Question

The calorific value $H_2$(g) at STP is 12.78 kJ/L. The approximate standard enthalpy of formation of $H_{2}O$(l) is –

• A. -143 kJ
• B. -286 kJ
• C. Zero
• D. +286 kJ

Answer 1

The correct option is B -286 kJ

1 L $H_2\left(g\right)$ at STP = $\frac{1}{22.4}$ mol
$\therefore$ Heat released due to combustion of $\frac{1}{22.4}$ mol $H_2\left(g\right)=12.78$ kJ
Heat released due to combustion of 1 mol of $H_2\left(g\right)=12.78\times 22.4=286.27$ kJ
$\therefore$ approximate standard enthalpy of formation of $H_{2}O\left(l\right)=-286$ kJ