3 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 3
The diagram shows a galvanic cell; the arrows show the direction of electron flow in the wire. As the cell operates, which statement describes the movement of charged particles in the salt bridge?
Answer and reasoning
AK⁺ ions move toward the Zn half-cell, and NO₃⁻ ions move toward the Cu half-cell A student who thinks cations are drawn toward the zinc electrode by the electrons it releases picks this. Ion movement keeps each solution neutral: the Zn half-cell gains Zn²⁺, so anions (NO₃⁻) move in, and the Cu half-cell loses Cu²⁺, so cations (K⁺) move in.
BElectrons move through the salt bridge from the Cu half-cell to the Zn half-cell A student who thinks electrons complete a loop through the salt bridge picks this. Electrons travel only through the electrodes and the wire; in the salt bridge, charge is carried by K⁺ and NO₃⁻ ions.
CNO₃⁻ ions move toward the Zn half-cell, and K⁺ ions move toward the Cu half-cellCorrect Electrons leave the Zn electrode, so zinc is oxidized, adding Zn²⁺ ions to the left solution, and Cu²⁺ ions are removed from the right solution as copper is deposited. NO₃⁻ ions move toward the Zn half-cell and K⁺ ions toward the Cu half-cell to keep both solutions neutral.
DCu²⁺ ions move through the salt bridge from the Cu half-cell to the Zn half-cell A student who thinks the reacting ions must travel to the zinc picks this. Cu²⁺ ions are reduced at the Cu electrode in their own half-cell; electrons reach them through the wire, and the salt bridge carries K⁺ and NO₃⁻ ions.
The reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) is thermodynamically favored. In the cell shown, a power supply drives electrons through the external circuit in the directions of the arrows. Which statement describes what happens at the Zn electrode while the power supply operates?
Answer and reasoning
AZn²⁺ ions are reduced, so zinc metal builds up on the electrodeCorrect The arrows show electrons being pushed into the Zn electrode, so reduction occurs there: Zn²⁺(aq) + 2 e⁻ → Zn(s). The power supply drives the unfavored reverse of the galvanic reaction, and the Zn electrode gains mass.
BZn atoms are oxidized and leave the electrode as Zn²⁺ ions A student who thinks the favored reaction occurs whatever power supply is connected picks this. The power supply pushes electrons into the Zn electrode, so the half-reaction there is the reverse of the favored one: Zn²⁺ is reduced.
CZn²⁺ ions are reduced, so the electrode becomes smaller A student who links reduction with a decrease in size picks this. Reduction of Zn²⁺ deposits zinc metal on the electrode, so it grows and its mass increases.
DCu²⁺ ions that arrive through the salt bridge plate copper onto it A student who thinks Cu²⁺ ions travel through the salt bridge to the other electrode picks this. The salt bridge carries K⁺ and NO₃⁻; the ions reduced at the Zn electrode are the Zn²⁺ ions in its own solution.
Which statement about the electrodes of galvanic and electrolytic cells is correct?
Answer and reasoning
AOxidation occurs at the anode in galvanic cells and in electrolytic cellsCorrect For all electrochemical cells, oxidation occurs at the anode and reduction occurs at the cathode; the definitions do not change between galvanic and electrolytic cells.
BOxidation occurs at the anode in galvanic cells but at the cathode in electrolytic cells A student who thinks the electrode roles reverse in an electrolytic cell picks this. The reaction is reversed compared with a galvanic cell, but the anode is still where oxidation occurs.
CThe anode is the electrode that gains mass in both galvanic and electrolytic cells A student who links oxidation with gaining something picks this. Oxidation of a metal anode sends its atoms into solution as ions, so a metal anode loses mass.
DThe cathode is where cations are produced in galvanic cells and in electrolytic cells A student who links 'cathode' with 'cation' picks this. Reduction occurs at the cathode, where metal cations are usually consumed; cations of an oxidized metal are produced at the anode.
In preparation: 0 of 3 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
9.8.A.1 Electrode Fix
Electrode
A conductor (often a metal strip, or an inert conductor such as platinum or graphite) in contact with a half-cell solution, at whose surface a half-reaction occurs and through which electrons pass to or from the external circuit.
Half-cell
One electrode and the solution in contact with it; oxidation occurs in one half-cell and reduction in the other.
Salt bridge
A tube or strip containing a solution of an unreactive salt (such as KNO₃) that connects the half-cells. Its ions move to keep each half-cell's solution electrically neutral: anions toward the anode half-cell and cations toward the cathode half-cell. It carries ions, not electrons, and completes the circuit.
External circuit
The wire and measuring device (voltmeter or ammeter) or power supply connecting the electrodes. Electrons flow through it from the anode to the cathode.
Electrode mass change
A metal electrode that is oxidized loses mass as its atoms enter the solution as cations; an electrode on which metal cations are reduced gains mass as metal is deposited. The amounts are related by the mole ratio of the balanced cell reaction.
Students often think Cations in the salt bridge move toward the anode half-cell, attracted by the electrons that the anode releases into the wire, and anions move toward the cathode half-cell. In fact Toward the cathode half-cell. Reduction there removes cations (such as Cu²⁺) from the solution, so cations from the salt bridge move in, and anions move toward the anode half-cell, where oxidation adds cations to the solution.
Students often think Electrons pass through the salt bridge (and the solutions) from one half-cell to the other, completing a loop of electron flow. In fact No. Electrons travel only through the electrodes and the external circuit. In the salt bridge and the solutions, charge is carried by moving ions.
9.8.A.2 Galvanic (voltaic) cell Fix
Galvanic (voltaic) cell
An electrochemical cell in which a thermodynamically favored redox reaction occurs and drives electrons through the external circuit.
Electrolytic cell
An electrochemical cell in which an external power supply drives a thermodynamically unfavored redox reaction.
Students often think A cell built from two different metal electrodes joined by a salt bridge is a galvanic cell, whatever reaction occurs in it. In fact No. Whether a cell is galvanic or electrolytic depends on whether its reaction is thermodynamically favored or unfavored, not on its parts; the same two half-cells form an electrolytic cell when a power supply drives the unfavored reaction.
Students often think Electrons flowing through the external circuit show that a cell is galvanic, because only a galvanic cell produces a current. In fact No. Electrons flow through the external circuit in both galvanic and electrolytic cells; in an electrolytic cell they are pushed by the power supply.
9.8.A.3 Anode Fix
Anode
The electrode at which oxidation occurs, in galvanic and electrolytic cells alike. Electrons leave the cell through the anode into the external circuit.
Cathode
The electrode at which reduction occurs, in galvanic and electrolytic cells alike. Electrons enter the cell through the cathode from the external circuit.
Students often think Reduction occurs at the electrode that gets smaller, because 'reduction' means a decrease. In fact No. Reduction is a gain of electrons. A metal electrode at which metal cations are reduced gains mass; the electrode that loses mass is the one at which the metal is oxidized.
Students often think Oxidation occurs at the electrode that gains mass, because oxidation means gaining something, as when a metal gains oxygen. In fact No. Oxidation is a loss of electrons. A metal electrode that is oxidized loses mass as its atoms enter the solution as cations.
4 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 4
A galvanic cell is built from a Zn electrode in Zn(NO₃)₂(aq) and an Ag electrode in AgNO₃(aq). The cell reaction is Zn(s) + 2 Ag⁺(aq) → Zn²⁺(aq) + 2 Ag(s). The table shows the masses of the electrodes before and after the cell operated for some time. If all the silver formed stays on the Ag electrode, what is the missing mass of the Ag electrode after operation?
Answer and reasoning
A22.16 g A student who thinks one Ag⁺ ion is reduced for each Zn atom oxidized picks this: 0.0200 mol Ag = 2.16 g. Each Zn atom releases 2 electrons and each Ag⁺ ion gains 1, so 2 mol Ag form per mol Zn.
B24.32 gCorrect The Zn electrode lost 1.31 g, or 0.0200 mol of Zn. Each Zn atom gives 2 electrons and each Ag⁺ ion takes 1, so 0.04007 mol Ag (4.32 g) is deposited, and the electrode's mass becomes 20.00 + 4.32 = 24.32 g.
C21.31 g A student who thinks the mass gained by one electrode equals the mass lost by the other picks this. Zn and Ag have different molar masses, and 2 mol Ag form per mol Zn, so 4.32 g of Ag is deposited.
D22.62 g A student who applies the 2:1 coefficient ratio directly to masses picks this: 2 × 1.31 g = 2.62 g. Coefficients give mole ratios; the 1.31 g of Zn must be converted to moles first.
Working Mass of Zn oxidized = 25.00 g − 23.69 g = 1.31 g; n(Zn) = 1.31 g ÷ 65.38 g/mol = 0.02004 mol. From the equation, 2 mol Ag form per mol Zn: n(Ag) = 0.04007 mol; mass of Ag = 0.04007 mol × 107.87 g/mol = 4.32 g. Mass after = 20.00 g + 4.32 g = 24.32 g.
A cell contains a Cu electrode in Cu(NO₃)₂(aq) and a Ni electrode in Ni(NO₃)₂(aq), joined by a salt bridge and by wires. While the cell operates, the reaction Cu(s) + Ni²⁺(aq) → Cu²⁺(aq) + Ni(s) occurs; this reaction is thermodynamically unfavored. Which classification of the cell, with its justification, is correct?
Answer and reasoning
AGalvanic, because it has two different metal electrodes joined by a salt bridge A student who classifies a cell by its components picks this. The same two half-cells can form a galvanic or an electrolytic cell; what decides is whether the reaction that occurs is favored or unfavored.
BGalvanic, because electrons flow through the wires from one electrode to the other A student who thinks electron flow shows that a cell is galvanic picks this. Electrons flow through the external circuit in electrolytic cells too, pushed by the power supply.
CElectrolytic, because both of its half-cells contain solutions of dissolved electrolytes A student who takes 'electrolytic' to mean 'containing electrolytes' picks this. The classification is right but the reason is not: galvanic cells contain electrolyte solutions too. The cell is electrolytic because its reaction is unfavored and must be driven.
DElectrolytic, because the unfavored reaction is driven by an outside power supplyCorrect Galvanic cells use thermodynamically favored reactions; electrolytic cells use unfavored reactions driven by external electrical energy. Since the unfavored reaction occurs, the wires must connect the cell to a power supply.
A galvanic cell is built from a Zn electrode in Zn(NO₃)₂(aq) and a Cu electrode in Cu(NO₃)₂(aq). The electrodes are connected by wires to a voltmeter, and the solutions are joined by a salt bridge containing KNO₃(aq). The voltmeter shows a steady reading. The student then lifts the salt bridge out of both beakers. How does the voltmeter reading change, and why?
Answer and reasoning
AIt falls to zero, because electrons can no longer pass between the beakers through the salt bridge A student who thinks electrons travel through the salt bridge picks this. The reading does fall to zero, but because ions, not electrons, can no longer move between the half-cells.
BIt stays the same, because the salt bridge only keeps the two solutions from mixing A student who thinks the salt bridge only separates the solutions picks this. The salt bridge also lets ions move to keep each solution neutral; without it the circuit is broken.
CIt falls to zero, because ions can no longer move between the two half-cells to complete the circuitCorrect Current flows only around a complete circuit: electrons through the wire and ions through the solutions and salt bridge. Without the salt bridge, no charge can pass between the solutions, so the reading falls to zero.
DIt stays the same, because electrons can still flow through the wire from Zn to Cu A student who thinks the wire alone completes the circuit picks this. Electrons cannot keep flowing through the wire unless ions can move between the half-cells to balance the charge.
In a galvanic cell built from a Zn electrode in Zn(NO₃)₂(aq) and a Cu electrode in Cu(NO₃)₂(aq), the zinc electrode loses mass and the copper electrode gains mass as the cell operates. Which half-reaction occurs at the anode?
Answer and reasoning
AZn²⁺(aq) + 2 e⁻ → Zn(s) A student who thinks reduction occurs where the mass is reduced picks this. Reduction of Zn²⁺ would deposit zinc and increase the electrode's mass; the zinc electrode loses mass because zinc is oxidized.
BZn(s) → Zn²⁺(aq) + 2 e⁻Correct The zinc electrode loses mass because zinc atoms lose electrons and enter the solution as Zn²⁺ ions. This oxidation occurs at the anode.
CCu(s) → Cu²⁺(aq) + 2 e⁻ A student who thinks oxidation occurs at the electrode that gains mass picks this. Oxidizing copper would remove copper from the electrode; the copper electrode gains mass because Cu²⁺ is reduced there.
DCu²⁺(aq) + 2 e⁻ → Cu(s) A student who thinks cations are produced at the cathode, and so consumed at the anode, picks this. Cu²⁺ is reduced at the cathode; the anode is where oxidation occurs, here of zinc.
Working The zinc electrode loses mass, so zinc atoms are leaving it as Zn²⁺ ions: Zn(s) → Zn²⁺(aq) + 2 e⁻. This is an oxidation (loss of electrons), and oxidation occurs at the anode. (At the cathode, Cu²⁺(aq) + 2 e⁻ → Cu(s), which is why the copper electrode gains mass.)
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
