Unit 2 – Shapes & Bond Angles
Unit 2 - Jan 2009
Q1.
This question is about boron and nitrogen compounds.
(a)  Draw and name the shape of a boron trifluoride, BF3, molecule.  Suggest the FBF bond angle.
(3)

Name of shape
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FBF bond angle
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(b)  Ammonia has the formula NH3.  Its HNH bond angle is less than the FBF bond angle in boron trifluoride.
(i)   Estimate the HNH bond angle in NH3.
(1)
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(ii)   Explain why the HNH bond angle is less than that for FBF.
(1)
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 (iii)   Name the strongest intermolecular force between BF3 molecules.
(1)
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(iv)   Name the strongest intermolecular force between NH3 molecules.
(1)
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(c)  Ammonia will react with oxygen in the presence of a platinum catalyst at 750°C
       forming water and nitrogen(II) oxide, NO.
(i)   What is the oxidation number of nitrogen in ammonia, NH3?
(1)
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(ii)   The diagram below shows the distribution of molecular energies in the reaction at 750°C.  On the same diagram, draw a curve to show the distribution at 500°C and explain what effect this change in temperature would have on the rate of the reaction.
(3)

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*(d)  Explain how a catalyst speeds up the rate of a reaction.
(3)
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(Total for question = 14 marks)

Unit 2 - Jun 2009
Q2.
 Which of these bond angles is the smallest?
   A     HNH in NH3
   B     HCH in CH4
   C     HOH in H2O
   D     OCO in CO2
(Total for question = 1 mark)

Q3.
 Which statement best describes the shape and bond angles in the molecule SF6?
   A     Octahedral, 90° and 180°
   B     Trigonal bipyramidal, 90° and 180°
   C     Octahedral, 90° and 120°
   D     Trigonal bipyramidal, 90° and 120°
(Total for question = 1 mark)

Unit 2 - Jan 2010
Q4.
 Which of the following molecules is linear?
   A     Carbon dioxide, CO2
   B     Sulfur dioxide, SO2
   C     Water, H2O
   D     Methanal, HCHO

(Total for question = 1 mark)

Unit 2 - Jun 2010
Q5.
 This question is about bond angles.
A    90°
B    104°
C    107°
D    109.5°
Select, from A to D above, the most likely value for the bond angle of
(a) HCH in methane, CH4.
(1)
   A
   B
   C
   D
(b) FSF in sulfur hexafluoride, SF6.
(1)
   A
   B
   C
   D
(c) FOF in oxygen difluoride, OF2.
(1)
   A
   B
   C
   D
(Total for question = 3 marks)

Unit 2 - Jan 2011
Q6.
 This question is about methanol, CH3OH, and ethanol, CH3CH2OH.
(a) (i) Draw a dot and cross diagram for methanol, showing outer electrons only.
(1)

     (ii) Give the approximate values for the HCH and COH bond angles in methanol.
                  Justify your answers.
(4)
HCH angle .........................
Justification
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COH angle .........................
Justification
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     (iii) Using displayed formulae, draw a diagram to show a hydrogen bond between two methanol molecules. On your diagram, show the bond angle around the hydrogen atom of the hydrogen bond and give its value.
(2)

(b) Methanol reacts with sodium.
(i) State what you would observe in this reaction.
(2)
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(ii) Write the equation for this reaction. State symbols are not required.
(1)

(c) Ethanol can be used to make ethanal.
(i) Identify, by name or formula, the two chemicals you would use to make ethanal from ethanol in the laboratory.
(2)
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(ii) Draw a diagram of the apparatus you would use to prepare ethanal from ethanol in the laboratory and collect the product.
(2)

(iii) Both ethanal and propane have a molar mass of 44 g mol−1, but their boiling temperatures are different.

Suggest which substance has the higher boiling temperature. Justify your answer by comparing the intermolecular forces in each compound.
(2)
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 (Total for question = 16 marks)

Unit 2 - Jun 2011
Q7.
 What is the FBF bond angle in boron trifluoride, BF3?
   A    180°
   B    120°
   C    109.5°
   D    90°
(Total for Question = 1 mark)

Q8.
This question is about ethanethiol, CH3CH2SH. Thiols are like alcohols, but the oxygen atom has been replaced by a sulfur atom. They react in a similar way to alcohols.
(a) (i)   Draw a dot and cross diagram for ethanethiol, showing outer electrons only.
(2)

    (ii)  Give the value for the CSH bond angle in ethanethiol. Justify your answer.
(3)
CSH angle .........................
Justification
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(b) There are hydrogen bonds between ethanol molecules but not between ethanethiol molecules.
      (i)   Explain why the bond angle around the hydrogen atom involved in a hydrogen bond is 180°.
(2)
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      (ii)  Explain why there are no hydrogen bonds between ethanethiol molecules.
(1)
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(c) (i)   Describe the formation of London forces.
(2)
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      (ii)  Explain why the London forces in ethanethiol are stronger than those in ethanol.
(1)
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(d) The reaction of sodium with ethanethiol, CH3CH2SH, is similar to its reaction with ethanol.
      (i)    Suggest one observation you would make when sodium is added to ethanethiol.
(1)
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      (ii)   Suggest a balanced equation for this reaction. State symbols are not required.
(1)

(e) Ethanol can be made from bromoethane by reaction with aqueous potassium hydroxide, KOH(aq), under suitable conditions.
      (i)   Write the equation for this reaction. State symbols are not required.
(1)

      (ii)  State the type and mechanism of this reaction.
(2)
Type
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Mechanism
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     (iii)  Suggest the formula of a suitable chemical to make ethanethiol from bromoethane.
(1)
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(f) When ethanethiol undergoes complete combustion in air, a gas is produced which is not formed on the complete combustion of ethanol. Identify the gas and suggest why it is damaging to the environment.
(2)
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(Total for Question = 19 marks)

Unit 2 - Jan 2012
Q9.
This question concerns the shapes of molecules and ions:
A      linear
B      trigonal planar
C      pyramidal
D      tetrahedral
Select from A to D the shape of

 (a) boron trichloride, BCl3
(1)
   A
   B
   C
   D

(b) the ammonium ion, NH4+
(1)
   A
   B
   C
   D

(c) carbon dioxide, CO2
(1)
   A
   B
   C
   D

(Total for question = 3 marks)

Unit 2 - Jun 2012
Q10.
The oxygen atom in a molecule of water has two bonding pairs and two lone pairs of electrons. Based on the electron-pair repulsion theory, the H–O–H bond angle is most likely to be
   A       180°
   B       109.5°
   C       107°
   D       104.5°
(Total for question = 1 mark)

Q11.
 The diagram below shows a dot and cross diagram of nitric acid.

      (a) Identify which of the labelled sets of electrons represents a dative covalent bond.
(1)
   A  
   B  
   C  
   D  

      (b) In terms of orbital overlap, the double bond is
(1)
   A      A π bond.
   B      two σ bonds.
   C      two π bonds.
   D      A σ bond and a π bond.
(Total for question = 2 marks)

Q12.
The shape of a molecule of boron trifluoride, BF3, is
   A       trigonal planar.
   B       pyramidal.
   C       tetrahedral.
   D       T–shaped.
(Total for question = 1 mark)

Unit 2 - Jun 2013
Q13.
Which of the following molecules has a linear shape and bond angles of 180°?
   A    CH4
   B    H2O
   C    CO2
   D    SF6

(Total for Question = 1 mark)
Q14.
Nanorockets have generated a lot of excitement due to their potential uses in the medicinal field, such as in the delivery of drugs around the body.

Some bacteria have the ability to move at speeds of 100 times their body length per second. Scientists in one study made nanorockets that reached speeds of up to 200 times their length per second. These scientists made their rockets on a nano scale (10−9) from nanotubes with platinum coated on the inside as a catalyst. The fuel used to power these tiny rockets was hydrogen peroxide, H2O2, which forms water and oxygen gas when undergoing decomposition.

Other forms of nanotechnology are already being used. For example, some sun creams use nanoparticles of titanium(IV) oxide which form an invisible protective layer against UV radiation.

(a)  Write an equation for the catalytic decomposition of hydrogen peroxide. State symbols are not required.
(1)

(b)  Draw a dot and cross diagram to show the electronic configuration of the oxygen gas produced in the breakdown of the hydrogen peroxide (only outer electrons should be shown).
(1)

(c)  Suggest a dot and cross diagram for the hydrogen peroxide molecule in which each oxygen atom is covalently bonded to one hydrogen atom (only outer electrons should be shown).
(2)

*(d)  The bond angles in hydrogen peroxide are similar to those in a water molecule. Suggest a bond angle for hydrogen peroxide and reasons for your value.
(3)
Bond Angle
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Reasons
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(e)  In the future, the aim is to develop a nanorocket that can use a fuel such as glucose rather than hydrogen peroxide. Suggest an advantage of using glucose and a disadvantage of using hydrogen peroxide.
(2)
Glucose advantage
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Hydrogen peroxide disadvantage
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(f)  The boiling temperature of hydrogen peroxide is relatively high, about 150°C, for such a small molecule. Explain fully why this is the case.
(2)
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(g)  It is suggested that hydrogen peroxide is a polar liquid.
Describe how you might carry out an experiment to test whether a liquid is polar.
(3)
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*(h)  The speed of the nanorockets is controlled by the rate of decomposition of hydrogen peroxide. The speed at the body temperature of 37 °C is faster than under normal laboratory conditions. Draw Maxwell-Boltzmann distribution curves on the axes below. Label your diagram and use it to explain why the increase in the speed of the rockets occurred.
(4)

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(i)  The scientists used platinum in their nanorockets. Explain the catalytic role of the platinum in the reaction.
(2)
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(j)  Nanotubes can be made from carbon. These carbon nanotubes can be good electrical conductors in a similar way to graphite.
Explain why they are able to conduct electricity.
(2)
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(k)  Some scientists are concerned that the use of nanoparticles in cosmetic products, such as sun cream, could pose a health hazard. Suggest why this might be the case.
(1)
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(Total for Question = 23 marks)

Q15.
Which of the following molecules has the smallest bond angle?
   A    H2O
   B    NH3
   C    CH4
   D    SF6

(Total for Question = 1 mark)

Unit 2 - Jun 2014
Q16.
This question concerns the shapes of the hydrides of Group 5 elements.
(a)  What is the approximate H—N—H bond angle in the ammonium ion, NH?
(1)
   A    90°
   B    104.5°
   C    107°
   D    109.5°

(b)  Suggest the shape of the phosphine molecule, PH3.
(1)
   A    Trigonal planar
   B    Trigonal pyramidal
   C    Trigonal bipyramidal
   D    Octahedral
 (Total for question = 2 marks)

Q17.
Spontaneous combustion is often a subject of fantasy in movies, but it does actually happen with some chemical compounds. One such compound is silane, SiH4, which is analogous to methane, CH4. Methane is the main gas that is used in school and college laboratories with Bunsen burners, but it requires a spark or a lighted splint to ignite. Silane does not require any such ignition, and at room temperature is spontaneously flammable. This can make an interesting chemical demonstration.
One method of making silane is by mixing together two solids, silicon dioxide and magnesium, with the magnesium being in excess. The two chemicals are thoroughly mixed together. This mixture is then heated to red-heat and initially produces powdered silicon. The silicon then reacts further with the excess magnesium powder and forms magnesium silicide, Mg2Si. The two reactions which occur are shown in the equations below.
SiO2 + 2Mg → 2MgO + Si
Si + 2Mg → Mg2Si
The magnesium silicide formed is then reacted with hydrochloric acid to form silane, SiH4.

(a)  Complete and balance the equation for this reaction. State symbols are not required.
(2)
Mg2Si + ............HCl → ................................................. + .................................................
(b)  Bubbles of silane rise to the surface in the reaction mixture and spontaneously combust with oxygen in the air.
Suggest the names or formulae of the products of the reaction between silane and oxygen.
(2)
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(c)  Predict the molecular shape of silane, SiH4, and suggest the bond angle.
(2)
Shape
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Bond angle
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(d)  Explain why the Si—H bond is longer than the C—H bond.
(2)
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*(e)  Identify the intermolecular forces present in pure samples of both silane and methane.
Explain why silane has a higher boiling temperature than methane and why both are gases at room temperature.
(4)
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(f)  (i)  Define the term electronegativity.
(2)
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*(ii)  Some Pauling electronegativity values for selected elements are given below.

Using the values in the table above, compare the polarity of the bonds in a molecule of methane with that found in a molecule of silane.
Comment on the significance of any difference.
(3)
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(iii)  Using the table in (f)(ii), choose an element which, when covalently bonded to hydrogen, forms a molecule containing bonds that are more polar than those in silane or methane. Give the formula of the hydride of your chosen element and state the electronegativity difference.
(2)
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(iv)  Explain why it is possible for the bonds within a molecule to be polar, but for the molecule itself to be non-polar. Give an example of such a molecule.
(2)
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(Total for question = 21 marks)