Question

2Mg(s) + O2(g) → 2MgO(s) ΔHrxn = -1204 kJ

Assume the heat given off from this reaction is used to heat a block of iron. Specific heat capacity of iron = 0.44 J g C

What mass of Mg must react with excess oxygen in order to warm a 2.60 kg block of iron from 20.0 C to 60.0 C?

Answer 1

mass of iron block = 2.60 kg = 2600 g

specific heat of iron = 0.44 J/g.^oC

initial temperature = 20.0 ^oC

final temperature = 60.0 ^oC

Heat gained by iron = (mass of iron block) * (specific heat of iron) * (final temperature - initial temperature)

Heat gained by iron = (2600 g) * (0.44 J/g.^oC) * (60.0 ^oC - 20.0 ^oC)

Heat gained by iron = 45760 J

heat lost by Mg reaction = -(Heat gained by iron)

heat lost by Mg reaction = -(45760 J)

heat lost by Mg reaction = -45760 J

heat lost by Mg reaction = -45.76 kJ

moles Mg = (heat lost by Mg reaction) / (H_rxn)

moles Mg = (-45.76 kJ) / (-1204 kJ / 2 moles Mg)

moles Mg = 0.076 mol

mass Mg = (moles Mg) * (molar mass Mg)

mass Mg = (0.076 mol) * (24.3 g/mol)

mass Mg = 1.85 g