What is the term used for a well insulated reaction vessel used to measure delta H of reactions?

May 15, 2017

Often the determination of the enthalpy of reaction can be done in a coffee-cup calorimeter, otherwise known as a constant-pressure calorimeter.

This device is often well-insulated so that we can rightfully say,

${q}_{s o \ln} \approx - {q}_{r x n}$

That is, ${q}_{s o \ln}$ and ${q}_{r x n}$ are opposite in sign!

At constant pressure, the heat flow through the solution, ${q}_{s o \ln}$ is equal to the enthalpy of solution, $\Delta {H}_{\text{soln}}$ in units of $\text{J}$.

This heat flow came out into the solution due to the reaction, so by conservation of energy, ${q}_{s o \ln} = - {q}_{r x n}$. We thus have in, $\underline{\text{J/mol}}$, that

$\textcolor{b l u e}{{q}_{s o \ln} / \left({n}_{\text{product}}\right) = - \Delta {\overline{H}}_{r x n}}$,

for the relationship through which one can find the enthalpy of a reaction that occurs in a solution inside the coffee-cup calorimeter, where ${n}_{\text{product}}$ is the mols of the reaction product.

For the solution heat flow, we write:

${q}_{s o \ln} = {m}_{s o \ln} {C}_{s o \ln} \Delta T$,

and we often approximate the solution heat capacity ${C}_{s o \ln}$ as the heat capacity of water, i.e. ${C}_{s o \ln} \approx \text{4.184 J/g"^@ "C}$.

The mass of the solution, ${m}_{s o \ln}$, is generally approximated by knowing its volume (by measurement!) and assuming the density of the solution is that of water, $\text{1 g/mL}$.

Thus, by measuring the temperature change in the solution, and knowing how much solution was measured out, one can determine ${q}_{s o \ln}$ and consequently determine $\Delta {H}_{\text{rxn}}$.