Electrochemistry Questions and Answers

Answer: C

Correct answer: cathode potential minus anode potential

The cell EMF is

with both taken as reduction potentials. For the Daniell cell that is \( 0.34 - (-0.76) = 1.10 \) V. A positive result confirms the reaction as written is spontaneous; a negative one means the cell would run the other way.

Answer: D

The group 2.303RT over F evaluates to 0.0591 volts at 298 kelvin, and dividing by the number of electrons gives the change in potential per decade of concentration. For a two-electron electrode the figure is about 30 millivolts.

Answer: A

Correct answer: more negative

The Nernst equation for a metal electrode is

and diluting reduces the ion concentration, making the logarithm more negative and the potential with it. Physically, fewer ions in solution means less tendency for the metal to be deposited and more for it to dissolve. This is also why a concentration cell generates an EMF at all.

Answer: B

Correct answer: identical but in solutions of different concentration

A concentration cell has chemically identical electrodes, so the standard potentials cancel and the whole EMF comes from the concentration difference through the Nernst equation:

The cell runs so as to equalise the two concentrations, and its EMF falls to zero when they become equal.

Answer: C

A positive EMF therefore corresponds to a negative free energy change and a spontaneous reaction. The maximum electrical work obtainable equals the free energy decrease, which is why a cell is measured at zero current.

Answer: D

Correct answer: only then is it the reversible thermodynamic value

Drawing current makes the reaction proceed irreversibly: the electrodes polarise, the internal resistance produces a voltage drop, and the terminal voltage falls below the true EMF. Only at essentially zero current is the measured potential the reversible value related to the free energy by \( \Delta G = -nFE \). This is why EMF is measured by a potentiometer, which balances the cell against a known voltage and draws no current at balance.

Answer: A

The reaction has no further tendency to proceed in either direction, so no work can be extracted and the free energy change is zero. Setting the Nernst equation to zero yields the equilibrium constant from the standard EMF.

Answer: B

Mercury in contact with mercurous chloride and a fixed chloride concentration gives a stable known potential, so it can replace the awkward hydrogen electrode. Its value depends on the chloride concentration chosen, which is stated with it.

Answer: C

Correct answer: a thin glass membrane between two solutions

The glass electrode is a bulb of special glass containing a solution of fixed pH with an internal reference electrode. A potential develops across the thin membrane that depends on the difference in hydrogen ion activity between the two sides, so with the inside fixed the potential measures the outside. The membrane is fragile and must be kept hydrated for the ion exchange layer to function.

Answer: D

Correct answer: 59

The Nernst slope at 25 degrees is

per decade of activity, and one pH unit is one decade, so the electrode gives about 59 millivolts per pH unit. The slope varies with absolute temperature, which is why a pH meter has a temperature compensation and why calibration must be done at the sample temperature.