Question

A 0.1326 g sample of magnesium was reacted with oxygen (a combustion reaction) in a bomb calorimeter. The total heat capacity of the calorimeter is 5,760 J/C. The temperature rise of the calorimeter was 0.570C.

a) ) Write the balanced thermochemical reaction equation.

b) Calculate the Molar Enthalpy of Combustion, ΔH_comb of magnesium.

Answer 1

To find the enthalpy change per mole of a substance A in a reaction between two substances A and B, the substances are separately added to a calorimeter and the initial and final temperatures (before the reaction has started and after it has finished) are noted. Multiplying the temperature change by the mass and specific heat capacities of the substances gives a value for the energy given off or absorbed during the reaction. Dividing the energy change by how many moles of A were present gives its enthalpy change of reaction.

q = Cv (Tf - Ti)

here q is the amount of heat according to the change in temperature measured in joules and Cv is the heat capacity of the calorimeter which is a value associated with each individual apparatus in units of energy per temperature (Joules/Kelvin or Joules/deg celcius) and Temp change is Final minus Initial.

Reaction of Magnesium burning =

2Mg + O₂ --------------> 2MgO + Energy released

Now from above equation Energy will be

q = 5760 J/C X 0.570 Deg-Cel

q = 3283.2 Joules

Enthalpy = q / sample weight = 3283.2 /0.1326 = 24760 Joules/Gram

Now if we require enthalpy in kJ/mol multiply above value with Molecular weight of Mg as 24.305 gm/mol

= 24760 X 24.305 = 601792 Joules/mole (or 601.8 Kjoules/mole)