Question

The heat for a reaction,
$C_{10}{H}_8\left(s\right)+12O_2\left(g\right)\to 10C{O}_2\left(g\right)+4H_{2}O\left(l\right)$
at constant volume is given as $-1228.2kcal$ at ${25}^{o}C$. The heat of the reaction at constant pressure and same temperature is:

• A. $-1345.5kcal$
• B. $-1229.3kcal$
• C. $-1156kcal$
• D. $-1380.2kcal$

Answer 1

The correct option is B $-1229.3kcal$

Given that, $△U=-1228.2kcal=-1228.2\times {10}^{3}cal$

we know that:
$△H=△U+△n_{g}RT$
$△n_g=10-12=-2$
$T=25+273=298K$
$R=2cal/K.mol$
so,
$△H=-1228.2\times {10}^3+(-2)\times \left(2\right)\times 298$ $=-1229.39kcal$