10.1. Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds
A subsection of Chemistry, 9701, through 10. Group 2
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Chemists recognise that atoms are made of three types of particle. Complete the following table with their names and properties. name of particle relative mass relative charge +1 1/1836 Most elements exist naturally as a mixture of isotopes, each with their own relative isotopic mass. The mass spectrum of an element reveals the abundances of these isotopes, which can be used to calculate the relative atomic mass of the element. Magnesium has three stable isotopes. Information about two of these isotopes is given. isotope relative isotopic mass percentage abundance 24Mg 24.0 79.0 26Mg 26.0 11.0 Defi ne the term relative isotopic mass. The relative atomic mass of magnesium is 24.3. Calculate the percentage abundance and hence the relative isotopic mass of the third isotope of magnesium. Give your answer to three signifi cant fi gures percentage abundance = isotopic mass = Magnesium can be produced by electrolysis of magnesium chloride in a molten mixture of salts. Give equations for the anode and cathode reactions during the electrolysis of molten magnesium chloride, MgCl 2. anode cathode The electrolysis is carried out under an atmosphere of hydrogen chloride gas to convert any magnesium oxide impurity into magnesium chloride. An investigation of the reaction between magnesium oxide and hydrogen chloride gas showed that an intermediate product was formed with the composition by mass Mg, 31.65%; O, 20.84%; H, 1.31% and Cl, 46.20%. Calculate the empirical formula of this intermediate compound. empirical formula The acid/base behaviour of the oxides in the third period varies across the period. Describe this behaviour and explain it with reference to the structure and bonding of sodium oxide, Na2O, aluminium oxide, Al 2O3, and sulfur trioxide, SO3. Write equations for reactions of these three oxides with hydrochloric acid and/or sodium hydroxide as appropriate.
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THEORY
2015
Paper 2, Variant 2
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The elements in Group II, and their compounds, show a variety of trends in their properties. Magnesium, calcium and barium all react with cold water to form hydroxides. Describe and explain the trend in reactivity of these three elements with cold water. Give the equation for the reaction of magnesium with cold water. Suggest why the water eventually turns cloudy during the reaction of magnesium with cold water. Suggest the equation for the reaction of hot magnesium with steam. The oxides of magnesium, calcium and barium all react with dilute nitric acid to form nitrates. Give the equation for the reaction of magnesium oxide with nitric acid. State the trend in thermal stability of the nitrates of Group II. Give the equation for the thermal decomposition of magnesium nitrate. Apart from lithium nitrate, the nitrates of the Group I elements decompose in a different way to those of the Group II elements. The equation for the thermal decomposition of potassium nitrate is 2KNO3 → 2KNO2 + O2 By identifying any changes in oxidation number, explain which element is reduced and which is oxidised in this decomposition. A refractory material is one that does not decompose or melt at very high temperatures. Over 50% of magnesium oxide production is for use as a refractory material. Explain why magnesium oxide has a very high melting point. The word ‘lime’ is usually used to refer to a range of calcium-containing compounds that have a range of uses. Write equations to show how calcium carbonate can be converted into calcium hydroxide by a two-step process. A garden pond, with a total volume of 8000 dm3, has been contaminated in such a way that its pH has fallen to 4. This means that the concentration of hydrogen ions, H+, in the water is 1 × 10–4 mol dm–3. Write an ionic equation for the neutralisation reaction that occurs between hydrogen ions and carbonate ions, CO3 2–. Use your equation to calculate the mass of powdered calcium carbonate that would need to be added to the pond to neutralise the acidity. mass = g
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THEORY
2015
Paper 2, Variant 3
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The mass spectrum of the element magnesium is shown below. m / e relative abundance (%) From the mass spectrum, complete the table with the relative abundances of the three isotopes. isotope relative abundance 24Mg 25Mg 26Mg Use your values in to calculate the relative atomic mass, Ar, of magnesium to two decimal places. Ar (Mg) = Describe and explain the trend in the thermal stabilities of the nitrates of the Group II elements down the group. When lithium nitrate, LiNO3, is heated, it readily decomposes giving off a brown gas. This reaction is similar to that which occurs when magnesium nitrate is heated, but it does not occur with other Group I nitrates. Suggest an equation for the action of heat on LiNO3. Suggest why the Group I nitrates other than LiNO3 do not decompose in this way when heated.
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THEORY
2015
Paper 4, Variant 1
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The mass spectrum of the element magnesium is shown below. m / e relative abundance (%) From the mass spectrum, complete the table with the relative abundances of the three isotopes. isotope relative abundance 24Mg 25Mg 26Mg Use your values in to calculate the relative atomic mass, Ar, of magnesium to two decimal places. Ar (Mg) = Describe and explain the trend in the thermal stabilities of the nitrates of the Group II elements down the group. When lithium nitrate, LiNO3, is heated, it readily decomposes giving off a brown gas. This reaction is similar to that which occurs when magnesium nitrate is heated, but it does not occur with other Group I nitrates. Suggest an equation for the action of heat on LiNO3. Suggest why the Group I nitrates other than LiNO3 do not decompose in this way when heated.
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THEORY
2015
Paper 4, Variant 3
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The elements in Group 2, and their compounds, show many similarities and trends in their properties. Magnesium, calcium, strontium and barium all react with cold water. Describe what you would see when some calcium is added to cold water. Write an equation for the reaction taking place in . Describe how the reaction of barium with cold water would differ from the reaction of calcium in in terms of what you would see. Magnesium oxide can be formed by the reaction of magnesium and oxygen in the air. Draw a fully labelled reaction pathway diagram for the reaction between magnesium and oxygen. energy reaction pathway Explain why there is no visible reaction when a piece of magnesium ribbon is exposed to the air. Magnesium oxide is used to manufacture heat-resistant bricks for furnace linings in the steel-making industry. State and explain the property of magnesium oxide that makes it suitable for this use. Suggest a reason why magnesium oxide cannot be used as a lining for any furnaces containing acidic materials. The nitrates and carbonates of the Group 2 elements, from magnesium to barium, decompose when heated. State the trend in the temperature of thermal decomposition of these Group 2 nitrates and carbonates. Give the equation for the thermal decomposition of magnesium carbonate. Give the equation for the thermal decomposition of calcium nitrate.
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THEORY
2016
Paper 2, Variant 1
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An experiment was carried out to determine the percentage of iron in a sample of iron wire. A 3.35 g piece of the wire was reacted with dilute sulfuric acid, in the absence of air, so that all of the iron atoms were converted to iron(ions. The resulting solution was made up to 250 cm3. Write a balanced equation for the reaction between the iron in the wire and the sulfuric acid. A 25.0 cm3 sample of this solution was acidified and titrated with 0.0250 mol dm–3 potassiumdichromate(. 32.0 cm3 of the potassium dichromate(solution was required for complete reaction with the iron(ions in the sample. The relevant half-equations are shown. Cr2O7 2– + 14H+ + 6e– → 2Cr3+ + 7H2O Fe2+ → Fe3+ + e– Use the half-equations to write an equation for the reaction between the iron(ions and the acidified dichromate(ions. Calculate the amount, in moles, of dichromate(ions used in the titration. amount = mol Calculate the amount, in moles, of iron(ions in the 25.0 cm3 sample of solution. amount = mol Calculate the amount, in moles, of iron in the 3.35 g piece of wire. amount = mol Calculate the mass of iron in the 3.35 g piece of wire. mass = g Calculate the percentage of iron in the iron wire. percentage = % Some electronegativity values are shown. element electronegativity aluminium 1.5 chlorine 3.0 iron 1.8 Use the data to suggest the nature of the bonding in iron(chloride. Explain your answer. Suggest an equation for the reaction between iron(chloride and water.
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THEORY
2016
Paper 2, Variant 3
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