Chapter 10, Problem 7PEB

To determine

To wite: The balanced chemical equation for each of the following decomposition reaction.

1. When solid zinc carbonate is heated, solid zinc oxide and carbon dioxide gas are produced.

2. Liquid hydrogen peroxide decomposes to liquid water and oxygen gas.

3. Solid ammonium nitrite decomposes into liquid water and ammonia gas.

Answer to Problem 7PEB

Solution:

1. The balanced chemical equation for the reaction is ${\text{ZnCO}}_{\text{3}}(s)\stackrel{\text{Heat}}{\to }\text{ ZnO(}s{\text{) + CO}}_{\text{2}}\uparrow$

2. The balanced chemical equation for the reaction is ${\text{2H}}_{\text{2}}{\text{O}}_{\text{2}}(l)\stackrel{\text{Heat}}{\to }{\text{ 2H}}_{\text{2}}\text{O(}l{\text{) + O}}_{\text{2}}\uparrow$

3. The balanced chemical equation for the reaction is ${\text{NH}}_4{\text{NO}}_{\text{2}}(s)\stackrel{\text{Heat}}{\to }{\text{ 2H}}_{\text{2}}\text{O(}l{\text{) + N}}_2\uparrow$

Explanation of Solution

Introduction:

A decomposition reaction, as the term states, is the one in which a single compound is decomposed or broken down into different chemical substances which may be simpler compounds, elements or a combination of both.

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) 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
Zinc Carbonate	${\text{ZnCO}}_{\text{3}}$
Zinc Oxide	$\text{ZnO}$
Carbon Dioxide	${\text{CO}}_{\text{2}}$

The chemical equation for the reaction is: ${\text{ZnCO}}_{\text{3}}\stackrel{\text{Heat}}{\to }{\text{ ZnO + CO}}_{\text{2}}$.

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
Zinc	Zn	1	1
Carbon	C	1	1
Oxygen	O	3	3

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: ${\text{ZnCO}}_{\text{3}}\stackrel{\text{Heat}}{\to }{\text{ ZnO + CO}}_{\text{2}}$

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:

${\text{ZnCO}}_{\text{3}}(s)\stackrel{\text{Heat}}{\to }\text{ ZnO(}s{\text{) + CO}}_{\text{2}}\uparrow$

(b) 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
Hydrogen Peroxide	${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$
Water	${\text{H}}_{\text{2}}\text{O}$
Oxygen	${\text{O}}_{\text{2}}$

The chemical equation for the reaction is: ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}\stackrel{\text{Heat}}{\to }{\text{ H}}_{\text{2}}{\text{O + O}}_{\text{2}}$.

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
Hydrogen	H	2	2
Oxygen	O	2	3

In this reaction, from the inventory it is clear that the number of atoms for hydrogen are same on each side but the oxygen atoms 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 O atoms are less on the reactants side. Therefore, multiply the reactants side by 2.

Now the equation becomes ${\text{2H}}_{\text{2}}{\text{O}}_{\text{2}}\stackrel{\text{Heat}}{\to }{\text{ H}}_{\text{2}}{\text{O + O}}_{\text{2}}$

Now again check the inventory for each atom for this equation.

Name of element	Symbol	Number of atoms in reactants	Number of atoms in products
Hydrogen	H	4	2
Oxygen	O	4	3

Now, both the hydrogen and oxygen atoms are lesser on the products side. Therefore, multiply the product ${\text{H}}_{\text{2}}\text{O}$ by 2. The chemical equation for this reaction now becomes: ${\text{2H}}_{\text{2}}{\text{O}}_{\text{2}}\stackrel{\text{Heat}}{\to }{\text{ 2H}}_{\text{2}}{\text{O + O}}_{\text{2}}$

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
Hydrogen	H	4	4
Oxygen	O	4	4

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

Hence, the chemical equation is now balanced.

The balanced chemical equation for this reaction is ${\text{2H}}_{\text{2}}{\text{O}}_{\text{2}}\stackrel{\text{Heat}}{\to }{\text{ 2H}}_{\text{2}}{\text{O + O}}_{\text{2}}$.

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:

${\text{2H}}_{\text{2}}{\text{O}}_{\text{2}}(l)\stackrel{\text{Heat}}{\to }{\text{ 2H}}_{\text{2}}\text{O(}l{\text{) + O}}_{\text{2}}\uparrow$.

(c) 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
Ammonium Nitrite	${\text{NH}}_4{\text{NO}}_{\text{2}}$
Water	${\text{H}}_{\text{2}}\text{O}$
Nitrogen	${\text{N}}_{\text{2}}$

The chemical equation for the reaction is: ${\text{NH}}_4{\text{NO}}_{\text{2}}\stackrel{\text{Heat}}{\to }{\text{ H}}_{\text{2}}{\text{O + N}}_2$.

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
Nitrogen	N	2	2
Hydrogen	H	4	2
Oxygen	O	2	1

In this reaction, from the inventory it is clear that the number of atoms for nitrogen are balanced but the number of atoms for hydrogen and oxygen 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 atoms are less on the products side. Therefore, multiply the products side by 2.

Now, the equation becomes ${\text{NH}}_4{\text{NO}}_{\text{2}}\stackrel{\text{Heat}}{\to }{\text{ 2H}}_{\text{2}}{\text{O + N}}_2$

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
Nitrogen	N	2	2
Hydrogen	H	4	4
Oxygen	O	2	2

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 ${\text{NH}}_4{\text{NO}}_{\text{2}}\stackrel{\text{Heat}}{\to }{\text{ 2H}}_{\text{2}}{\text{O + N}}_2$.

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:

${\text{NH}}_4{\text{NO}}_{\text{2}}(s)\stackrel{\text{Heat}}{\to }{\text{ 2H}}_{\text{2}}\text{O(}l{\text{) + N}}_2\uparrow$.