This diagram is used by a delivery company to compare the molecules in two bottled-fuel compounds. Propane is placed in one cylinder and butane in another. ...
Question 1 Report
This diagram is used by a delivery company to compare the molecules in two bottled-fuel compounds. Propane is placed in one cylinder and butane in another. Both are alkanes, but the company needs to state which cylinder contains the gas that is more easily liquefied under pressure. Fig. 1 shows displayed formulae for the two hydrocarbons. The carbon chain is longer in butane, so its molecules have stronger intermolecular forces than propane molecules. Neither compound reacts with bromine water in the dark, unlike ethene made by cracking.
(a) Give the molecular formula of propane. [1]
(b) Complete the molecular formula of butane: C4H__. [1]
(c) Which compound, propane or butane, has the higher boiling temperature? [1]
(d) Explain why the compound chosen in (c) has a higher boiling temperature. [2]
(e) State the observation when bromine water is added to either compound and kept away from ultraviolet light. [1]
(f) Name the unsaturated hydrocarbon with formula C2H4. [1]
Answer Details
(a) Propane has three carbon atoms and eight hydrogen atoms: \(\mathrm{C_3H_8}\). [1]
(b) Butane is an alkane, so it follows \(\mathrm{C_nH_{2n+2}}\). For \(n=4\), \(H=2(4)+2=10\). The missing number is 10. [1]
(c) Butane has the higher boiling temperature. [1]
(d) Butane molecules are larger and have more electrons than propane molecules. This gives stronger intermolecular forces, so more energy is needed to separate the molecules when boiling. [2]
(e) Both compounds are saturated alkanes, so under these conditions bromine water remains orange/orange-brown. [1]
(f) \(\mathrm{C_2H_4}\) is ethene. It is an alkene because it contains a carbon-carbon double bond. [1]
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