Displacement reaction occurs when a metal which is situated at a higher position in the electrochemical series displace a metal below it from its salt solution.
The heat of displacement is the energy change when one mole of metal is displaced from its salt solution by a more electropositive metal
Displacement reaction is always an exothermic reaction.
The example below shows how the heat of displacement can be calculated.

Example:

A student pours 50cm³ hydrochloric acid, 2 mol/dm³ into a plastic beaker, and records its temperature. Some zinc powder (excess) is added into the acid. The solution is stirred by using a glass rod, and the highest temperature that reaches by the mixture is recorded.

Initial temperature of the solution: 27.0°C

Highest temperature of the solution: 57.0°C

Find the heat of displacement of the reaction.

[Specific heat capacity of the solution = = 4.2 Jg^-1°C^-1, density of the solution =1 g/cm³ ]

Answer:

2HCl(ak) + Zn(p) → ZnCl₂ (ak) + H₂(g)

Number of mole of hydrochloric acid
\begin{gathered}
n = \frac{{MV}}{{1000}} \hfill \\
n = \frac{{(2.0)(50)}}{{1000}} \hfill \\
n = 0.1mol \hfill \\
\end{gathered}

Number of mole of hydrogen gas produced
= 0.1/2=0.05mol

Amount of heat released

Q = mcθ
Q = (50)(4.2)(57-27)
Q = 6300J

\[\begin{gathered}
\Delta H = \frac{{{\text{Heat Change}}}}{{{\text{Number of mole of precipitate form}}}} \hfill \\
= \frac{{6300}}{{0.05}} \hfill \\
= 126,000Jmo{l^{ – 1}} = 126kJmo{l^{ – 1}} \hfill \\
\end{gathered} \]

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