Introductory Chemistry

Quiz: Ions in solution.
Answer key.

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1. a. ii is best. (iii includes an incorrect ion, and i has an incorrect ratio of ions.)

b. iv and v are correct. One of those is qualitative; the other is quantitative.

c. iv and v are correct. One of those is qualitative; the other is quantitative.

d. Ag2SO4 (s) --> 2 Ag+ (aq) + SO42- (aq)

An important issue here is that the four parts of this question all deal with exactly the same issue. They all express the point that one formula unit of Ag2SO4 dissociates into its ions when dissolved in water. They express this in different ways. Of particular note, parts b and c above are identical -- except for the symbolism.

2. CH3OH. Methanol is a polar molecule, and does not change in any way when it dissolves in water. The question tells you all that.

An equation for what happens in #2 would simply be CH3OH (l) --> CH3OH (aq).

Questions 1 and 2 contrast how ionic and covalent compounds behave when they dissolve in water. Ionic compounds typically dissociate into their ions, with each ion being solvated. Covalent compounds are molecular, and the molecules typically remain intact when they dissolve. For polar covalent solutes dissolving in water, the solute molecules are solvated. (The behavior of acids and bases, Ch 18, introduces an additional complexity. But #2 specified that the solute was neither of those.)

3. Write both equations in complete ionic form, then cancel out all the spectator ions. That leaves you with the net ionic equations. For both, you will get the same net ionic equation:
2 H+ (aq) + 2 OH- (aq) --> 2 H2O (l)
If they have the same net ionic equation, then they are really the same reaction. Thus they have the same heat of reaction (ΔH): -114 kJ.

Should you simplify the net ionic equation written above, by dividing through by 2? Probably not in this case. If your goal was simply to write the net ionic equation for the original reaction, then simplifying it would be a good idea. However, in this case the question is about ΔH, the heat of the reaction. Sometimes ΔH is given per mole of a particular chemical, but it is often given "per reaction". The latter is true in the question. The given ΔH is for the reaction, as written. You are asked for the ΔH for the second reaction, as written. Thus dividing by 2 would not be helpful.

Another way to look at it... Both net ionic equations involve only the formation of water. If you do simplify, you find that ΔH is -57 kJ per mole of water. That value holds equally for the second reaction. Since that reaction shows 2 mol water, then ΔH for the reaction is -114 kJ.

If you answered the question with ΔH = -57 kJ per mole water, that is a true statement, but it is not the answer to what was asked. If you answered -57 kJ, then you somehow missed that both reactions involve 2 mol water.

This quiz is based, in part, on one by Dr. Joe Ledbetter, Chemistry Department, Contra Costa College.

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Last update: September 23, 2014