Chemistry_Grade_10-12 (1).pdf

Ion dipole forces exist between ions and dipole

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Ion-dipole forces exist between ions and dipole molecules . The ion is attracted to the part of the molecule that has an opposite charge to its own. An example of this is when an ionic solid such as sodium chloride dissolves in water. Momentary dipole forces occur between two non-polar molecules , where at some point there is an uequal distribution of charge in the molecule. For example, there are London forces between two molecules of carbon dioxide. Hydrogen bonds occur between hydrogen atoms and other atoms that have a high electronegativity such as oxygen, nitrogen and fluorine. The hydrogen atom in one molecule will be attracted to the nitrogen atom in another molecule, for example. There are hydrogen bonds between water molecules and between ammonia molecules. Intermolecular forces affect the properties of substances. For example, the stronger the intermolecular forces, the higher the melting point of that substance, and the more likely that substance is to exist as a solid or liquid. Its boiling point will also be higher. In liquids , properties such as surface tension , capillarity and evaporation are the result of intermolecular forces. Exercise: Summary Exercise 1. Give one word or term for each of the following descriptions: (a) The tendency of an atom in a molecule to attract bonding electrons. (b) A molecule that has an unequal distribution of charge. (c) A charged atom. 2. For each of the following questions, choose the one correct answer from the list provided. (a) The following table gives the melting points of various hydrides: Hydride Melting point ( 0 C) HI -50 NH 3 -78 H 2 S -83 CH 4 -184 In which of these hydrides does hydrogen bonding occur? i. HI only ii. NH 3 only iii. HI and NH 3 only iv. HI, NH 3 and H 2 S (IEB Paper 2, 2003) (b) Refer to the list of substances below: HCl, Cl 2 , H 2 O, NH 3 , N 2 , HF Select the true statement from the list below: i. NH 3 is a non-polar molecule ii. The melting point of NH 3 will be higher than for Cl 2 iii. Ion-dipole forces exist between molecules of HF iv. At room temperature N 2 is usually a liquid 98
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CHAPTER 5. INTERMOLECULAR FORCES - GRADE 11 5.4 3. The respective boiling points for four chemical substances are given below: Hydrogen sulphide -60 0 C Ammonia -33 0 C Hydrogen fluoride 20 0 C Water 100 0 C (a) Which one of the substances exhibits the strongest forces of attraction between its molecules in the liquid state? (b) Give the name of the force responsible for the relatively high boiling points of ammonia and water and explain how this force originates. (c) The shapes of the molecules of hydrogen sulphide and water are similar, yet their boiling points differ. Explain. (IEB Paper 2, 2002) 99
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5.4 CHAPTER 5. INTERMOLECULAR FORCES - GRADE 11 100
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Chapter 6 Solutions and solubility - Grade 11 We are surrounded by different types of solutions in our daily lives. Any solution is made up of a solute and a solvent . A solute is a substance that dissolves in a solvent. In the case of a salt (NaCl) solution, the salt crystals are the solute. A solvent is the substance in which the solute dissolves. In the case of the NaCl solution, the solvent would be the water. In most cases, there
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