A galvanic cell differs from an electrolytic cell in that a galvanic cell
Answer: A
The chemical reaction in a galvanic cell is thermodynamically downhill and its free energy appears as electrical work. An electrolytic cell runs the reverse reaction and must be pushed by an external supply larger than the cell's own opposing EMF.
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In any electrochemical cell, oxidation occurs at the
Answer: B
Correct answer: anode
Oxidation is loss of electrons and it always occurs at the anode, in a galvanic cell and an electrolytic one alike. What changes between the two is the sign: the anode is negative in a galvanic cell, where the reaction is spontaneous and pushes electrons into the external circuit, and positive in an electrolytic cell, where the supply draws them out. Defining the electrodes by the reaction rather than by the sign avoids the confusion.
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In the cell notation Zn | Zn\( ^{2+} \) || Cu\( ^{2+} \) | Cu, the double vertical line represents
Answer: C
Correct answer: the salt bridge or liquid junction
A single vertical line marks a phase boundary within a half cell, such as metal against its solution; the double line marks the salt bridge or liquid junction separating the two half cells. The bridge completes the circuit by carrying ions while preventing the two solutions from mixing, and it keeps the liquid junction potential small by using an electrolyte whose cation and anion have nearly equal mobilities, such as potassium chloride.
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The function of a salt bridge in a galvanic cell is to
Answer: D
It carries the ionic current and neutralises the charge that would otherwise build up in each compartment, so the cell can keep working. Potassium chloride is used because its cation and anion have almost equal mobilities.
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Potassium chloride is the usual salt bridge electrolyte because its ions have nearly equal
Answer: A
Equal mobilities mean the two ions diffuse across the junction at the same rate, so almost no charge separation and hence almost no liquid junction potential develops. A salt whose ions moved at different rates would introduce an unwanted EMF.
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A liquid junction potential arises at the boundary between two solutions because the ions
Answer: B
The faster ion runs ahead and leaves a charge separation that opposes further diffusion, and the resulting potential adds an unknown term to the measured EMF. A salt bridge of potassium chloride reduces it to a fraction of a millivolt.
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The standard hydrogen electrode is assigned a potential of
Answer: C
It is an arbitrary reference against which every other electrode potential is measured, since a single electrode's absolute potential cannot be determined. It consists of platinised platinum in acid of unit activity under one bar of hydrogen.
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The standard electrode potential of an electrode is measured with the ion at
Answer: D
Correct answer: unit activity and 25 degrees Celsius
The standard state is unit activity for every species involved, one atmosphere for any gas, and a stated temperature of 25 degrees. All tabulated standard electrode potentials are measured against the standard hydrogen electrode under those conditions, which is what makes them comparable. Away from unit activity the potential must be corrected by the Nernst equation.
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A metal high in the electrochemical series, with a large negative standard potential, is a
Answer: A
Correct answer: strong reducing agent
A large negative potential means the metal loses electrons readily, so it is easily oxidised and therefore a good reducing agent - it gives electrons to something else. Sodium, potassium and calcium sit at that end of the series. The other end, with large positive potentials, holds the strong oxidising agents such as fluorine, which take electrons readily.
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A metal will displace another from its salt solution if the first metal's standard potential is
Answer: B
Correct answer: more negative than the second’s
Displacement requires the first metal to be the better reducing agent, that is to lose electrons more readily, which means the more negative standard potential. Zinc at \( -0.76 \) V displaces copper at \( +0.34 \) V from its sulphate, and the reverse cannot happen. This is precisely what the electrochemical series is for: any metal displaces any metal below it in the list.
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