At which electrode of a galvanic cell does oxidation take place?
Anode
✓
Cathode
Salt bridge
External wire
Correct Answer
Anode
Oxidation always occurs at the anode, where the species being oxidized releases electrons into the external circuit. The cathode is where reduction occurs. The salt bridge carries ions, not electrons, and is not an electrode. The external wire only conducts electrons between the electrodes.
Category: Galvanic Cells
Question 2
What is the role of the salt bridge that connects the two half-cells of a galvanic cell?
It carries electrons from the anode to the cathode
It lets ions migrate so each half-cell stays electrically neutral
✓
It supplies the reactants consumed at the electrodes
It catalyzes the transfer of electrons between metals
Correct Answer
It lets ions migrate so each half-cell stays electrically neutral
As the anode produces cations and the cathode consumes them, charge would build up and stop the reaction; ions moving through the salt bridge prevent this. It carries electrons from the anode to the cathode is wrong because electrons travel through the external wire. It supplies the reactants consumed at the electrodes is wrong because the electrolyte in the bridge is chosen to be unreactive. It catalyzes the transfer of electrons between metals is wrong because the bridge has no catalytic role.
Category: Galvanic Cells
Question 3
By convention, what standard reduction potential is assigned to the standard hydrogen electrode?
1.00 V
+0.76 V
0.00 V
✓
-0.34 V
Correct Answer
0.00 V
The standard hydrogen electrode is the reference against which all other half-cell potentials are measured, so its potential is defined as exactly 0 V at all temperatures. A value of 1.00 V is not used for any reference electrode. A value of +0.76 V is the magnitude of the zinc half-cell potential measured against hydrogen. A value of -0.34 V is the copper half-cell potential with the wrong sign.
Category: Cell Potentials
Question 4
A galvanic cell uses the reaction Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s). The standard reduction potentials are +0.34 V for Cu2+/Cu and -0.76 V for Zn2+/Zn. What is the standard cell potential?
-1.10 V
+0.42 V
-0.42 V
+1.10 V
✓
Correct Answer
+1.10 V
The standard cell potential equals E cathode minus E anode, and copper is reduced at the cathode while zinc is oxidized at the anode: 0.34 V - (-0.76 V) = +1.10 V. The value -1.10 V results from subtracting in the wrong order. The value -0.42 V results from adding the two potentials as written, 0.34 V + (-0.76 V). The value +0.42 V results from flipping the sign of the zinc potential and then subtracting the copper potential, 0.76 V - 0.34 V.
Category: Cell Potentials
Question 5
A cell reaction transfers 2 moles of electrons and has a standard cell potential of +1.50 V. Using F = 96,485 C/mol, what is the standard free energy change, rounded to three significant figures?
-289 kJ
✓
+289 kJ
-145 kJ
-579 kJ
Correct Answer
-289 kJ
The relationship is delta G = -nFE, so delta G = -(2)(96,485 C/mol)(1.50 V) = -289,455 J, which rounds to -289 kJ; the negative sign shows a spontaneous reaction. The value +289 kJ omits the negative sign in the equation. The value -145 kJ uses 1 mole of electrons instead of 2. The value -579 kJ uses 4 moles of electrons instead of 2.
Category: Free Energy and Equilibrium
Question 6
A redox reaction at 25 degrees C transfers 2 moles of electrons and has a standard cell potential of +0.0592 V. Using E = (0.0592 V/n) log K, what is the equilibrium constant K?
10
100
✓
2
0.01
Correct Answer
100
Rearranging gives log K = nE/0.0592 V = (2)(0.0592)/0.0592 = 2, so K = 10^2 = 100. The value 10 results from using n = 1. The value 2 is log K, not K itself. The value 0.01 results from using a negative exponent, which would apply only to a negative cell potential.
Category: Free Energy and Equilibrium
Question 7
A redox reaction has a positive standard cell potential. Which pair of values describes its standard free energy change and equilibrium constant?
Positive delta G and K greater than 1
Negative delta G and K less than 1
Negative delta G and K greater than 1
✓
Positive delta G and K less than 1
Correct Answer
Negative delta G and K greater than 1
Because delta G = -nFE, a positive cell potential gives a negative standard free energy change, and a negative delta G corresponds to K greater than 1, so products are favored at equilibrium. Positive delta G and K greater than 1 is inconsistent because a positive delta G means K is less than 1. Negative delta G and K less than 1 is inconsistent for the same reason in reverse. Positive delta G and K less than 1 describes a reaction with a negative cell potential.
Category: Free Energy and Equilibrium
Question 8
Given the standard reduction potentials F2/F- +2.87 V, Cl2/Cl- +1.36 V, Ag+/Ag +0.80 V and Cu2+/Cu +0.34 V, which species is the strongest oxidizing agent?
Cl2
Ag+
Cu2+
F2
✓
Correct Answer
F2
An oxidizing agent is itself reduced, so the species with the most positive reduction potential, F2 at +2.87 V, has the greatest tendency to gain electrons. Cl2 is a strong oxidizing agent but its potential is lower at +1.36 V. Ag+ is weaker still at +0.80 V. Cu2+ is the weakest of the four at +0.34 V.
Category: Cell Potentials
Question 9
What is the oxidation number of manganese in the permanganate ion, MnO4-?
+7
✓
+6
+4
+2
Correct Answer
+7
Each oxygen is assigned -2, giving -8 for four oxygens, and the sum of oxidation numbers must equal the ion's charge of -1, so manganese is +7. A value of +6 would make the overall charge -2. A value of +4 is the oxidation number of manganese in MnO2. A value of +2 is the oxidation number of manganese in the Mn2+ ion.
Category: Redox Balancing
Question 10
When a half-reaction in acidic solution is balanced by the half-reaction method, which species is added to balance oxygen atoms?
H+
H2O
✓
OH-
O2
Correct Answer
H2O
Oxygen atoms are balanced by adding water molecules to the side that needs oxygen, and hydrogen is then balanced with H+. H+ is added to balance hydrogen, not oxygen. OH- is used only when the reaction occurs in basic solution. O2 cannot be added because it is not part of the reaction being balanced.
Category: Redox Balancing
Question 11
In the cell notation Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s), what does the double vertical line represent?
A phase boundary
An inert electrode
A salt bridge
✓
The external wire
Correct Answer
A salt bridge
Cell notation lists the anode on the left and the cathode on the right, with a double line marking the salt bridge or porous barrier between the two half-cells. A phase boundary is shown by a single vertical line, such as the one between solid zinc and its ions. An inert electrode would appear as its own symbol, such as Pt(s), within its half-cell. The external wire is not shown in cell notation at all.
Category: Galvanic Cells
Question 12
Given the standard reduction potentials Zn2+/Zn -0.76 V, Cu2+/Cu +0.34 V, Ag+/Ag +0.80 V and Au3+/Au +1.50 V, which metal will spontaneously reduce Cu2+ ions to copper metal under standard conditions?
Silver
Gold
Copper
Zinc
✓
Correct Answer
Zinc
A metal can reduce Cu2+ only if its own reduction potential is lower than that of copper, which makes the overall cell potential positive; zinc at -0.76 V meets this. Silver has a higher reduction potential than copper, so it is not oxidized by Cu2+. Gold has an even higher potential and is harder still to oxidize. Copper cannot reduce its own ion because the cell potential would be zero.
Category: Cell Potentials
Question 13
For the cell reaction Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s), how does the cell potential change if the Cu2+ concentration is raised while the Zn2+ concentration stays the same?
The cell potential increases
✓
The cell potential decreases
The cell potential stays the same
The cell potential drops to zero
Correct Answer
The cell potential increases
In the Nernst equation, Q = [Zn2+]/[Cu2+], so raising the reactant Cu2+ lowers Q, and a smaller Q makes the cell potential larger. The cell potential decreases would require raising the product concentration instead. The cell potential stays the same ignores the concentration dependence that the Nernst equation describes. The cell potential drops to zero happens only when the reaction reaches equilibrium.
Category: Free Energy and Equilibrium
Question 14
What is the cell potential of a galvanic cell whose reaction has reached equilibrium?
The standard cell potential
Zero volts
✓
0.0592/n volts
The negative of the standard potential
Correct Answer
Zero volts
At equilibrium Q equals K and delta G is zero, so there is no driving force and the cell potential is zero, which is why a dead battery delivers no current. The standard cell potential is the value only when every species is in its standard state. The value 0.0592/n volts is only the coefficient in the Nernst equation. The negative of the standard potential has no special meaning for a cell at equilibrium.
Category: Free Energy and Equilibrium
Question 15
Which rechargeable battery is built from a lead electrode and a lead dioxide electrode immersed in dilute sulfuric acid?
Nickel-cadmium battery
Alkaline battery
Lead acid battery
✓
Lithium-ion battery
Correct Answer
Lead acid battery
In a lead acid battery, lead is oxidized and lead dioxide is reduced, and both form lead(II) sulfate during discharge; six cells of about 2 V each make a 12 V car battery. A nickel-cadmium battery uses cadmium and nickel oxide electrodes in potassium hydroxide. An alkaline battery uses zinc and manganese dioxide in potassium hydroxide. A lithium-ion battery moves lithium ions between a graphite electrode and a lithium cobalt oxide electrode.
Category: Batteries and Fuel Cells
Question 16
What distinguishes a fuel cell from an ordinary battery?
It runs a nonspontaneous reaction using external power
It stores all of its reactants inside a sealed case
It produces electricity without any redox reaction
Its reactants are supplied continuously from outside the cell
✓
Correct Answer
Its reactants are supplied continuously from outside the cell
A fuel cell is a galvanic cell that keeps producing current as long as fuel, such as hydrogen, and oxygen are fed into it. It runs a nonspontaneous reaction using external power describes an electrolytic cell. It stores all of its reactants inside a sealed case describes an ordinary battery. It produces electricity without any redox reaction is wrong because a fuel cell oxidizes its fuel and reduces oxygen.
Category: Batteries and Fuel Cells
Question 17
Which metal, wired to an underground iron pipe, would protect the pipe by corroding in its place?
Magnesium
✓
Copper
Silver
Lead
Correct Answer
Magnesium
Cathodic protection uses a metal that is more easily oxidized than iron, with a lower reduction potential, so it becomes the anode and is slowly consumed while the iron acts only as the cathode. Magnesium fits because its reduction potential is far below that of iron. Copper has a higher reduction potential than iron and would speed the iron's corrosion. Silver is even harder to oxidize than copper. Lead also has a higher reduction potential than iron, so it cannot protect it.
Category: Corrosion
Question 18
During the electrolysis of molten sodium chloride, what product forms at the cathode?
Chlorine gas
Sodium metal
✓
Hydrogen gas
Oxygen gas
Correct Answer
Sodium metal
In the molten salt, Na+ ions gain electrons at the cathode to form liquid sodium metal, which is how sodium is produced commercially. Chlorine gas forms at the anode, where Cl- ions are oxidized. Hydrogen gas cannot form because no water is present in the melt. Oxygen gas cannot form for the same reason.
Category: Electrolysis
Question 19
A current of 3.00 A is passed through a Cu2+ solution for 965 s. Using F = 96,500 C/mol and a molar mass of 63.55 g/mol for copper, what mass of copper is deposited, to three significant figures?
1.91 g
0.477 g
0.953 g
✓
0.0150 g
Correct Answer
0.953 g
The charge is Q = It = (3.00 A)(965 s) = 2895 C, which is 2895/96,500 = 0.0300 mol of electrons; each Cu2+ needs 2 electrons, giving 0.0150 mol Cu, or 0.953 g. The value 1.91 g treats each copper ion as needing only 1 electron. The value 0.477 g divides the moles of electrons by 4 instead of 2. The value 0.0150 g reports the moles of copper as if they were grams.
Category: Electrolysis
Question 20
When an aqueous solution of NaCl is electrolyzed, which substance is produced by reduction at the negative electrode?
Sodium metal
Chlorine gas
Oxygen gas
Hydrogen gas
✓
Correct Answer
Hydrogen gas
Water is much easier to reduce than Na+ ions, so at the cathode water gains electrons to form hydrogen gas and hydroxide ions, the basis of the chlor-alkali process. Sodium metal does not form because Na+ has a far more negative reduction potential than water. Chlorine gas forms at the anode, not the cathode. Oxygen gas, when it forms, also appears at the anode because it comes from oxidation.
Category: Electrolysis