(a)(i) Name and explain the common defects of a primary cell. (ii) State two advantages of a secondary cell over a primary cell. (b) Draw a labelled diagram...
(a)(i) Name and explain the common defects of a primary cell.
(ii) State two advantages of a secondary cell over a primary cell.
(b) Draw a labelled diagram to show the essential parts of a dry leclanche cell.
(c)(i) Explain why six accumulators each of e.m.f 2V connected in series can be used to start the engine of a car whereas eight dry cells each of e.m.f 1.5 V connected in series cannot be used.
(ii) Name the materials used for the positive terminal, the negative terminal and the electrolyte in a
I. leclanche cell;
II. charged lead acid accumulator.
(a)(i) Common defects of a primary (simple) cell:
Polarisation: hydrogen gas liberated at the positive (copper) plate collects there as a layer of bubbles. This layer increases the internal resistance of the cell and sets up a back e.m.f. that opposes the working e.m.f., so the current falls rapidly. It is reduced by adding a depolariser (e.g. potassium dichromate or manganese(IV) oxide) which oxidises the hydrogen to water.
Local action: impurities (carbon, iron) in the zinc plate form many tiny short-circuited local cells with the zinc, so the zinc is slowly eaten away and current is wasted even when the cell is not in use. It is prevented by amalgamating the zinc, i.e. coating it with a thin film of mercury so that the impurities are covered and cannot act.
(a)(ii) Two advantages of a secondary cell over a primary cell:
A secondary cell can be recharged and used again many times, whereas a primary cell cannot be recharged.
A secondary cell has a very low internal resistance, so it can deliver a large current and supply current for a much longer working life than a primary cell.
(b) Labelled diagram of a dry Leclanche cell:
The essential parts are the outer zinc case (negative electrode), the central carbon rod (positive electrode) surrounded by a paste of manganese(IV) oxide and powdered carbon (the depolariser), the ammonium chloride paste (electrolyte), and the pitch/wax seal with a brass cap on top.
Cross-section of a dry Leclanche cell showing the zinc case (negative electrode), central carbon rod (positive electrode), manganese(IV) oxide and carbon depolariser, ammonium chloride paste electrolyte, and pitch seal with brass cap.
(c)(i) The eight dry cells and the six accumulators give the same total e.m.f.: \(8 \times 1.5 = 12\ \text{V}\) and \(6 \times 2 = 12\ \text{V}\). What differs is the internal resistance. A dry (Leclanche) cell has a very high internal resistance, so eight of them in series have a large total internal resistance. When such a battery is connected to the very small resistance of a starter motor, most of the e.m.f. is dropped across the cells' own internal resistance and only a small current \(\left(I = \dfrac{E}{R + r}\right)\) flows, too small to turn the engine. Each accumulator has a very low internal resistance, so six in series can force the very large current needed through the low-resistance starter motor and start the engine.
(c)(ii) Materials used:
Positive terminal
Negative terminal
Electrolyte
I. Leclanche cell
Carbon rod (surrounded by manganese(IV) oxide, \(\text{MnO}_2\), as depolariser)
Polarisation: hydrogen gas liberated at the positive (copper) plate collects there as a layer of bubbles. This layer increases the internal resistance of the cell and sets up a back e.m.f. that opposes the working e.m.f., so the current falls rapidly. It is reduced by adding a depolariser (e.g. potassium dichromate or manganese(IV) oxide) which oxidises the hydrogen to water.
Local action: impurities (carbon, iron) in the zinc plate form many tiny short-circuited local cells with the zinc, so the zinc is slowly eaten away and current is wasted even when the cell is not in use. It is prevented by amalgamating the zinc, i.e. coating it with a thin film of mercury so that the impurities are covered and cannot act.
(a)(ii) Two advantages of a secondary cell over a primary cell:
A secondary cell can be recharged and used again many times, whereas a primary cell cannot be recharged.
A secondary cell has a very low internal resistance, so it can deliver a large current and supply current for a much longer working life than a primary cell.
(b) Labelled diagram of a dry Leclanche cell:
The essential parts are the outer zinc case (negative electrode), the central carbon rod (positive electrode) surrounded by a paste of manganese(IV) oxide and powdered carbon (the depolariser), the ammonium chloride paste (electrolyte), and the pitch/wax seal with a brass cap on top.
Cross-section of a dry Leclanche cell showing the zinc case (negative electrode), central carbon rod (positive electrode), manganese(IV) oxide and carbon depolariser, ammonium chloride paste electrolyte, and pitch seal with brass cap.
(c)(i) The eight dry cells and the six accumulators give the same total e.m.f.: \(8 \times 1.5 = 12\ \text{V}\) and \(6 \times 2 = 12\ \text{V}\). What differs is the internal resistance. A dry (Leclanche) cell has a very high internal resistance, so eight of them in series have a large total internal resistance. When such a battery is connected to the very small resistance of a starter motor, most of the e.m.f. is dropped across the cells' own internal resistance and only a small current \(\left(I = \dfrac{E}{R + r}\right)\) flows, too small to turn the engine. Each accumulator has a very low internal resistance, so six in series can force the very large current needed through the low-resistance starter motor and start the engine.
(c)(ii) Materials used:
Positive terminal
Negative terminal
Electrolyte
I. Leclanche cell
Carbon rod (surrounded by manganese(IV) oxide, \(\text{MnO}_2\), as depolariser)
