the reaction between 0.0689 g of magnesium with an excess of HBr was carried out in an ice calorimeter. The volume of water in the calorimeter decreased by 0.19 mL during the reaction. 

a) write the equation for the reaction which occurs. What type of reaction is taking place?

b) calculate the enthalpy of the reaction (in Kj/mol). Show all steps of the calculation.