Question

The Heat required for reaction of one litre of $H_2\left(g\right)$ with sufficient $O_2\left(g\right)$ at STP is $12.78kJ$. Find the approximate value of standard enthalpy of formation of $H_{2}O\left(l\right)$.

• A. $+186kJ$
• B. $-286kJ$
• C. $-480kJ$
• D. None of these

Answer 1

The correct option is B $-286kJ$

The reaction taking place is:
$H_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to H_{2}O\left(l\right)$

Considering,
$1L{H}_2\left(g\right)atSTP=\frac{1}{22.4}mol$
$\therefore$ Heat released due to reaction of $\frac{1}{22.4}$ mol of $H_2\left(g\right)=12.78kJ$
But for formation one mole is reacting, so
Heat released due to reaction of 1 mol of $H_2\left(g\right)$
$=12.78\times 22.4=286.27kJ$
$\therefore$ standard enthalpy of formation of $H_{2}O\left(l\right)=-286kJ$

Standard enthalpy of formation (Δf H°)

It is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements in their most stable forms (Standard states 1 bar and usually 298 K).

It is denoted by ${\Delta }_{f}H°$

Example:

Standard enthalpy of formation of $HCl\left(g\right)$

Chemical equation

$\frac{1}{2}{H}_2\left(g\right)+\frac{1}{2}C{l}_2\left(g\right)\to HCl\left(g\right)$
Standard enthalpy of reaction in this case is the standard enthalpy of formation of $HCl\left(g\right)$

${\Delta }_{f}H°(HCl,g)=H_m°(HCl,g)-\frac{1}{2}{H}_m°(H_2,g)-\frac{1}{2}{H}_m°(C{l}_2,g)$