system—that is, the total energy contained in the system—will change. The potential energy of
the system during the phase change is the same as the internal energy. Therefore, when a solid
melts into a liquid, the potential energy of the substance will change. Solids have a defined
temperature, known as the melting point, at which they change to liquid. At this temperature,
any energy added to the solid will go toward changing the phase, not changing the
temperature, until all the solid has changed to liquid. We have already shown that the phase
change occurs at a constant temperature, and because a change in kinetic energy is associated
with a temperature change, the kinetic energy in the solid-to-liquid transition will remain the
same.63 . E
Reading from left to right, the NH 4 + loses a proton to become NH 3 while the H 2 O picks up a
proton to become H 3 O+. Thus, one proton is transferred from NH 4 + to H 2 O. Choices A and D are
wrong because the oxidation states of the atoms are not changed. Double displacement would
require that two species be exchanged; however, only one species, the proton, is being
exchanged. Neutralization is a special case of a double displacement reaction.
64 . C
The equilibrium constant expression shows the product of the product concentrations raised
to their stoichiometric coefficients over the product of the reactant concentrations raised to
their stoichiometric coefficients. For heterogeneous equilibria, pure solids and liquids are not
included in this equation, but gases and aqueous components are included. So for this
question you need to construct the correct equilibrium constant expression and calculate the
equilibrium constant: The concentration of ammonia squared over the product of the
nitrogen concentration and the cube of the hydrogen concentration. Plugging in the
appropriate values, you get 15^2 over the product of 1 times 2^3 . This gives an equilibrium
constant of 28.
65 . C
To answer this question, you must balance the equation given to find the stoichiometric
coefficient for hydrochloric acid. Start with oxygen, since it’s the only element that is present
in only one compound on each side of the equation. Since there are three oxygens on the right
side of the equation, you have to place a “3” before the water molecule on the left. There are
now six hydrogen atoms on the left side of the equation, so a “5” must be placed before the
HCl on the right side for a total of six hydrogens on that side also. Before jumping right to the
answer, we should balance the entire equation, since the answer might be five or some