1. Given the lead-acid battery reaction:

Pb + PbO₂ + H₂SO₄ Discharge --> Charge<-- 2PbSO₄ + 2H₂O

Which unit is oxidized during battery discharge?

A)    SO₄^2-

B)    2H₂O

C)    Pb

D)    PbO₂

Correct Answer: Option C - Pb

In the discharge mode, as represented by the total reaction, Pb and PbO₂ are depleted, and the concentration of H₂SO₄ is diminished. Since the density of solution has a bearing on the concentration of H₂SO₄ measurement of the density can be used as a simple way to enumerate how far the cell has undergone discharge. The battery can be recharged and thus resorbed to its original condition. To perform this, a direct current is passed through the cell in the reverse direction and this reverses the reactions.

At the anode Pb is oxidized to PbSO₄ and the oxidation number of Pb changes from 0 to +2. This happens when the battery is discharging.

Hence, options A, B and D are incorrect.

2. Given the balanced equation representing the reaction occurring in a voltaic cell:

In the completed external circuit, the electrons flow from:

A)    Pb(s) to Zn(s)

B)    Pb₂+(aq) to Zn₂+(aq)

C)    Zn(s) to Pb(s)

D)    Zn₂+(aq) to Pb₂+(aq)

Correct Answer: Option C - Zn(s) to Pb(s)

A voltaic cell spontaneously converts chemical energy to electrical energy. In voltaic cells the two half-reactions of a redox reaction, in an aqueous neutral solution, are separated but connected between each electrode with a conductive wire and between the two solutions with a salt bridge.

Hence, options A, B and D are incorrect.

3. Which balanced equation represents a redox reaction?

A)                 CuCO₃(s) → CuO(s) + CO₂(g)

B)                  2KClO₃(s) → 2KCl(s) + 3O₂(g)

C)                  AgNO₃(aq) + KCl(aq) → AgCl(s) + KNO₃(aq)

D)                 H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

Correct Answer: Option B - 2KClO₃(s) → 2KCl(s) + 3O₂(g)

Redox (shorthand for reduction-oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. The term redox comes from the two concepts of reduction and oxidation. It can be explained in simple terms:

• Oxidation describes the loss of electrons / hydrogen or gain of oxygen / increase in oxidation state by a molecule, atom or ion.
• Reduction describes the gain of electrons / hydrogen or a loss of oxygen / decrease in oxidation state by a molecule, atom or ion.

Hence, options A, C and D are incorrect.

4. What is the oxidation number of nitrogen in NO(g)?

Correct Answer: +2

Oxygen can donate 2 electrons.

Hence, to bond with oxygen the nitrogen atom has to accept the 2 electrons of the oxygen atom, making the oxidation number of nitrogen in NO (Nitric oxide) +2.

Base your answers to questions 5 and 6 on the information below.

For the process of electroplating an electroplating cell includes a battery connected to a silver bar at one end and a metal spoon on the other end; with the bar and spoon submerged in a beaker containing AgNO₃(aq).

5. Explain why AgNO₃ is a better choice than AgCl for use in this electrolytic process.

Correct Answer:

Silver nitrate produces more ions than silver chloride in water, since it slightly dissolves in water; hence, AgNO₃ is a better choice than AgCl for use in this electrolytic process.

Base your answers to questions 5 and 6 on the information below.

For the process of electroplating an electroplating cell includes a battery connected to a silver bar at one end and a metal spoon on the other end; with the bar and spoon submerged in a beaker containing AgNO₃(aq).

6. Explain the purpose of the battery in this cell.

Correct Answer:

The battery provides the electrical energy necessary for the reaction to occur; which acts as the external stimulus providing energy required for the bonds in the compound to break and help the electrolytic process.

7. Which substance functions as the electrolyte in an automobile battery?

A)                 H₂SO₄

B)                  PbO₂

C)                  H₂O

D)                 PbSO₄

Correct Answer: Option A - H₂SO₄

A car battery is a type of rechargeable battery that supplies electric energy to an automobile. An electrolyte is any substance containing free ions that behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible. Most vehicle batteries are lead-acid batteries. Each cell has an electrolytic solution of sulfuric acid (H₂SO₄) and water. During discharge, the concentration of sulfuric acid (H₂SO₄) decreases as lead sulfate slowly forms on the plates.

Hence, options B, C and D are incorrect.

8. Which half-reaction correctly represents reduction?

A)    Mn⁴⁺ → Mn³⁺ + e-

B)    Mn⁴⁺ → Mn⁷⁺ + 3e-

C)    Mn⁴⁺ + 3e- → Mn⁷⁺

D)    Mn⁴⁺ + e- → Mn³⁺

Correct Answer: Option D - Mn⁴⁺ + e- → Mn³⁺

A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.

In reduction half reactions, the oxidation number is lowered because electrons are added. When Mn⁴⁺ gains electrons, the negative charge of the electron cancels out the positive charge and Mn³⁺ atom results.

Hence, options A, B and C are incorrect.

9. Given the balanced equation representing a reaction:

2KClO₃(s) → 2KCl(s) + 3O₂(g)

The oxidation state of chlorine in this reaction changes from

A)    -1 to +1

B)    +1 to -1

C)    -1 to +5

D)    +5 to -1

Correct Answer: Option D - +5 to -1

Oxidation state shows the total number of electrons which have been removed from an element (a positive oxidation state) or added to an element (a negative oxidation state) to get to its present state. Oxidation involves an increase in oxidation state. The oxidation state of an uncombined element is zero. That is obviously so, because it hasn't been either oxidised or reduced yet. This applies whatever the structure of the element - whether it is, for example, Xe or Cl₂ or S₈, or whether it has a giant structure like carbon or silicon.

In the above reaction the oxidation state of chlorine in KClO₃ is +5 i.e. potassium has an oxidation number of +1 and oxygen has a oxidation number of -2

= +1 plus (-2) 3 = -5, hence, making the oxidation number of chlorine +5

In KCl the oxidation number of chlorine is -1; i.e. chlorine has to bond with one potassium atom having an oxidation number +1, which makes the oxidation number of chlorine -1.

Hence, options A, B and C are incorrect.

10. Given the balanced equation representing a reaction occurring in an electrolytic cell:

2NaCl(l) → 2Na(l) + Cl₂(g)

Where is Na(l) produced in the cell?

A)                 At the anode, where oxidation occurs

B)                  At the cathode, where reduction occurs

C)                  At the anode, where reduction occurs

D)                 At the cathode, where oxidation occurs

Correct Answer: Option B - At the cathode, where reduction occurs

Oxidation-reduction or redox reactions take place in electrochemical cells. There are two types of electrochemical cells. Spontaneous reactions occur in galvanic (voltaic) cells; non spontaneous reactions occur in electrolytic cells. Both types of cells contain electrodes where the oxidation and reduction reactions occur. Oxidation occurs at the electrode termed the anode and reduction occurs at the electrode called the cathode.

Hence, options A, C and D are incorrect.