Question

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

Answer 1

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_(soln) ~~ -q_(rxn)`

That is, `q_(soln)` and `q_(rxn)` are opposite in sign!

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At constant pressure, the heat flow through the solution, `q_(soln)` is equal to the enthalpy of solution, `DeltaH_"soln"` in units of `"J"`.

This heat flow came out into the solution due to the reaction, so by conservation of energy, `q_(soln) = -q_(rxn)`. We thus have in, `ul"J/mol"`, that

`color(blue)(q_(soln)/(n_("product")) = -DeltabarH_(rxn))`,

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_("product")` is the mols of the reaction product.

For the solution heat flow, we write:

`q_(soln) = m_(soln)C_(soln)DeltaT`,

and we often approximate the solution heat capacity `C_(soln)` as the heat capacity of water, i.e. `C_(soln) ~~ "4.184 J/g"^@ "C"`.

The mass of the solution, `m_(soln)`, is generally approximated by knowing its volume (by measurement!) and assuming the density of the solution is that of water, `"1 g/mL"`.

Thus, by measuring the temperature change in the solution, and knowing how much solution was measured out, one can determine `q_(soln)` and consequently determine `DeltaH_"rxn"`.