O-LevelChemistryBonding & Structure5070/21 · Oct/Nov 2014 · Q610 Marks

Parts of the structures of silicon dioxide and poly(ethene) are shown below. [Figure showing the structure of silicon dioxide as a giant molecular network, with oxygen atoms and silicon atoms labelled.] [Figure showing the structure of poly(ethene) as a long chain of repeating -CH2-CH2- units, with H and C atoms labelled.]

(a)The melting point of silicon dioxide is 1610°C. Poly(ethene) starts to melt at 130°C. Explain, in terms of structure and bonding, the difference between the melting points of these two substances.[4]
(b)What type of polymerisation is used to make poly(ethene)?[1]
(c)Poly(ethene) is made from ethene monomers. Explain why ethene is both a hydrocarbon and an unsaturated compound.[2]
(d)Silicone fluids are polymers. Part of the structure of a silicone fluid is shown below. C2H5 C2H5 C2H5 -O-Si—O—Si—O-Si-O- C2H5 C2H5 C2H5 The monomer used in making this silicone fluid is a saturated compound with two –OH groups. Deduce the structure of this monomer.[1]
(e)The compound used to make the monomer of the silicone fluid has the following composition by mass. C = 18.6g, Cl = 55.0g, H = 4.65g, Si = 21.7g Deduce the empirical formula of this compound. empirical formula[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[4] (a) silicon dioxide is giant covalent structure/has a continuous structure of covalent bonds all linked in 3-dimensions (1) all bonds are strong/all bonds need high temperature to break/all bonds need a lot of energy to break (1) poly(ethene) has weak forces between the molecules/weak intermolecular forces (1) not much energy required to overcome weak forces/weak forces easily broken/small amount of energy required to separate molecules (1)
[1] (b) addition (polymerisation) (1)
[2] (c) hydrocarbon because contains carbon and hydrogen only/contains carbon and hydrogen and no other element (1) unsaturated because it has a (C=C) double bond (1)
[1] (d) [Structure showing Si bonded to two C2H5 groups and two OH groups, e.g., (C2H5)2Si(OH)2] (1)
[2] (e) C, H, Si, Cl ratios... (1) C2H6SiCl2 (1)

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