Enthalpy of reaction (Δ_rH) refers to the enthalpy change for an entire chemical system as reactants change to products. However, sometimes knowing the enthalpy change per mole of a specified entity in the reaction is useful. This quantity, designated (Δ_rH_m), is known as the "molar enthalpy of reaction" and has the units kJ/mol. How can calorimetry investigations assist us in calculating this quantity?

In the previous section, you learned that Δ_rH = Q. In other words, the enthalpy change of a system is equal to the total thermal energy change of the water in the calorimeter (as reflected by the temperature change of the water).

You learned also that the total thermal energy change of a system can be calculated using the formula, Q = mcΔt.

Since Δ_rH = Q

\( \mathrm { \Delta_rH = mc\Delta t } \)

Now, recall from Module 3 that Δ_rH = n Δ_rH_m. Note the subscript "m", which indicates that the enthalpy change is per mole.

Therefore, nΔ_rH_m = mcΔt

In summary then, to solve for the molar enthalpy of reaction (for a specific substance in that reaction), we divide mcΔt by the number of moles of that specific substance using the following:

Δ_rH_m = mcΔt/n

Although calorimetric data can be used to calculate molar enthalpy change, a certain degree of experimental error is inevitable. As you learned earlier in the module, no calorimeter is 100% efficient. We assume that the water absorbs all the heat, but some heat is invariably absorbed by the calorimeter itself. The degree to which calorimeter inefficiency affects experimental results is reflected in a measure known as "percent difference".

Percent difference is a measure of how closely an experimental value compares with an accepted (predicted) value. The predicted value is the widely accepted value. Sometimes percent difference is referred to as percent error.

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