Question

1150 kcal heat is released when following reaction is carried out at constant volume ${{27}^{\circ }}C$
${{C}_{7}}{{H}_{16}}(l)+11{{O}_{2}}(g)\to 7C{{O}_{2}}(g)+8{{H}_{2}}O(l)$
Find the heat change at constant pressure.

Answer 1

Hint: Different reactions have different energy requirements. Some reactions release energy while taking up energy in different forms either it be heat or light or any other.

Complete step by step answer:
Depending upon whether the reaction takes up or releases energy it can endothermic or exothermic reaction.
The amount of heat transferred is equal to the change in internal energy.
The mass of substance and temperature is directly proportional to the heat. The heat change is also dependent on the phase of either liquid, solid or gas.
To calculate the heat released in a chemical reaction we use the equation $Q=mc\Delta T$
where,
m is the mass of the liquid being heated in kilograms
c is the specific heat capacity of the liquid in joules per kilogram degree celsius
and $\Delta T$ is the change in temperature in degrees Celsius.
As we already know
Heat change at constant pressure = $\Delta H$
and heat change at constant volume = $\Delta E$
${{C}_{7}}{{H}_{16}}(l)+11{{O}_{2}}(g)\to 7C{{O}_{2}}(g)+8{{H}_{2}}O(l)$
Here, $\begin{align}
  & \Delta {{n}_{g}}=7-11=-4 \\
 & \Delta H=\Delta U+\Delta {{n}_{g}}RT \\
 & \Delta H=1150000+(-4)(2)(300) \\
 & \Delta H=1150000-2400 \\
 & \Delta H=1147.6kJ \\
\end{align}$
Therefore the heat change at constant pressure is $1147.6kJ$.

Note: Since we were given $1150kJ$ we converted it into joules which is $1150000$J.
Always make sure while calculating that you keep all the quantities in their standard SI units and accordingly answer( mostly we answer in the unit we were given in the question or you can refer to the options if given).
The change in number of moles are calculated only for the substance in gaseous state.