Chapter 10, Problem 8PEB

To determine

To write: The complete as well as the balanced chemical equation for the following replacement reaction and if reaction is not predicted as replacement reaction, no reaction is to be written as the product.

1. $\text{Zn }\left(s\right){\text{+ FeCl}}_{\text{2}}\left(aq\right)\to$

2. $\text{Zn(}s{\text{) + AlCl}}_3(aq)\to$

3. $\text{Cu(}s{\text{) + HgCl}}_2(aq)\to$

4. $\text{Al(}s\text{) + HCl}(aq)\to$

Answer to Problem 8PEB

Solution:

1. The complete and balanced chemical equation for the reaction is

   $\text{Zn(}s{\text{) + FeCl}}_{\text{2}}(aq)\to {\text{ ZnCl}}_{\text{2}}\text{(}aq\text{)+ Fe(}s\text{)}$.

2. The complete and balanced chemical equation for the reaction is

   $\text{Zn(}s{\text{) + AlCl}}_3(aq)\to \text{No reaction}\text{.}$

3. The complete and balanced chemical equation for the reaction is

   $\text{Cu(}s{\text{) + HgCl}}_2(aq)\to {\text{ CuCl}}_2\text{(}aq\text{) + Hg(}l\text{)}$

4. The complete and balanced chemical equation for the reaction is

   $\text{2Al(}s\text{) + 6HCl}(aq)\to {\text{ 2AlCl}}_3\text{(}aq{\text{) + 3H}}_2\uparrow$

Explanation of Solution

Introduction:

When an atom or ion is replaced or substituted by another atom or ion in a chemical compound, the reaction is known as a replacement or substitution reaction. This reaction occurs primarily due to difference in the ability of two elements to hold electrons. The element or ion which is chemically more active replaces the other.

A replacement reaction occurs when a more chemically active element replaces a lesser chemically active element from a chemical compound. The chemical activity of elements is determined by their ability to lose or gain electrons.

Usually the metals have been arranged according to their ability to lose electrons in a series called as the activity series.

The activity series of metals is given below:

Higher the position of a metal in the activity series, higher is its chemical activity.

A chemical equation is a way of depicting a chemical reaction using chemical symbols and formulae.

A balanced chemical equation is the one in which the number of atoms for each element and the total charge are same on both the sides of the equation. This is done to avoid the violation of the conservation of mass principle.

Generally, the balanced chemical equation also uses certain symbols to describe the physical state of a substance, whether it is a solid, liquid, gas, aqueous solution or a precipitate.

The symbols used to represent different physical states are given in the table below:

Physical State	Symbol
Solid	($s$)
Liquid	($l$)
Aqueous Solution	($aq$)
Gas	($g$) or $\uparrow$
Precipitate	$\downarrow$

Explanation:

(a) From the activity series, it is clear that zinc is more active than iron as it is placed higher in the activity series. So, it will displace iron and the replacement reaction will take place.

Step 1: Write the unbalanced chemical equation for the reaction.

The chemical formulae for each of the substance involved in the reaction are given in the following table:

Name of substance	Chemical formula
Zinc	$\text{Zn}$
Iron Chloride	${\text{FeCl}}_{\text{2}}$
Zinc Chloride	${\text{ZnCl}}_{\text{2}}$
Iron	$\text{Fe}$

The chemical equation for the reaction is: $\text{Zn }\left(s\right){\text{+ FeCl}}_{\text{2}}\left(aq\right)\to {\text{ ZnCl}}_{\text{2}}\left(aq\right)\text{+ Fe}\left(s\right)$.

Step 2: Now, make an inventory for the number of atoms of each element on both sides of the equation.

Name of element	Symbol	Number of atoms in reactants	Number of atoms in products
Zinc	Zn	1	1
Iron	Fe	1	1
Chlorine	Cl	2	2

Step 3: Balance the number of atoms on each side of the equation by using appropriate coefficients.

In this reaction, from the inventory it is clear that the number of atoms for each of the element are equal on both sides of the equation. Hence, the equation is automatically balanced.

The balanced chemical equation for this reaction is therefore, given as: ${\text{Zn + FeCl}}_{\text{2}}\to {\text{ ZnCl}}_{\text{2}}\text{+ Fe}$

Step 4: Denote the respective physical states of each of the substances (elements or compounds) involved in the reaction by using appropriate notations for each physical state in the balanced chemical equation.

Generally solids are denoted by ($s$), liquids by ($l$), aqueous solutions by ($aq$), gases by ($g$) or $\uparrow$ and precipitates by $\downarrow$.

The final balanced chemical equation now therefore becomes:

$\text{Zn(}s{\text{) + FeCl}}_{\text{2}}(aq)\to {\text{ ZnCl}}_{\text{2}}\text{(}aq\text{)+ Fe(}s\text{)}$

(b) A replacement reaction occurs when a more chemically active element replaces a lesser chemically active element from a chemical compound. The chemical activity of elements is determined by their ability to lose or gain electrons.

Higher the chemical activity of the metal, higher is its rank in the activity series.

It is clear from the activity series that Zinc is less reactive than Aluminum. Therefore, it cannot displace Aluminum and hence the reaction is not possible.

So, the chemical equation for the reaction is $\text{Zn(}s{\text{) + AlCl}}_3(aq)\to \text{No reaction}\text{.}$

(c) From the activity series, it is clear that copper is more active than mercury as it is placed higher in the activity series. So, it will displace mercury and the replacement reaction will take place.

Step 1: Write the unbalanced chemical equation for the reaction.

The chemical formulae for each of the substance involved in the reaction are given in the table below:

Name of substance	Chemical formula
Mercuric Chloride	${\text{HgCl}}_2$
Copper	$\text{Cu}$
Mercury	$\text{Hg}$
Copper Chloride	${\text{CuCl}}_2$

The chemical equation for the reaction is: ${\text{Cu + HgCl}}_2\to {\text{ CuCl}}_2\text{ + Hg}$.

Step 2: Now make an inventory for the number of atoms of each element on both sides of the equation.

Name of element	Symbol	Number of atoms in reactants	Number of atoms in products
Copper	Cu	1	1
Mercury	Hg	1	1
Chlorine	Cl	2	2

Step 3: Balance the number of atoms on each side of the equation by using appropriate coefficients. Repeat this until the number of atoms for each element are equal on both sides of the equation.

As seen from the inventory made in Step 2, number of atoms for each of the elements is already balanced.

Therefore, the chemical equation is already balanced.

Hence, the balanced chemical equation for this reaction is ${\text{Cu + HgCl}}_2\to {\text{ CuCl}}_2\text{ + Hg}$.

Step 4: Denote the respective physical states of each of the substances (elements or compounds) involved in the reaction by using appropriate notations for each physical state in the balanced chemical equation.

Generally solids are denoted by ($s$), liquids by ($l$), aqueous solutions by ($aq$), gases by ($g$) or $\uparrow$ and precipitates by $\downarrow$.

The final balanced chemical equation now therefore becomes:

$\text{Cu(}s{\text{) + HgCl}}_2(aq)\to {\text{ CuCl}}_2\text{(}aq\text{) + Hg(}l\text{)}$.

(d) From the activity series, it is clear that aluminum is more active than hydrogen as it is placed higher in the activity series. So, it will displace hydrogen and the replacement reaction will take place.

Step 1: Write the unbalanced chemical equation for the reaction.

The chemical formulae for each of the substance involved in the reaction are given in the table below:

Name of substance	Chemical formula
Aluminum	$\text{Al}$
Hydrogen Chloride	$\text{HCl}$
Aluminum Chloride	${\text{AlCl}}_3$
Hydrogen gas	${\text{H}}_2$

The chemical equation for the reaction is: $\text{Al + HCl}\to {\text{ AlCl}}_3{\text{+ H}}_2$.

Step 2: Now, make an inventory for the number of atoms of each element on both sides of the equation.

Name of element	Symbol	Number of atoms in reactants	Number of atoms in products
Aluminum	Al	1	1
Hydrogen	H	1	2
Chlorine	Cl	1	3

In this reaction, from the inventory it is clear that the number of atoms for aluminum are balanced but the number of atoms for hydrogen and chlorine are unbalanced.

Step 3: Balance the number of atoms on each side of the equation by using appropriate coefficients. Repeat this until the number of atoms for each element are equal on both sides of the equation.

As seen from the inventory made in Step 2, the number of H and Cl atoms are less on the reactants side. Therefore, multiply the reactant atoms of hydrogen and chlorine by 6 and the product atoms of chlorine by 2.

Now, the equation becomes $\text{Al + 6HCl}\to {\text{ 2AlCl}}_3{\text{ + H}}_2$

And now, make another inventory to check if the equation is balanced or not.

Name of element	Symbol	Number of atoms in reactants	Number of atoms in products
Aluminum	Al	1	2
Hydrogen	H	6	2
Chlorine	Cl	6	6

Now, the number of atoms for chlorine are balanced whereas the number of atoms for hydrogen and aluminum are unbalanced. Therefore, multiply the reactant side atoms of Aluminum by 2 and multiply the number of hydrogen atoms on product side by 3.

The chemical equation now becomes $\text{2Al + 6HCl}\to {\text{ 2AlCl}}_3{\text{ + 3H}}_2$

Name of element	Symbol	Number of atoms in reactants	Number of atoms in products
Aluminum	Al	2	2
Hydrogen	H	6	6
Chlorine	Cl	6	6

Now number of atoms for each of the elements is same on both sides of the equation.

Hence, the chemical equation is now balanced.

Hence, the balanced chemical equation for this reaction is $\text{2Al + 6HCl}\to {\text{ 2AlCl}}_3{\text{ + 3H}}_2$.

Step 4: Denote the respective physical states of each of the substances (elements or compounds) involved in the reaction by using appropriate notations for each physical state in the balanced chemical equation.

Generally solids are denoted by ($s$), liquids by ($l$), aqueous solutions by ($aq$), gases by ($g$) or $\uparrow$ and precipitates by $\downarrow$.

The final balanced chemical equation now therefore becomes:

$\text{2Al(}s\text{) + 6HCl}(aq)\to {\text{ 2AlCl}}_3\text{(}aq{\text{) + 3H}}_2\uparrow$.