Question

The combustion of magnesium used to be used in flash bulbs because of the brightness of its flash.

What is the heat of combustion of magnesium anyway?

a) (4pts) To determine the heat of combustion, I burned a 0.532 g sample of magnesium in a constant volume bomb calorimeter and the temperature of the water increased by 6.318 ^oC. Calculate the heat of      combustion of my sample of Mg, in kJ/mol, given that the heat capacity of the calorimeter is 2085 J/ ^oC . Show all work.

b) (3pts) Given the combustion of magnesium is Mg(s) + ½ O₂(g) à MgO(s) Calculate the heat of combustion of magnesium from Appendix C in your text. Show all work. Compare to your previous answer

Answer 1

Heat absorbed by calorimeter = - heat given in combstion reaction

heat absorbed by calorimeter = heat capacity x rise in temperature

= 2085J /g.K x 6.318 K

= 13173.03 J

Thus 0.532 g of Mg on combustion gave 13173.03 J of heat

one of Mg (24g/Mol) on combustion gives = 24g/mol x13173.03 J/0.532g

=594272 J /mol

= 594.272 kJ/mol

b) Mg(s) + 1/2 O2 (g) ----------> MgO(s)  

The heat of this reaction = heat of combustion

We can compute the heat of reaction as

Delta H rxn = sum of heats of formation of products - sum of heats of formation of reactants

= delta Hf (MgO) - {delt aHf Mg + delta Hf of O2}

= -601.70 kJ - [0 +0]

= -601.70 kJ

Comparing the experimental value of 594.272 kJ , this value is approximately close.