Question 47: [Physical > Chemical Energetics]
Gaseous phosphorus pentachloride can be decomposed into gaseous phosphorus trichloride and
chlorine by heating. The table below gives the bond energies.
|
Bond
|
Bond
energy / kJ mol–1
|
|
P-Cl
(in both chlorides)
|
330
|
|
Cl-Cl
|
240
|
What is the enthalpy change in the decomposition of PCl5 to PCl3 and Cl2?
A –420 kJ mol–1
B –90 kJ mol–1
C +90 kJ mol–1
D +420 kJ mol–1
Reference: Past Exam Paper - June 2003 Paper 1 Q7
Solution 47:
Answer: D.
Form a balanced equation.
PCl5(g)-->PCl3(g)+Cl2(g)
Enthalpy Change = Bond Broken - Bonds Formed
2 P-Cl bonds were broken and a Cl-Cl bond was formed
= (2*330) - 240 = +420 kJ/mol
Question 10: [Physical
> Chemical Energetics]:
At 600°C oxides of
nitrogen react with un-burnt hydrocarbons in a catalytic converter in a car exhaust.
The equation using methane as representative of a hydrocarbon molecule would be
as follows.
4NO + CH4 à
2N2 + CO2 + 2H2O
Which statement is
likely to be true about the energy change of this reaction?
A It is endothermic as heat energy is converted into
chemical energy.
B It is exothermic as a high temperature is required.
C It is exothermic as the triple bond NO is broken.
D It is exothermic as the products have large negative
enthalpy changes of formation.
Reference: Past Exam Paper – November 2002 Paper 1
Q10
Solution 10:
Answer: D.
A cannot be answer because conversion of energy is not endothermic
B cannot be an answer because exothermic reactions do not require a high temperature to start unlike endothermic reactions because exothermic reactions have a smaller activation energy compared to endothermic reactions . Exothermic reactions have s smaller activation energy and therefore do not require high temperature and therefore exothermic reactions occur more readily and at a faster rate than endothermic reactions
C cannot be answer because breaking of a bond is endothermic
Question 9: [Physical
> Chemical Energetics]:
The following energy
cycle represents the enthalpy changes in the formation of carbon dioxide from
its constituent elements in their standard states.
What substances are present at level Y in this diagram?
A C(g) + 2O(g)
B C(g) + O2(g)
C C(s) + O2(g)
D CO2(g)
Reference: Past Exam Paper – November 2002 Paper 1 Q9
Solution 9:
Answer: A.
At the zero level are the starting elements in their
standard states, so this immediately eliminates option C as the correct answer.
Option D is carbon dioxide which is the final product of the reaction thus will
be eliminated as level Y is an intermediate step, not the final. In option B
the carbon changes form from solid to gas. This would require energy to break
the weak forces of attraction between carbon atoms. In option A the strong
covalent bond within the oxygen molecule is being broken. This will require a
significantly higher amount of energy to overcome. Hence the energy level of A
must be higher than the energy level of B, making option A the correct answer.
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