Question

When $Zn$ dust is added to a sufficiently large volume of aqueous solution of copper sulphate. 3.175 g of copper metal and 20 J of heat is evolved. The $\Delta H$ for the reaction is:
$Zn\left( s \right) + CuS{O_4} \to ZnS{O_4}(aq) + Cu\left( s \right)$
(Atomic weight of Zn$ = 65.3$, atomic weight of Cu$ = 63.5$)
A. 20 J
B. 200 J
C. 400 J
D. 65.3 J

Answer 1

Hint: A solution in which the solvent is water is called an aqueous solution. In an aqueous solution, the solute or dissolved ions or molecules are surrounded by the water molecules and make several bonds within the water. The dissolved ions or molecules can spread throughout the water. The reactions which occur in aqueous solution are mostly double-displacement or metathesis reactions i.e. when a cation is displaced to form an ionic bond with the other anion. The cation which is bonded with the latter anion will dissociate and make a bond with the other anion. The aqueous solution which conducts strong electric current contains strong electrolytes and the aqueous solution which conducts poor electric current contains weak electrolytes. The strong electrolytes get completely dissociated in an aqueous solution while the weak electrolytes do not completely dissociate in an aqueous solution.

Complete step by step answer:
The given reaction is:

 $Zn\left( s \right) + CuS{O_4} \to ZnS{O_4}(aq) + Cu\left( s \right)$

In this reaction, we can see zinc displaces copper from the solution so we can say that the given reaction is an example of a displacement reaction. As the heat is evolved in the reaction, it is an exothermic reaction.

In the given question we have found the change in the heat for the whole reaction.

Given,

Mass of Copper $ = 3.175$g

Molecular mass of copper $ = 63.5$

Now,

Number of moles of copper $ = \dfrac{m}{M}$

$ = \dfrac{{3.175}}{{63.5}}$

$ = 0.05$ mol

(Since the mass of copper and molecular mass of copper is given, using this we have calculated
number of moles of copper)

We know,

$\Delta H = {H_{product}} - {H_{react.}}$

$\Rightarrow \Delta H = 20 - 0$

$\Delta H = 20J = {H_{product}}$

(We have calculated the $\Delta H$ value and is equal to the heat evolved in the product)

As we can see that $\Delta H$ is equal to the heat evolved in the product so we can say that

0.05 mole of copper evolves heat $ = 20J$
1 mole of copper evolves heat $ = \dfrac{{20}}{{0.05}}$
$ = 400J$

Therefore, the $\Delta H$ value for the reaction is 400 J.

So, the correct answer is Option C .

Note: A reaction in which an atom or set of atoms gets displaced by another atom in a molecule is known as a displacement reaction. In general, the reaction can be written as:

$A + BC \to AC + B$

The above reaction shows that A is more reactive than B.

Displacement reactions are of two types: Single Displacement reaction and Double Displacement reaction.