AQA A-LEVEL CHEMISTRY Group 2 Questions and Answers

AQA A-LEVEL CHEMISTRY Group 2 Questions and Answers GROUP 2 What are the group 2 elements? Magnesium Calcium Strontium Barium Radium [[NOT BERYLLIUM]]. PROPERTIES What is the general trend about electrons? There are two electrons in the outer s-orbital which goes further. PROPERTIES What are the size of the atoms in group 2? The atoms get bigger down the group. Each element has an extra filled main level of electrons compared with the one above it. PROPERTIES What are the melting points throughout group 2? Other than magnesium which has the lowest boiling point: The strength of metallic bonds decreases down the group because the sea of delocalised electrons are further away from the nucleus. PROPERTIES What happens with the ionisation energies down group 2? Total ionisation = ionisation energy 1 + IE 2 They decrease down the group as the electrons are further from the nucleus and have shielding. PROPERTIES What happens with their reactivities down group 2? They become more reactive as they can lose their outer electrons to form ionic compounds faster. REACTIONS How do the metals in group 2 react with oxygen? Burns in oxygen. 2Mg + O₂ - 2MgO MgO is a white solid with a high melting point due to its ionic bonding. Mg will also react slowly with oxygen without a flame. Mg ribbon will often have a thin layer of magnesium oxide on it formed by reaction with oxygen. This needs to be cleaned off by emery paper before doing reactions with Mg ribbon. If testing for reaction rates with Mg and acid, an un-cleaned Mg ribbon would give a false result because both the Mg and MgO would react but at different rates. Such as: Mg + 2HCl - 2MgCl₂ + H₂ MgO + 2HCl - MgCl₂ + H₂O REACTIONS How does magnesium react with steam vs warm water? Magnesium reacts in steam to produce magnesium oxide and hydrogen. The Mg would burn with a bright white flame. Mg (s) + H₂O (g) - MgO(s) + H₂ (g) ----------- Mg will also react with warm water, giving a different magnesium hydroxide product Mg (s) + 2H₂O (l) - Mg(OH)₂ (aq) + H₂ (g) This is a much slower reaction than the reaction with steam and there is no flame. REACTIONS How do group 2 elements react with cold water? The other group 2 metals will react with cold water with increasing vigour down the group to form hydroxides - fizzing, (more vigorous down group) - the metal dissolving, (faster down group) - the solution heating up (more down group) - with calcium a white precipitate appearing (less precipitate forms down group with other metals) Ca + 2H₂O (l) - Ca(OH)₂ (aq) + H₂ (g) The hydroxides produced make the water alkaline (if they are soluble in water) Keep in mind these are redox reactions: the hydrogen is oxidised and the calcium is reduced. REACTIONS How can you use magnesium to extract titanium? Titanium is a very useful metal because it is abundant, has a low density and is corrosion resistant - it is used for making strong, light alloys for use in aircraft for example. Titanium cannot be extracted with carbon because titanium carbide (TiC) it is formed rather than titanium . Titanium cannot be extracted by electrolysis because it has to be very pure. Titanium is extracted by reaction with a more reactive metal (e.g. Mg). 1. TiO2 (solid) is converted to TiCl4 (liquid) at 900C 2. The TiCl4 is purified by fractional distillation in an argon atmosphere. 3. The Ti is extracted by Mg in an argon atmosphere at 500C TiO₂ + 2Cl₂ + 2C - TiCl₄ + 2CO TiCl₄ + 2Mg - Ti + 2MgCl₂ TiO₂ is converted to TiCl₄ as it can be purified by fractional distillation, TiCl₄ being molecular (liquid at room temperature) rather than ionic like TiO₂ (solid at room temperature). REACTIONS Why is titanium expensive? The expensive cost of the Mg This is a batch process which makes it expensive because the process is slower (having to fill up and empty reactors takes time) and requires more labour and the energy is lost when the reactor is cooled down after stopping The process is also expensive due to the argon, and the need to remove moisture (because TiCl4 is susceptible to hydrolysis). High temperatures required in both steps This all makes titanium expensive even though it is a relatively abundant metal. It is only therefore used to a limited amount even though it has useful properties. HYDROXIDES What are the properties of the hydroxides? Group II hydroxides become more soluble down the group. All Group II hydroxides when not soluble appear as white precipitates. HYDROXIDES What are the properties of magnesium hydroxide? Classed as insoluble in water Mg²⁺(aq) + 2OH⁻(aq) - Mg(OH)₂ (s). A suspension of magnesium hydroxide in water will appear slightly alkaline (pH 9) so some hydroxide ions must therefore have been produced by a very slight dissolving Magnesium hydroxide is used in medicine (in suspension as milk of magnesia) to neutralise excess acid in the stomach and to treat constipation Mg(OH)₂ + 2HCl - MgCl₂ + 2H₂O It is safe to use because it is so weakly alkaline. It is preferable to using calcium carbonate as it will not produce carbon dioxide gas. HYDROXIDES What are the properties of calcium hydroxide? Calcium hydroxide is classed as partially soluble in water and will appear as a white precipitate It is used in agriculture to neutralise acidic soils. A suspension of calcium hydroxide in water will appear more alkaline (pH 11) than magnesium hydroxide as it is more soluble so there will be more hydroxide ions present in solution. An aqueous solution of calcium hydroxide is called lime water and can be used a test for carbon dioxide. The limewater turns cloudy as white calcium carbonate is produced. Ca(OH)₂ (aq) + CO₂ (g) - CaCO₃ (s) + H₂O(l) HYDROXIDES What are the properties of barium hydroxide? Barium hydroxide would easily dissolve in water. The hydroxide ions present would make the solution strongly alkaline Ba(OH)₂ (s) + aq - Ba²⁺ (aq) + 2OH⁻(aq) SULFATES What are the properites of group 2 sulfates? Group II sulfates become less soluble down the group BaSO₄ is the least soluble. SULFATES What is the equation for forming sulfates? SrCl₂(aq) + Na₂SO₄ (aq) - 2NaCl (aq) + SrSO₄ (s) Sr²⁺ (aq) + SO₄²⁻ (aq) - SrSO₄ (s) SULFATES What is Barium Sulfate used for? is used in medicine as a 'Barium meal' given to patients who need x-rays of their intestines. The barium absorbs the x-rays and so the gut shows up on the x-ray image. Even though barium compounds are toxic, it is safe to use here because its low solubility means it is not absorbed into the blood. SULFATES What happens if barium reacts with sulfuric acid? If barium metal is reacted with sulfuric acid it will only react slowly as the insoluble barium sulfate produced will cover the surface of the metal and act as a barrier to further attack. The same effect will happen to a lesser extent with metals going up the group as the solubility increases. The same effect does not happen with other acids like hydrochloric or nitric as they form soluble group 2 salts. SULFATES How can you test the presence of sulfate ions? BaCl2 solution acidified with hydrochloric acid is used as a reagent to test for sulphate ions. If acidified barium chloride is added to a solution that contains sulfate ions a white precipitate of barium sulfate forms. Other anions should give a negative result which is no precipitate forming. Simplest ionic equation: Ba²⁺ (aq) + SO₄²⁻(aq) - BaSO₄ (s). The hydrochloric acid is needed to react with carbonate impurities that are often found in salts which would form a white barium carbonate precipitate and so give a false result [it is also a white ppt, making it indistinguishable from barium sulfate's ppt]. You could not use sulfuric acid because it contains sulfate ions and so would give a false positive result. Fizzing due to CO2 would be observed if a carbonate was present. 2HCl + Na₂CO₃ - 2NaCl + H₂O + CO₂ SOLUBLE SALTS How can an insoluble salt (preciptation reactions) be formed? Insoluble salts can be made by mixing appropriate solutions of ions so that a precipitate is formed barium nitrate (aq) + sodium sulfate (aq) barium sulfate (s) + sodium nitrate (aq) These are called precipitation reactions. A precipitate is a solid SOLUBLE SALTS Which salts are soluble vs insoluble? Soluble: All sodium, potassium and ammonium salts All nitrates Most chlorides, bromides, iodides Most sulfates Sodium, potassium and ammonium carbonates Sodium, potassium and ammonium hydroxides Insoluble: Silver, lead chlorides, bromides iodides Lead, strontium and barium sulfates Most other carbonates Most other hydroxides