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ACS II

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Question 1

Antimony reacts with sulfur according to the following reaction:

2Sb(s) + 3S(s) → Sb2S3(s)

What is the mass percent yield from 2.57 g of Sb in excess S, if 1.56 g of Sb2S3 is obtained?

Sb2S3 is 340. g/mol

Correct

 

Explanation

actual yield / theoretical yield * 100


2.57 g /121.7 g/mol   * 1/2  = 0.01055 mol * 340 = 3.5899 g


1.56 g


1.56 g/ 3.68899 = .435*100 = 43.5%


 

Question 2

For the following reaction at 25 oC, ΔHo = -1371 kJ and ΔSo = 345 J/K

A +4B → 3C + 3D

What is ΔGo at 121 oC in kJ?

Correct

 

Explanation

Did you change to K and use consistent units?

ΔG=  ΔH - TΔS

=(-1371)-(121+273)(.345)

=1506

 

Question 3

What is the oxidation state of Mn in KMnO4?

Correct

 

Explanation

K   = 1+

Mn=

O4 = 4(2-)


1-8=0

-7 + x =0

7=x

 

Question 4

Balance the following half-reaction in base, using smallest whole number coefficients. What is the coefficient for OH- in the balanced equation?

CrO42-(aq) → Cr(OH)3(s)

(Hint, technically only H2O and OH- are available. However, you may first balance using H2O and H+ and then convert to only H2O and OH-. . .)

Correct

 

Explanation

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Question 5

Balance the following reaction in acid, using smallest whole number coefficients. What is the coefficient for H2O in the balanced equation?

Ga(s) + MnO4-(aq) → Ga3+(aq) + Mn2+(aq)

(Hint, first balance each half-reaction separately.)

Correct

 

Explanation

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Question 6

A stock solution of NaOH is 0.800 M. What volume of this solution (in mL) is needed to prepare 2.00 L of 0.091 M NaOH?

Correct

 

Explanation

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Question 7

What mass of copper (in mg) could be plated on an electrode from the electrolysis of a Cu(N03)2 solution with a current of 0.600 A for 47.1 min?

Correct

 

Explanation

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Question 8

Consider the following half-reaction.

4H+(aq) + O2(g) + 4e- → 2H2O(l)  Eo = 1.23 V 

What is E, if the pressure of oxygen gas is 1.0 atmosphere and the pH is 6.03?
USE 2 DECIMAL PLACES. (Hint, are solids and pure liquids included in Q? Using LeChatelier's principle is E going to be larger or smaller?)

Correct

 

Explanation

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Question 9

The activation energy for a particular reaction is 32 kJ/mol. By what factor will the rate constant increase when the temperature is increased from 50 oC to 67 oC?

Correct

 

Explanation

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Question 10

What is Kc for the reaction:2NO2(g) → N2O4(g)?

Correct

 

Explanation

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Question 11

Mg2C2O4(s) → 2Mg2+(aq) +C2O42-(aq) Ksp = 8.6 x 10-5

What is Qsp if 100 mL of 0.062 M Mg(N03)2 is added to 50 mL of 0.030 M Na2C2O4?

Correct

 

Explanation

Recall that if Qsp is greater than Ksp, a precipitate forms.

 

Question 12

Pb(OH)2(s) → Pb2+(aq) + 2OH-(aq) Ksp = 1.4 x 10-20

What is the solubility of lead hydroxide in pure water? (Simplify by neglecting the OH- from water.)

Correct

 

Explanation

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Question 13

2 L of an unknown concentration of the weak base ammonia are standardized. 50.0 mL of the basic solution are titrated with 0.200 M HCl. The end point occurs after 62.8 mL of acid are added. What is the concentration of ammonia in the 2 L flask?

Correct

 

Explanation

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Question 14

These experiments were preformed:

I. A copper wire was placed in a solution of AgNO3(aq). The copper dissolved and filments of silver metal grew on the remaining copper wire.
II. A copper wire was placed in a solution of Zn(NO3)2(aq). No reaction was detected.
III. An aluminum wire was placed in a solution of Zn(NO3)2(aq). The aluminum dissolved and Zn metal formed.

Rank the activity of these metal.

Cu Correct
Al Correct
Zn Correct
Ag Correct

 

Explanation

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Question 15

Match Correctly

AgNO3(aq) + NaOH(aq) → AgOH(s) + NaNO3(aq) Correct
Ag+(aq) + OH-(aq) → AgOH(s) Correct
NO3-(aq). Na+ Correct
AgNO3(aq), NaOH(aq) Correct

 

    

    

 

Question 16

Match the type of reaction

AgNO3(aq) + NaOH(aq) → AgOH(s) + NaNO3(aq) Correct
C3H7OH(l) + 4.5O2(g) → 3CO2(g) + 4H2O(g) Correct
H2SO4 + 2NaOH(aq) → H2O(l) + Na2SO4(aq) Correct
AgNO3(aq) + Cu(s) → Ag(s) + Cu(NO3)3(aq) Correct

 

    

    

 

Question 17

Which of the relationships is true for the transition from ice to water for an ice water mixture at 0 oC and 1 atm?

Select one:
 Correct

 

Explanation

At transition states ΔG=0

ice to water is endothermic so ΔH>0

 

Question 18

A particular chemical reaction has a positive ΔH and a positive ΔS. Which statement is correct?

Select one:
 Correct

 

Explanation

ΔG=  ΔH - TΔS

Kelvin is always positive

= (+) - (+)(+)

and in order for a reaction to be spontaneous or exergonic we need to ΔG to be negative

so temperature has so increase in order to have an overall negative equation

 

Question 19

For which of these processes is the value of ΔS expected to be negative?

Select one:
 Correct

 

Explanation

The side with more moles of gas has the greater entropy.

 

Question 20

In which process is entropy decreased?

Select one:
 Correct

 

Explanation

entropy decreases as there is more order and solids have more order than liquids

 

Question 21

For an endothemic process to be spontaneous which of the following conditions is correct?

Select one:
 Correct

 

Explanation

ΔG= ΔH - TΔS

(-)=(+)-TΔS

high T to make it overall negative and positive ΔS to keep it overall negative

 

Question 22

Which laboratory measurement is expressed to the appropriate number of significant figures?

Select one:
 Correct

 

Explanation

You always express one more sig fig than is clearly labeled

 

Question 23

Which statement is true for the following unbalanced reaction?

Cu1+(aq) + MnO4-(aq) → Cu2+(aq) + Mn2+(aq)

Select one:
 Correct

 

Explanation

Reduction is Losing (RIG) in terms of electrons

 

Question 24

Which change requires a reducing agent to produce the indicated product in an acidic solution? (Half-reactions not balanced.)

Select one:
 Correct

 

Explanation

A reducing agent allows something to be reduced, that is, it provides electrons.

 

Question 25

A student wanted to make compound D using the following reaction which takes place in water:

A(s) +O2(g) → D(s) + H+(aq)

The student used atomsperic air (0.21 atm oxygen) and 1.0 M HCl (the presence of the acid increases the rate of the reaction). Unfortunately, the equilibrium yield of D was too low to be useful. Which of the following changes will improve the yield of D?

Select one:
 Correct

 

Explanation

(This is easy if you remember LeChatelier's principle.)


Use less of the acid because in the presence of water it will fully dissociate and increase the H+ concentration which will actually push the reaction to the left which is the opposite of what is wanted

 

Question 26

Under certain conditions some substances, like MnO2, can act as an oxidizing agent; under other conditions, as a reducing agent. What is true for these substances?

Select one:
 Correct

 

    

    

 

Question 27



Standard reduction potentials,  Eo

Fe3+(aq) + e- → Fe2+(aq)   +0.77 V
Cu2+(aq) + e- → Cu1+(aq)   -0.15 V


Which of these ions is the best oxidizing agent?

Select one:
 Correct

 

Explanation

The best oxidizing agent is the one with the highest reduction potential

 

Question 28



Standard reduction potentials,  Eo

Cu2+(aq) + 2e- → Cu(s)   +0.34 V
Cu2+(aq) + e- → Cu1+(aq)   -0.15 V
Ni2+(aq) + 2e- → Ni(s)   -0.25 V



A quantity of a powdered mixture of copper and nickel is added to a solution containing Cu2+ and Ni2+ ions, each at unit activity (1 M). What reaction will occur?

Select one:
 Correct

 

Explanation

The reaction giving the largest voltage occurs.

 

Question 29



Standard reduction potentials,  Eo

Al3+(aq) + 3e- → Al(s)   -1.66 V
Ni2+(aq) + 2e- → Ni(s)   -0.25 V



What is the standard electrode potential for a voltaic cell constructed in the appropriate way from the above two half-cells?

Select one:
 Correct

 

Explanation

Voltage gives tendency for something to occur and is independent of the number of electrons (doubling the number of electrons, that is doubling each half-reaction and adding gives the same voltage). This is not true about energy or enthalpy. Twice as much material, doubles the energy produced or the enthalpy change.

 

Question 30

What is the correct procedure for diluting an acid?

Select one:
 Correct

 

Explanation

Acid is added to water to avoid splattering. More dense acid sinks as it is diluted and as heat is released. Less dense water stays on top, is less dispersed, and may end up boiling and causing splattering.

 

Question 31

Under certain conditions, the average rate of appearance of the metal oxide in the reaction 4M(s) +3O2(g) → 2M2O3((s)
is 0.20 mol/s. What is the average rate for the disappearance of O2? (Hint, for some of you it may be easier to start by writing the rate of reaction in terms of oxygen and the metal oxide.)

Select one:
 Correct

 

Explanation

rate of appearance of the metal oxide is 0.20

0.2 * 3 O2/2  M2O3 =  0.30 mol/s

 

Question 32

What is a correct description of chemical equilibrium?

Select one:
 Correct

 

    

    

 

Question 33

A gas-phase reaction, A + B +M → AB + M, proceeds by a rare trimolecular collision between A, B, and M where M is a second A or B which takes away excess energy. If the concentrations of both A and B are doubled, the reaction rate will change by a factor of? (Hint, when A goes from 1 to 2 and B goes from 1 to 2, what happens to M?)

Select one:
 Correct

 

Explanation

M doubles (goes from 2 to 4). Rate increases by 2x2x2.

 

Question 34

The value of an equilibrium constant can be used to predict each of these except the

Select one:
 Correct

 

    

    

 

Question 35

Consider the reaction: 2KClO3(s) → KClO4(s) + KCl(s) + O2(g)

What is the correct expression for Kc?

Select one:
 Correct

 

Explanation

Remember that solids and liquids should not be included in the equilibrium expression and that the proper format is products over reactants

 

Question 36

Consider the reaction: 2CO(g)+ O2(g) → 2CO2(g) 

What is the correct expression for Kp?

Select one:
 Correct

 

Explanation

Products over reactants raised to their coefficents

 

Question 37

For the reaction: 2NO2(g) → N2O4(g), ΔH = -57 kJ. Which reaction characteristics will be affected by a change in temperature?

1. Value of equilibrium constant  2. Equilibrium concentrations

Select one:
 Correct

 

    

    

 

Question 38

For the reaction: A +2B +2C → 2D +E, the following table gives the initial rates of reactions for various concentrations. What is the experimental rate law?

Select one:
 Correct

 

Explanation

Remember when trying to find the order of A, B, or C to if possible hold them constant so for A  use rows 1 and 2 because B and C both don't change their molarities and you can clearly see that (1/2)^n = (1/2) that n is equal to 1

 

Question 39

Consider this reaction, carried out at constant volume.

SO2(g) + O2(g) → 2SO3(g)   ΔH =-198 kJ

The concentration of SO2(g) at equilibrium increases if

Select one:
 Correct

 

Explanation

LeChatelier's Principle (easy). The ACS Guide had many questions of this type.


In order to increase the concentration of SO2 you can lessen O2 or add more 2SO3 or even add heat


And remember adding an inert gas is useless

 

Question 40

The reversible reaction:2H2(g) + CO(g) → CH3OH(g) + heat

is carried out by mixing carbon monoxide and hydrogen gases in a closed vessel under high pressure with a small amount of a suitable catalyst. After equilibrium is established at high temperature, all three substances are present. Which of the following will increase the yield of product?

Select one:
 Correct

 

Explanation

LeChatelier's Principle (medium hard).


Increase H2

Increase CO 

Decrease Heat

Catalysts only increase the rate but not the yield

 

Question 41

Consider the reaction: AB2 → A + 2B
What is the equilibrium constant expression if the initial concentration of AB2 is 1.0 M, the initial product concentrations are 0, and the equilibrium concentration of A is represented by x?

Select one:
 Correct

 

Explanation

A=X

B=2x

AB2=1-x

products/reactants

(x)(2x)^2/(1-x)

 

Question 42

What is the intermediate, if any, for the following proposed mechanism?

Select one:
 Correct

 

Explanation

An intermediate is a compound that shows up in the middle of the reaction but not in the end or the beginning (aka is not a reactant or a product) in this case the intermediate is  N2O2 

 

Question 43

What is the rate law for the above proposed mechanism? (Remember, intermediates are not included in the final rate law.)

Select one:
 Correct

 

Explanation

2NO + Cl2  ------> 2 NOCl 


rate = k [NO]^2[Cl2]

 

Question 44

Which of the following has an effect on the rate constant?

Select one:
 Correct

 

Explanation

The above change the rate, but not the rate constant. The rate constant is independent of amounts of reactant.

 

Question 45

Two reactions with different activation energies have the same rate at 1000 oC. Whic statement correctly describes the rates of these two reactions at the 950 oC?

Select one:
 Correct

 

Explanation

The larger the Ea the faster the rate constant increases as T increases. Conversely, the larger the Ea the faster it decreases as T decreases.

 

Question 46

The reaction rate is k[Ce4+][Mn2+] for the following reaction: 

2Ce4+(aq) + Tl+(aq) + Mn2+(aq) → 2Ce3+(aq) + Tl3+(aq) + Mn2+(aq

What is the catalyst?

Select one:
 Correct

 

Explanation

M doubles (goes from 2 to 4). Rate increases by 2x2x2.

 

Question 47

What is the overall reaction order for the above reaction?

Select one:
 Correct

 

Explanation

M doubles (goes from 2 to 4). Rate increases by 2x2x2.

 

Question 48

What is activation energy for the following reaction? Is it exothermic or endothermic?

Select one or more:
 Correct
 Correct

 

Explanation

    

 

Question 49

For the reaction: A + 2B → C if the concentration of A is doubled the rate increases by a factor of 4. If the concentration of both A and B are doubled, the rate increases by a factor of 4.

Select one:
 Correct

 

Explanation

M doubles (goes from 2 to 4). Rate increases by 2x2x2.

 

Question 50

A plot of the log of the reactant concentration as a function of time gives a straight line. What is the order of the reaction for this reactant?

Select one:
 Correct

 

Explanation

Plotting using log or ln both give a straight line (different slope).

 

Question 51

Which molecule has the greatest entropy?

Select one:
 Correct

 

Explanation

The molecule with the highest PE or the least stability will have the greatest entropy typically this is indicated by the molecule size


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