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Form 4 Chemistry Paper 2 Sample Revision Questions With Answers

Given that:
Enthalpy of combustion of Carbon (graphite) = $-393kJmol^-1$
Enthalpy of combustion of carbon (II) oxide = $-283 kJmol^-1$
(a) Write down the thermo-chemical equations for the combustion of:
(i) Carbon (graphite)
(ii) Carbon (II) oxide
(b) Determine the enthalpy of formation of carbon (II) oxide.
(c) Draw an energy level diagram for the processes in (a) and (b) above.
(d) State two properties of Carbon (IV) Oxide that make it suitable for use in fire extinguishers.
(e) When 8.0 g of sulphur is completely burned in oxygen in a calorimeter, the heat evolved raises the temperature of $500 cm^3$ of water by $35^0C$ . Calculate the heat of combustion of sulphur. (The specific heat capacity of water = $4.2 kJKg^-1K^-1, S = 32$ ).

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Answer Text:
a)i). $C_(s) + O_(2 (g))$ → $CO_(2(g))$ $DeltaH = -393 kJmol^-1$
ii) $CO_(g) +1/2 O_(2(g)) → CO_(2(g)) DeltaH = -283 kJmol^-1$
b). $C_(s) + 1/2 O_(2(g)) → CO_(g) DeltaHf =?$
$CO_(g) + 1/2 O_(2(g)) → CO_(2(g)) DeltaHc = -283 kJmol^-1$
$C_(s)+ O_(2(g)) → CO DeltaHc = -393 kJmol^-1$
$DeltaH_f (CO) = DeltaH C (s ) - DeltaHC(CO)$
= $-393 – (-283) = -110 kJmol^-1$
c).
d). Denser than air / does not support combustion / does not burn.
e). $DeltaH = mcDeltaT = 500/1000 times 4.2 times 35 = 73.5$
Heat of combustion =
$32/8 times 73.5= 294 kJmol^-1$