240 ❯ STEP 4. Review the Knowledge You Need to Score High
(d) Since the cadmium ion concentration increases, the equilibrium must shift to the right.
Endothermic processes shift to the right when they are heated. This is in accordance
with Le Châtelier’s principle.
Give yourself 1 point for endothermic. Give yourself 1 point for mentioning
Le Châtelier’s principle.
(e) There are three acceptable resonance forms. It is only necessary for you to draw one
of them correctly. For an atom, the formal charge = valence electrons – nonbonding
electrons – ½ (bonding electrons).
Each of the resonance forms is shown below with the formal charge for each atom
calculated:OC−
N OC−
N OC−
NO 6 – 4 – ½ (4) = 0 6 – 6 – ½ (2) = –1 6 – 2 – ½ (6) = + 1
C 4 – 0 – ½ (8) = 0 4 – 0 – ½ (8) = 0 4 – 0 – ½ (8) = 0
N 5 – 4 – ½ (4) = –1 5 – 2 – ½ (6) = 0 5 – 6 – ½ (2) = –2
Total –1 –1 –1The total is a check in case of a calculation error. The total must equal the charge
on the ion (0 for a molecule).
You get 1 point for a correct Lewis structure and 1 point for a correct set of formal
charges.
Total your points. There are 10 points possible. Subtract 1 point if any numerical
answer has an incorrect number of significant figures.❯ Rapid Review
● A chemical equilibrium is established when two exactly opposite reactions occur in the
same container at the same time and with the same rates of reaction.
● At equilibrium the concentrations of the chemical species become constant, but not
necessarily equal.
● For the reaction aA + bB cC + dD, the equilibrium constant expression would be:K =[C][D]
c [A][B]cd
ab.
Know how to apply this equation.● Le Cha^telier’s principle says that if an equilibrium system is stressed, it will reestablish
equilibrium by shifting the reactions involved. A change in concentration of a species
will cause the equilibrium to shift to reverse that change. A change in pressure or tem-
perature will cause the equilibrium to shift to reverse that change.