2. 1 Energetics Test Markscheme

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  • Created on: 30-04-13 06:30
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2.1 TEST MARK SCHEME
1. (a)
Enthalpy (Energy) to break a (covalent) bond (1) OR dissociation
energy
Varies between compounds so average value used (1) QL mark
OR average of dissociation energies in a single molecule / e.g.
CH4
Do not allow mention of energy to form bonds
but with this case can allow second mark otherwise 2nd mark
consequential on first
2
(b) (i)
1/2 N2 + 3/2 H2 NH3 (1)
Ignore s s
(ii) H = ()bonds broken ­ ()bonds formed (1)
= 1/2 × 944 + 3/2 × 436 ­ 3 × 388 (1)
= ­38 kJ mol­1 (1)
Ignore no units, penalise wrong units
Score 2/3 for 76
1/3 for +38
Allow 1/3 for +76
4
(c) 4 (C­H) + (C=C) + (H­H) ­ (6 (C­H) + (C­C)) = ­136 (1)
(C=C) + (H­H) ­ ((C­C) + 2 (C­H)) = ­136
2 (C­H) = 836 (1)
(C­H) = 418 (kJ mol­1) (1)
Note: allow (1) for ­836
another (1) for ­418
3
[9]
2. (a)
H = (bonds broken) ­ (bonds formed) (or cycle)
1

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­ 496/2 (or 2 × 463 + 146 ­(2 × 463 + 496/2)
1
= ­ 102 (kJ mol­1) (1)
1
(accept no units, wrong units loses a mark
+102 scores (1) only)
(b) C(s) + 2H2(g) CH4(g) equation (1) Correct state symbols (1)
2
(c) (i)
Macromolecular
1
(accept giant molecule or carbon has many
(4) bonds)
(ii) H = Hf(products) ­ Hf (reactants) (or cycle)
1
= 715 + 4 × 218 ­ (­74.…read more

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C2H6 + 3½O2 2CO2 + 3H2O (1)
(ii) H = 2 × H (CO2) + 3 × H (H2O) ­ H (C2H6) (1)
= ­ 788 ­ 858 ­ (­85)
= ­ 1561 kJ mol­1
(1) (1)
4
[6]
4. (b)
H = Hc (reactants) ­ Hc (products)
1
= (7x ­ 394) + (4 x ­ 286) ­ (­ 3909)
1
= + 7 kJmol­1
1
(b) q = mcT = 250 x 60 x 4.18 = 62700J = 62.…read more

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Allow 1/3 for +773
[9]
5.…read more

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CE
for /2
Note various combinations of answers to part (c) score as follows:
­89 to ­91 kJ (8) (or ­89000 to 91000J)
no units (7)
+89 to +91 kJ (7) (or + 89000 to +91000J)
no units (6)
­44 to ­46 kJ (5) (or 44000 to 46000J)
no units (4) if units after 6.72 or 6720 (5)
+44 to +46 kJ (4) (or +44000 to + 46000)
if no units and
if no units after 6.…read more

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