Presentation on theme: "Equilibrium A state in which opposing processes of a system are occurring at the same rate. 1.Physical (a) Saturated Solution – dissolution and crystallization."— Presentation transcript:
Equilibrium A state in which opposing processes of a system are occurring at the same rate. 1.Physical (a) Saturated Solution – dissolution and crystallization (b) Contained Liquid – vaporization and condensation 2.Chemical (a) Reaction – Forward and reverse reactions occurring at the same rate.
A true equilibrium can be reached from either direction. H 2(g) + I 2(g) ↔ 2HI (g) Starting with reactants Starting with products
Haber Process Fritz Haber Perfected the synthesis of ammonia from hydrogen (H 2 ) and nitrogen (N 2 ) – 1913 N 2(g) + 3H 2(g) ↔ 2NH 3(g) (1) Why is this difficult?. &. (2) Why was this important? (1) Nitrogen is triple bonded and a very stable element. :NΞN: (2) Ammonia needed for making explosives on a large scale (2) Now needed for the production of fertilizers to feed the world. Haber used equilibrium concepts to determine what set of conditions would cause the greatest amount of ammonia to be produced.
Law of Mass Action a.k.a. Equilibrium Expression Expresses concentrations, [ ]’s, of reactants and products @ equilibrium in terms of a constant, K, the equilibrium constant. K = [products] / [reactants]
.... Equilibrium expression For reaction: j A + k B ↔ p R + q S K = [R] p x [S] q / [A] j x [B] k
Characteristics of “K” 1.Determined experimentally 2.Value changes with ΔT 3.Must be determined at equilibrium 4.If K is large, >1, reaction favors products, equilibrium lies to the “right”. 5.If K is small, <1, reaction favors the reactants, equilibrium lies to the “left”. 6.K for the reverse reaction = 1/K
..... K continued 7. In heterogeneous systems, involving solids, liquids, and/or gases, solids and liquids are omitted from the expression. CaCO 3(s) ↔ CaO (s) + CO 2(g) K = ?? K = [CO 2 ] 1
..... K continued 8.K can be calculated by using molarity or atm’s, if a gas phase reaction. if molarity: K expressed as K c if atm: K expressed as K p Also K p = K c (RT) Δn R =.0821 atm x L / K x mol Δn = # gas mol products - # gas mol reactants ??? When will K p = K c ?
ICE Calculations / Tables You are often given incomplete information about an equilibrium system and asked to calculate K. I = initial C = change E = equilibrium
Example A mixture of 1.50 mol of SO 2 and 1.00 mol of O 2 is placed in a 2.00 L vessel. At equilibrium 0.828 mol SO 3 has formed. 2SO 2(g) + O 2(g) 2SO 3(g) I 1.50 mol 1.00 mol ? C ? ? ? E ? ? 0.828 mol K c =
Factors affecting equilibrium Changes in: 1.Temperature 2.Pressure 3.Concentrations Le Chatelier’s Principle “When a stress is applied to a system, the system behaves in a way to reduce that stress.”
Stresses applied at equilibrium 1.Δ[ ] of a reactant or product * (a) Addition of a reactant or product System/reaction will shift to consume that substance (b) Removal of a reactant or product System/reaction will shift to form more of the substance N 2(g) + 3H 2(g) ↔ 2NH 3(g) *Changing [ ] of solids and liquids does not effect equililbrium.
... More stresses 2.ΔPressure by changing volume* (a) V↓ then P↑ - how can we reduce P? Shift in direction of fewer gas moles (b) V↑ then P↓ - What happens? Shift in direction of more gas moles N 2(g) + 3H 2(g) ↔ 2NH 3(g) *no shift when # of gas moles equal on both sides
... Still more stresses 3.ΔT Almost always changes value of K (a) Addition of heat causes a shift in the endothermic direction. (b) Removal of heat causes a shift in the exothermic direction 4. Addition of a catalyst NOT A STRESS only gets you to equilibrium faster.
Quiz 1. When a chemical reaction reaches equilibrium we say that __________ are equal and ______________ are constant. 2. The Law of Mass Action is also known as the _____. 3.Write the law of mass action for the following rxn: H 2(g) + Cl 2(g) ↔ 2HCl (g) 4. What concentration units are used to calculate K c ? 5. What concentration units are used to calculate K p ? 6. What does the magnitude of K tell you about the equilibrium that has been reached?
... Quiz continued 7. Write the equation relating K p to K c. 8. When does K p = K c ? 9.If heat is added to a system at equilibrium, what will the system do? 10. What is the name of the principle that describes what reactions will do when they come under a stress?
Solubility Equilibria (K sp ) Solubility The amount of substance that will dissolve in a given amount of solvent at a given temperature. Units: grams/100 g water or molarity (molar solubility) Can be affected by adding other substances to the solution.
Solubility Equilibria cont….. BaSO 4 considered insoluble according to solubility rules........ But........ Even salts considered insoluble in water will, to a very small degree, dissociate and establish an equilibrium. For BaSO 4 in water: BaSO 4(s) ↔ Ba +2 (aq) + SO 4 -2 (aq)
Write the Equilibrium Expression: K sp = [Ba +2 ] x [SO 4 -2 ] K sp
Solubility Equilibria problem types: 1.Calculate K sp from molar solubility 2.Calculate molar solubility from K sp 3.Will a precipitate form?