IGCSE Science Past Paper - Gases in Atmosphere (Double) PDF

IGCSE Science C2 Inorganic Chemistry 2C Gases in the Atmosphere (Double)  Learning Outcomes Know the approximate percentages by volume of the four most abundant gases in dry air Understand how to determine the percentage by volume of oxygen in air using ex...

IGCSE Science C2 Inorganic Chemistry 2C Gases in the Atmosphere (Double)  Learning Outcomes Know the approximate percentages by volume of the four most abundant gases in dry air Understand how to determine the percentage by volume of oxygen in air using experiments involving the reactions of metals (e.g. iron) and non-metals (e.g. phosphorus) with air Describe the combustion of elements in oxygen, including magnesium, hydrogen and sulfur Describe the formation of carbon dioxide from the thermal decomposition of metal carbonates, including copper(II) carbonate Know that carbon dioxide is a greenhouse gas and that increasing amounts in the atmosphere may contribute to climate change Practical: determine the approximate percentage by volume of oxygen in air using a metal or a non-metal  The Big The Composition of Air Picture Global Warming Combustion of and Greenhouse Elements Effect Scan here for YouTube Video Thermal Decomposition of Properties of Carbonates Oxides  The Composition of Air The approximate percentages (by volume) of the four most abundant gases in unpolluted, dry air are: Amount in Air Gas Amount in Air (%) (Fraction) Nitrogen (N2) 78.1 About 4/5 Oxygen (O2) 21.0 About 1/5 Argon (Ar) 0.9 Carbon Dioxide (CO2) 0.04 The remainder is made up of other noble gases in very small amounts (helium, neon, krypton, xenon and radon) Measuring the Percentage of Oxygen - Practicals There are three key experiments that are used to measure the percentage of oxygen in air: ○ Using copper ○ Using the rusting of iron ○ Using phosphorus All depend on the reaction of a substance with oxygen in the air We then look at how much the volume decreases as the oxygen is removed Measuring the Percentage of Oxygen - Practicals The following YouTube links show each of the practicals being carried out: Using copper ○ https://www.youtube.com/watch?v=-28uuBcBK_E&list=LL&index=17 Using the rusting of iron ○ https://www.youtube.com/watch?v=yBM6Yg2xZlo Using phosphorus ○ https://www.youtube.com/watch?v=2yge15dp9rk  Using Copper - Apparatus This method uses the reaction of copper with the oxygen in the air Equation: 2Cu (s) + O2 (g) → 2CuO (s) The pink-brown copper turns black as the copper(II) oxide is formed  Using Copper - Method 1. The plunger on one of the gas syringes is set to 0 cm3 and the other is set to 100 cm3 – this means the apparatus contains 100 cm3 2. The silica tube is heated strongly using a roaring Bunsen flame 3. The plunger in the left-hand gas syringe is pushed in to allow the air to pass over the copper and the plunger on the right-hand side moves out 4. The plungers are pushed in sequence (left, right, left, right and so on) to pass the air over the copper consistently 5. The Bunsen burner is moved periodically to ensure all the copper reacts 6. The volume of gas in the syringes falls as the oxygen is used 7. Once the volume stops moving, set one plunger back to zero and allow the apparatus cool. The final gas reading can then be taken  Using Copper - Calculation Table of Results Initial volume of air in the apparatus (cm3) 100 Final volume of air in the apparatus (cm3) 79  Using Copper – Key Points We must ensure that all the oxygen reacts to get an accurate reading so the copper must be in excess The apparatus must be cooled before taking the final reading as gases will expand when they are hot Taking the reading too early will cause the volume of oxygen removed to be lower than expected The volume is rarely exactly 21% - any value between 18% to 24% is generally accepted  Using Rusting of Iron - Apparatus Iron rusts in damp air, using up the oxygen as it does so Equation: 4Fe (s) + 3O2 (g) + 6H2O (l) → 4Fe(OH)3 (s) The black iron filings turns red-brown as the rust is formed  Using Rusting of Iron - Method 1. Find the volume of air present by filling the conical flask and connecting tube with water and then transferring to a measuring cylinder 2. Set up the apparatus shown in the diagram 3. Put wet iron filings in to the conical flask 4. Record the initial reading on the gas syringe 5. Leave the apparatus for approximately a week, until the reading on the gas syringe stops changing 6. Record the final reading on the gas syringe Using Rusting of Iron - Calculation Table of Results Initial volume of air in the conical flask (cm3) 130 Initial volume of air in the connecting tube (cm3) 12 Initial volume of air in the gas syringe (cm3) 92 Final volume of air in the gas syringe (cm3) 43  Using Rusting of Iron – Key Points The total volume used in the equation must also include the volume of the conical flask and the connecting tube We must ensure that all the oxygen reacts to get an accurate reading so the iron must be in excess This experiment takes at least a week to see a reading – if the value is much less then expected, then it was not to react for long enough The volume is rarely exactly 21% - any value between 18% to 24% is generally accepted  Using Phosphorus - Apparatus Phosphorus is very reactive with oxygen and formed phosphorus oxide Equation: 4P (s) + O2 (g) → 2P2O5 (aq) Phosphorus oxide formed is very soluble in water and as the oxygen is used up the water level will rise  Using Phosphorus - Method 1. Set up the apparatus shown in the diagram with the phosphorus floating on the water in an evaporating basin 2. The initial level of the water is marked on the side of the bell jar using a waterproof pen or sticker 3. The bung is removed from the bell jar and the phosphorus is touched with hot metal wire to ignite it 4. The bung is quickly replaced to avoid loss of gas 5. The water level will rise as the oxygen is removed and reacts with the phosphorus and the phosphorus oxide formed will dissolve in the water 6. Mark the final level of the water with a waterproof pen or sticker 7. Determine the volume of water by filling the jar to the marks and transferring to a measuring cylinder Using Phosphorus - Calculation Table of Results Initial volume of water in the bell jar (cm3) 1000 Final volume of water in the bell jar (cm3) 790  Using Phosphorus – Key Points We must ensure that all the oxygen reacts to get an accurate reading so the phosphorus must be in excess For safety, this would only be carried out by a teacher as the initial pressure increase and reactivity of phosphorus would make it too dangerous for students A similar experiment can be carried out by students using a candle, a petri dish and a measuring cylinder of water The volume is rarely exactly 21% - any value between 18% to 24% is generally accepted  Combustion of Elements in Oxygen Some elements burn in oxygen and these reactions are known as combustion reactions (or sometimes oxidation reactions) Elements will burn more brightly and rapidly in pure (100%) oxygen than in air (21% oxygen) There are three key combustion reactions you should know: magnesium, sulfur and hydrogen We can then make some generalisations about the properties of the oxides formed from these reactions  Combustion of Magnesium Magnesium (a silver ribbon of metal) burns in oxygen (a colourless gas) to form magnesium oxide (a white powder) 2Mg(s) + O2(g) → 2MgO(s) This white solid can then be dissolved in water to form magnesium hydroxide MgO(s) + H2O(l) → Mg(OH)2(aq) https://www.youtube.com/watch?v=Ozq6RxdnZkc  Combustion of Sulfur Sulfur (a yellow powder) burns in oxygen (a colourless gas) with a blue flame to form poisonous sulfur dioxide (a colourless gas) S(s) + O2(g) → SO2(g) The sulfur dioxide can be dissolved in water to form an acidic solution of sulfurous acid SO2(g) + H2O(l) → H2SO3(aq) https://www.youtube.com/watch?v=V1sQO91UvFI  Combustion of Hydrogen Hydrogen (a colourless gas) burns in oxygen (a colourless gas) with a pale blue flame to form water (a colourless liquid) 2H2(g) + O2(g) → 2H2O(l) If the mixture is ignited, it will explode. This is the basis of the squeaky pop test for hydrogen https://www.youtube.com/watch?v=XlmtcsSJr7Q  Properties of Oxides Metal Oxides Non-metal Oxides ionic compounds containing O2- ions covalent compounds basic oxides which means they will acidic oxides, which react with alkalis or react with acids to form salts bases to form salts typically insoluble in water however if they dissolve, they will form alkaline often soluble in water forming acidic solutions which are metal hydroxides solutions containing hydrogen ions, H+ containing hydroxide ions, OH-  Thermal Decomposition Carbon dioxide is a colourless gas which is generally formed by the thermal decomposition of metal carbonates such as copper carbonate or calcium carbonate The metal carbonate will decompose to form a metal oxide and carbon dioxide Thermal decomposition is the breaking up of something by heating it  Thermal Decomposition Copper carbonate (CuCO3) Calcium carbonate (CaCO3) Copper carbonate (CuCO3) is a green Calcium carbonate (CaCO3) will only powder decompose at very high temperatures It will decompose on heating into black It will decompose on heating calcium copper(II) oxide and carbon dioxide oxide and carbon dioxide gas gas CaCO3(s) → CaO(s) + CO2(g) CuCO3(s) → CuO(s) + CO2(g) This reaction converts limestone (CaCO3) in to quicklime (CaO)  The Greenhouse Effect Carbon dioxide forms when fossil fuels are burned as well as during thermal decomposition of metal carbonates in industry Carbon dioxide is a greenhouse gas which contributes to the greenhouse effect The greenhouse effect happens when high energy UV and visible light pass in to the atmosphere and warm up the Earth The build up of CO2 in the atmosphere causes some of this heat to be trapped instead of being radiated back out to space The Greenhouse Effect The Greenhouse Effect  The Greenhouse Effect The increasing levels of CO2 in the atmosphere is causing the greenhouse effect to become more widespread and contributes to climate change The climate change effects caused by the greenhouse effect are: Melting polar ice caps Rising sea levels Extreme weather patterns (floods, droughts and heat waves)