Chemistry_Grade_10-12 (1).pdf

In order for a solid to melt the energy of the

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In order for a solid to melt, the energy of the particles must increase enough to overcome the bonds that are holding the particles together. It makes sense then that a solid which is held together by strong bonds will have a higher melting point than one where the bonds are weak, because more energy (heat) is needed to break the bonds. In the examples we have looked at, metals, ionic solids and some atomic lattices (e.g. diamond) have high melting points, whereas the melting points for molecular solids and other atomic lattices (e.g. graphite) are much lower. Generally, the intermolecular forces between molecular solids are weaker than those between ionic and metallic solids. 2. Boiling point Definition: Boiling point The temperature at which a liquid changes its phase to become a gas . 28
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CHAPTER 2. WHAT ARE THE OBJECTS AROUND US MADE OF? - GRADE 10 2.5 When the temperature of a liquid increases, the average kinetic energy of the particles also increases, and they are able to overcome the bonding forces that are holding them in the liquid. When boiling point is reached, evaporation takes place and some particles in the liquid become a gas. In other words, the energy of the particles is too great for them to be held in a liquid anymore. The stronger the bonds within a liquid, the higher the boiling point needs to be in order to break these bonds. Metallic and ionic compounds have high boiling points while the boiling point for molecular liquids is lower. The data in table 2.2 below may help you to understand some of the concepts we have explained. Not all of the substances in the table are solids at room temperature, so for now, let’s just focus on the boiling points for each of these substances. Of the substances listed, ethanol has the weakest intermolecular forces, and sodium chloride and mercury have the strongest. What do you notice? Substance Melting point ( 0 C ) Boiling point ( 0 C ) Ethanol ( C 2 H 6 O ) -114,3 78,4 Water 0 100 Mercury -38,83 356,73 Sodium chloride 801 1465 Table 2.2: The melting and boiling points for a number of substances You will have seen that substances such as ethanol, with relatively weak intermolecular forces, have the lowest boiling point, while substances with stronger intermolecular forces such as sodium chloride and mercury, must be heated much more if the particles are to have enough energy to overcome the forces that are holding them together in the liquid or solid phase. Exercise: Forces and boiling point The table below gives the molecular formula and the boiling point for a number of organic compounds called alkanes . Refer to the table and then answer the questions that follow. Organic compound Molecular formula Boiling point ( 0 C) Methane CH 2 -161.6 Ethane C 2 H 6 -88.6 Propane C 3 H 8 -45 Butane C 4 H 10 -0.5 Pentane C 5 H 12 36.1 Hexane C 6 H 14 69 Heptane C 7 H 16 98.42 Octane C 8 H 18 125.52 Data from: (a) Draw a graph to show the relationship between the number of carbon atoms in each alkane, and its boiling point (Number of carbon atoms will go on the x-axis and boiling point on the y-axis).
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