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Chemical Equilibrium

Chemical Equilibrium. A Review. Questions about reactions. ?. What’s happening?. The chemical equation. 2 IO 3 - + 5 HSO 3 -  I 2 + 5 SO 4 2- + 3 H + + H 2 O. 2. How fast is it happening?. Kinetics. H 2 O 2 (aq)  H 2 O(l) + O 2 (g) -- slow.

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Chemical Equilibrium

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  1. Chemical Equilibrium A Review

  2. Questions about reactions ? • What’s happening? The chemical equation 2 IO3- + 5 HSO3- I2 + 5 SO42- + 3 H+ + H2O 2. How fast is it happening? Kinetics H2O2(aq)  H2O(l) + O2(g) -- slow H2O2(aq) + catalyst  H2O(l) + O2(g) -- FAST 3. To what extent does it occur? Equilibrium Chapters 15-18 HC2H3O2 + H2O C2H3O2- + H3O+

  3. H2O(l) H2O(g) I2(H2O) I2(CCl4) H2O NaCl(s) NaCl(aq) CO(g) + 2 H2(g) CH3OH(g) Dynamic Equilibrium • Equilibrium – opposite chemical processes taking place at equal rates.

  4. N2O4 (g, colorless) 2 NO2 (g, brown) kfwd [NO2]2 krev [N2O4] = Dynamic Equilibrium For the dinitrogen tetroxide - nitrogen dioxide equilibrium: At equilibrium: ratefwd = raterev kfwd[N2O4] = ratefwd raterev = krev[NO2]2 kfwd[N2O4] = krev[NO2]2 = Kc

  5. CH3CO2H (aq) + H2O(l) H3O+(aq) + CH3CO2- 1) Small Kc 2) Large Kc 2 CO(g) + O2 (g) 2 CO2 (g)Kc = 2.2 x 1022 3) Intermediate Kc 2 BrCl(g) Br2 (g) + Cl2 (g)Kc = 5 Magnitude of K Kc = 1.8 x 10 -5

  6. C(s) + H2O(g) CO(g) + H2(g) [CO][H2] Kc = [H2O] Pure Liquids and Solids • Equilibrium constant expressions do not contain concentration terms for pure solid or pure liquid components

  7. Altering Equilibrium Conditions: Le Châtelier’s Principle • If an external stress (such as change in concentration, temperature, etc.) is applied to a system in a state of dynamic equilibrium, the equilibrium shifts in the direction that minimizes the effect of the stress. - Henri Louis LeChâtelier in 1888

  8. CH4 (g) + NH3 (g) HCN(g) + 3 H2 (g) CH4 (g) + NH3 (g) HCN(g) + 3 H2 (g) The Effect of a Change in Concentration Given the system at equilibrium: Adding ammonia to the reaction mixture at equilibrium will force the reaction to produce more product. Likewise, removing ammonia from the equilibrium mixture will cause the reverse reaction to produce more reactants. System restores equilibrium by producing products Equilibrium shifts to the left to replace some reactant Add NH3 Remove NH3

  9. Problem: Carbon will react with water to yield carbon monoxide and and hydrogen, in a reaction called the water gas reaction that can be used to convert coal into a useful fuel. C(s) + H2O (g) CO(g) + H2 (g) Predicting Changes in Concentration • What happens to: • [CO] if C is added? • [CO] if H2O is added? • [H2O] if H2 is added? • [H2O] if CO is removed? (a) No change, since carbon is a solid, as long as some carbon is present to allow the reaction to proceed in the first place. (b) The reaction moves to the product side, and [CO] increases. (c) The reaction moves to the reactant side, and [H2O] increases. (d) The reaction moves to the product side, and [H2O] decreases.

  10. Fe3+(aq) + SCN-(aq) FeSCN2+(aq) Lab This Week Yellow Colorless Red

  11. Lab Continued • Establish a new equilibrium for each stress • Observe and record the effect • Justify/explain changes before leaving lab • Note that balanced equations should be part of the ‘Calculations’ section in the report • Hints: • Only Fe3+ reacts with SCN- to give color • Sn2+ is a reducing agent, Fe2+ doesn’t react with SCN- • SCN-ppts with Ag+ • PO43- forms a soluble colorless complex with Fe3+ • Ammonia is a base. What do bases do with transition metal ions?

  12. Procedure Proposal Predict the effect of each of the stresses and provide a 1 sentence justification for each prediction.

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