Chapter 4: Chemical bonding
2 Aluminium chloride, AlCl3, and ammonia, NH3, are both covalent molecules. a i Draw a diagram of an ammonia molecule, showing its shape. Show any lone pairs of electrons. ii State the bond angle
N H
H
[3]
in the ammonia molecule.
[1]
b Explain why ammonia is a polar molecule. [2] c An ammonia molecule and an aluminium chloride molecule can join together by forming a co-ordinate bond. i Explain how a co-ordinate bond is formed. [1] ii Draw a dot-and-cross diagram to show the bonding in the compound formed between ammonia and aluminium chloride, H3NAlCl3. (Use a for a nitrogen electron, a for an aluminium electron and an for the hydrogen and chlorine electrons.) [3] d Aluminium chloride molecules join together to form a compound with the formula Al2Cl6. Draw a displayed formula (showing all atoms and bonds) to show the bonding in one Al2Cl6 molecule. Show the dative covalent bonds by arrows. [2] Total = 12 3 Electronegativity values can be used to predict the polarity of bonds. a Explain the term electronegativity. b The electronegativity values for some atoms are given below:
[2]
H = 2.1, C = 2.5, F = 4.0, Cl = 3.0, I = 2.5
c
Use these values to predict the polarity of each of the following bonds by copying the bonded atoms shown below and adding δ+ or δ– above each atom. i H I ii F I iii C Cl The shape of iodine trichloride, ICl3, is shown below. Cl
69
[2]
Cl I
Cl
i Describe the shape of this molecule. ii Explain why the ICl3 molecule has this shape. iii Suggest a value for the Cl I Cl bond angle. d The boiling points of the hydrogen halides are shown in the table. Hydrogen halide
HF
HCl
HBr
HI
Boiling point / °C
+20
–85
–67
–35
i Explain the trend in boiling points from HCl to HI. ii Explain why the boiling point of HF is so much higher than the boiling point of HCl. e Tetrachloromethane, CCl4, is a non-polar molecule. i Draw a diagram to show the shape of this molecule. ii Explain why this molecule is non-polar.
[2] [2] [1]
[2] [3] [2] [1] Total = 17