50 . C
In an electrochemical cell, oxidation (increase in oxidation number) occurs at the anode. The
oxidation number of Cl increases from −1 in Cl− to 0 in Cl 2 . In every pair except C, the oxidation
number of an element is decreasing, not increasing; every other set is an example of a
reduction.51 . D
The Lewis structure of H 2 O 2 cannot be drawn without an O–O bond. In each of the other cases,
another element is the central atom and there are no O–O bonds.
52 . B
Substances that are liquids or gases at room temperature are generally molecular substances.
(One of the few exceptions to this is mercury metal.) Ionic and network covalent substances
tend to be brittle, often crystalline solids at room temperature, with almost zero vapor
pressure.
53 . B
Since elements experience a change in oxidation number, this is a redox reaction. H changes
from +1 to 0; it is reduced, and therefore it must have acted as an oxidizing agent.
54 . E
In general, the amount of energy involved in nuclear reactions is orders of magnitude greater
than in chemical reactions.
55 . A
Lead chloride, PbCl 2 , is insoluble in water and will form a precipitate. The reactions of HCl with
Ba(OH) 2 and NaCH 3 COO are acid-base neutralizations and do not form insoluble salts. Though
HCl can oxidize some active metals (e.g., zinc), it is not a strong enough oxidizing agent to
dissolve silver metal. The reaction with Br 2 , if any, would be a redox reaction and not a
precipitation, but a look at the table of reduction potentials shows that Br 2 is not a strong
enough oxidizing agent to oxidize Cl− anyway.
56 . E
Before anything else, you need to write the formula for potassium sulfate: K 2 SO 4 . The
concentration of K+ ions in the solution is therefore