3.6. Intermolecular forces, electronegativity and bond properties
A subsection of Chemistry, 9701, through 3. Chemical bonding
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The strength of interaction between particles determines whether the substance is a solid, liquid or gas at room temperature. Lithiumsulfide, Li2S, is a crystalline solid with a melting point of 938 °C. It conducts electricity when it is molten. Give the formulae of the particles present in solid lithiumsulfide. Explain, in terms of the structure of the crystalline solid, why lithiumsulfide has a high melting point. Carbon monoxide, CO, is a gas at room temperature and pressure. It contains a coordinate bond. Explain what is meant by coordinate bond. Draw a ‘dot-and-cross’ diagram to show the arrangement of outer electrons in CO. Show the electrons belonging to the C atom as ×. Show the electrons belonging to the O atom as ●.  Nitrogen, N2, is also a gas at room temperature and pressure. Neither CO nor N2 is an ideal gas. State two assumptions that are made about the behaviour of particles in an ideal gas.  Explain why N2 does not behave as an ideal gas at very high pressures. Complete the table by naming all the types of intermolecular forces (van der Waals’) in separate samples of N2and CO. N2COnumber of electrons per molecule presence of a dipole moment   boiling point / °C –195.8 –191.5 intermolecular forces (van der Waals’)  Suggest why the bond in a molecule of CO contains a dipole moment. 
9701_s21_qp_22
THEORY
2021
Paper 2, Variant 2
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The melting points of some solids are shown in Table 1.1. Table 1.1 solid melting point / K magnesium phosphorus sodium chloride sulfur State the type of bonding present in magnesium and in sodium chloride. bonding in magnesium bonding in sodium chloride Explain the difference in the melting points of magnesium and sodium chloride. Explain the difference in the melting points of phosphorus and sulfur in terms of structure and bonding. Define electronegativity. Explain why electronegativity increases across a period. Name the strongest intermolecular force that exists between NH3molecules. Draw a diagram to show the formation of the strongest intermolecular force between two molecules of NH3. Include any relevant lone pairs of electrons and dipoles. The melting points of ice and ammonia are shown in Table 1.2. Table 1.2 solid melting point / K ice ammonia Suggest two reasons for the difference in the melting points of ice and ammonia.
9701_s23_qp_22
THEORY
2023
Paper 2, Variant 2
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