Unit 2 - Equilibria

What is Le Chatelier’s principle?

A stress imposed on a system will be counteracted by an equilibrium shift.

What is the only thing that changes the Equilibrium constant?

temperature

What will happen if the temperature is increased for an exothermic reaction? What happens to K?

It will shift to reactants (reverse rxn consumes heat)

The equilibrium constant will be smaller

How do changes in pressure/volume affect a system?

They affect gaseous systems:

Increased pressure/smaller volume will cause a shift to consume gas and counteract the change.

Decreased pressure/larger volume will cause a shift to produce more moles of gas

How does the addition of an inert gas affect the system?

It doesn’t affect the equilibrium since the gas is not part of the reaction, however the total pressure of the system increases.

Would adding a liquid or solid affect the equilibrium?

no, they do not have concentrations

what does a large K mean for a reaction at equilibrium?

The products are heavily favoured (near completion)

what will happen if Q>K?

Q<K? Q=K?

Q>K means there are too many products and the reaction will shift to the reactants to reach equilibrium

Q<K indicates there are not enough products: shift forward to equilibrium

Q=K means the system is at equilibrium

What is K₂ if:

1. N₂O + O₂ <—> NO₂ + NO (K₁= 3)

2. 2N₂O + 2O₂ <—> 2NO₂ + 2NO

9 (3²)

Because if K₂= [NO₂]²[NO]² / [N₂O]²[O₂]²

the exponent can be moved out: K₂= ( [NO₂] [NO] / [N₂O] [O₂] )² 

which is equivalent to K2= (K1)²

what are the approximations made in small K problems?

That adding or subtracting x (a very small value) will not impact the equilibrium concentrations.

define an Arrhenius acid and base

Arrhenius Base = a substance that dissociates to produce OH- in solution

Arrhenius acid = a substance that dissociates to produce H3O+ in solution

define an Bronsted-Lowry base

Base = accepts a proton

Acid = proton donor

define an Lewis base

Lewis base = Donates a pair of electrons

Lewis acid = Accepts a pair of electrons

metal cations and electron deficient atoms are Lewis ______

acids (accept Electrons)

All Arrhenius acids and bases are also _________

bronsted-lowry acids and bases

are Metal oxides acids or bases? Write the reaction of CaO with water

They are bases

CaO + H2O —> Ca(OH)₂ (aq)

are non-metal oxides acids or bases? Write the reaction of P4O10 with water

they are acids

P4O10 + 6H2O —> 4H₃PO₄

And then H₃PO₄ dissociates to form protons in water.

compare amphiprotic and amphoteric

Amphiprotic = can accept or donate H+

Amphoteric = act as an acid or a base

All amphiprotic compounds are amphoteric but not the other way

what is a buffer?

a solution with appreciable amounts of a weak acid and its conjugate base

When does an optimal buffer occur? (where buffer capacity is greatest)

at the ½ equivalence point when [HA] = [A-]

what is buffer range?

The pH range over which a buffer can effectively neutralise acids and bases

what is a common ion?

An ion that comes from 2 sources (produced by 2 different reactions)

what are 3 ways to make a buffer?

1. Add a conjugate base to a solution of its weak acid

2. Add a conjugate acid to a solution of its weak base

3. Partial neutralisation by addition of a strong acid or base

to use the H-H equation, [A-]/[HA] must be in what range?

0.1 < [A-]/[HA] < 10

define solubility

The maximum amount of solid that can dissolve to produce a saturated solution

how does the common ion effect influence solubility?

Decreases solubility

what are 4 groups that make up the sparingly soluble salts? what are exceptions?

Sulfides (S^2-)

Hydroxides (OH-)

Carbonates (CO₃^2-)

Phosphates (PO₄^3-)

Exception = unless with group 1 elements or NH4+

When does a precipitate form?

when Q>K or the solution is “oversaturated”

A salt with a ______ Ksp will precipitate first

smaller

What is the effect on solubility of adding a basic salt to an acidic solution? explain.

In an acidic solution, basic anions from the salt will react with protons to form a weak acid. This pulls the reaction forward and increases the solubility of the salt.

would an acidic solution make PbCl2 more soluble? why or why not?

No because Cl- is the conjugate of a strong acid and has very weak to no basic properties. It will not react with H+ to be pulled out of solution.