Expansion of a gas in a vacuum is called free expansion. Calculate the work done and the change in internal energy when $1$ litre of ideal gas expands isothermally into vacuum until its total volume is $5$ litre? 
 

The net enthalpy change of a reaction is the amount of energy required to break all the bonds in reactant molecules minus amount of energy required to form all the bonds in the product molecules. What will be the enthalpy change for the following reaction. 

${\mathrm{H}}_2\left(\mathrm{g}\right) + {\mathrm{Br}}_2\left(\mathrm{g}\right) \to 2\mathrm{HBr}\left(\mathrm{g}\right)$

Given that Bond energy of ${\mathrm{H}}_2, {\mathrm{Br}}_2$ and $\mathrm{HBr}$ is $435 \mathrm{kJ} {\mathrm{mol}}^{-1}, 192 \mathrm{kJ} {\mathrm{mol}}^{-1}$ and $368 \mathrm{kJ} {\mathrm{mol}}^{-1}$ respectively. 
 

The enthalpy of reaction for the reaction:

$2{\mathrm{H}}_2\left(\mathrm{g}\right) + {\mathrm{O}}_2\left(\mathrm{g}\right) \to 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$ is ${∆}_{\mathrm{r}}{\mathrm{H}}^{\circ }=– 572 \mathrm{kJ} {\mathrm{mol}}^{-1}$. What will be standard enthalpy of formation of ${\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right)$?  

What will be the work done on an ideal gas enclosed in a cylinder, when it is compressed by a constant external pressure, ${\mathrm{p}}_{\mathrm{ext}}$ in a single step as shown in Figure? Explain graphically.