5 Steps to a 5 AP Chemistry

Entering these values into the Kdexpression gives: (0.013)(0.052)^4 /(0.087) =1.1 × 10 −^6.

Give yourself 1 point for this answer. You can also get 1 point if you correctly put your

values into the wrong equation.

b. The table in part achanges to the following:

CdI 4

2 −

(aq) Cd^2 +(aq) I−(aq)

Initial 0.100 M 0 0.400

Change −x +x + 4 x

Equilibrium 0.100 − xx 0.400 + 4 x

Kd=[Cd^2 +][I]^4 /[CdI 42 −] =1.1 × 10 −^6 =(x)(0.400 + 4 x)^4 /(0.100 −x)

=(x)(0.400)^4 /(0.100)

x=4.3 × 10 −^6 M =[Cd^2 +]

Give yourself 1 point for the correct setup and 1 point for the correct answer. If you

got the wrong Kin part a, you can still get one or both points for using it correctly.

c. The equilibrium is Cd(OH) 2 Cd^2 +(aq) +2 OH−(aq)

The dilution reduces both the Cd2+and OH−concentration by a factor of 2. This gives:

[Cd2+] = 4.3 × 10 −^6 /2 =2.2 × 10 −^6 and [OH−] = 2.0 × 10 −^5 /2 = 1.0 × 10 −^5

The reaction quotient is: Q =[Cd^2 +][OH−^2 ] =(2.2 × 10 −^6 )(1.0 × 10 −^5 )^2

= 2.2 × 10 −^16

This value is less than Ksp, so no precipitate will form.

Give yourself 1 point for a correct calculation, and another point for the correct con-

clusion. If you correctly use a wrong [Cd2+] you calculated in part b, you can still get

both points.

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 Cha^telier’s principle.

Give yourself 1 point for endothermic. Give yourself 1 point for mentioning Le

Cha^telier’s principle.

Your score is based on a total of 8 points. Subtract one point if any 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 nec-

essarily equal.

 For the reaction aA +bB cC +dD, the equilibrium constant expression would be:

Kc. Know how to apply this equation.

cd

ab

CD

AB

=

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238  Step 4. Review the Knowledge You Need to Score High