Q4. Name four chemicals that NCERT says are produced by electrochemical methods, and give two reasons why electrochemistry matters for new technologies.
Sodium hydroxide, chlorine and fluorine, along with many metals. Reactions carried out electrochemically can be energy efficient and less polluting (NCERT page 31).
Q5. NCERT gives the Daniell cell’s potential as 1.1 V “when concentration of Zn2+ and Cu2+ ions is unity”. What does its footnote add?
Strictly, activity should be used instead of concentration (NCERT footnote, page 32). NCERT adds that it is proportional to concentration and equal to it in dilute solutions; more precisely, dimensionless activity approaches concentration divided by the standard concentration in the ideal dilute limit (reasoned).
Q6. An opposing voltage on a Daniell cell is raised slowly from 0 to 1.5 V. Describe what happens to the current in the wire.
Below 1.1 V the current flows from copper to zinc, getting smaller as the voltage rises. At 1.1 V it is zero. Above 1.1 V it flows from zinc to copper as the reaction runs backwards (NCERT Figure 2.2, page 32; size reasoned).
Q7. Assertion: in a Daniell cell opposed by 1.5 V, zinc is deposited on the zinc rod. Reason: the zinc rod is now the cathode. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: reduction happens at the cathode, and here it is Zn2+ + 2e− → Zn (NCERT page 32).
Q8. In a Daniell cell driven as an electrolytic cell, 0.381 g of copper dissolves from the copper rod. How many moles of electrons have passed, and how much zinc is deposited? (Zn 65.4 g mol−1, Cu 63.5 g mol−1.)
0.381/63.5 = 0.006 mol of copper, so 0.012 mol of electrons. They deposit 0.006 mol of zinc, 0.006 × 65.4 = 0.392 g.
Q9. Why is a galvanic cell a way of getting useful work out of a reaction that would otherwise just give heat?
If zinc is dipped straight into copper sulphate solution, the electrons pass directly and the energy comes out as heat. In the cell the two half reactions happen in separate places, so the electrons must travel through the wire, where they can run a device (NCERT Class 11 Chemistry, Section 7.4, page 250; reasoned).