Equilibrium Expressions mass-action expression

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When do you use this? You use Keq to find the concentration of a reversible reaction at equilibrium. You use Q to find the concentration of a reversible.

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Presentation transcript:

Equilibrium Expressions mass-action expression Chemists use an equilibrium expression to mathematically describe a system undergoing a chemical change.

the General Expression Start with a balanced equation 3 2 N2 + H2 NH3 Coefficients become powers (exponents) Then . . . . [products] [NH3]2 [N2] x [H2]3 = Keq [reactants]

Significance of Keq equilibrium constant tells you if you have more products OR more reactants at equilibrium “favors” this means that at Eq, you have a greater concentration of that (those) species. A small constant favors reactants > 1 < 1 A large constant favors products

Practice Problems PCl5(g) ↔ PCl3(g) + Cl2(g) The equilibrium constant for this system equals 35.0 at a certain temperature. If the concentrations of the PCl5 and the PCl3 are 0.015 mol/L and 0.78 mol/L respectively, what is the concentration of the Cl2? 0.67 mol/L Answer worked out

Practice Problems PCl5(g) ↔ PCl3(g) + Cl2(g) The equilibrium constant for this system equals 35.0 at a certain temperature. If the concentrations of the PCl5 and the PCl3 are 0.015 mol/L and 0.78 mol/L respectively, what is the concentration of the Cl2?

Use an ICE table and MOLES to help you figure it out! …………….. Now A total of 3.50mol of PCl5 is placed into a 0.500L container and heated to 250oC. At equilibrium, the container holds 0.270 mol of Cl2. Calculate the equilibrium constant. Use an ICE table and MOLES to help you figure it out!

This is what an ICE table looks like

Subtract because reactants are being used up. Add because products are being formed.

This Keq makes sense because it is < 1 and therefore favors the reactants at equilibrium. Go back and look at the last line of the ICE table. There is a greater concentration of reactants than products.

2SO2 (g) + O2 (g) ⇄ 2SO3 (g) Consider the above reaction. When 0.600 moles of SO2 and 0.600 moles of O2 are placed into a 1.00 liter container and allowed to reach equilibrium, the equilibrium [SO3] is to be 0.250M. Calculate the Keq value.

All these Ks which is which????????? Ksp = solubility product constant for “insoluble” substances Ka = relative strength of ionization of an acid HI = H+ + I- Ka= “very large”/strong H2S = 2H+ + S-2 Ka=9.5x10-8 /weak Keq = equilibrium constant; calculated from mass action expression Larger numbers favor products

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