1 1. a. An ammonium ion, made by the reaction between ammonia molecule and a hydrogen ion, can be represented as shown in the diagram below. (i) Name the type of bond represented in the diagram N-H Covalent bonding (ii) Name the type of bond represented in the diagram by N→H Co-ordinate bond (iii) In terms of electrons, explain why an arrow is used to represent this N → H bond It’s because both electrons used in the bonding come from nitrogen. (iv) In terms of electron pairs, explain why the bond angles in the NH4+ ion are all 109O 28’? It’s because the bonding electron pairs they repel equally therefore they will be apart as far as possible to experience little repulsion between them hence forming the tetrahedral shape. b) Define electronegativity? This is the ability of an atom or element to attract an electron in a covalent bond. c) A bond between nitrogen and hydrogen can be represented as δN - δH (i) In this representation, what is the meaning of the symbol δ +? It means electron deficient (ii) From this bond representation, what can be deduced about the electronegativity of hydrogen relative to that of nitrogen? Electronegativity of hydrogen is less than that of Nitrogen. 2 2. a) Both HF and HCl are molecules having a polar covalent bond. Their boiling points are 293 K and 188 K respectively. (i) State which property of the atoms involved causes a bond to be polar. This is the electronegativity difference between the atoms. (ii) Explain, in terms of the intermolecular forces present in each compound, why HF has a higher boiling point than HCl. This is because HF has hydrogen bonding and HCl has van der waals’ forces hence the hydrogen bond is stronger and more energy is required to break the bonds. b) When aluminium chloride reacts with chloride ions, as shown by the equation below, a co-ordinate bond is formed. AlCl3 + Cl- AlCl-4 Explain how this co-ordinate bond is formed. In a co-ordinate bond both shared electrons are donated by the same atom hence the electron lone pair is donated from the chloride ion to the aluminium atom. c) Draw the shape of the PCl5 molecule and of the PCl4+ ion. State the values of the bond angles. PCl5 has a bond angle of 90O and 120O and PCl4+ has a bond angle of 109.5O 3 3. Lithium hydride, LiH, is an ionic compound containing the hydride ion, H- The reaction between LiH and aluminium chloride, AlCl3, produces the ionic compound LiAlH4 a) Balance the equation below which represents the reaction between LiH and AlCl3 4LiH + AlCl3 LiAlH4 + 3LiCl b) Give the electronic configuration of the hydride ion, HH- = 1s2 c) Predict the shape of the AlH4- ion. Explain why it has this shape. It has a tetrahedral shape since there is equal repulsion between the bonding pairs d) A bond can be represented by H Al, Name this type of bond and explain how it is formed. This is a co-ordinate bond and the lone pair of electrons both they are donated from H- to Al. 4 4. The table below shows some values of melting points and some heat energies needed for melting Substance I2 Melting point/ K Heat energy for melting/kJ mol-1 387 HF HCl HI NaCl 158 1074 190 7.9 222 2.0 28.9 3.9 2.9 a) Name three types of intermolecular forces Hydrogen bonding, van der waals’ and dipole – dipole forces. b) (i) Describe the bonding in a crystal of iodine There is covalent bonding within the molecule and there are van der waals’ forces between the molecules (ii) Name the crystal type which describes an iodine crystal Molecular structure (iii) Explain why heat energy is required to melt an iodine crystal. This is because the van der waals’ forces between the molecules must be weakened or broken. c) In terms of intermolecular forces involved, suggest why (i) Hydrogen fluoride requires more heat energy for melting than does of hydrogen chloride. This is because HF has hydrogen bonding between its molecules and HCl has van der waals’ forces hence the hydrogen bond is stronger than van der waals’ (ii) Hydrogen iodide requires more heat energy for melting than does hydrogen chloride. This is because Hydrogen iodide is a larger molecule than hydrogen chloride and so has more electrons in its structure. This means that it has stronger van der waals’ forces between its molecules hence more heat energy is required. d) Explain why the : (i) heat energy required to melt sodium chloride is large 5 This is because it has an ionic structure therefore has strong forces between its ions hence heat energy required is high. (ii) The heat energy needed to vaporise one mole of sodium chloride (171KJ mol-1) is much greater than the heat energy required to melt one mole of sodium chloride. This is because when vaporising all bonds they must be broken whereas as melting it only breaks the structure and leave a liquid form hence much greater energy required when vaporising. e) In terms of structure and bonding, suggest why graphite has a very high melting and boiling point Its’ because graphite has a giant covalent structure and has strong covalent bonds between its carbon atoms hence large energy required to break the bonds. 6 5. The diagram below represents a section of a crystal of silicon dioxide a) Name an element which has a structure similar to this. Carbon b) Name the type of bonding between silicon and oxygen in this crystal Covalent bonding c) Name the type of structure illustrated by this diagram. Macromolecule structure. d) Describe the motion of the atoms in this crystalline solid They vibrate about a fixed position. e) In terms of structure and bonding, describe what happens to the atoms in this crystal when it melts The bonds they break and the molecules or atoms they then become mobile being able to move about. f) Explain why this crystal is a non-conductor of electricity in the solid state and why graphite is a good conductor. This is because electrons are not free in silicon dioxide whereas there are delocalised electrons in graphite therefore it can easily conduct electricity. 7 REFERENCES: Harden, H., 2011. Chemistry. London: Hodder Education. Kostiner, E. and Jespersen, N., 2003. Chemistry. Hauppauge, N.Y.: Barron's. Lister, T. and Renshaw, J., 2000. Understanding Chemistry For Advanced Level. Cheltenham: Stanley Thornes. NORRIS, R., 2017. Essential Chemistry For Cambridge Igcse(R) Print And Online Student Book Pack. [Place of publication not identified]: OXFORD UNIV Press. Older, J. and Smith, M., 2015. OCR A Level Chemistry 2. Hodder Education Group. Vokins, M., 2000. Chemistry. Harlow: Longman.
