Ethane reacts with chlorine when a mixture of the two gases is exposed to ultraviolet light. Fig. 8.1 shows the displayed formula of the main organic produc...
Question 1 Report
Ethane reacts with chlorine when a mixture of the two gases is exposed to ultraviolet light. Fig. 8.1 shows the displayed formula of the main organic product.
Fig. 8.1
Use these relative atomic masses where needed: Ar(H) = 1, Ar(C) = 12, Ar(Cl) = 35.5.
(a) State what is meant by a photochemical reaction. [1]
(b) Write the balanced symbol equation for the reaction of ethane with chlorine to form the compound shown in Fig. 8.1. [2]
(c) Name the type of reaction taking place and explain why ethane reacts in this way rather than by an addition reaction. [3]
(d) Name the inorganic product formed in part (b) and give its formula. [2]
(e) Give the molecular formula and a name of the organic compound formed when two hydrogen atoms on the same carbon atom of ethane are replaced by chlorine atoms. [2]
(f) 3.0 g of ethane reacts completely so that one hydrogen atom per molecule is replaced. Calculate the maximum mass of the compound shown in Fig. 8.1 that could be formed. [3]
(g) Ethene, C2H4, also reacts with chlorine. State the type of reaction, name the organic product, and explain why ethene reacts differently from ethane. [3]
Answer Details
The displayed formula in Fig. 8.1 shows chloroethane, C2H5Cl: an ethane molecule with one hydrogen replaced by a chlorine atom. This is a substitution reaction driven by ultraviolet light.
(a) A photochemical reaction is one that only takes place, or is started, when (ultraviolet) light energy is supplied [1].
(b) Balanced equation forming chloroethane:
C2H6 + Cl2 → C2H5Cl + HCl [2] (1 formulae, 1 balancing). One Cl ends up in the organic product and one in HCl.
(c) The reaction is substitution [1]; ethane is saturated and contains only single C-C bonds / no C=C double bond [1], so no atoms can add across a double bond; instead a hydrogen atom is replaced by a chlorine atom [1].
(d) The inorganic product is hydrogen chloride [1], formula HCl [1].
(e) Replacing two hydrogens on the same carbon gives C2H4Cl2 [1], named 1,1-dichloroethane (accept dichloroethane) [1].
(f) Mass calculation (Ar: H = 1, C = 12, Cl = 35.5):
- \(M_r(\text{C}_2\text{H}_6) = 2(12) + 6(1) = 30\); moles of ethane = \(3.0 / 30 = 0.10\) mol [1];
- the ratio ethane : chloroethane is 1 : 1, so moles of chloroethane = 0.10 mol; \(M_r(\text{C}_2\text{H}_5\text{Cl}) = 2(12) + 5(1) + 35.5 = 64.5\) [1];
- mass = \(0.10 \times 64.5 = 6.45\) g [1].
(g) Ethene reacts by addition [1]; the organic product is 1,2-dichloroethane (C2H4Cl2) [1]; ethene is unsaturated with a C=C double bond that opens so chlorine atoms add on, whereas ethane is saturated and can only react by substitution [1].
Exam tip: saturated (single bonds) means substitution + UV light + a hydrogen halide by-product; unsaturated (C=C) means addition across the double bond with no by-product.
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