 # Heat of Displacement

Heat of displacement is the energy change when one mole of metal is displaced from its salt solution by a more electropositive metal
1. 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.
2. The heat of displacement is the energy change when one mole of metal is displaced from its salt solution by a more electropositive metal
3. Displacement reaction is always an exothermic reaction.
4. 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 ]

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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