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Hints for HW7 Q15. (c) In an experiment a piece of magnesium ribbon is burned in air in a crucible. The mass of the mixture of MgO and magnesium nitride after burning is 0.470 g. Water is added to the crucible, further reaction occurs, and the crucible is heated to dryness until the final product is 0.486 g of MgO. What was the mass percentage of magnesium nitride in the mixture obtained after the initial burning? Step 1. Start with facts we really know. How many moles of Mg atoms are in this system? Step 2. What are the reactions for the burning? What fraction of the Mg atoms end up in each product? How can you adjust the fraction of Mg atoms in each product and get the known mass of products? Q15. (e) The standard enthalpy of formation of Mg3N2(s) is -461.08 kJ mol-1. Calculate the standard enthalpy change for the reaction between magnesium metal and ammonia gas. You will need the deltaH value for ammonia converting to N2 and H2. That value is +92.38 kJ/mole, as listed here: 3 Mg + N2 ----> Mg3N2 deltaH = -461.08 kJ 3 Mg + 2 NH3 ---> 1 Mg3N2 + 3 H2 deltaH = ???
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This work is licensed under a Creative Commons License.