C5 Revision

So what’s in air? Oxygen. Nitrogen. Carbon dioxide. Water. Argon. What about dry air? The same apart from water.

Elements or compounds? Oxygen. Nitrogen. Carbon dioxide. Argon. Sort the above into elements and compounds? Elements contain only one type of atom; Oxygen, nitrogen and argon. Compounds contain at least 2 different types of atom in a fixed ratio; Carbon dioxide.

Molecules. A molecule is any substance that contains more than one atom. It can be either a compound and sometimes an element. Which is not a molecule? O2. N2. CO2. Ar. Ar – it exists as an atom on its own.

It’s good to share. The other way that atoms can gain a full outer shell of electrons is by sharing their outer electrons. This happens between non-metal atoms.

Click for another example Hydrogen and Hydrogen H H H2 Click for another example Click for animation

Simple covalent compounds. Covalent bonds are strong, but there are weak forces of attraction between individual molecules.

Explaining the properties Why are a lot of simple covalent compounds gases and liquids at room temperature? They have low melting and boiling points. Why do they have low melting and boiling points. This is because of the weak intermolecular forces between covalent compounds.

Explaining the properties Why can’t covalent compounds conduct electricity? This is because there are no free electrons, they have no charge. Now let’s look at how covalent bonds are formed, remember the non-metal atoms share electrons to gain a full outer shell.

Giant covalent structure.

Giant covalent structures Giant covalent structures contain strong covalent bonds between all atoms. Very high melting points. In diamond, each carbon atom forms four covalent bonds In graphite, each carbon atom forms three covalent bonds to form layers which can slide over each other. Free electrons in graphite allow it to conduct electricity. Giant covalent structures

Diamond Diamond

Graphite Graphite

Comparing giant and simple covalent structures. The many covalent bonds in the giant structure make it hard and also explains why it has a high melting point.

Physical properties of metals. Malleable – can be shaped. High tensile strength. High melting and boiling points. Good conductors of heat and electricity. Different forms of Zinc.

Metallic bonding. Metallic bonding is the strong electrostatic attraction between a sea of delocalised electrons and close packed positive metal ions. This is why metals have high melting and boiling points and high tensile strength.

Metallic Bonding Metallic Bonding

Metallic Bonding Metallic Bonding

Explaining the properties of metals. Metals are malleable because the delocalised electrons allow the atoms to slide over each other so that they can be shaped.

How could you find out what is in this white powder?

Precipitates. Precipitates are insoluble solids formed when some ionic solutions are mixed together.

Testing for metal ions. Some of metal ions give distinctive precipitates when sodium hydroxide solution is added. Copper Cu2+ – blue Iron II Fe2+ – dirty green Iron III Fe3+ - rusty brown.

Testing for halides (group 7) Silver nitrate can be used to detect the following halide ions; Chloride ions – white precipitate. Bromide ions – cream precipitate. Iodine ions – pale yellow precipitate.

Testing for sulfates. When barium nitrate or barium chloride is added to a solution containing sulfate ions (SO42-) a white precipitate is formed.

Testing for carbonates. When an acid react with a carbonate what gas is formed? Carbon dioxide. How do we test for carbon dioxide? It turns limewater milky (a precipitate is formed)

Keyword - definitions Ion – charged particle. Precipitate – insoluble solid. Qualitative test – tells you what is present but not how much. Cation – positive metal ion. Anion – negative non-metal ion.

What is an ore? Ores are rocks that contain varying amounts of minerals from which metals can be extracted. Still need to separate the metal from other elements in the compound.

Extracting copper. Why can copper be found as copper in the ground (native)? It is not very reactive. There are also ores that contain copper. Native copper Malachite – copper carbonate.

The reactivity series. Copper is low down in the reactivity series. It is not very reactive. The more reactive a metal is, the more difficult it is to extract from its ore.

Heating with carbon. Why was carbon used? What type of reaction is this an example of? Suggest what products were formed.

Answers. Carbon is more reactive than copper and so removes the oxygen. Removing oxygen is called reduction. The carbon gains oxygen and this is called oxidation. copper carbonate  copper + carbon oxide dioxide copper oxide + carbon  copper + carbon dioxide

Copper mining. For every 2kg of copper 1000kg of copper ore needs to be extracted. What impact will this have on the local environment?

Keywords Electrolysis – the breakdown of a substance by electricity. Electrolytes – the substance that is broken down, it must be molten or dissolved. Electrodes – the rods that conduct electricity. Anode – positive electrode. Cathode – negative electrode.

Basic electrolysis set-up.

Electrolysis Electrolysis is the process of splitting up compounds using electricity Electrolysis only works if the substance is molten or dissolved (so the ions are free to move) Metals form at the negative electrode and non-metals form at the positive electrode. Electrolysis

Electrolysis Electrolysis

= bromide ion = lead ion Anode Cathode

Compound Anode Cathode Sodium Bromide Potassium Iodide Calcium Fluoride Magnesium Oxide Lithium Chloride

Properties and uses of aluminium. Conducts heat and electricity well. It has a low density for a metal. It does not corrode. Uses: Aeroplanes, window frames, foil, drinks cans, electricity cables and pans.

Where does aluminium come from? Aluminium is extracted from its ore bauxite. Bauxite is mainly aluminium oxide. Formula Al2O3 A bauxite mine in western Australia

Key features of the electrolysis of aluminium oxide. Aluminium oxide is molten. Oxygen is formed at the graphite anode. The anodes are gradually worn away by oxidation. Aluminium is formed at the graphite cathode. The process has a high electrical energy requirement.

Electrolysis of aluminium oxide

Electrolysis of Aluminium Oxide

Understanding the process. Why must the aluminium oxide be molten? Why are the anodes gradually worn away? Why is cryolite used? Why is aluminium expensive?

Answers The aluminium oxide must be molten for electrolysis to take place. When molten the ions are free to move, in the solid form they are fixed. The oxygen formed at the anode reacts with the carbon anode to from carbon dioxide.

Answers Cryolite lowers the melting point and so saves energy. The electrolytic process requires a lot of electrical energy. Most aluminium extraction plants are located near power stations, usually hydroelectric plants.

Calculating relative formula mass. Top Tips. 1. The relative atomic mass is usually at the top – it is always the highest number. 2. When calculating the relative formula mass, a) Write out all the different atoms (Capital letter). b) Write down the number of each atom. c) Multiply the number of each type of atom by its atomic mass. d) Add all the masses together.

An example. Calculate the RFM – for calcium carbonate. Formula CaCO3 Type, number and atomic mass of each atom; Ca x 1 x 40 = 40 C x 1 x 12 = 12 O x 3 x 16 = 48 Total = 100 – Relative formula mass of CaCO3.

Relative formula mass Relative formula mass

Relative formula mass, Mr 28/04/2017 The relative formula mass of a compound is blatantly the relative atomic masses of all the elements in the compound added together. Relative atomic mass of O = 16 E.g. water H2O: Relative atomic mass of H = 1 Therefore Mr for water = 16 + (2x1) = 18 Work out Mr for the following compounds: HCl NaOH MgCl2 H2SO4 K2CO3 H=1, Cl=35 so Mr = 36 Na=23, O=16, H=1 so Mr = 40 Mg=24, Cl=35 so Mr = 24+(2x35) = 94 H=1, S=32, O=16 so Mr = (2x1)+32+(4x16) = 98 K=39, C=12, O=16 so Mr = (2x39)+12+(3x16) = 138

Calculating percentage mass 28/04/2017 If you can work out Mr then this bit is easy… Percentage mass (%) = Mass of element Ar Relative formula mass Mr x100% Calculate the percentage mass of magnesium in magnesium oxide, MgO: Ar for magnesium = 24 Ar for oxygen = 16 Mr for magnesium oxide = 24 + 16 = 40 Therefore percentage mass = 24/40 x 100% = 60% Calculate the percentage mass of the following: Hydrogen in hydrochloric acid, HCl Potassium in potassium chloride, KCl Calcium in calcium chloride, CaCl2 Oxygen in water, H2O