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Equilibrium

Equilibrium. Chapter 16. Reversible Reactions – A chemical reaction in which the products can regenerate the original reactants. Forward reaction – the formation of products from the reactants

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Equilibrium

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  1. Equilibrium Chapter 16

  2. Reversible Reactions – A chemical reaction in which the products can regenerate the original reactants. • Forward reaction – the formation of products from the reactants • Smog – 2NO2 N2O4 as N2O4 is being formed in one direction, NO2 is being formed in the other… • NO2 to N2O4 is the forward reaction • N2O4 to NO2 is reverse reaction • Both are reactants and both are products!

  3. Chemical Equilibrium – the rate at which the forward reaction is equal to the reverse reaction • It is the state in which the concentrations of reactants and products remain constant • rate they are formed in each reaction equals the rate they are consumed in the opposite reaction. • ***At equilibrium the reaction does NOT stop****

  4. Factors affecting reaction rates • Concentrations – when the concentrations of the substances reacting are higher, the reaction is faster. • Ex. Zn + 2HCl  ZnCl2 + H2 (1.0m) • Ex. Zn + 2HCl  ZnCl2 + H2 (2.0m) Faster • Temperature – higher temp. speed up endothermic reactions.

  5. 16-2 The law of chemical equilibrium • Equilibrium constant – equal to the ratio of the product concentrations (raised to the power indicated by the coefficients) to the reactant concentrations (raised to the power of the coefficients) • Law of Chemical Equilibrium – Every reversible reaction proceeds to an equilibrium state.

  6. aA + bB  cC + dD • Keq = [C]c[D]d[products]coefficients • [A]a[B]b [reactants]coefficients • **When writing Keq only use the elements in the GAS state** • [ ] mean concentration

  7. Keq is a measure of the extent to which a reaction proceeds to completion. • If Keq is >>1, a lot of the products were formed. (remember products over reactants) • If Keq is <<1, the reaction did not really get going. • If Keq = 1, the reaction is at equilibrium when the products and reactants are equal

  8. Homogeneous equilibria – equilibrium conditions that deal with one state of matter. • Heterogeneous equilibria – equilibrium conditions that deal with more than one state of matter.

  9. We only use gases because liquids and solids concentrations do not significantly change • The Reaction Quotient..(Q) is used to determine if a reaction is at equilibrium. • Solve it the same way you solve Keq but put in the numbers.

  10. After you solve for Q, compare the Keq value with the Q value… • If Q > Keq The reaction will proceed to the left • (Because the denominator of the Q is too small and the numerator is too large IE there’s too many products and not enough reactants) • If Q < Keq the reaction will proceed to the Right • (because the denominator is too large and the numerator is too small… There are too many reactants so more products need to be formed to reach equilibrium • If Q = Keq the system is at equilibrium. There is no shift.

  11. Consider the reaction: COCl2 CO + Cl2 Keq =170 • If the concentrations of CO and Cl2 are each 0.15M and the concentration of COCl2 is 1.1 x 10-3 M, is the reaction at equilibrium? If not, in which direction will it proceed?

  12. 1st write the Keq • COCl2 (g) > CO (g) + Cl2 (g) Keq = 170 • Keq= [CO][Cl2] = Q = (0.15)(0.15) = 20 • [COCl2] 1.1 x 10-3 • Q < Keq • The reaction proceeds to the right because Q < Keq

  13. 16-3 Le Chatelier’s Principle • When a stress is applied to a reaction at equilibrium, the reaction will shift to relieve that stress.

  14. Stresses • 1. Changing the amounts of reactants or products • If you add more reactant, shift to right to use up reactant • If you add more product, shift to left to use up product

  15. Stresses • 2. Change in pressure • If the pressure is increased, the reaction will shift to the side with less moles. • Ex. 2NO2 N2O4 • The reaction will proceed to the right because there are half the amount of moles

  16. Change in temp. • Reversible reactions are endothermic one way and exothermic the other. • If you add heat, and the reaction is exothermic, the reaction will go toward completion • If you add heat, and the reaction is endothermic, the reaction will go toward the reactants.

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