Question
Geological transformations produce fossil fuels.
(a) Combustion of coal emits particulates into the atmosphere.
(i) Outline why this affects global warming.
(ii) State the major form of energy produced by the combustion of coal.
(b) Conversion of petroleum to petrol (gasoline) involves fractional distillation and cracking.
Distinguish between these processes.
Fractional distillation:
Cracking:
(c) The equation for the combustion of octane is:
\(C_6H_{18}(l) + 12 \frac{1}{2} O_2 \rightarrow 8CO_2(g) + 9H_2O(l)\)
(i) Determine the mass of carbon dioxide, in g, produced when 1kJ of energy is produced. Use section 13 of the data booklet.
(ii) Suggest a piece of evidence that leads some people to not accept a causal link between the industrial emission of greenhouse gases, such as \(CO_2\), and global warming.
Answer/Explanation
Answer:
(a) (i) reflects «sun» light
(ii) electrical/electricity
(b) Fractional distillation (Any one of):
separates compounds according to boiling points/vapor pressure
OR
physical process
OR
involves breaking intermolecular forces
OR
separates based on molar masses
OR
does not use catalyst
Cracking (Any one of):
shorter hydrocarbon chains formed/lower molar masses
OR
increased branching formed
OR
increased aromatic ring formation
OR
produces alkenes/unsaturated hydrocarbons
OR
chemical process
OR
new compounds formed/breaking «and remaking»/changing covalent bonds
OR
uses catalyst
(c) (i) ALTERNATIVE 1
5470 «kJ» produced when 8 «mol» produced
«8 x 44.01 =» 352 «g» to produce 5470 «kJ»
1 «kJ» would release «352/5470 =» 0.0644 «g»
ALTERNATIVE 2
1 «kJ»/5470«kJ=» \(1.828×10^{-4}\)
8/5470 «mol =» 0.001463 mol
«8/5470 «mol» x 44.01 «g\(mol^{-1}\) =» 0.0644 «g»
(ii) many fluctuations of temperature have occurred in the «geological» past
OR
different global models produce different outcomes
OR
industrial emissions are less than natural ones
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