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Covalent substances Covalent compounds - simple molecules Covalent bonds form between non-metal atoms. Each bond consists of a shared pair of electrons, and is very strong. Covalently bonded substances fall into two main types: 1. simple molecules and 2. giant covalent structures. Simple molecules you draw a water molecule A molecule of carbon dioxide These contain only a few atoms held together by strong covalent bonds. Properties of simple molecular substances Low melting and boiling points - This is because the weak intermolecular forces break down easily. That is the slight attraction each molecule has with each other Non-conductive - Substances with a simple molecular structure do not conduct electricity. This is because they do not have any free electrons or an overall electric charge. Higher Tier only Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points. Covalent bonding - giant covalent structures Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds. The atoms are usually arranged into giant regular lattices - extremely strong structures because of the many bonds involved. From left to right - graphite, diamond, silica Properties of giant covalent structures Very high melting points - Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Variable conductivity - Diamond does not conduct electricity. Graphite contains free electrons, so it does conduct electricity. Silicon is semi-conductive - that is, midway between non-conductive and conductive. Silica Silica, which is found in sand, has a similar structure to diamond. Buckminsterfullerene a giant molecule where carbon atoms form pentagons and hexagons - in a similar way to a leather football. It is used in lubricants. Practise questions: 4. When a substance with a simple molecular structure boils, which bonds are broken? covalent bonds ionic bonds intermolecular bonds 5. Which of the following statements is true of substances with a simple molecular structure? They have low melting points and conduct electricity. They have low melting points and do not conduct electricity. They have high melting points and do not conduct electricity. 6. Which of the following statements is true of diamond? Each carbon atom is covalently bonded to three other carbon atoms. Each carbon atom is covalently bonded to four other carbon atoms. There are free electrons. 7. Which of the following statements is true of graphite? Each carbon atom is covalently bonded to three other carbon atoms. Each carbon atom is covalently bonded to four other carbon atoms. There are no free electrons. 8. Explain why most covalent substances are gases [4] .............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................ ............................................................................................................................................. ............................................................................................................................................ Salts are formed as the product of an acid reaction with an alkali. Soluble salts dissolve in water. Insoluble salts do not dissolve in water. Nitrates and most chlorides are soluble. This is why many of the chemicals you use in the laboratory are nitrates or chlorides. To make an insoluble salt, make two soluble salts react them together in a precipitation reaction. Precipitations means tiny particles appear in the reaction mixture: it forms a precipitate. Flame tests Metals change the colour of a flame when they are heated in it. Different metals give different colours to the flame, so flame tests can be used to identify the presence of a particular metal in a sample. This is how you would carry out a typical flame test: 1. 2. 3. 4. Dip a clean flame test loop in the sample solution Hold the flame test loop at the edge of a Bunsen burner flame Observe the changed colour of the flame, and decide which metal it indicates Clean the loop in acid and rinse with water, then repeat steps 1 to 3 with a new sample Learn these Metal Barium Calcium Copper Lithium Sodium Potassium Testing for the sulfate ion Flame test colour Pale green Yellow-red Green-blue Red Orange Lilac You can test to see if a solution contains You can test to see if a solution contains sulfate ions by using barium sulfate. If barium chloride solution is added to a sample of water containing sulfate ions, barium sulfate is formed. Barium sulfate is insoluble in water, and will be seen as a white precipitate. chloride, bromide or iodide ions by using silver nitrate. If silver nitrate solution is added to a sample of water containing halide ions the silver halide is precipitated. This is because the silver halides are all insoluble in water. The results look like this: Silver chloride is a white precipitate Silver bromide is a cream precipitate Silver iodide is a pale yellow precipitate 1. Which of the following salts are soluble? Potassium hydroxide Silver chloride Lead sulfate 2. Barium sulfate is toxic. Why is it given to patients? They only rub it on their body The cure outweighs the risk It is insoluble so doesn't dissolve in the blood 3. What is the colour of copper in a flame test? Green-blue Yellow Red 4. What is barium chloride used to test for? Carbonate ions Sulfate ions Halides 5. Which of the following salts are insoluble? Sodium sulfate Sodium carbonate Calcium sulfate 6. Which of the following does not produce a red colour in a flame test? Lithium Potassium Calcium 7. What colour precipitate does silver iodide form in a halide test? White Cream Pale yellow 8. Explain what a precipitation reaction is [2] ........................................................................................................................................... .......................................................................................................................................... .......................................................................................................................................... ......................................................................................................................................... Spectroscopy All atoms give off light when heated, although sometimes this light is not visible to the human eye. A prism can be used to split this light to form a spectrum, and each element has its own distinctive line spectrum. This technique is known as spectroscopy. Chemists use spectroscopy to detect very small amounts of an element. Some examples of what line spectra look like are shown here: A prism can be used to split light Scientists have used line spectra to discover new elements. In fact, the discovery of some elements, such as rubidium and caesium, was not possible until the development of spectroscopy. The element helium was discovered by studying line spectra emitted by the Sun. Which element was discovered using spectroscopy? Gold Caesium Iron Explain spectroscopy in your own words (read the about 2 or 3 times – then without looking at it write down what you remember) .................................................................................................................................................. ................................................................................................................................................. ............................................................................................................................................... .................................................................................................................................................. ............................................................................................................................................... .............................................................................................................................................. Catalysts Effect of catalysts A catalyst is a substance that can increase the rate of a reaction. The catalyst remains unchanged. Only a very small amount is needed to increase the rate of reaction between large amounts of reactants. Different catalysts catalyse different reactions. The table summarises some common catalysts used in industry and the reactions they catalyse: Catalyst Reaction catalysed Iron Making ammonia from nitrogen and hydrogen Platinum Making ammonia from nitrogen and hydrogen Vanadium(V) oxide Making sulfuric acid Catalytic converters Modern cars have a catalytic converter to help reduce the toxic gases. Catalytic converters use a platinum and rhodium catalyst with a high surface area. This increases the rate of reaction of carbon monoxide and unburnt fuel from exhaust gases with oxygen from the air. The product from this is carbon dioxide and water, which is less harmful to the environment. Manganese dioxide is a black powder that catalyses the breakdown of hydrogen peroxide to water and oxygen. Which of the following statements is correct? The mass of manganese dioxide will stay the same during the reaction. The catalysed reaction will produce more oxygen than the uncatalysed reaction. The particles in the catalysed reaction will have more energy than in the uncatalysed reaction. Explain how a catalyst can change the rate of a reaction (in your own words) .............................................................................................................................................. ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... .............................................................................................................................................. Endo and exothermic reactions Chemical reactions transfer energy to, or from, the surroundings. They often cause a temperature change. For example, when a bonfire burns, it transfers heat energy to the surroundings. Objects near a bonfire become warmer. The temperature rise can be measured with a thermometer. Exothermic reactions Transfer energy to the surroundings. Some examples of exothermic reactions are: Burning (combustion) Neutralisation reactions between acids and alkalis The reaction between water and calcium oxide Explosions This graph shows you the amount of energy the bonds have before and after – if less after then that energy has been ‘given out’ Endothermic reactions Take in energy from the surroundings. The surroundings get colder. Some examples of endothermic reactions are: Electrolysis and Photosynthesis The reaction between ethanoic acid and sodium carbonate The reaction between ammonium nitrate and water This graph show the amount of energy the bonds have before and after – if more then, the energy has been ‘taken in’ from the surroundings Bonds and energy changes Bonds in the reactants are broken New bonds are made in the products Energy is absorbed to break bonds. Bond-breaking is an endothermic process. Energy is released when new bonds form. Bond-making is an exothermic process. +0–0 Methane + Oxygen + Carbon dioxide + water Put a circle round the bonds that are broken and a square round the bonds that are formed Y11 Revision ideas 1.BBCbitesize – gcse – science – additional - edexcel – any of the chemistry sections 2.Skoool.co.uk – double click on London Grid for learning – ks4 tab – chemistry – choose any topics 3.School intranet – W drive – Mrs Moon – chemistry revision ppt 4.EDEXCEL GCSE Science - Doc Brown's Chemistry Clinic (google search) Scroll down to EDEXCEL GCSE Additional Science = a Double Award Science with GCSE Science 5.Create full colour diagrams of topics on revision cards – see Mrs Moon for cards 6.Use past paper questions to work out the ‘perfect’ answer 7. half term revision on Friday from 9 – 11 in IRC with Mrs Moon 8. Do corrections from mock exam 9. use your revision guide – there are question pages at the end of each section 10. Create a revision power point for your self/others Y11 Revision ideas 1.BBCbitesize – gcse – science – additional - edexcel – any of the chemistry sections 2.Skoool.co.uk – double click on London Grid for learning – ks4 tab – chemistry – choose any topics 3.School intranet – W drive – Mrs Moon – chemistry revision ppt 4.EDEXCEL GCSE Science - Doc Brown's Chemistry Clinic (google search) Scroll down to EDEXCEL GCSE Additional Science = a Double Award Science with GCSE Science 5.Create full colour diagrams of topics on revision cards – see Mrs Moon for cards 6.Use past paper questions to work out the ‘perfect’ answer 7. half term revision on Friday from 9 – 11 in IRC with Mrs Moon 8. Do corrections from mock exam 9. use your revision guide – there are question pages at the end of each section 10. Create a revision power point for your self/others
