Chemistry Exp. To Oxidise Phenylmethanol – Flashcards

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question
How is Benzoic acid prepared in the lab?
answer
Oxidising phenylmethanol using potassium permanganate solution in the *presence of sodium carbonate*
question
Equation for this reaction And explaining of colour change observed
answer
3C₆H₅CH₂OH + 4KMnO₄ (purple) → 3C₆H₅COOH + 4MnO₂(brown) + H₂O + 4KOH
question
In what apparatus is the phenylmethanol reacted with potassium manganate
answer
Phenylmethanol = known mass/vol. Potassium permanganate = saturated Sodium carbonate = solution In conical flask, in boiling water bath
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Why is the potassium permanganate solution saturated?
answer
To ensure sufficient is present to oxidise all of the phenylmethanol
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What is the function of the sodium carbonate?
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Ensures pH stays at an alkaline level, as this oxidation reaction works best in alkaline conditions
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What further steps are taking in isolating the benzoic acid?
answer
Conical flask taken out of water bath when brown, run under tap to cool. Placed in fume cupboard, HCl added Dip glass rod in solution, then on litmus paper to make sure environment is acidic Sodium sulphite solution added until brown precipitate disappears Placed in ice Crystals formed filtered in Buchner flask Conical flask rinsed out with solution in Buchner Crystals washed with ice-cold water Crystals left dry overnight Weight taken
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Why is HCl added?
answer
To convert the intermediate compound sodium benzoate to benzoic acid
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Why is the flask cooled?
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Benzoic acid is only slightly soluble in cold water, causes benzoic acid to be precipitated out
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Use of HCl during cooling process
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Neutralises any excess sodium carbonate added and potassium hydroxide produced in reaction Provides an acidic medium for Mn⁴⁺ ions to be reduced Mn⁴⁺ + 2e⁻ → Mn²⁺ (in the presence of H⁺)
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Chemical formula for sodium sulfite
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Na₂SO₃
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Why is sodium sulphite added to the conical flask once crystals are formed?
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Reducing agent; Reduces Mn⁴⁺ + 2e⁻ → Mn²⁺ Mn⁴⁺ is insoluble, and would contaminate the benzoic acid, so reduced to Mn²⁺ which is soluble
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Half reaction for conversion of sulphite to sulfate
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SO₃²⁻ + H₂O → SO₄²⁻ + 2H⁺ + 2e⁻ (provides electrons and protons, I think)
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Why is the conical flask placed in ice?
answer
Benzoic acid low solubility in water => maximise yield of crstals
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2 benefits of suction filtration? (using a Buchner flask)
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Dries crystals Speeds up filtration process
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Why is the conical flask rinsed with the filtrate from the Buchner flask?
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To get rid of any remaining crystals
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Why are the crystals washed in ice-cold water?
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Remove any soluble impurities
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