Download april test

School of Chemistry and Physics, Howard College Campus
University of KwaZulu-Natal
CHEMICAL ENGINEERING CHEMISTRY 1 (CHEM161)
Class Test 2: APRIL 2014
Duration: 40 minutes
Marks: 25
Student Number
Student Name
Tutorial Venue
Tutor
Question 1
(a)
Calculate the frequency (in s-1) of the electromagnetic radiation emitted by a hydrogen atom when its
electron undergoes a transition from the n = 4 level to the n = 2 level.
(2)
(b)
Consider the following diagram:
E3
E2
E
E1
(i)
Give the type of electromagnetic radiation shown in the diagram above.
(½)
(ii)
How many lines would you expect in the spectrum for the above transitions?
(½)
(iii)
Draw a fully labeled diagram showing the spectral lines.
(1½)
Question 2
(a)
Write two Lewis structures for cyanamide, NH2CN. Choose the more plausible structure (Use C as the
central atom).
(4)
2
(b)
Explain, in detail, whether you expect carbon tetrachloride (CCl4) to be polar or nonpolar (Hint. Your
answer must include Lewis structures, molecular geometry and net dipole moment).
(4)
Question 3
(a)
For the reaction
C6H14(l) + O2(g)  CO2(g) + H2O(l)
ΔHcombustion = -3246 J mol-1
Calculate the enthalpy of formation of n-hexane (C6H14) given ΔHfo of CO2 and water are -393.5 and
-285.8 kJ mol-1 respectively.
(2)
3
(b)
Use Hess’s law to calculate the standard enthalpy of formation of carbon disulfide (CS2) from its
elements, given that
C(graphite) + O2(g)  CO2(g)
ΔHrxno = - 393.5 kJ mol-1
S(rhombic) + O2(g)  SO2(g)
ΔHrxno = - 296.4 kJ mol-1
CS2(l) + 3O2(g)  CO2(g) + 2SO2(g)
ΔHrxno = - 393.5 kJ mol-1
(3½)
Question 4
(a)
The density of an unknown gas, at STP, is 1.429 g L-1. Calculate the molar mass of the gas (R = 0.0821
atm L mol-1 K-1).
(2)
4
(b)
In an experiment to determine the atomic weight of Al, 1.349 of Al metal was allowed to react with
excess dilute H2SO4 and 1.910 L of H2 gas was evolved and collected over water at 23°C and 746 mm of
Hg. Calculate the atomic weight of Al. (Vapour pressure of H2O at 23°C = 21.0 mmHg)
Al(s) + H2SO4(aq)  Al2(SO4)3(aq) + H2(g) (unbalanced)
5
(5)