Unit 9 check

Unit 9 check: Thermodynamics and Electrochemistry

11 questions from 11 lessons in Unit 9. Answer each one, and if you miss, follow the link back to where it is taught, then try again. Your score counts first tries.

Question 1 · from 9.1 · Rank it

Each sample is 1.0 mol of water. Rank the samples by their entropy.

Highest entropy first

Give each one a place, 1 for first. Give two the same place if they are equal.

Steam at 100 °C in a 40 L container
Ice at 0 °C
Steam at 100 °C in a 120 L container
Liquid water at 0 °C

Your ranking: none yet

Question 2 · from 9.2 · Calculate

Calculate ΔS° for N₂(g) + 3H₂(g) → 2NH₃(g), in J/(mol·K), with its sign. The standard molar entropies are N₂(g) 191.6, H₂(g) 130.7 and NH₃(g) 192.8 J/(mol·K). Give your answer to one decimal place.

Your answer J/(mol·K)

Question 3 · from 9.3 · Calculate

For N₂(g) + 3H₂(g) → 2NH₃(g) at 298 K, ΔH° = −92.2 kJ/mol and ΔS° = −198.1 J/(mol·K). What is ΔG°, in kJ/mol? Give your answer to one decimal place.

Your answer kJ/mol

Setup for question 4

A chemist sorts what she sees in four situations at 25 °C. In each, ΔG° tells whether the process is favored, and the observation tells whether it occurs at a measurable rate.

One of the favored changes is a mixture of hydrogen and oxygen in a sealed flask at 25 °C, where 2H₂(g) + O₂(g) → 2H₂O(l) has ΔG° = −474.2 kJ/mol. The flask shows no change over a year.

Question 4 · from 9.4 · Sort it

Tap each situation, then the bin that describes it.

Favored and occurring

Favored but under kinetic control

Not favored

Setup for question 5

Reactions in the body run at about 310 K, not 298 K. One step in a metabolic pathway has ΔG° = +7.50 kJ/mol at 310 K. These numbers are a teaching example.

Question 5 · from 9.5 · Sort it

Each card gives ΔG° for a reaction at 298 K, where RT is about 2.5 kJ/mol. Tap a card, then the bin that describes K.

K is much less than 1

K is close to 1

K is much greater than 1

Setup for question 6

A student compares three salts, each made of one cation and one anion, at 298 K. Salt Z is a made-up salt. These numbers are a teaching example. Use R = 8.314 J/(mol·K).

Dissolution data at 298 K
Salt	ΔH° (kJ/mol)	ΔS° (J/(mol·K))
NaCl	+3.9	+43.4
AgBr	+84.4	+47.7
Salt Z	−12.0	−30.0

Question 6 · from 9.6 · Calculate

Using the table: what is ΔG° for dissolving salt Z at 298 K, in kJ/mol? Give your answer to the nearest 0.1 kJ/mol, with its sign.

Your answer kJ/mol

Setup for question 7

A student studies three reactions in a model cell. Reaction 3 shares no species with the other two. These numbers are a teaching example.

Reactions in a model cell
Reaction	Equation	ΔG° (kJ/mol)
1	Y + HPO₄²⁻ → Y-P + H₂O	+22.0
2	ATP + H₂O → ADP + HPO₄²⁻	−30.5
3	S → T + U	−45.0

Question 7 · from 9.7 · Multiple choice

Using the table: which statement about the reactions is supported?

Setup for question 8

A student builds a cell from a lead electrode in 1.0 M Pb(NO₃)₂(aq) and a silver electrode in 1.0 M AgNO₃(aq). A wire with a voltmeter joins the electrodes, and a salt bridge of KNO₃(aq) joins the solutions. The voltmeter shows a steady reading while the cell runs for 30 minutes. The student weighs each electrode before and after. These numbers are a teaching example.

Electrode masses before and after 30 minutes
Electrode	Mass before (g)	Mass after (g)	Change (g)
Lead	12.450	11.820	−0.630
Silver	9.800	10.456	+0.656

Question 8 · from 9.8 · Decide and justify

Using the table: in the wire, which way do the electrons travel?

Which reason supports your decision?

Setup for question 9

A student has the four standard half-cells in the table. Each is at standard conditions. In every cell the student builds, the half-reaction with the more positive potential is the one reduced.

Standard reduction potentials at 25 °C. This is a teaching table; the exam gives its own.
Reduction half-reaction	E°red (V)
Ag⁺(aq) + e⁻ → Ag(s)	+0.80
Cu²⁺(aq) + 2e⁻ → Cu(s)	+0.34
Fe²⁺(aq) + 2e⁻ → Fe(s)	−0.44
Mg²⁺(aq) + 2e⁻ → Mg(s)	−2.37

Question 9 · from 9.9 · Select all

Using the table: select the three true statements about the reaction Fe(s) + Mg²⁺(aq) → Fe²⁺(aq) + Mg(s) at standard conditions.

Select 3 answers, then check.

Question 10 · from 9.10 · Match

Match each value of Q to what it means for the cell potential, E, compared with E°. Every cell has a positive E° as written, so K is greater than 1.

Question 11 · from 9.11 · Fill the statement

Complete the pathway from a mass of metal to the charge that was needed.

Moles of metal = mass ÷ . Moles of electrons = moles of metal × . Charge = moles of electrons × .

0 of 11 answered.