Question

MRE’s are military meals that can be heated oba flame less heater. Mg(s) + 2H2O(l) = Mg(oh)2(s) +H2(g). Calculate the standard enthalpy for the reaction. Calculate the number of grams of mg needed fir this reaction

Answer 1

For the given reaction of magnesium,

Mg (s) + 2H2O (l) <==> Mg(OH)2 (s) + H2 (g)

The standard enthalpy change for the reaction dHrxn would be,

dHrxn = dH(products) - dH(reactants)

taking values from literature,

dHrxn = (-924.5 + 0) - (0 + 2 x -285.83)

           = -352.84 kJ/mol

enthalpy of compounds in standard state are zero.

So the standard enthalpy of the reaction is -352.84 kJ/mol.

Lets say, we need 1 g of Mg(OH)2 formation.

Then amount of Mg needed = (1 g/58.32 g/mol) x 24.3 g/mol x 1000 = 416.67 mg

molar mass of Mg(OH)2 = 58.32 g/mol

molar mass of Mg = 24.3 g/mol

moles = grams/molar mass