# How to evaluate enthalpy variation of combustion of sucrose in a constant volume calorimeter?

Consider combustion reaction (with the formation of $H_2O$ and $CO_2$) of a mass $m$ of sucrose in a constant volume calorimeter with thermal capacity $C$. This reaction increases the temperature of $\Delta T$.

How do I find $\Delta H$ of this reaction?

Since $$\Delta U= C \Delta T= \Delta H+\Delta n_{gas} R T$$

I would use this formula, but what is $\Delta n_{gas}$ in this case? It should be zero (from textbook) but I do not undersand why it is zero. Furthermore, can I really use this formula even if $T$ is not a constant and there is a variation in temeperature in the reaction?

• $\ce{C6H12O6 (s) + 6 O2 (g) -> 6 CO2 (g) + 6 H2O (l)}$. 6 moles of gas on the left and 6 on the right, so $\Delta n_g = 0$. – orthocresol Jul 24 '17 at 11:31