Chem 201 ACS Review

Solutions and Colligative Properties

100

This what happens to the vapor pressure of the solution compared to pure water when a nonvolatile solute is added to water. 

What happens when vapor pressure decreases?
_{Adding a nonvolatile solute lowers the mole fraction of solvent, reducing the escaping tendency of solvent molecules and therefore lowering vapor pressure.}

200

This is what happens to the molality when a student doubles the mass of a solvent while keeping moles of solute constant.

What is the molality being cut in half?

_{Molality is moles of solute per kilogram of solvent, so doubling solvent mass halves the molality.}

300

This is the proper arrangement of the following in order of increasing boiling point: 

CH₄, CH₃OH, C₆H₁₄

What is: CH₄ < C₆H₁₄ < CH₃OH

400

Assuming complete dissociation, this is the freezing point of a solution containing 0.400 mol of MgCl₂ dissolved in 1.00 kg of water and has a K_f = 1.86 °C·m⁻¹.

What is -2.23 °C?

MgCl₂ → 3 ions → i = 3
m = 0.400 / 1.00 = 0.400 m
ΔT_f = (i)(m)(K_f) = (3)(0.400)(1.86) = 2.23 °C
Freezing point = 0 – 2.23 = –2.23 °C

500

This is the vapor pressure (in torr) of a solution that is made by mixing 4.00 mol benzene (P° = 100 torr) and 1.00 mol nonvolatile solute?

What is 80.0 torr?

X_solvent = mol solute/(mol solute + mol solvent)
X_solvent = 4/(4+1) = 0.800
P = X_solvent × P° = 0.800(100) = 80.0 torr

Kinetics & Rate Laws

100

This is the reaction order of a linear plot of ln[A] vs. time.

_{Add 100 if you can give confirm the (+) or (-) slope of all 3 orders?}

What is a 1^st Order reaction?

_{2nd Order = 1/[A] vs time
0th Order = A vs time}

200

For the reaction:
2 N₂O₅ → 4 NO₂ + O₂
This is the rate of disappearance of N₂O₅ if NO₂ is forming at 0.40 M·s⁻¹.

What is 0.20 M·s⁻¹?

_{Reaction stoichiometry dictates that N₂O₅ disappears half as fast as NO₂ appears due to the 2:4 ratio.}

300

This determines the rate law if the first step of a mechanism is the slow step.

_{Add 150 if you can tell me what happens if the 2^nd step is slow?}

What is the molecularity of the slow (rate-determining) step?
_{The slowest step acts as the “bottleneck,” so its molecularity determines the rate law.} 

_{If the second step is the slow step, the rate law is determined by the molecularity of the slow second step, except that any intermediates must be substituted out using equilibrium expressions from earlier fast steps.} 

400

This is how long it takes for the concentration to drop to 12.5% of its initial value, of a first order reaction, with a k = 2.50 × 10⁻³ s⁻¹. This can be determined by using the follow formula: t_1/2 = 0.693/k

What is 831s?

12.5% = 1/8 → three half-lives.
t½ = 0.693/k = 0.693 / (2.50×10⁻³) = 277 s
Three half-lives → 3 × 277 = 831 s

_{First-order reactions have a constant half-life, so reaching 12.5% requires three half-lives.  12.5/100 = 1/8, and 1/2 > 1/4 > 1/8}

500

This is the rate law for the following data:
  [A]      [B]      Rate (M/s)
0.10    0.10        0.020
0.20    0.10        0.040
0.20    0.20        0.160

What is the Rate = k[A][B]²?

Doubling [A] (B constant) → rate doubles → first order in A.
Doubling [B] (A constant) → rate × 4 → second order in B.

Equilibrium, Acid/Base, Ksp, and Buffers

100

Increasing pressure on:
N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g)
shifts the reaction in this direction.

Where is, toward the right (products)?

_{Increasing pressure favors the side with fewer gas moles, which is ammonia in the Haber process.}

200

This is the order of these acids from strongest to weakest: 

HF , K_a = 3.5 x 10^-4
HCN , K_a = 4.9 x 10^-10
HNO₃ , K_a = 24

What is HNO₃ > HF > HCN?

_{Acid strength correlates with Ka values, and nitric acid is strong while HF is weak and HCN is very weak.} 

300

This is whether a precipitate will form if Q < K_sp for a salt.

What is no, the solution is unsaturated?

_{Q < Ksp means ions are below saturation, so no solid forms.} 

400

This is the pH of a buffer is made with 0.300 M HA and 0.120 M A⁻, and a K_a = 5.0 × 10⁻⁵.

What is a pH of 3.90?

pK_a = –log(5.0×10⁻⁵) = 4.30
pH = pK_a + log([A⁻]/[HA])
= 4.30 + log(0.120 / 0.300)
= 4.30 + log(0.40) = 4.30 – 0.40 = 3.90

_{The Henderson–Hasselbalch equation relates pH to pKa and the ratio of conjugate base to acid.} 

500

This is the solubility of CaF₂ in water, with a K_sp = 3.9 × 10⁻¹¹.

What is a molarity of 2.1 × 10⁻⁴ M

CaF₂ → Ca²⁺ + 2 F⁻
K_sp = (s)(2s)² = 4s³
s = (K_sp/4)^1/3
s = (3.9×10⁻¹¹ / 4)^1/3
s ≈ 2.1 × 10⁻⁴ M

_{Ksp expressions depend on stoichiometry, and CaF₂ dissolves into one Ca²⁺ and two F⁻ ions, giving Ksp = 4s³.}

Thermodynamics

100

This phase transition always increases entropy.

What is transitioning from a solid → liquid
Or a solid → gas

_{Liquids and gases have more molecular disorder than solids, so entropy increases during melting or vaporization.} 

200

This type of reaction has ΔH < 0 and ΔS > 0 under specific temperature conditions.

What is being spontaneous at all temperatures.

_{A negative ΔH and positive ΔS guarantee ΔG < 0 for all temperatures.} 

300

The following are all at standard states except for this one:

CO₂ at 0.50 atm
H₂O(l)
O₂ at 1 atm
C(graphite)

What is CO₂ at 0.50 atm?

_{Standard state for gases is defined as 1 atm, so CO₂ at 0.50 atm is not in standard state.} 

400

This is the change in free energy of a reaction at 350 K, that has an change in enthalpy of = 125 kJ and a change in entropy of +320 J/K.

What is 13 kJ?

Convert ΔS: 0.320 kJ/K
ΔG = ΔH – TΔS
= 125 – (350)(0.320)
= 125 – 112 = +13 kJ

_{ΔG = ΔH – TΔS; if TΔS nearly equals ΔH, the sign of ΔG depends on their difference.} 

500

This is the equilibrium constant of a reaction that has ΔG° = –42.0 kJ at 298 K.

What is 2.4 × 10⁷?

ΔG° = –RT ln(K)
–42000 = –(8.314)(298) ln(K)
lnK = 42000 / (8.314×298) = 17.0
K ≈ e¹⁷ ≈ 2.4 × 10⁷

_{A large negative ΔG° corresponds to a large equilibrium constant because 
K = e}^{(–ΔG°/RT)}_.

Electrochemistry and Nuclear Chemistry

100

Oxidation occurs at this electrode in a galvanic cell.

What is the anode?

_{Oxidation is defined as the loss of electrons, which occurs at the anode in a galvanic cell. 
LEO/GER}

200

This radiation increases the atomic number by 1.

What is beta (β⁻) decay?

_{Beta decay converts a neutron to a proton, increasing the atomic number by one.} 

300

Zn(s) | Zn²⁺ || Cu²⁺ | Cu(s)
This species is the oxidizing agent in this cell.

What is Cu²⁺?

_{The oxidizing agent is the species that gets reduced, and Cu²⁺ gains electrons to form Cu(s).} 

400

This is the E°_cell for:
Cu(s) + 2 Ag⁺ → Cu²⁺ + 2 Ag(s)
Given:
Cu²⁺ + 2e⁻ → Cu(s)   E° = +0.34 V
Ag⁺ + e⁻ → Ag(s)     E° = +0.80 V

What is +0.46 V?

E°_cell = E°_cathode – E°_anode
Cathode = Ag = +0.80
Anode = Cu = +0.34
E°_cell = 0.80 – 0.34 = +0.46 V

_{Cell potential is the difference between cathode and anode reduction potentials, reflecting electron flow from anode to cathode.} 

500

A reaction that transfers 3 electrons and has E° = +0.65 V, contains this change in standard free energy expressed in kJ.

What is -188 kJ?

ΔG° = –nFE°
= –(3)(96485)(0.65)
= –1.88×10⁵ J = –188 kJ

_{ΔG° = –nFE°, so a positive cell potential produces a negative free energy change indicating spontaneity.}