Q4. Assertion: the voltage of a mercury cell stays nearly constant during its life. Reason: its overall reaction involves no ion in solution whose concentration can change. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion (NCERT page 55).
Q5. What happens to the ammonia formed at the cathode of a dry cell?
It forms a complex with the zinc ions, [Zn(NH3)4]2+ (NCERT page 54).
Q6. One lead storage cell is discharged until 0.200 mol of electrons has passed. What mass of PbSO4 has formed on the two plates together? (PbSO4 303.3 g mol−1.)
The overall reaction makes 2 mol of PbSO4 for 2 mol of electrons, so 0.200 mol, 0.200 × 303.3 = 60.7 g, half on each plate (NCERT page 55; reasoned).
Q7. In NCERT’s equation for the nickel-cadmium cell, give the change in oxidation state of nickel and of cadmium, and check that the electrons balance.
Nickel goes from +3 in Ni(OH)3 to +2 in Ni(OH)2, gaining 1 electron per atom, 2 for the two nickel atoms. Cadmium goes from 0 to +2 in CdO, losing 2. The electrons balance (NCERT page 56; reasoned).
Q8. Which cell does NCERT describe as suitable for hearing aids, and what property makes it suitable?
The mercury cell: it suits low-current devices such as hearing aids and watches, and its potential, about 1.35 V, remains constant during its life, because the overall reaction involves no ion in solution whose concentration can change (NCERT pages 54–55). Strictly, the voltage is nearly rather than exactly constant (reasoned; see the body).
Q9. At least how much charge must be passed to recharge a lead storage battery that delivered 9.65 A for 1.00 hour?
At least the charge it delivered, 9.65 × 3600 = 34 740 C, since charging must reverse the same amount of reaction; in practice a little more is needed (reasoned from NCERT pages 52 and 55).
Q10. During discharge of a lead storage battery, which plate is negative, and what reaction occurs there?
The lead plate, the anode: Pb(s) + SO42−(aq) → PbSO4(s) + 2e− (NCERT pages 33 and 55).