Question

The amount of the heat released when $20mL$ $0.5MNaOH$ is mixed with $100mL0.1MHCl$ is $xkJ$. The heat of neutralization is

• A. $-100xkJ/mol$
• B. $-50xkJ/mol$
• C. $+100xkJ/mol$
• D. $50xkJ/mol$

Answer 1

Answer: (A)

Millimoles (or milliequivalents) of
$NaOH=20\times 0.5=10$
Millimoles (or milliequivalents) of
$HCl=100\times 0.1=10$
$\therefore \underset{\stackrel{10}{\mathrm{milliequivalents}}}{\mathrm{NaOH}}+\underset{\stackrel{10}{\mathrm{milliequivalents}}}{\mathrm{HCl}}\to NaCl+H_{2}O$
Thus, heat released when 10 milliequivalents of $HCl$ are neutralised by 10 millimoles of $NaOH=\times kJ$. But heat of neutralisation is heat released when 1 equivalent of $HCl$ is neutralised by $1$ equivalent of $NaOH$.
$\therefore {\Delta }_{\text{neu }}H=-\frac{x}{10\times 10^{-3}}$ or $-100\times kJ/mol$