Barnard Chemistry BC2001x
Chang: General Chemistry: The Essential Concepts, 5th Edition
Comments and corrections about the textbook will be posted here.
If you find an error or something that you think is confusing, please tell me about it.

Every textbook has strengths and weaknesses.  Much as I admire Chang's book, there are a few things that I disliked from the beginning.  The first two points are repeated in another handout:

• The periodic table and chart on the inside front cover round atomic weights to four digits. The logic behind this is that this is sufficient precision for many chemical calculations.  But for some elements many more digits are significant, and for some applications more careful attention to significant figures is important.  You will be given a Fisher Scientific periodic chart with atomic weights with correct significant figures.  Always use the Fisher chart for atomic weights in problems and lab reports.

• Chang makes a clear and careful distinction between three ways of writing equations for chemical reactions (pages 90-100): molecular equations, ionic equations, and net ionic equations.  Later in both the text and problems he uses all three.  In this course, only net ionic equations are permitted in your answers.  Thus while HCl(aq) is a fine way to label a bottle, a chemical equation containing HCl(aq) will be considered incorrect: because this is a strong acid, molecules of HCl(aq) do not exist in solution, only ions H⁺(aq) and Cl^-(aq).

• Problem 3.74  It is much easier to understand what is happening if you look at the net ionic equation:
            4 Au(s) + 8 CN^-(aq) + O₂(g) + 2 H₂O(l) → 4 Au(CN)₂^- (aq) + 4 OH^-(aq)
  This shows that the gold, initially mixed in the rock, goes into solution (as a complex ion) so that it can be filtered and recovered.  Gold metal is very hard to ionize -- it is low on the activity series -- but cyanide ion helps this happen.  If you live in the western U.S., you probably have read that cyanide pits from old mines are a serious environmental problem.  It is also used in silver mining, for a similar reason.

• Problems 3.93 and 3.94.  These are schematic pictures which represent poorly the relative volume of a gas occupied by the molecules.  Gases are typically 99.9% empty space, so if drawn to scale, the molecules should be much smaller!  Conversely, figure 4.3 shows empty space between water molecules in a solution.  In a condensed phase, the molecules are in contact with each other.  Think of figures like these as rough cartoons rather than accurate pictures. 

• Problem 3.102   This problem would be easier to understand if it said "obtained from a 2.62 x 10³ kg sample of iron ore".  In other words, the sample is not pure Fe₂O₃, but rather Fe₂O₃(s) mixed with rock.  The purity of the ore is the fraction by weight that is Fe₂O₃(s): (grams Fe₂O₃)/(grams ore)

• Problem 8.90  This problem is ambiguous: in many cases there are multiple oxides (CO, CO₂; NO₂, N₂O₅; etc.)  I do not expect you to be able to predict when oxides are amphoteric (you will become familiar with some examples later in lab.)

• Problem 9.30, page 295 in 4th Ed.  The answer book shows resonance structures for ClO₄^-, all of which involve expanded valence shells.  There is NO GOOD REASON to do this.  One can write a good single Lewis structure with the octet rule satisfied for each atom.  For simplicity, use expanded shells only when forced to do so!  The sulfate anion is another example where some books insist on using expanded valence shells and writing resonance structures when there is no need to do so.

• Lewis structure/VSEPR problems (Chapter 10, 4th Ed., Chapter 9, 5th Ed.)  Chang includes some examples where the substance is ionic (metal with non-metal).  Lewis and VSEPR theory are really designed for and work best for covalent bonding in molecular substances, so skip the ionic examples, like 10.7.d, 10.66: a, e, and f, and similar examples.  Also: our focus is in this chapter is geometry via VSEPR, not hybridization.

• Table 13.2 (page 439 of 5th Ed.)  Units of colligative property constants K_f  and K_b.  Chang uses (^oC/m).  Other books and course and lab materials use (^oK/m) or (^oK-kg/mole)  or (deg-kg/mole).  The values are the same!  This constant relates a temperature change delta-T to the molality (moles solute/kg solvent).  The size of a degree is the same in kelvin or centigrade, so delta-T is the same number, whether the two T's are both in ^oC or both in ^oK.

• Example 13.6 (page 436, 5th Ed.):  The density of water (not of the solution) is 1.0 g/mL.  [added 10/8/08]Answer to problem 15.52.  These reactions would be easier to understand if written as net ionic reactions:
  e) CO₂(g) + OH^-(aq) → HCO₃^-(aq)      {thus adding hydroxide decreases P_CO2}
  f) CaCO₃(s) + 2 H⁺(aq) → Ca²⁺(aq) + CO₂(g) + H₂O(l)   {thus adding acid increases P_CO2}

• Answer to problem 18.48: Incorrect P_CO2, final answer should be 1.2 x 10¹⁴

• Page 49: in list of oxyacids, phosphoric acid is H₃PO₄, not H₃NO₄ (which does not exist).