Unit 9 practice test: Thermodynamics and Electrochemistry

In ion-dipole attraction, oxygen (δ−) points toward cations and hydrogen (δ+) points toward anions.

• No: the oxygen should face the anion, because both are electron-rich.: Like charges repel; the partly negative oxygen is attracted to the cation, not the anion.
• No: the hydrogens should face the cation, because hydrogen bonds form to cations.: Partly positive hydrogens are repelled by a cation. They point toward anions.
• Correct: Yes: δ− oxygen faces the cation and a δ+ hydrogen faces the anion.: Right: opposite partial charges attract, so this is the ion-dipole orientation.
• Yes, because water molecules point the same way around any ion.: The drawing shows different orientations around the two ions, which is what makes it right.

Dissolving has three steps: separating solute particles (absorbs), separating solvent particles (absorbs) and forming solute-solvent attractions (releases).

• It absorbs energy, because water has to be pulled toward the ions.: Attractions forming release energy; separating particles is what absorbs it.
• It has no energy effect, because no chemical bonds form.: Ion-dipole attractions are intermolecular, but forming them still releases energy.
• Correct: It releases energy, as opposite partial charges come together.: Right: forming any attraction lowers the potential energy and releases energy (the third step of dissolving).
• It absorbs energy, because the ions break apart from the crystal at that moment.: Separating the ions from the crystal is a different step, and that one absorbs energy.

Coulombic attraction grows with charge and falls with distance. Mg²⁺ (2+, smaller) beats Na⁺ (1+, larger).

• Correct: Mg²⁺ has a larger charge and smaller radius, so its Coulombic attraction is greater.: Right: both the charge and the distance favor Mg²⁺.
• Mg²⁺ has a larger molar mass, so it attracts water more strongly.: Mass does not set the strength of an electrostatic attraction; charge and distance do.
• Mg²⁺ has more electrons than Na⁺, so it is more polarizable.: Mg²⁺ and Na⁺ have the same number of electrons (10), and the key is charge and size.
• Mg²⁺ forms covalent bonds with water, while Na⁺ forms ionic bonds.: Both form ion-dipole attractions with water; the strength differs because of charge and size.

Hydration orders the water (S down) while the ions disperse (S up). The net sign depends on which is larger.

• It raises ΔS°soln, because water molecules move in closer to the ions.: Being held close to an ion restricts the water molecules, which lowers their entropy.
• It has no effect on ΔS°soln, because the water was already a liquid.: The water molecules in the shells lose freedom compared with bulk liquid, so the entropy changes.
• It raises ΔS°soln, because ion-dipole attractions release heat.: Releasing heat affects ΔH°, and through the surroundings, not the ordering effect asked about here.
• Correct: It lowers ΔS°soln: water held around ions has fewer arrangements.: Right: hydration shells reduce the freedom of the water, an entropy decrease that offsets the ions dispersing.

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s): zinc goes from 0 to +2 (oxidized).

• Correct: Zn(s): Right: Zn atoms lose electrons and become Zn²⁺(aq), while Cu²⁺ gains them and plates out.
• Cu²⁺(aq): Cu²⁺ gains electrons to become the copper coating: it is reduced.
• NO₃⁻(aq): Nitrate is a spectator ion here; its oxidation number does not change.
• Cu(s): Copper metal is a product in Trial 1, not something that reacts.

The cell runs the favored reaction from Trial 3. Copper is oxidized, so the copper electrode is the anode.

• Silver, because Trial 4 shows silver does not react with Cu²⁺: Trial 4 shows that silver is not oxidized by Cu²⁺; silver ions are reduced instead, at the cathode.
• Silver, because silver is the more valuable metal: The cost of a metal has nothing to do with which electrode is the anode.
• Copper, because copper has the larger mass of the two metals: Atomic mass does not decide which metal is oxidized; the observed reaction does.
• Correct: Copper, because in Trial 3 Cu(s) gives electrons to Ag⁺: Right: the favored reaction is Cu + 2 Ag⁺ → Cu²⁺ + 2 Ag, so copper is oxidized at the anode.

A metal that coats with another metal from solution is being oxidized by those ions. Zn reacts with both ions, Cu with one, Ag with none.

• Ag > Cu > Zn: This is reversed: silver never reacts, so it is the least easily oxidized.
• Correct: Zn > Cu > Ag: Right: Zn gives electrons to Cu²⁺ and Ag⁺; Cu gives electrons to Ag⁺ but not to Zn²⁺; Ag gives electrons to neither.
• Cu > Zn > Ag: Copper does not react with Zn²⁺ (Trial 2), so zinc is more easily oxidized than copper.
• Zn > Ag > Cu: Silver does not react with Cu²⁺ (Trial 4), so copper is more easily oxidized than silver.

An electrolytic cell uses an external energy source to drive a thermodynamically unfavored redox reaction.

• Correct: An electrolytic cell: outside energy drives an unfavored reaction: Right: Trial 6 shows Zn²⁺ is not reduced by Ag on its own, so the power supply must drive it.
• A galvanic cell, because a metal plates onto an electrode: Plating happens in both kinds of cell. What decides the type is whether the reaction is favored or driven.
• A galvanic cell, because the power supply produces a current: In a galvanic cell the reaction itself produces the current; here the current is supplied from outside.
• Neither, because a reaction that is not favored will not happen: Outside energy can drive an unfavored reaction (topic 9.7); that is what an electrolytic cell does.