- A Don’t fall into the temptation trap and pick (B). Temperature differences
can indicate the direction of heat flow. However, temperature is not a
direct measure of heat energy. Instead, associate heat energy with
enthalpy.
- E On this test, the easiest way to balance an equation is by plugging in the
answers. The test writer tells you that the coefficient in front of H 2 O is 6,
so put that in there. Now, see which of the answer choices, if placed in
front of NF 3 , would result in a balanced equation. If, for instance, we try
choice (B), we’d have 1 mole of N on the left, which would give us 3
moles of F on the left. In order to have 3 moles of F on the right we’d
have to put a 3 in front of the HF on the right. That means we’d have 3
moles of H on the right and 12 moles of H on the left. This is way out of
balance.
Suppose we try option (D) and put a 3 in front of the NF 3 on
the left. That gives us 9 moles of F on the left, which means
we’d have to put a 9 in front of the HF on the right. That in
turn would provide 9 moles of H on the right when we have
12 moles of H on the left. Once again, this is out of balance.
Now, let’s try choice (E). We put a 4 in front of NF 3 on the
left, which gives us 12 F on the left and means we must put a
12 in front of the HF on the right. That gives us 12 H on the
right, balanced by 12 H on the left. The coefficients for both
NO and NO 2 would be 2, which would balance both the
nitrogens (N) and the oxygens (O).
- B Make sure you can distinguish between an acid and a base. A base can
donate an unshared electron pair, according to the Lewis definition, so
(B) is correct. Choices (A), (D), and (E) are characteristics of acids. And
what about (C)? Not all bases contain the OH− ion in their structure—
NH 3 is an example.
- C The substance described has some metallic characteristics (shiny and
high melting point) and some nonmetallic ones (brittle and poor electrical
conductivity). It sounds like something that’s between a metal and a
nonmetal such as a metalloid or semimetal. This is a general description
