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C4.1 Endothermic and exothermic reactions AQA AS Chemistry C4 Energetics And Kerboodle Answers : Page No. 74
Summary questions
1 Natural gas, methane, CH4, gives out 890 kJ when one mole is burnt completely.
CH4(g) + 202(g) -C02(g) + 2H20(I)
Calculate how much heat would be given out when 8 g of methane is burnt completely.
445 kJ
2 The following reaction does not take place under normal conditions. C02(g) + 2H20(l)-CH4(g) + 202(g)
If it did, would you expect it to be exothermic or endothermic?
Endothermic
C4.2 Enthalpy AQA AS Chemistry C4 Energetics And Kerboodle Answers: Page No. 76
Summary questions
CH4(g) + 2O2(g) → CO2(g) + 2H2O(1) ∆H298 = -890 kJ mol-1.
Change
Enthalpy (heat energy)
c State what the 298 indicates.
That the enthalpy change is measured at 298 K.
That heat is given out.
Exotherrnic
C4.3 Measuring enthalpy changes AQA AS Chemistry C4 Energetics And Kerboodle Answers: Page No. 81
Summary questions
-672 kJ mo1-1
-46.2 kJ mo1-1
It will be smaller (less nega1ive).
This is caused by hea1 loss.
The enthalpy change.
b State what the term m represents
The mass of water (or other 5ubstancc in which the heat is collected).
c State what the term c represents
The specific heat capaci1y or 1hc wa1cr (or other substance).
d State what the term A T represents.
The temperature change.
C4.4 Hess’s law AQA AS Chemistry C4 Energetics And Kerboodle Answers: Page No. 84
a -70 kJ mol-1
b-217 kJ mol-1
c-97 kJ mol-1
d -191 kJ mol-1
e +301 kJ mol-1
C4.5 Enthalpy changes of combustion AQA AS Chemistry C4 Energetics And Kerboodle Answers: Page No. 86
Summary questions
Via ∆cH-85 kJ mol-1 difference is due to rounding errors in the data
C4.6 Representing thermochemical cycles: Page No. 90
a -70 kJ mol-1
b217 kJ mol-1
c -97 kJ mo1-1
d -191 kJ mol-1
e + 301 kJ mol-1
4.7 Bond enthalpies AQA AS Chemistry C4 Energetics And Kerboodle Answers : Page No. 93
Summary questions
1 x C-C, 6 x C-H, 1 x Br-Br
3018 kJ mol1
1 x C-C, 5 x C-H. 1 x C-Br, 1 x H-Br
b How much energy does this take?
3063 kJ mol-1
45 kJ mol-1
a State what is AH for the reaction (this requires a sign)•
-45 kJ mo1-1
b Identify if the reaction in part a is endothermic or exothermic.
Exothermic
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Practice questions: Page No. 94-95
(a) State Hess’s Law. (1 mark)
The enthalpy change / heat (energy) change (at constant pressure) in a reaction is independent of the route / path taken (and depends only on the initial and final states).
M1 q = m c ΔT OR calculation (25.0 × 4.18 × 14.0)
M2 = 1463 J OR 1.46 kJ (This also scores M1)
M3 must have both the correct value within the range specified and the minus sign
For 0.0210 mol, therefore
ΔH1 = – 69.67 to – 69.52 kJ mol-1
OR ΔH1 = – 69.7 to – 69.5 kJ mol-1
The idea of heat loss.
(e) Suggest one reason why the value for AH exp cannot be measured directly. AQA. 2013
Impossible to add / react the exact / precise amount of water.
3N2H4 → 4NH3 + N2.
(b) State the meaning of the term mean bond enthalpy.
M1 enthalpy / heat (energy) change / required / needed to break / dissociate a covalent bond (or a specified covalent bond).
M2 average / mean over different molecules / compounds / substances.
(c) Some mean bond enthalpies are given in the table.
M1
⅀ (bonds broken) – ⅀ (bonds formed) = ΔH.
Heat (energy) change at constant pressure.
The enthalpy change / heat (energy) change (at constant pressure) in a reaction is independent of the route / path taken (and depends only on the initial and final states).
ΔH + 963 = –75 – 432 OR ΔH + 963 = – 507 (M1)
ΔH = –75 – 432 – 963 (M1 and M2)
ΔH = –1470 kJ mol–1
Award 1 mark for + 1470
4 (a) Define the term standard enthalpy of combustion, AcH9. (3 marks)
Enthalpy change / heat energy change when1 mol of a substance is completely burned in oxygen at 298 K and 100 kPa or standard conditions
ΔH = Σ(bonds broken) − Σ(bonds formed)
= (6 × 412) + 612 + 348 + (4.5 × 496) − [(6 × 743) + (6 × 463)]
= −1572 kJ mol−1
(c) State why the standard enthalpy of formation, AcH6, of oxygen is zero. (1 mark)
By definition ΔfH is formation from an element
ΔcH =ΣΔfH(products) – ΣΔfH (reactants or cycle)
= (3 × −394) + (3 × −242) − (+20)
= −1928 kJ mol−1
Bond enthalpies are mean / average values from a range of compounds.
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