Q4. Assertion: a fuel cell can run for as long as its reactants are supplied. Reason: its reactants are fed in, and its products removed, continuously. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: the reactants are fed in and the products removed continuously, so the cell runs as long as the reactants are supplied (NCERT page 56). In practice its electrodes and catalyst must also stay in working order (reasoned).
Q5. Write the electrode reactions of the hydrogen–oxygen fuel cell and show that the sodium hydroxide is not used up.
Cathode: O2 + 2H2O + 4e− → 4OH−; anode: 2H2 + 4OH− → 4H2O + 4e− (NCERT page 56). The 4OH− made at the cathode are used at the anode, so adding the two gives 2H2 + O2 → 2H2O with no net change in hydroxide; the water formed must be removed (reasoned).
Q6. A fuel cell gives 1.0 kWh of electrical energy at 70% efficiency. How much energy would the hydrogen have released if burnt? And how much would a thermal plant at 40% need to burn to give the same 1.0 kWh?
1.0/0.70 = 1.4 kWh of chemical energy for the fuel cell; 1.0/0.40 = 2.5 kWh for the thermal plant (efficiencies from NCERT pages 56–57).
Q7. How many moles of oxygen does a hydrogen–oxygen fuel cell use while 4.0 mol of electrons pass through the external circuit?
The cathode reaction takes 4 electrons per O2, so 1.0 mol of O2 (and 2.0 mol of H2) (NCERT page 56).
Q8. In the hydrogen economy, why does the source of the energy used to produce hydrogen matter?
In NCERT’s words, hydrogen production must come from splitting water using solar energy; then hydrogen is a renewable and non-polluting source of energy (page 58). Beyond the NCERT text: hydrogen made from fossil fuels releases carbon dioxide when it is made, even though its use in a fuel cell gives only water (US Energy Information Administration, “Production of hydrogen”); wind power can drive the electrolysis as well as solar (US Department of Energy, “Hydrogen Fuel Basics”).
Q9. Which way do electrons move in the external circuit of a hydrogen–oxygen fuel cell, and which way do hydroxide ions move inside it?
Electrons go through the wire from the hydrogen electrode (anode) to the oxygen electrode (cathode); hydroxide ions move through the electrolyte from the cathode, where they are made, to the anode, where they are used (reasoned from NCERT page 56).