Chemical Systems and Equilibrium -Equilibrium Calculations

reaction quotient (Q)

a numerical value determined by using the same formula as the equilibrium constant (data for a reversible reaction) that may or may not be at equilibrium

using Q to det the dir of a rxn

rxn would move in the dir from Q to K

Q < K

more reactants than products at the current concentrations, therefore the rxn must proceed RIGHT

Q > K

more products than reactants at the current concentrations, therefore the rxn would proceed left

calc eq concentrations from K and initial concentrations steps

1. balanced chem eq

2. calc initial concentrations if necessary

3. CALC Q and det the concen shift dir so you know what is pos and neg on the ICE table

4. make ICE table

5. place all eq expressions for each substance in the K expression and isolate for x

three ways to calc eq concens

1. perfect square (no quad as you sqr root both sides at some point)

2. small k simplication (100 rule)

3. quad formula

small k simplication

• when k is vv small so you can assume the rxn hardly proceeds and can assume the eq concen is almost the same as initial concens

• remove x term associated w the SMALLEST NON-ZERO CONCEN

• use 100 rule

small k simplification : 100 rule

is the ratio of [initial concen of reactant'] / K is greater than 100, then x is vv small so it can be removed from that part of the expression

K constant for solubility

look at the coefficients in the BALANCED NIE WITH THE IONS

molar solubility

• the value of x in K solubility solving questions

• recall the definition of solubility

solubility

the max amount of an ionic compound that will dissolve in a given volume of solvent at a particular temp

(solubility) use Q and K to det if a precipitate forms : Q < K

equilibrium shifts right therefore no precipitate bc less ions are present than what is needed to saturate the solution

(solubility) use Q and K to det if a precipitate forms : Q > K

equil shifts left; more ions are present than what is required to saturate the solution therefore it is supersaturated to produces a precipitate

common ion effect

a DECREASE in solubility of an ionic compound due to the presence of a common ion in solution

• consistent with Le Chatelier’s Principle

AgCl(s) ⇌ Ag+ (aq) + Cl- (aq)

• Adding NaCl decreases the solubility (ie. quantity of solute that dissolves) of AgCl because there are now more Cl- (chloride anions) in the solution – reaction shifts to the left

another definition of solubility

quantity of solute that dissolves

the guy who made that one principle

Le Chatelier

eq for K constant

K constant would change according to the

temp of the system

homogeneous equilibria

equilibria where the reactants and products are all in the same phase.

heterogeneous equilibria

reactions and products are in diff phases

concentration of liquids and solids are constant bc

their concentrations are fixed and equal to its density

EX: Since a litre of water at SATP is 1.00kg, which is equal to 55.5 mol, water has a “concentration” of 55.5mol/L

super large Keq would

favour products

very small Keq would

favour reactants

K approx = 0

favours neither products or reactants

Keq’

equilibrium constant for the reverse rxn (1 / Keq)

le chatelier’s principle

“...when a chemical system at equilibrium is disturbed by a change in a property, the system adjusts in a way that OPPOSES the change.”

• factors like temp, pressure, and concen can affect equilibrium therefore causing an equilibrium shift

addition of a reactant causes the rxn to

proceed in the direction that consumes the added substance (go towards the prods aka the right, other way if it was the addition of products)

removing a substance causes the rxn to

proceed in the direction that produces the missing substance

when adding a substance

the rate of the forward and reverse rxns are slightly faster than the initial eq bc more particles would be present

when adding a reactant

the forward rxn (rxn shifts right) increases which causes the reverse rxn to also pick up, and the forward rxn will start to slow as the reactants are used up

temp shifting a rxn

if mixture is cooled, rxn goes towards the side that is producing heat (energy)

if mixture is heated, rxn goes towards the side that is NOT producing heat (energy)

le chatelier and gas volume changes

• only for eqns that are all gases

• decrease in volume (increase in pressure) causes rxn to proceed in the dir of FEWER # OF MOLES

• increase in volume causes rxn to proceed in the dir of more moles

• IF THERE IS THE SAME # OF MOLES ON BOTH SIDES, the rxn does not shift

if there’s a decrease in V, then both the forward and reverse rxns should

increase in rate due to increased concens on both sides

changes that do not affect the posi of equilibrium sys’s

• adding catalysts (would speed up both the reverse and forward rxn)

• adding inert gases (noble gases)

less volume

more concen