11282 12 Ammonium dichromate is used in the “volcano” experiment. When heated, it decomposes to produce a vast amount of green chromium oxide and gases which push out the green “ash” to form a pile of “lava”. (NH4)2Cr2O7 → Cr2O3 + N2 + 4H2O The water forms steam because of the heat of the reaction. (a) Write the equation for the reaction, with state symbols, for the reactants and products. [1] (b) Ammonium dichromate is very soluble in water. At room temperature 10.0 g of ammonium dichromate dissolve in 25.0 cm3 of water. The orange solution can be tested for the presence of ammonium ions. (i) Calculate the solubility of the ammonium dichromate in g dm−3 to 3 significant figures. [1] (ii) Calculate the solubility of the ammonium dichromate in mol dm−3 to 3 significant figures. [1] (iii) Explain how you would show that the orange solution contains ammonium ions. [3] 11282 [Turn over (c) The nitrogen given off in the reaction consists of two isotopes, nitrogen-14 and nitrogen-15. The percentage abundance of nitrogen-14 is 99.632%. (i) Explain what is meant by the term isotopes. [2] (ii) Calculate the percentage abundance of nitrogen-15 given off. [1] (iii) Calculate the relative atomic mass of nitrogen to three decimal places. [2] (iv) Explain why there is a difference between the calculated relative atomic mass and the one provided in the data sheet. [1] . 11282 (d) The dichromate ion is a very strong oxidising agent. The half-equation which shows its oxidising ability is: Cr2O7 2− + 14H+ + 6e− → 2Cr3+ + 7H2O (i) Use this equation to explain, in terms of oxidation numbers, why the dichromate ion is an oxidising agent. [2] (ii) Use this equation to explain, in terms of electrons, why the dichromate ion is an oxidising agent. [1] (e) Dichromates react with chlorides in the presence of concentrated sulfuric acid to produce chromyl chloride, CrO2Cl2, which is a deep red liquid with a boiling point of 117 °C. Using this information, explain whether chromyl chloride is ionic or covalent. [2] 11282 BLANK PAGE DO NOT WRITE ON THIS PAGE (Questions continue overleaf) [Turn over