2. 1 Energetics Assessed Homework Ms

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2.1 ENERGETICS ASSESSED HOMEWORK MS
1. (a)
2220 × (1)
=1 510 000 kJ (unit must be correct for value) (1)
ignore ­ve sign
do not allow kJ mol­1 for unit
penalise significant figures (other than 3 or 4) once in (a) and (b) together
2
(b) (i)
the energy / enthalpy needed to break 1 mol of bonds (1)
averaged over a number of molecules containing that bond
or `different environments for the bond' (1)
reference to gas phase or to produce gaseous atoms (1)
ignore reference to s.t.p. or standard conditions
3
(ii) bonds broken: (2 × 348) + (8 × 412) + (5 × 496) = 6472 (1)
(condone wrong sign)
bonds formed: (6 × 743) + (8 × 463) = 8162 (1)
(condone wrong sign)
give one mark for correct 2:8:5 or 6:8 ratio if both totals above are
wrong
enthalpy change = ­1690 (ignore units) (1)
3
(iii) value in (c)(ii) used average / nonspecific values (1)
H2O is
in different states in the two equations / is a gas in (c) / is a liquid in
(b) (1)
2
[10]
2. (a)

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Enthalpy change when 1 mol of compound
1
Is formed from it's elements
1
All substances in their standard state
1
(b) H = Hc (reactants) ­ Hc (products)
1
= (7x ­ 394) + (4 x ­ 286) ­ (­ 3909)
1
= + 7 kJmol­1
1
(c) Heat change = m c T
1
= 250 × 4.18 × 60 = 62700J = 62.7kJ
1
Moles C7H8 = 2.5 /92 = 0.0272
1
H = 62.7 / 0.…read more

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K and 100 kPa or standard conditions 1
(b) H = bonds broken ­ bonds formed
1
= (6 × 412) + 612 +348 + (4.…read more

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­ 145 (1)
4
(c) H = 3×H (CO2) + 25 × H (H2O) ­H (N.G) (1)
= 3 × ­394 + 25 × ­242 ­ (­145) (1)
= ­1642 (1)
(or H = H (N.G) ­3 × H ( ½N2) (1)
= ­1540 ­3 × 34 (1) = ­1642 (1) )
3
(d) does not require Oxygen (1)
(or lower Ea)
1
(e) +44 (1)
energy needed to break bonds (1)
2
[12]
5.…read more

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J mol­1
Mark one : 2 × (answer to (c))
Mark two : Dividing by answers to (b)
Allow 133 ­ 134
Penalise incorrect units
Mark conseq to equation in (a) for full marks, also to that in (c)
If No working is shown and answer is incorrect zero
2
(e) Incomplete reaction or Heat loss (1)
1
[8]…read more

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