Chem II Ch. 16, 17, 18, 19

Qsp < Ksp

No precipitate

Qsp = Ksp

No precipitate

Qsp > Ksp

Yes precipitate

Common Ion Effect

Influence (usually a decrease in solubility) an ion already in solution has on the solubility of an ionic compound

• Usually decreases solubility if the acid shares a common ion with the salt

Solving For Molar Solubility w/ Common Ion Effect

1. Identify the common ion between the two compounds

   • the given concentration of the other compound (not the one you find Ksp for) will be the concentration of the common ion

2. Write rxn of main compound breaking down into its ions

3. Make RICE table (main compound = solid) & put concentration of common ion

   • if main compound has OH, always assume the OH came from H₂O ( [OH] from H₂O = 1 × 10^-7 M)

4. Find Ksp and plug in equilibrium expression to find s

Metal hydroxides & compounds with strong or weak base anions have …

Higher solubility in acidic solutions

Mg/Ca/Sr/Ba with (OH)₂ have …

Lower solubility in basic solutions because of the common ion effect

Most other metal hydroxides have …

Increased solubility in basic solutions due to the formation of a complex ion

Salts that contain a basic anion are …

More soluble in strong acids and the anion will react with H⁺ (aq) to form its weak conjugate acid

Steps for Expecting Solubility to be More, Less, or Same as in Pure Water Solution

1. Write what ions each salt breaks into when dissolved in H₂O

   a) If the salt shares a common ion with the acid (common ion effect), solubility will be less than in pure water solution

   b) If above doesn’t apply, continue to Step 2

2. Write eqn of salt + strong acid

• if it forms a weak conjugate acid, its solubility is more than in pure water solution

Selective Precipitation Definition

Using differences in solubility to separate mixtures of ions

Steps for Determining Concentration and Whether There is Precipitation

1. Ignoring neutral ions, write out what precipitate is formed when combining solutions

   • Ex: NaCO₃ + MgNO₃: MgCO₃ (s) → Mg²⁺ (aq) + CO₃^2- (aq)

2. Write out Ksp eqn and find its table value

   • You need to compare Q with Ksp, but need to find Q first

3. Calculate concentration of both ions, plug them into Q, and compare with Ksp

• Qsp > Ksp = a precipitate forms

• Qsp < Ksp = no precipitate forms

4. Create RICE Table

5. Solve for concentration of each ions as normal

1st Law of Thermodynamics

1) the energy of the universe is conserved (constant)

2) the energy of an isolated system is conserved

3) the change in internal energy for a closed system is equal to the heat and work exchanged

Entropy, S - Information Theory Perspective

Allows us to calculate the absolute entropy of a system

2nd Law of Thermodynamics

A spontaneous process in an isolated system always increases the entropy of the system

Entropy, S General Definition

A measure of how much you don’t know about a macrostate

Factors that Increase Entropy

• Increased Volume of gas at constant temp

• More Matter on Product Side of Rxn

• Increased Temperature

• S_solid < S_liquid < S_gas

3rd Law of Thermodynamics

The entropy of a perfect crystal at 0 K is zero

Entropy - Thermodynamic Perspective

The change in entropy is the reversible heat exchanged at temperature, T

Formula for Finding △S°_universe

If △S°_universe is positive …

Then is product favored aka thermodynamically favored to occur

If △S°_universe is negative …

Then is not product favored

At low temp …

△H°_rxn has the most effect on sign of △S°_universe

At high temp …

△S°_rxn has the most effect on sign of △S°_universe

Reversible

1) forward & backward change follows the same path

2) equations of state are always obeyed

3) the reversible path as infinitesimally small steps so that the system & surroundiings are always in equilibrium

Formula for Finding △S°_rxn

Factors in Determining Entropy

1) Phase

• S_solid < S_liquid < S_gas

2) Molar Mass

• Heavier = higher entropy

3) Molecular Complexity

• more complex = higher entropy

Formula for Finding △H°_rxn

Formula for Finding △S_surroundings

If Endothermic Rxn, Entropy of Surroundings …

Decreases

If Exothermic Rxn, Entropy of Surroundings …

Increases

When △S_univ > 0 and △G°_rxn < 0 …

Spontaneous, K > 1; product favored

When △S_univ < 0 and △G°_rxn > 0 …

Non spontaneous, K < 1; reactant favored

When △S°_rxn = + and △H°_rxn = + …

Spontaneous at High Temp

When △S°_rxn = - and △H°_rxn = - …

Spontaneous at Low Temp

When △S°_rxn = + and △H°_rxn = - …

Spontaneous at All Temp

When △S°_rxn = - and △H°_rxn = + …

Spontaneous at No Temp

Formula for Finding T_switch

Formula for Finding △G°_rxn

△G°_rxn Definition

Change in free energy to create 1 mol of compound from its elements in their standard state

If Reverse a Rxn …

Flip sign of △G°_rxn

If multiply a rxn by a factor …

Multiply △G°_rxn by the factor

If add two rxns …

Add △G°_rxn together

Formula for Finding △G

R = 8.314 × 10^-3

The better the agent …

the worse the conjugate

Balancing Redox Rxns in Pure H₂O

1. Look at the main chemical rxn and identify what is oxidized and reduced

2. Write out the oxidation half rxn and write out how many electrons is added and on which side of rxn

3. Write out the reduction half rxn and do same thing

4. Combine them back

Balancing Redox Rxns in Acidic Solutions

1. Balance the electrons of the non-oxygen / central atoms

2. Balance O atoms with H₂O by adding however H₂O on the other side to satisfy the amount of needed O atoms

3. Balance H atoms with H⁺ by looking adding however H⁺ atoms on other side to satisfy the amount of needed H atoms

4. Balance charge of e^- by adding necessary electrons on either side

Balancing Redox Rxns in Basic Conditions

1. Balance central atoms

2. Balance O atoms with OH^-

3. Balance H atoms with H⁺

4. Add OH^- to both sides to turn H⁺ and OH^- into H₂O

5. Balance charge with e^-

Oxidation is always on …

The left

Reduction is always on …

The right

Reference Cell

Standard hydrogen electrode (SHE)

• H₂ (g, 1 atm)

Formula for Finding E°_cell

E°_cathode - E°_anode

If E°_cell > 0 and △G < 0

Spontaneous in the direction written, Q < K

If E°_cell < 0 and △G > 0

Non spontaneous, Q < K, backward rxn

Candidates for Reduction

The more positive reduction potential

Candidates for What Gets Oxidized

The more negative reduction potential

If Atomic # > amu

Have too many neutrons, use β^- - decay to get rid of a neutron

If Atomic # < amu

Don’t have enough neutrons, so use β⁺ (positron emission) or electron capture to make a neutron

If both heavy AND too few neutrons

Use α - decay

S° > 0

Have more mol of gas on product side

Product Favored

S° < 0

Have more mol of gas on reactant side

Reactant favored

s → l

l → g

g → s

△S > 0, spontaneous

l → s

g→ l

△S < 0, nonspontaneous

Formula for Finding Total Charge Passed (in coulombs)

Q = I * t

• I = the current in amperes (C/s)

• t = time in seconds

Formula for Converting Charge to Moles of Electrons

mol e^- = Q/F

• F = faraday’s constant

Steps for Finding Amount of Grams Able to be Electroplated

1. Use Q = I*t to find total charge passed (answer will be in C units)

2. Convert charge to mole e^- by using mol e^- = Q/F

3. Write down half reaction including electrons added

4. Divide answer from Step 2 by the amount of electrons

5. Convert mol to grams

If Asked What Will Reduce H⁺ Spontaneously

Look at table with standard reduction potentials

• metals above hydrogen

If Asked to Find Change in Gibbs free energy

Use △G = -nFE_cell

• n = number of electrons transferred

• F = faraday’s constant

• E_cell = E_cathode - E_anode

Good Candidates for being a Reducing Agent

More negative standard reduction potential

Good Candidates for being an Oxidizing Agent

Less negative standard reduction potential

Use Nernst Equation When …

The electrochemical cell is not at standard conditions

• not at 1 M, 1 atm, or 298 K

Equation for Q in lnQ

[products / [reactants]

Form of electrochemical cell notation

reactant | product || reactant | product

The salt bridge completes the circuit by allowing __ to flow towards the anode cell and __ to flow towards the cathode cell

Anions, cations

In Chemical Rxn, what can be oxidized and reduced/ oxidizing and reducing agents?

Only species on the reactant side

For alpha decay …

Mass # decreases by 4, atomic # decreases by 2

For beta^- decay …

Mass # stays the same, atomic # increases

For positron emission (beta⁺) and electron capture …

Mass # stays the same, atomic # decreases

in E_binding = -△mc² / A

△m = actual mass - sum of subatomic particle mass

Steps for Solving for E_binding per nucleon

1. Determine the number of protons and neutrons

• M - A = N

2. Multiply the number of protons and neutrons to the mass numbers given accordingly

3. Find △m (actual mass - sum of subatomic particle mass) and convert to kg/mol by adding x 10^-3 to the end of the number

4. Plug numbers into equation where A = atomic number