📋 Structured Questions
Complete each question on paper, then check your answers against the mark scheme.
Question 1: Production of Ethanol
5 marks(a) Write equations for the fermentation of glucose, and the subsequent complete combustion of the ethanol produced. [2]
(b) Fermentation of glucose is often described as producing a "carbon-neutral" biofuel. Support this statement by writing an overall equation showing the balance of carbon dioxide taken in and released, assuming photosynthesis forms the glucose. [2]
(c) Give one reason why this process is not strictly carbon-neutral in practice. [1]
Show Mark Scheme
(a)
- Fermentation: C6H12O6 -> 2C2H5OH + 2CO2 [1]
- Combustion: C2H5OH + 3O2 -> 2CO2 + 3H2O [1]
(b)
- Photosynthesis: 6CO2 + 6H2O -> C6H12O6 + 6O2 (takes in 6 CO2) [1]
- Fermentation + Combustion: (2 CO2 from fermentation) + (2 x 2 CO2 from burning 2 molecules of ethanol) = 6 CO2 released. Net balance is zero. [1]
(c)
- Carbon dioxide is released by burning fossil fuels to power machinery for farming, transporting crops/fuel, and heating the fractional distillation column to purify the ethanol. [1]
Question 2: Oxidation and Chemical Tests
5 marksA student has three unlabelled bottles containing propan-1-ol, propan-2-ol, and 2-methylpropan-2-ol.
(a) Describe a chemical test (including reagents and observations) that could identify which bottle contains 2-methylpropan-2-ol. [2]
(b) The student oxidizes the other two alcohols. Describe how the student could use Tollens' reagent or Fehling's solution to distinguish between the oxidation products of propan-1-ol and propan-2-ol. Include expected observations. [3]
Show Mark Scheme
(a)
- Add acidified potassium dichromate(VI) (K2Cr2O7/H2SO4) and warm. [1]
- The bottle containing 2-methylpropan-2-ol (a tertiary alcohol) will show no reaction and remain orange. The other two bottles (containing primary/secondary alcohols) will turn from orange to green. [1]
(b)
- Oxidation of propan-1-ol yields propanal (an aldehyde). Oxidation of propan-2-ol yields propanone (a ketone). [1]
- Add Tollens' reagent (or Fehling's solution) and warm. [1]
- Observations: The oxidation product of propan-1-ol (propanal) forms a silver mirror (or red precipitate with Fehling's). The oxidation product of propan-2-ol (propanone) shows no reaction. [1]
Question 3: Dehydration Mechanism
4 marksWrite the mechanism for the dehydration of cyclohexanol to form cyclohexene in the presence of a concentrated phosphoric acid catalyst (H3PO4, acting as a source of H+ ions). Include all curly arrows, lone pairs, and formal charges. [4]
Show Mark Scheme
- Step 1: Curly arrow from the lone pair on the oxygen atom of the cyclohexanol -OH group to the catalyst H+ proton. [1]
- Intermediate 1: Correct structure of protonated cyclohexanol (with -OH2+ group, positive charge on oxygen). [1]
- Step 2: Curly arrow from the C-O bond to the positive oxygen atom, representing loss of a water molecule. [1]
- Step 3: Correct carbocation intermediate (cyclohexyl ring with C+). Curly arrow from a C-H bond on an adjacent carbon atom into the adjacent C-C bond to form the C=C double bond (cyclohexene) and release H+. [1]
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