Principles of Chemistry Exam 1

What are the three characteristics of equilibrium?

Opposing processes occur at the same rate, there is no net change in the concentration of reactants and products, and it can be approached from either direction using any amount of reactants and products

Equilibrium position depends on…

concentration, temperature, and pressure

Law of mass action

For aA + bB <=> cC + dD, K = ([C]^c[D]^d)/([A]^a[B]^b)

If K>1, the equilibrium…

lies to the right (more products than reactants at equilibrium)

If K<1, the equilibrium…

lies to the left (more reactants than products at equilibrium)

Equilibrium units for species in solution

mol/L or M

Equilibrium units for gases

mol/L or atm (for pressure)

Equilibrium units for pure solids, liquids, or solvents

They do NOT appear

Units for K

No units

homogeneous equilibrium

all species are in the same physical state

heterogeneous equilibrium

species are in different states of matter, so not all enter into K

Calculating Δn

Δn = #moles of gas product - #moles of gas reactant (find moles in chemical equation)

If K ≥ 10^4, we say…

the reaction has gone to completion

Reaction quotient (Q)

Set up identically to K, but with inputs not at equilibrium

Q=K

Reaction is at equilibrium

Q>K

initial [products] is too large; reaction moves to left

Q<K

Initial [products] is too small; reaction moves to right

Le Chatlier’s Principle

When a stress is applied to a system in dynamic equilibrium, the equilibrium tends to adjust to minimize the effect of the stress

Effect of pressure increase caused by compressed volume on equilibrium position

favors side of equilibrium with fewer moles of gas

Effect of dilution on equilibrium position

favors side with more moles dissolved

Effect of increased amount of product/reactant on equilibrium position

favors the side that consumes the added species

Effect of increased temperature in exothermic reaction on equilibrium position

favors reactants

Effect of decreased temperature in endothermic reaction on equilibrium position

favors products

Solubility

the number of moles that dissolve to reach equilibrium, or x

Lower Ksp means…

Lower solubility

Ksp

For aA(s) <=> bB (aq) + cC(aq), Ksp = [B]^b[C]^c

Qsp<Ksp

Unsaturated solution, capable of dissolving more

Qsp=Ksp

Saturated solution

Qsp>Ksp

Precipitate forms until Q=K

Adding equilibrium expressions means…

Multiplying Ks

Reversing the positions of the products/reactants means…

Taking the inverse of K

Arrhenius acid

when dissolved in water, changes [H+]

Arrhenius base

when dissolved in water, increases [OH-]

Bronsted-Lowry acid

proton donor

Bronsted-Lowry base

proton acceptor

Lewis acid

Electron pair acceptor

Lewis base

acid and conjugate base

an acid and its deprotonated base

Ka=

([H+][A-])/[HA]

If Ka is large…

the acid is strong

If Ka is smaller…

the acid is weak

If pKa is smaller…

the acid is stronger

Kw=

[H3O+][OH-] = 1.0×10^14

Relationship between Ka, Kb, and Kw

Ka*Kb = Kw

electron withdrawing groups

elements with high electronegativity

Effect of electronegativity on pH

Elements with higher electronegativity weaken the bond between the acidic proton and its acceptor. Proximity of EN atoms to a labile H increases acidity

When to consider autoionization of water

If [strong acid/base] < 1×10^-7 M

Strong acids to know!!

HCl, HBr, HI, H2SO4, HNO3, HCl4

Strong bases to know!

NaOH and KOH

How to find the pH of weak acids

1. Identify major species in solution

2. Identify species that can produce H+

3. Decide which equilibrium dominates (larger Ka)

4. Make an ICE table

5. Determine pH

Kb =

([BH+][OH-])/[B]

Buffer definition

a solution that has significant concentrations of a weak acid and its conjugate base

Henderson-Hasselbach Equation

pH=pKa + log([base]/[acid])

Equilibrium point pH for a strong acid titrated by a strong base, or a strong base titrated by a strong acid

pH = 7

Equilibrium point pH for a weak acid titrated by a strong base

pH > 7

Equilibrium point pH for a weak base titrated by a strong acid

pH < 7

Halfway point

Point at which [HA] = [A-], so pH = pKa

Equivalence point

point at which moles of original acid = moles of base added

How to find pH at the equivalence point

Use an ICE table and Kb (weak base problem)

Amount of titrant > amount of initial reactant

Use unreacted moles of titrant to find pH