Electrochem practice AP questions

S°=1/2 [187+(2×257)−205]J/(mol⋅K)

2 SO2 (g) + O2 (g)→2 SO3 (g) ΔS°=−187 J/(molrxn⋅K) SubstanceApproximate S°(J/(mol⋅K))SO2(g)?O2(g)205SO3(g)257 The reaction between SO2 and O2 is represented by the chemical equation above. The table provides the approximate absolute entropies, S°, for O2(g) and SO3(g). Which of the following mathematical expressions can be used to correctly calculate S° for SO2(g) ?

Only PbSO4(s) will precipitate.

A 1.0 L solution AgNO3(ag) of and Pb(NO3)2(aq) has a Ag+ concentration of 0.020 M and a Pb2+ concentration of 0.0010 M. A 0.0010 mol sample of K2SO4(s) is added to the solution. Based on the information in the table above, which of the following will occur? (Assume that the volume change of the solution is negligible.)

Energy Entropy Remains constant Increases

A cube of ice is added to some hot water in a rigid, insulated container, which is then sealed. There is no heat exchange with the surroundings. What has happened to the total energy and the total entropy when the system reaches equilibrium?

[Ag+]>1M and [Mg2+]=1M in the second cell, resulting in Q<1, and Q=1 for the first cell.

Ag+(aq)+e−→Ag(s)+0.80Mg2+(aq)+2e−→Mg(s)−2.37 The galvanic cell shown above generates a cell potential of +3.17V when operated under standard conditions. A second galvanic cell is made from the same two metals, and the measured cell potential is +3.25V. Which of the following could be the reason for the second cell having a greater cell potential?

The cell potential decreases because the reduction of Pb2+ is less thermodynamically favorable than the reduction of Ag+.

Ag+(aq)+e−→Ag(s)+0.80Pb2+(aq)+2e−→Pb(s)−0.13Zn2+(aq)+2e−→Zn(s)−0.76 The cell potential for the standard galvanic cell shown above is +1.56V. If AgNO3(aq)|Ag(s) is replaced with 1M Pb(NO3)2(aq) solution and a Pb electrode, which of the following describes what happens to the operation of the cell, and why?

The value obtained for Ksp would be too small because less AgCl(s) would dissolve because of the common ion effect due to the Cl−(aq) already in the water.

AgCl(s) ⇄ Ag+(aq)+Cl−(aq) Ksp=1.8×10−10 Shown above is information about the dissolution of AgCl(s) in water at 298K. In a chemistry lab a student wants to determine the value of s, the molar solubility of AgCl, by measuring [Ag+] in a saturated solution prepared by mixing excess AgCl and distilled water. How would the results of the experiment be altered if the student mixed excess AgCl with tap water (in which [Cl−]=0.010M) instead of distilled water and the student did not account for the Cl− in the tap water?

2

Al (s) → Al3+(aq)+3 e− Zn2+(aq) + 2 e−→Zn(s) The half-reactions for the oxidation-reduction reaction between Al(s) and Zn2+(aq) are represented above. Based on the half-reactions, what is the coefficient for Al(s) if the equation for the oxidation-reduction reaction is balanced with the smallest whole-number coefficients?

+2.46 V

Al3+(aq) + 3 e- → Al(s) E° = -1.66 V Ag+(aq) + e- → Ag(s) E° = +0.80 V According to the standard reduction potentials given above, what is the standard cell potential for the reaction represented below? 3 Ag+(aq) + Al(s) → 3 Ag(s) + Al3+(aq)

275 years

An archaeologist graduate student found a leg bone of a large animal during the building of a new science building. The bone had a carbon-14 decay rate of 14.8 disintegrations per minute per gram of carbon. Living organisms have a decay rate of 15.3 disintegrations per minute. How old is the bone? The half-life of C-14 is 5730 years.

193,000 sec

An electric current of 1.00 ampere is passed through an aqueous solution of Ni(NO3)2. How long will it take to plate out exactly 1.00 mol of nickel metal, assuming 100 percent current efficiency? (1 faraday = 96,500 coulombs = 6.02 X 1023 electrons)

2.32x109 yr

An igneous rock contains a Pb-206/U-238 mass ratio of 0.372. How old is the rock? (U-238 decays into Pb-206 with a half-life of 4.5x109 yr.)

83

Atoms with Z > ________ are radioactive and decay in one or more steps involving mostly alpha and beta decay.

4S3=4×10−9 M

Ba(IO3)2(s) ⇄ Ba2+(aq) + 2 IO3−(aq) Ksp=4×10−9 According to the information about the dissolution of Ba(IO3)2(s) shown above, the correct value of S, the molar solubility of Ba(IO3)2(s), can be calculated using with of the following mathematical relationships?

PbCl2

Based on the Ksp values in the table above, a saturated solution of which of the following compounds has the highest [CI-]?

14N

Beta decay of 14C produces a beta particle and

It must be positive, since ∆G° is negative and ∆H° is positive.

CH3OH->CO+2H2 The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K What can be inferred about ∆S° for the reaction at 600 K?

Adding NaF(s), because the reaction will proceed toward reactants

CaF2(s) ⇄ Ca2+(aq) + 2 F-(aq) ΔH>0 Dissolution of the slightly soluble salt CaF2 is shown by the equation above. Which of the following changes will decrease [Ca2+] in a saturated solution of CaF2 , and why? (Assume that after each change some CaF2(s) remains in contact with the solution.)

0.025M

CaF2(s)⇄Ca2+(aq)+2 F−(aq) Ksp=4.0×10−11 The concentration of F−(aq) in drinking water that is considered to be ideal for promoting dental health is 4.0×10−5M. Based on the information above, the maximum concentration of Ca2+(aq) that can be present in drinking water without lowering the concentration of F−(aq) below the ideal level is closest to

-24.1 kJ

Calculate the change in internal energy (ΔE) for a system that is giving off 25.0 kJ of heat and is changing from 12.00 L to 6.00 L in volume at 1.50 atm pressure. (Remember that 101.3 J = 1 L∙atm)

0.812 amu

Calculate the mass defect in Mo-96 if the mass of a Mo-96 nucleus is 95.962 amu. The mass of a proton is 1.00728 amu and the mass of a neutron is 1.008665 amu.

7.83 %

Carbon-11 is used in medical imaging. The half-life of this radioisotope is 20.4 min. What percentage of a sample remains after 75.0 min?

The molar solubility of CdF2 in pure water is 0.0585M, and adding NaF decreases this solubility because the equilibrium shifts to favor the precipitation of some CdF2.

CdF2(s)⇄Cd2+(aq)+2F−(aq) A saturated aqueous solution of CdF2 is prepared. The equilibrium in the solution is represented above. In the solution, [Cd2+]eq=0.0585M and [F−]eq=0.117M. Some 0.90MNaF is added to the saturated solution. Which of the following identifies the molar solubility of CdF2 in pure water and explains the effect that the addition of NaF has on this solubility?

E˚ is positive and ∆G˚ is negative.

Cu(s) + 2 Ag+ → Cu2+ + 2 Ag(s) If the equilibrium constant for the reaction above is 3.7 x 1015, which of the following correctly describes the standard voltage, E˚, and the standard free energy change, ∆G˚, for this reaction?

NOT: Oxidation Half-reactionReduction Half-reactionCu(s)→Cu+(aq)+e−Ag2+(aq)+2e−→Ag(s)

Cu(s) + 2 AgNO3(aq)→ Cu(NO3)2(aq) + 2 Ag(s) The reaction between solid copper and aqueous silver nitrate produces solid silver and a blue solution, as represented by the balanced equation shown above. Based on the balanced equation, which of the following identifies the oxidation and reduction half-reactions?

The initial E° for the second cell is the same as for the first cell, because the overall chemical reaction that occurs in the cell does not change.

Cu2+(aq)+Zn(s)→Cu(s)+Zn2+(aq) E°=+1.10V The galvanic cell illustrated above generates a potential of +1.10V. For the construction of a second galvanic cell (not shown), only one modification was made: the Cu electrode has double the mass of the Cu electrode in the first cell. Which of the following correctly compares the initial E° for the second cell to that of first cell at 298K, and why?

The presence of additional Br− ions already in the solution means equilibrium will be reached when much less CuBr has dissolved.

CuBr(s)⇄Cu+(aq)+Br−(aq) Shown above is the chemical equation for the dissolution of the slightly soluble salt CuBr(s). Its Ksp value in pure water was experimentally determined. CuBr was found to be much less soluble in a 0.001MNaBr solution than in pure water. Which of the following correctly explains the decrease in solubility of CuBr in 0.001 M NaBr?

The mass number and atomic number decrease.

Describe what changes occur during alpha decay.

The mass number is unchanged and the atomic number decreases.

Describe what changes occur during electron capture.

The mass number and atomic number do not change.

Describe what changes occur during gamma ray emission.

The mass number is unchanged and the atomic number decreases.

Describe what changes occur during positron emission.

chemical energy

Energy that is associated with the relative positions of electrons and nuclei in atoms and molecules is called

The added KSCN(s) dissolves, causing the reaction system to respond by producing more product to partially consume SCN−(aq) and reduce its concentration.

Fe3+(aq) + SCN−(aq) ⇄ FeSCN2+(aq) Colorless Colorless Red When colorless solutions containing Fe3+(aq) ions and SCN−(aq) ions are combined, a deep-red complex ion, FeSCN2+(aq) quickly forms, as shown in the net ionic equation above. Which of the following explains the observation that adding a few additional crystals of KSCN(s) results in the red color of the solution becoming deeper?

1.74 × 102 minutes

Fluorine-18 undergoes positron emission with a half-life of 1.10 × 102 minutes. If a patient is given a 248 mg dose for a PET scan, how long will it take for the amount of fluorine-18 to drop to 83 mg? (Assume that none of the fluorine is excreted from the body.)

II and III only

For which of the following processes would ΔS have a negative value? I. 2 Fe2O3(s) → 4 Fe(s) + 3 O2(g) II. Mg2+ + 2 OH- → Mg(OH)2(s) III. H2(g) + C2H4(g) → C2H6(g)

Because ΔG°>>0 , Ka<<1 , and HA only partially dissociates.

HA(aq)+H2O(l)⇄A−(aq)+H3O+(aq)ΔG°=+35 kJ/molrxn Based on the chemical equation and ΔG° given above, which of the following justifies the claim that HA(aq) is a weak acid?

2Sc(s) + 3Mn2+(aq)⇄ 2 Sc3+(aq)+3Mn(s) E°=+0.90V

Half-Reaction E°(V)Mn2+(aq)+2e−→Mn(s) −1.18Sc3+(aq)+3e−→Sc(s) −2.08 Based on the reduction potentials given in the table above, which of the following gives the balanced chemical equation and the correct standard cell potential for a galvanic cell involving Sc3+(aq) and Mn2+(aq) ?

The cell potential decreases because the reduction of Pb2+ is less thermodynamically favorable than the reduction of Ag+.

Half-Reaction Standard Reduction Potential, E°(V)Ag+(aq)+e−→Ag(s)+0.80Pb2+(aq)+2e−→Pb(s)−0.13Zn2+(aq)+2e−→Zn(s)−0.76 The cell potential for the standard galvanic cell shown above is +1.56V. If AgNO3(aq)|Ag(s) is replaced with 1M Pb(NO3)2(aq) solution and a Pb electrode, which of the following describes what happens to the operation of the cell, and why?

0.20 mole

How many moles of NaF must be dissolved in 1.00 liter of a saturated solution of PbF2 at 25˚C to reduce the [Pb2+] to 1 x 10-6 molar? (Ksp PbF2 at 25˚C = 4.0 x 10-8)

The voltage will increase in cell Y and decrease in cell Z.

If the half-cell containing 1.00 M Fe(NO3)2(aq) in galvanic cells Y and Z is replaced with a half-cell containing 5.00 M Fe(NO3)2(aq), what will be the effect on the cell voltage of the two galvanic cells?

4.0 x 10-18

In a saturated solution of Zn(OH)2 at 25°C, the value of [OH-] is 2.0 x 10-6 M. What is the value of the solubility-product constant, Ksp, for Zn(OH)2 at 25°C ?

5.87 g

In the electroplating of nickel, 0.200 faradays of electrical charge is passed through a solution of NiSO4. What mass of nickel is deposited?

Cl2(g)→Cl2(l)

In which of the following processes will ΔS° be negative?

-1.66 V

M(s) + 3 Ag+(aq) → 3 Ag(s) + M3+(aq) E° = + 2.46 V Ag+(aq) + e- → Ag(s) E° = + 0.80 V According to the information above, what is the standard reduction potential for the half-reaction M3+(aq) + 3 e- → M(s) ?

Decreasing the pH

Mg(OH)2(s) ⇄ Mg2+(aq) + 2 OH- (aq) The exothermic dissolution of Mg(OH)2(s) in water is represented by the equation above. The Ksp of Mg(OH)2 is 1.8 x 10-11. Which of the following changes will increase the solubility of Mg(OH)2 in an aqueous solution?

The molar solubility of MgCO3 increases with increasing acidity (lower pH ).

Moles of HCl addedMolar solubility of MgCO302.6×10−32.7×10−43.3×10−34.6×10−45.5×10−36.0×10−46.9×10−3 MgCO3(s)⇄Mg2+(aq)+CO32−(aq) A saturated solution of MgCO3 at equilibrium is represented by the equation above. Four different saturated solutions were prepared and kept at the same temperature. A given amount of HCl was added to each solution and data were collected to calculate the molar solubility of MgCO3 as shown in the table above. Which of the following can be concluded from the data?

ΔG°=−RTlnK<0 because K>>1

N2 (g)+3 H2(g) ⇄ 2 NH3 (g) K=5.6×105at298K ΔH°rxn = −91.8kJ/molrxn The synthesis of NH3 is represented by the equation above. Based on the equilibrium constant, K, and ΔH°rxn given above, which of the following can best be used to justify that the reaction is thermodynamically favorable at 298K and constant pressure?

31.2 days

Neptunium-239 has a half-life of 2.35 days. How many days must elapse for a sample of 239Np to decay to 0.0100% of its original quantity?

2Ag+(aq)+Ni(s)→2Ag(s)+Ni2+(aq)

Ni(s)→Ni2+(aq)+2e− Ag+(aq)+e−→Ag(s) Which of the following is the balanced net ionic equation for an oxidation-reduction reaction between Ag+(aq) and Ni(s) based on the half-reactions represented above?

4 La(s) + 3 O2(g) → 2 La2O3(s)

Of the following reactions, which involves the largest decrease in entropy?

Ca(OH)2 will not precipitate because Q < Ksp .

Reaction 1: CaC2(s) + 2 H2O(l) → C2H2(g) + Ca(OH)2(s) Reaction 2: NaOCl(aq) + 2 HCl(aq) → Cl2(g) + NaCl(aq) + H2O(l) Reaction 3: C2H2(g) + Cl2(g) → C2H2Cl2(g) Ca(OH)2(s) precipitates when a 1.0 g sample of CaC2(s) is added to 1.0 L of distilled water at room temperature. If a 0.064 g sample of CaC2(s) (molar mass 64 g/mol) is used instead and all of it reacts, which of the following will occur and why? (The value of Ksp for Ca(OH)2 is 8.0 x 10-8.) Ca(OH)2 will precipitate because Q < Ksp .

In galvanic cells Y and Z, which of the following takes place in half-cell 3 ?

Reduction occurs in cell Y, and oxidation occurs in cell Z.

At 25°C, ∆H° for the reaction is negative.

Sr2+(aq) + F-(aq) ⇄ SrF+(aq) At 25°C, the equilibrium constant for the reaction represented above has a value of 1.3. At 50°C, the value of the equilibrium constant is less than 1.3. Based on this information, which of the following must be correct?

The number of neutrons equals the number of protons.

Stable isotopes, with low atomic numbers, have a N/Z ratio of 1. What does that imply?

1.56 V

Standard reduction potentials for the half-reactions associated with the electrochemical cell shown above are given in the table below. Half-ReactionE°(V)Ag+(aq)+e−→Ag(s)+0.80Zn2+(aq)+2e−→Zn(s)−0.76 What is the value of E° for the cell?

8.92 × 103 years

The age of an ancient tree trunk is estimated using radiocarbon dating. If the trunk has a C-14 decay rate that is 34% of what it is in living plants, how old is the trunk? The half-life of C-14 is 5730 years.

nuclear fusion

The combination of two light nuclei to form a heavier nuclei is called

ΔH°ΔS°ΔG°---

The combustion of water is 2 H2(g) + O2(g) → 2 H2O(g) For the reaction represented above at 25°C, what are the signs of ΔH°, ΔS°, and ΔG°?

ΔS°rxn is negative because the number of molecules in the gas phase decreases as the reaction proceeds.

The diagram above represents the gas-phase reaction of NO2(g) to form N2O4(g) at a certain temperature. Based on the diagram, which of the following best predicts and explains the sign of the entropy change for the reaction, ΔS°rxn?

0.733 mg

The half-life of cobalt-60 is 5.20 yr. How many milligrams of a 2.000-mg sample remain after 7.55 years?

271 g

The nuclide As-76 has a half-life of 26.0 hours. If a sample of As-76 weighs 344 g, what mass of As-76 remains after 538 minutes?

It must be positive, since ∆G° is negative and ∆H° is positive.

The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K What can be inferred about ∆S° for the reaction at 600 K?

3.6 x 10-4

The solubility of silver sulfate (Ag2SO4) in water at 100oC is approximately 1.4 g per 100 mL. What is the solubility product of this salt at 100oC?

Ksp =[Sn2+][OH-]2

The solubility product expression for tin(II) hydroxide, Sn(OH)2, is

nuclear fission

The splitting of a heavy nucleus to form two or more lighter ones is called

E°cell = +0.02 V

The standard reduction potentials for the half-reactions related to the galvanic cell represented above are listed in the table below. Which of the following gives the value of E°cell for the cell?

-1.92 × 103 kJ/mole

The temperature rises from 25.00°C to 29.00°C in a bomb calorimeter when 3.50 g of sucrose undergoes combustion in a bomb calorimeter. Calculate ΔErxn for the combustion of sucrose in kJ/mol sucrose. The heat capacity of the calorimeter is 4.90 kJ/°C. The molar mass of sugar is 342.3 g/mol.

4.0 × 10-2 M

The value of Ksp for PbCl2 is 1.6 ×10-5. What is the lowest concentration of Cl-(aq) that would be needed to begin precipitation of PbCl2(s) in 0.010 M Pb(NO3)2 ?

Z < X < Y

Three saturated solutions (X, Y, and Z) are prepared at 25°C. Based on the information in the table above, which of the following lists the solutions in order of increasing [Ag+] ?

The cell would not operate because a current could not be conducted between the half-cells.

To construct the galvanic cell illustrated above, the salt bridge was prepared by soaking a piece of cotton in 5.0M NaNO3(aq) before placing it inside the U-shaped tube filled with distilled water. If the cotton was soaked in distilled water by mistake, which of the following best explains how the operation of the cell would be affected?

2-

What is the ion charge on a anion of hexachloroplatinate (IV)?

iron

What metal is in a complex ion in the blood that carries oxygen?

12.5%

What percentage of material will remain radioactive after 3 half-lives?

The standard reduction potential, E°, for Pb2+(aq) is greater than that for Mg2+(aq).

When a magnesium wire is dipped into a solution of lead (II) nitrate, a black deposit forms on the wire. Which of the following can be concluded from this observation?

ΔHΔSPositivePositive

When solid ammonium chloride, NH4Cl(s), is added to water at 25oC, it dissolves and the temperature of the solution decreases. Which of the following is true for the values of ΔH and ΔS for the dissolving process?

It is thermodynamically favorable and is driven by ΔH° only.

When water is added to a mixture of Na2O2(s) and S (s), a redox reaction occurs, as represented by the equation below. Which of the following statements about the thermodynamic favorability of the reaction at 298 K is correct?

CaCO3(s) → CaO(s) + CO2(g)

Which of the following equations represents a reaction for which the standard entropy change is positive (ΔS° > 0) ?

clear in two lines -> 2 together

Which of the following particle diagrams represents a process during which the entropy of the system increases?

GAMMA

Which particle has no mass and no charge?