1.This question is about the manufacture of ammonia. The equation for the formation of ammonia is N 2 (g) + 3H 2 (g) → 2NH 3 (g) The standard enthalpy of formation of ammonia is –46.2 kJ mol –1 . (a)(i) By reference to ammonia, define the term standard enthalpy of formation. (ii) Use the standard enthalpy of formation of ammonia to calculate the enthalpy change for the reaction shown in the equation above.
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1.This question is about the manufacture of
ammonia. The equation for the formation of
ammonia is N2(g) + 3H2(g) → 2NH3(g)
The standard enthalpy of formation of ammonia
is –46.2 kJ mol–1.
(a)(i) By reference to ammonia, define the term
standard enthalpy of formation.
(ii) Use the standard enthalpy of formation of
ammonia to calculate the enthalpy change for the
reaction shown in the equation above.
(iii) Use the bond enthalpies below to calculate
another value for the enthalpy change for the
reaction shown in the equation above. [Bond
enthalpies / kJ mol–1: N=N = +944; H—H = +436;
N—H = +388]
(iv) Suggest why the two values obtained for the
enthalpy change that you have calculated in
parts a) (ii) and iii are different, even though the
data in both cases are measured under standard
conditions.
(b)Draw an energy level diagram for the
formation reaction of ammonia.
2. Some standard enthalpy of combustion values are given below.
(i) Define standard enthalpy change of combustion of propane.(ii) Write a thermochemical equation for the standard enthalpy change of combustion of propane.
Substance ∆HCӨ/kJ mol─1
C (graphite) –393.5
H2(g) –285.8
propane C3H8(g) –2060.0
(iii) Use the data to calculate the standard
enthalpy change of propane. Draw a Hess’s Law
cycle as part of your answer. 3. Calculate the enthalpy of combustion of ane
by drawing an energy cycle and using the
following enthalpy of formation data:
Substance ∆Hf/kJ mol─1
methane ─75
Carbon dioxide ─394
water ─286
Relative abundance %
69 27 4
m/z 58 60 62
4. (a) Define the term relative atomic mass of an
element.
(b) A sample of nickel was analysed in a mass
spectrometer. Three peaks were observed with the
properties shown in the following table.
(i) Calculate the relative atomic mass of this
sample of nickel.
(ii) Identify species which was responsible for the
peak at m/z = 58.
(iii) What is the function of the electron gun and