Question

7.) Given the following reaction: 2 Mg (s)+ O2 (g) 2 MgO (s) AH 1204 kJ/mol a.) Calculate the mass of magnesium oxide formed if a reaction gives off 395 kJ of heat. b.) Draw a potential energy diagram for the reaction. Be sure to label each axis, the reactants, products, enthalpy of reaction, and activation energy on your diagram.

Answer 1

7

a)

The balanced reaction is

$2 Mg _{(s)} + O_2_{(g)} \rightarrow 2MgO_{(s)}$

The enthalpy change of the reaction is -1204 kJ/mol. Since it is negative, by convention it means energy is released.

Hence, energy released when 1 mole of MgO is formed is 1204 kJ.

Hence, when the energy released is 395 kJ, the number of moles of MgO that must have formed is

$\frac{1 \ mol }{1204 \ kJ } \times 395 \ kJ = 0.328 \ mol.$

Molecular mass of MgO is 40.30 g/mol.

Hence, mass of MgO formed is

$0.328 \ mol \times 40 \ g/ mol \approx 13.22 \ g$

b)

Note that the energy of product is lower than the energy of reactants. Hence, the enthalpy change is negative for this exothermic reaction.