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Equilibrium Calculations

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How can we describe an equilibrium system mathematically? reactants products ⇌ reactants The Keq is the equilibrium constant- a number that does not change. Providing the temperature is kept constant. products =3.0 Keq =

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Equilibrium Calculations An equilibrium system, at any given temperature, can be described by an equilibrium expression and equilibrium constant. aA+bB ⇌ cC+dD (aq) and (g) are included in the expression! (l) (pure liquids, meaning it is the only one in the equation) and (s) are not because they have constant concentration! [A] a [B] b [C] c [D] d Keq= Equilibrium Constant- a numberExpression- mathematical equation Keq= Products Reactants

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Only changes to (aq) and (g) reactants or products cause the equilibrium to shift. Solids (s) and pure liquids (l) have constant concentrations. Solids & Liquids have fixed densities, cannot be compressed, so their molar concentrations are constant (s) And (l) concentrations are already included in Keq value so we don’t include them in the equation. CaCO 3(s) + 2H + (aq) + 2Cl - (aq) ⇌ Ca 2+ (aq) + Cl 2 (g) + CO 2(g) + H 2 O (l) (s) and (l) do not! no shift rightleft rightno shift

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SO 3(g) + H 2 O (g) ⇌ H 2 SO 4(l) 1.at 25 o C, [SO 3 ] = M. [H 2 O] = M, and [H 2 SO 4 ] = 24 M. Calculate the Keq. The Keq has no units but concentration units that go in the expression must be M! =10.4 (0.200)(0.480) 1 [SO 3 ] [H 2 O] 1 don’t count (l)! Use 1Keq= = At equilibrium No ICE

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mole PCl 5, 0.40 mole H 2 O, mole HCl, and mole POCl 3 are found in a 2.0 L container at 125 o C. Calculate the Keq. PCl 5(s) + H 2 O (g) ⇌ 2HCl (g) + POCl 3(g) [HCl] = moles = 0.10 M 2.0 L [POCl 3 ] = moles = 0.20 M 2.0 L = 0.20 M 0.40 moles 2.0 L [H 2 O] = Keq = [HCl] 2 [POCl 3 ] [H 2 O] Keq = [0.10] 2 [0.20] [0.20] Keq = No ICE at equilibrium

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3. If mole of SO 3 and mole of SO 2 are found in a 2.00 L container at equilibrium at 25 o C. Calculate the [O 2 ]. [SO 3 ] = mole/2.00 L = M [SO 2 ] = mole/2.00 L = M Keq = [SO 3 ] 2 [SO 2 ] 2 [O 2 ] 798=(0.300) 2 (0.0100) 2 [O 2 ] (0.3) 2 = 798(0.01) 2 [O 2 ] [O 2 ] = (0.3) 2 798(0.01) 2 =1.14 M 2SO 2(g) + O 2(g) ⇌ 2SO 3(g) Keq = 798 1

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Size of Keq & Effect of Temperature on Keq

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products reactants Keq= Big Keq Keq= 10

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reactants products Keq= Little Keq Keq=0.1 Note that the keq cannot be a negative number!

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Keq=1 Keq= products reactants

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Effect of Temp on Keq

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Keq Only a temperature change can affect the value of Keq. Changes in concentrations, pressure or surface area have NO effect on Keq. –These changes correspond to increase in number of reacting molecules per liter. –Increased once and then equilibrium is re-established. –So ratios of products to reactants do not change.

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PBr 3(g) + Br 2(g) ⇋ PBr 5(g) o C Keq = 200 o C Keq = How can you tell if Keq gets bigger or smaller? Temperature increased Shifted left, Keq decreased Keq= [products] [reactants]

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More questions… If the value of K eq increases when the temperature decreases, is the reaction exothermic or endothermic?

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More questions… What will happen to the value of Keq in the following reaction if we added more [B]? A + B ⇌ C KJ.

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Hebden Practice Page 60: Exercises Page 62: Exercises 36-45

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