Structure 2: Models of bonding and structureIB Chemistry SL: Topic test

20 questions, 54 marks

IB Chemistry SL

Structure 2: Models of bonding and structure topic test

Total 54 marks

Name

Class

Date

1
A student is given an unlabelled white crystalline solid, compound Q. Q has a high melting point of 801 °C. When solid, Q does not conduct electricity, but it conducts well both when molten and when dissolved in water. Q is formed from calcium and bromine.
(a)
What is the formula of compound Q?
[1 mark]
ACaBr2
BCaBr
CCa2Br
DCaBr3
(b)
What is the correct name of Q?
[1 mark]
Acalcium(II) bromide
Bcalcium bromide
Ccalcium dibromide
Dcalcium bromate
(c)
Explain, in terms of the ionic model, why Q conducts electricity when molten or dissolved in water but not as a solid.
[2 marks]
Total for question 1: 4 marks
2
Consider two molecules: boron trichloride, BCl3, and nitrogen trichloride, NCl3.
(a)
What are the electron domain geometry and molecular geometry of NCl3?
[1 mark]
Atrigonal planar; trigonal planar
Btetrahedral; tetrahedral
Ctetrahedral; trigonal pyramidal
Dtrigonal pyramidal; trigonal pyramidal
(b)
Which best explains the overall polarity of BCl3?
[1 mark]
Apolar, because B–Cl bonds are polar
Bpolar, because boron has a lone pair
Cnonpolar, because chlorine is more electronegative than boron
Dnonpolar, because the bond dipoles cancel due to the symmetrical trigonal planar shape
(c)
Explain why NCl3 is polar overall even though, like BCl3, it contains three identical polar N–Cl bonds.
[2 marks]
Total for question 2: 4 marks
3
Potassium is a group 1 metal (K⁺ has charge 1+, ionic radius 138 pm) and calcium is a group 2 metal (Ca²⁺ has charge 2+, ionic radius 100 pm). Potassium melts at 63 °C and calcium melts at 842 °C.
(a)
Explain, using the metallic bonding model, why calcium has a much higher melting point than potassium.
[3 marks]
(b)
Explain, in terms of the metallic bonding model, why both potassium and calcium conduct electricity and are malleable, and explain why calcium is expected to conduct electricity better than potassium.
[4 marks]
Total for question 3: 7 marks
4
The Pauling electronegativity values of four elements are: sodium 0.9, aluminium 1.6, silicon 1.8 and chlorine 3.2. A materials scientist compares three chlorides: NaCl (melting point 801 °C), AlCl3 (sublimes at 178 °C) and SiCl4 (boils at 57 °C).
(a)
Using electronegativity differences, place NaCl, AlCl3 and SiCl4 in order from most ionic to most covalent, and explain the trend in terms of the bonding triangle model. State the type of structure (giant ionic or simple molecular) most likely for each compound, given the melting/boiling points above.
[6 marks]
(b)
SiCl4 boils at only 57 °C while silicon dioxide, SiO2, an oxide of the same element, does not melt until 1713 °C. Explain this large difference in terms of structure and bonding, and state the type of intermolecular force responsible for the boiling point of SiCl4.
[6 marks]
Total for question 4: 12 marks
5
A cleaning products company uses three white solids: washing soda (sodium carbonate, Na2CO3), milk of magnesia (magnesium hydroxide, Mg(OH)2) and sal ammoniac (ammonium chloride, NH4Cl).
(a)
What is the charge on the carbonate ion in Na2CO3?
[1 mark]
A2−
B1−
C3−
D2+
(b)
Which formula correctly represents magnesium hydroxide?
[1 mark]
AMgOH
BMg(OH)2
CMg2OH
DMg(OH)3
(c)
State the formula of the ammonium ion, and explain why NH4Cl is classified as an ionic compound even though the ammonium ion itself is held together internally by covalent bonds.
[2 marks]
Total for question 5: 4 marks
6
Boron trifluoride, BF3, reacts with ammonia, NH3, to form the compound F3B–NH3, in which a new bond forms between the boron and nitrogen atoms using a lone pair originally on nitrogen.
(a)
What type of bond forms between boron and nitrogen in F3B–NH3?
[1 mark]
Aionic bond
Bhydrogen bond
Ccoordination (dative covalent) bond, in which both electrons come from nitrogen
Dmetallic bond
(b)
What is the electron domain geometry around the boron atom in F3B–NH3, after the new bond forms?
[1 mark]
Atrigonal planar
Btrigonal pyramidal
Clinear
Dtetrahedral
(c)
Before it reacts with ammonia, BF3 has only three electron domains around boron and is described as an 'electron-deficient' molecule. State the electron domain geometry and molecular geometry of BF3 before the reaction, and explain why boron can accept a lone pair from nitrogen to form the coordination bond.
[2 marks]
Total for question 6: 4 marks
7
A forensic chemist separates a dye mixture by paper chromatography, using propanone (a non-polar organic solvent) as the mobile phase; the stationary phase is water held within the cellulose fibres of the paper. Component P, a polar molecule capable of hydrogen bonding with water, travels 3.6 cm from the baseline. Component Q, a non-polar molecule, travels 8.1 cm. The solvent front travels 9.0 cm in the same time.
(a)
Calculate the Rf values of components P and Q.
[3 marks]
(b)
Explain, in terms of intermolecular forces, why component P has a much lower Rf value than component Q on this chromatogram.
[4 marks]
Total for question 7: 7 marks
8
A bicycle frame is made from an aluminium alloy called duralumin, which is aluminium mixed with about 4 % copper and small amounts of magnesium and manganese. The handlebar grips are made from poly(chloroethene) (PVC), an addition polymer, and are coated with a thin layer of pure aluminium for decoration.
(a)
Explain, in terms of structure and bonding, why duralumin (an alloy) is harder and stronger than pure aluminium, and why pure aluminium is nonetheless described as malleable.
[6 marks]
(b)
The PVC coating is an addition polymer made from the monomer chloroethene, CH2=CHCl. Describe the repeating unit of poly(chloroethene) and how the polymer forms from the monomer, and explain, in terms of intermolecular forces, why PVC is a solid at room temperature while chloroethene itself is a gas.
[6 marks]
Total for question 8: 12 marks

End of questions