Question
(a) Photosynthesis enables green plants to store energy from sunlight as glucose.
(i) Write the equation for photosynthesis.
(ii) Identify the structural feature that allows chlorophyll to absorb light. Use section 35 of the data booklet.
(iii) Explain how photosynthesis is being employed to control global warming.
(b) Photovoltaic cells also convert sunlight into energy.
(i) State the form of energy produced by photosynthesis and photovoltaic cells.
Photosynthesis:
Photovoltaic:
(ii) Explain how a silicon-based photovoltaic cell brings about this conversion.
(c) Glucose can be converted to ethanol through fermentation:
\(C_6H_{12}O_6 (aq) → 2C_2H_5OH(aq) + 2CO_2 (g)\)
(i) Determine the energy efficiency of this conversion in terms of the enthalpies of combustion of the reactants and products. Use section 13 of the data booklet.
(ii) Suggest one reason, other than energy density and specific energy, why ethanol may be considered a more useful fuel than glucose.
(d) Both ethanol and glucose can be used to generate energy through fuel cells.
(i) Outline one way fuel cells differ from primary cells.
(ii) State one way to increase the maximum current of a voltaic cell.
Answer/Explanation
Answer:
(a) (i) \(6CO_2(g) + 6H_2O(l) → C_6H_{12}O_6(aq) + 6O_2(g)\)
(ii) conjugated «electronic» structure/delocalized «pi» electrons/alternate «single and» double bonds
(iii) reduces/sequesters \(CO_2\)/carbon dioxide «concentration from atmosphere»
«planting» more plants/trees
(b) (i) Photosynthesis: chemical
AND
Photovoltaic: electrical
(ii) Any three of:
n-type AND p-type «silicon layers»
OR
n-type doped with Gp 15 element/P AND p-type doped with Gp 13 element/B
potential difference/charge separation created between layers of silicon
«sunlight produces» free electrons that flow between layers «from p-type to n-
type»
OR
«sunlight produces» positive holes that flow between layers «from n-type to p-
type»
«excess» electrons move “from n-type to p-type” through an external circuit
(c) (i) «(2x-1367 / -2803) x 100 =» 97.54%
OR
2.46% loss «in energy efficiency»
(ii) liquid
OR
easier ignition
OR
more volatile
(d) (i) material provides energy in fuel cells
OR
fuel continually added in fuel cells
(ii) reduce «internal» resistance
Question
Photovoltaic cells often contain p- and n-type materials on either side of a p-n junction.
(a) Annotate the diagram to show the charge separation at the p-n junction and flow of electrons through the external circuit.
(b) Identify one element that could be used as the material for the p-n junction.
Answer/Explanation
Answer:
(a) negative above junction AND positive below junction
e- flow from n-type through wire to light globe
OR
from light globe to p-type through wire.
(b) silicon/Si OR germanium/Ge