When 1.40g of magnesium metal is allowed to react with 200ml of 6.00M aqueous HC

Question

When 1.40g of magnesium metal is allowed to react with 200ml of 6.00M aqueous HCl , the temperature rises from 25.0 C to 42.3C . Calculate delta H in kilojoules for the reaction, assuming that the heat capacity of the calorimeter is 776J/C , that the specific heat of the final solution is the same as that of water [(4.18 J/(gxC) ], and that the density of the solution is 1.00g/mL .

Explanation / Answer

Mg + 2 HCl => MgCl2 + H2. Moles of Mg = mass/molar mass of Mg = 1.40/24.305 = 0.05760 mol. Heat released by reaction = heat absorbed by solution + heat absorbed by calorimeter = mass x specific heat x temperature change of solution + heat capacty x temperature change of calorimeter = (200 + 1.40) x 4.18 x (42.3 - 25.0) + 776 x (42.3 - 25.0) = 27988.84 J. Delta H = -heat released/moles of Mg = -27988.84/0.05760 = -485917 J/mol = -486 kJ/mol.

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