCTS at equilibrium. Equilibrium Constant = Keq = [Products] [Reactants] Keq = [Products] [Reactants]

For a balanced reaction at equilibrium: For a balanced reaction at equilibrium: aA + bB ↔ cC + dD aA + bB ↔ cC + dD Keq = [C] c [D] d [A] a [B] b [A] a [B] b Equilibrium constants include all reaction species EXCEPT pure solids and liquid water (since their concentrations are really unchangeable )

Example CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (g) Write the equilibrium expression Keq = [CO 2 ][H 2 O] 2 [CH 4 ][O 2 ] 2 [CH 4 ][O 2 ] 2 Exponents come from coefficients in balanced chemical equation!

Example 2 HCl(aq) + NaOH(aq) H 2 O(l) + NaCl(aq) Write the Keq expresssion Keq = [NaCl] [HCl][NaOH] [HCl][NaOH] *Note: Liquid water is not included in Keq expressions. The concentration of pure liquids do not change.

Example 3 CaCO 3 (s) + 2HCl(aq) CaCl 2 (aq) + CO 2 (g) + H 2 O(l) Write Keq Expression Keq = [CaCl 2 ][CO 2 ] [HCl] 2 [HCl] 2 Solid CaCO 3 and liquid water are left out of the expression!

Equilibrium constants do not carry any units with them. They are seen as simple ratios whose numerical value indicates whether the forward or reverse reaction is favored in an equilibrium system. Equilibrium constants do not carry any units with them. They are seen as simple ratios whose numerical value indicates whether the forward or reverse reaction is favored in an equilibrium system. If the products are favored in an equilibrium reaction, then the Keq > 1 If the products are favored in an equilibrium reaction, then the Keq > 1 –If the Keq is very large, there will be much more products than reactants in the equilibrium system If the reactants are favored in an equilibrium reaction, then the Keq < 1 If the reactants are favored in an equilibrium reaction, then the Keq < 1 –If the Keq is very small, there will much more reactants than products.

K eq Calculations If you know the equilibrium concentrations, then you plug into the K eq expression and solve. If you know the equilibrium concentrations, then you plug into the K eq expression and solve. You must remember to raise the concentration values to the correct power in the expression. You must remember to raise the concentration values to the correct power in the expression.

K eq Calculations Remember: [X] means the concentration value of reactant or product “X” in molarity (moles/liter) Remember: [X] means the concentration value of reactant or product “X” in molarity (moles/liter) You might have to calculate the molarities with given data before plugging into the K eq expression. You might have to calculate the molarities with given data before plugging into the K eq expression.