A quality-control laboratory receives two clear liquid samples, P and Q, from a manufacturer of cleaning products. Both have molecular formula C3H8O and bot...
Question 1 Report
A quality-control laboratory receives two clear liquid samples, P and Q, from a manufacturer of cleaning products. Both have molecular formula C3H8O and both react with sodium metal to produce hydrogen gas. Fig. 1 shows the two possible alcohol structures. The chemist warms separate portions with acidified potassium dichromate solution, then adds sodium carbonate to the cooled products. Table 1 shows the observations.
| sample | result after oxidation and addition of sodium carbonate |
|---|---|
| P | effervescence; the gas turns limewater cloudy |
| Q | no effervescence |
(a) State the functional group present in each possible structure. [1]
(b) Give the general formula for the homologous series of alcohols. [1]
(c) Draw the displayed formula of propan-2-ol. [2]
(d) Use a balanced equation to show the reaction between propan-1-ol and sodium. [2]
(e) Calculate the volume of hydrogen, in cm3, made when 0.60 g of either alcohol reacts completely with excess sodium. Use molar gas volume = 24 000 cm3/mol. [3]
(f) Give two conditions needed for the oxidation reaction. [2]
(g) Explain the different observations for P and Q in Table 1. [4]
(h) Name the alcohol in sample P. [1]
Answer Details
(a) Both structures contain the hydroxyl group, −OH. [1]
(b) The general formula of alcohols is \(C_nH_{2n+1}OH\). [1]
(c) Propan-2-ol has a three-carbon chain with the hydroxyl group on the middle carbon: [2]
(d) \[2C_3H_7OH+2Na\rightarrow2C_3H_7ONa+H_2\] [2]
(e) \[n(\text{alcohol})=\frac{0.60}{60}=0.010\text{ mol}\] [1] Two moles of alcohol make one mole of hydrogen, so \[n(H_2)=\frac{0.010}{2}=0.0050\text{ mol}\] [1] \[V(H_2)=0.0050\times24000=120\text{ cm}^3\] [1]
(f) Use acidified potassium dichromate(VI) solution [1] and heat, typically under reflux. [1]
(g) P is a primary alcohol [1], so oxidation forms propanoic acid. [1] This acid reacts with sodium carbonate to make carbon dioxide [1], causing effervescence and making limewater cloudy. [1] Q is a secondary alcohol: it oxidises to a ketone, which does not react with sodium carbonate in this way.
(h) Sample P is propan-1-ol. [1]
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