Calculate the Molar Enthalpy of Reaction

Question

Calculate the molar enthalpy of reaction for Mg(s) for each of the 3 trials.

Trial 1: Mass of Mg(s)= 0.15g┃Change in Temp. 13.6°C

Trial 2: Mass of Mg(s)= 0.25g┃Change in Temp. 22.7°C

Trial 3: Mass of Mg(s)= 0.35g┃Change in Temp. 31.8°C

Answer 1

In order to calculate the molar enthalpy of a reaction, we need more information. The molar enthalpy of reaction (ΔH) depends not only on the change in temperature and the mass of the reactant but also on the specific heat capacity of the substance and the amount of substance in moles.

The general formula to calculate enthalpy is:

ΔH = mcΔT

Where:

m = mass

c = specific heat capacity

ΔT = change in temperature

However, we need to know the specific heat capacity (c) of the system (usually in J/g°C) to use this formula. We also need to divide the result by the number of moles of magnesium in each trial to get the molar enthalpy.

We can calculate the number of moles of magnesium (Mg) in each trial using its molar mass (about 24.305 g/mol), as follows:

Trial 1: 0.15g / 24.305 g/mol = 0.0062 mol

Trial 2: 0.25g / 24.305 g/mol = 0.0103 mol

Trial 3: 0.35g / 24.305 g/mol = 0.0144 mol

But without knowing the specific heat capacity, we cannot calculate the molar enthalpy of the reaction. If this reaction occurs in solution, for instance, it might also be useful to know the amount and specific heat capacity of the solution, because the solution usually absorbs or releases most of the heat from the reaction. If the reaction was performed in a calorimeter, we'd need the calorimeter constant. If you provide these details, I could assist you further with the calculations.