Lab 9 (U)

Question 1

During an oxidation-reduction reaction,

Answer 1

During an oxidation-reduction reaction, electrons are completely transferred from one entity to
another.

In an electrolytic cell, electrical energy is used to drive a non-spontaneous redox reaction.

Electrolytic processes follow laws first proposed by Michael Faraday.

These laws are able to quantify the chemical change that occurs within an electrolytic cell, provided that the amount of
charge passing through the cell is known.

Question 2

Electrolytic processes follow laws first proposed by Michael Faraday.

Answer 2

1. Electric current (I) is measured in amperes and is defined as the flow of charge (in coulombs) per second,
   I = Q/t (1)
   where I is current in amperes (A), Q is charge in coulombs (C), and t is time in seconds. Note that 1 A = C/s.
2. A coulomb of charge (C) is defined as the charge on 6.2415·1018 electrons. The charge on one mole of electrons (6.0221·1023 electrons) is called the Faraday (F). A simple calculation will
   show that the charge on one mole of electrons is 96485 C or 1 Faraday.
3. Ohm’s Law relates current (I), voltage (V), and resistance (R)
   V = IR where V is in volts (V), I is in amperes (A), and R is in ohms (Ω).

Question 3

In this experiment,

Answer 3

study a non-spontaneous redox reaction in which H+(aq) is reduced to H2(g) and Cu(s) is oxidized to Cu2+(aq).

Electrical energy from an external source is required to drive this non-spontaneous redox reaction.

You will measure the volume of hydrogen gas produced during the reaction at a known
temperature and pressure and calculate the number of moles of hydrogen gas formed.

Question 4

The apparatus

Answer 4

consists of an external DC electrical source, an ammeter to measure the current flow, and a variable resistance box to control the current flow (since I = V/R).

Electrolysis takes place in a beaker containing aqueous sulfuric acid.

Reduction occurs at the cathode which is an inert nichrome wire attached to the negative terminal of the power source.

Oxidation occurs at the anode, which is a piece of copper wire attached to the positive terminal of the power source.

this is opposite to the voltaic
cell.

Question 5

value of Faraday

Answer 5

If a relatively constant current is applied for a specific period of time, it is possible to calculate the
charge transferred in coulombs, Q = It.

By knowing the moles of gas and moles of electrons, the coulombs of charge per one mole of electrons can be calculated. This is the value of the Faraday.

Question 6

In this electrolysis,

Answer 6

electrons are stripped from copper metal in the anode and transferred to protons in the cathode.

This requires that the moles of Cu lost through the oxidation reaction (Cu → Cu2+ + 2 e–) must equal the moles of hydrogen gas formed by the reduction reaction (2H+(aq) +
2 e– → H2(g)).

Using this as a basis, you will be able to estimate the atomic mass of copper by knowing the mass lost during reaction.

As the oxidation of the copper wire proceeds, the concentration of Cu2+ ions in solution increases.

Thus, you must always start each run with a freshly prepared sulfuric acid solution. If this is not done, the Cu2+(aq), will be reduced in preference to the H+(aq) and the volume of H2(g) collected
will be erroneously low.

Question 7

As you set up your apparatus, keep the following information about electrical circuits in mind:

Answer 7

1. The resistance of a copper wire varies inversely with the square of the diameter of the wire.
2. The resistance of the electrical circuit also varies inversely with the surface area of the wire in contact with the solution.
3. The greater the separation between the electrodes in the solution, the greater the resistance
   in the circuit.

Question 8

If you are unable to obtain sufficient current flow,

Answer 8

it may be due to one of the factors described above.

Points 1 and 2 suggest that you should have as much of the copper wire into solution as possible.

Point 3 recommends that you should try to get the copper wire near the burette (within about 2 cm) and that the exposed nichrome wire should not be positioned too far inside the burette.

Question 9

The method for calculating the amount of hydrogen formed

Answer 9

formed is similar to the calculations in the “Determination of the Ideal Gas Constant” Experiment in the Chemistry 101/103 Laboratory
Manual.

Review this procedure prior to beginning this experiment.

Remember that your recorded
observations should reflect the precision of your measurements.

Record the time in minutes and
seconds.