Le Chatelier’s Principle Common Ion Effect Predicting Precipitates Chapter 17 & 18, Part III Le Chatelier’s Principle Common Ion Effect Predicting Precipitates

LeChatelier’s Principle Henri LeChatelier studied shifts in the equilibrium of reactions and came up with his principle. He said: If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.

LeChatelier’s Principle Let’s look at how several factors will shift the equilibrium point.

Concentration ↑ [ ] of reactant causes rxn to speed up to the right → ↑ [ ] of product - rxn speeds up toward reactant ← the opposite is also true . . . ↓[ ] of reactant causes rxn to make more reactant ← ↓[ ] of product - rxn tries to replace the lost product →

H2CO3 D H2O + CO2 Which way will equilibrium shift if adding: CO2 Which way will equilibrium shift if removing:

Temperature reversible rxns are exothermic one direction and endothermic the other direction ↓ in temp favors exothermic ↑ in temp favors endothermic

2SO2 + O2 D 2SO3 + heat Which way will equilibrium shift if heat is added? Which way will it shift if heat is removed?

Pressure (and volume) only affects gases ↓ volume results in ↑ pressure when pressure ↑ rxn proceeds toward fewer particles, compare # of particles (coefficients) of reactant gas(es) with # of particles (coefficients) of product gas(es)

N2 + 3H2 D 2NH3 Assuming all the products and reactants are gases, which way does equilibrium shift when pressure is added? Which way does it shift when pressure is relieved?

Reactions that run to Completion Some rxns never work in reverse. They don’t reach equilibrium b/c they only go one way. Reasons why: 1. one of the products is a gas, which escapes 2. a product precipitates (is insoluble) 3. one product won’t ionize enough to reform the reactant ex: 2HCl + 2NaOH  2NaCl + H2O

LeChâtelier’s Practice X(g) + 2Y(g)  Z(g) + heat How would each of the following changes shift equilibrium? a. remove Z by continuous condensation b. introduce an inert gas c. decrease volume of container d. add heat e. add extra Y

LeChâtelier’s Practice H2(g) + Cl2(g)  2HCl(g) ∆H = -185 kJ Predict how each of these would affect equilibrium a. increasing volume of container b. take away some chlorine c. what could you do to get more HCl?

Common Ion Effect When you combine two cmpds that have a “common ion” you actually make it seem like there is a huge concentration of that ion.

Common Ion Effect ex: HCl(aq) + NaCl(s) when dissolved in water will ionize to form H+ + Cl- + Na+ + Cl- that’s twice as much chlorine as sodium or hydrogen, remember ↑ concentration causes rxn to speed up →

Common Ion Effect ex: BaSO4(s) + Na2SO4(aq) →

Predicting Precipitates an insoluble solid that will separate from a solution during a reaction (double displacement). ~ using solubility rules, we can predict precipitates. Compound Solubility Exceptions Salts of group 1A and ammonia (NH3) Soluble (aq) Some lithium compounds Ethanoates, nitrates, chlorates, and perchlorates Few Exceptions Sulfates (SO4) Compounds of Pb, Ag, Hg, Ba, Sr, and Ca Chlorides, Bromides, and Iodides Compounds of Ag and some of Hg and Pb Sulfides and Hydroxides Insoluble (s) Group 1A compounds. Compounds of Ba, Sr, and Ca (slightly) Carbonates, Phosphates, and Sulfites Group 1A and ammonia (NH3) compounds

Predicting Precipitates Compound Solubility Exceptions Salts of group 1A and ammonia (NH3) Soluble (aq) Some lithium compounds Ethanoates, nitrates, chlorates, and perchlorates Few Exceptions Sulfates (SO4) Compounds of Pb, Ag, Hg, Ba, Sr, and Ca Chlorides, Bromides, and Iodides Compounds of Ag and some of Hg and Pb Sulfides and Hydroxides Insoluble (s) Group 1A compounds. Compounds of Ba, Sr, and Ca (slightly) Carbonates, Phosphates, and Sulfites Group 1A and ammonia (NH3) compounds Predict the precipitate… *precipitate* +1 -1 +1 -1 LiI (aq) + AgNO3 (aq) LiNO3 + AgI (aq) (s) *precipitate* +1 -1 +3 -2 NaOH (aq) + Cr2(SO4)3 (aq) Na2SO4 + Cr(OH)3 (aq) (s)

Net Ionic Equations a net ionic equation shows the chemical change in a solution (formation of precipitate). ~ all soluble (aq) compounds are separated into ions. ~ insoluble (s) compounds are written as compounds. To write… ~ write the complete ionic equation. separate (aq) compounds. leave (s) compounds intact. ~ cross out any spectator ions… ions that appear on both sides of equation.

Net Ionic Equations Ex… LiI (aq) + AgNO3 (aq) LiNO3 (aq) + AgI (s) write complete ionic equation… Li+1 + I-1 + Ag+1 + NO3-1 Li+1 + NO3-1 + AgI (s) cross out spectator ions… what is left is the net ionic equation! I-1 + Ag+1 AgI (s) Ex… NaOH (aq) + Cr2(SO4)3 (aq) Na2SO4 (aq) + Cr(OH)3 (s) Na+1 + OH-1 + Cr+3 + SO4-2 Na+1 + SO4-2 + Cr(OH)3 (s) OH-1 + Cr+3 Cr(OH)3 (s)