Chapter 7, Problem 35A

(a)

Interpretation Introduction

Interpretation:

The given chemical equation needs to be balanced.

  ${\text{H}}_2{\text{O}}_2\left(aq\right)\to {\text{H}}_2\text{O}\left(l\right)+{\text{O}}_2\left(g\right)$

Concept Introduction:

In a balanced equation, there are equal numbers of atoms of the elements on the reactant and the product side. This is due to the law of conservation of mass which states that matter can neither be created nor destroyed, but may be transformed from one form into another.

Answer to Problem 35A

The balanced chemical equation is

  ${\text{2 H}}_2{\text{O}}_2\left(aq\right)\to {\text{2 H}}_2\text{O}\left(l\right)+{\text{O}}_2\left(g\right)$

Explanation of Solution

The given reaction is a decomposition reaction.

  ${\text{H}}_2{\text{O}}_2\left(aq\right)\to {\text{H}}_2\text{O}\left(l\right)+{\text{O}}_2\left(g\right)$

It is seen that there are equal numbers of hydrogen (H) atoms on the two side, however, the product side has more oxygen (O) atoms.

It is known that atoms are indivisible; moreover, oxygen exists as a diatomic gas (O₂). Therefore, a trial approach must be employed to balance the numbers of O atoms. Since the product side has more O atoms, it will be useful to multiply ${\text{H}}_2{\text{O}}_2$ by 2 and try to balance the number of O atoms.

  ${\text{2 H}}_2{\text{O}}_2\left(aq\right)\to {\text{H}}_2\text{O}\left(l\right)+{\text{O}}_2\left(g\right)$

Since the number of H atoms has now doubled, hence, ${\text{H}}_2\text{O}$ must be multiplied by 2.

  ${\text{2 H}}_2{\text{O}}_2\left(aq\right)\to 2{\text{ H}}_2\text{O}\left(l\right)+{\text{O}}_2\left(g\right)$

A quick inspection indicates that all there are equal numbers of H and O atoms and thus, the reaction is balanced.

This trial and error approach works for simple equations; however, for more complex reactions, a proper method like the ion electron method must be used.

(b)

Interpretation Introduction

Interpretation:

The given chemical equation needs to be balanced.

  $\text{Ag}\left(s\right)+{\text{H}}_2\text{S}\left(g\right)\to {\text{Ag}}_2\text{S}\left(s\right)+{\text{H}}_2\left(g\right)$

Concept Introduction:

The law of conservation of mass indicates that all the atoms in a chemical reaction must be balanced.

Answer to Problem 35A

The balanced chemical equation is

  $\text{2 Ag}\left(s\right)+{\text{H}}_2\text{S}\left(g\right)\to {\text{Ag}}_2\text{S}\left(s\right)+{\text{H}}_2\left(g\right)$

Explanation of Solution

The unbalanced reaction is given as

  $\text{Ag}\left(s\right)+{\text{H}}_2\text{S}\left(g\right)\to {\text{Ag}}_2\text{S}\left(s\right)+{\text{H}}_2\left(g\right)$

In order to balance the number of silver (Ag) atoms on both sides, multiply $\text{Ag}$ on the left by 2.

  $\text{2 Ag}\left(s\right)+{\text{H}}_2\text{S}\left(g\right)\to {\text{Ag}}_2\text{S}\left(s\right)+{\text{H}}_2\left(g\right)$

There are equal numbers of atoms of all the elements on both sides and hence, the reaction is balanced.

(c)

Interpretation Introduction

Interpretation:

The given chemical equation needs to be balanced.

  $\text{FeO}\left(s\right)+\text{C}\left(s\right)\to \text{Fe}\left(l\right)+{\text{CO}}_2\left(g\right)$

Concept Introduction:As per the law of conservation of mass, there must be equal numbers of atoms of all the elements in a balanced chemical equation.

Answer to Problem 35A

The balanced chemical equation is

  $\text{2 FeO}\left(s\right)+\text{C}\left(s\right)\to \text{2 Fe}\left(l\right)+{\text{CO}}_2\left(g\right)$

Explanation of Solution

The unbalanced reaction is

  $\text{FeO}\left(s\right)+\text{C}\left(s\right)\to \text{Fe}\left(l\right)+{\text{CO}}_2\left(g\right)$

There are twice as many O atoms on the right as there are on the left. In order to balance the number of O atoms, multiply $\text{FeO}$ on the left by 2.

  $\text{2 FeO}\left(s\right)+\text{C}\left(s\right)\to \text{Fe}\left(l\right)+{\text{CO}}_2\left(g\right)$

The number of O atoms on both sides is equal; however, there are twice as many iron, $\text{Fe}$ atoms on the left as on the right. To balance the number of Fe atoms of both sides, multiply Fe on the right by 2.

  $\text{2 FeO}\left(s\right)+\text{C}\left(s\right)\to 2\text{ Fe}\left(l\right)+{\text{CO}}_2\left(g\right)$

The reaction has equal numbers of atoms of the elements on both sides and hence, the reaction is balanced.

(d)

Interpretation Introduction

Interpretation:

The given chemical equation needs to be balanced.

  ${\text{Cl}}_2\left(g\right)+\text{KI}\left(aq\right)\to \text{KCl}\left(aq\right)+{\text{I}}_2\left(s\right)$

Concept Introduction:

The law of conservation of mass states that there are equal numbers of atoms on the reactant and the product side of a balanced chemical equation.

Answer to Problem 35A

The balanced chemical equation is

  ${\text{Cl}}_2\left(g\right)+\text{2 KI}\left(aq\right)\to 2\text{ KCl}\left(aq\right)+{\text{I}}_2\left(s\right)$

Explanation of Solution

The unbalanced chemical equation is

  ${\text{Cl}}_2\left(g\right)+\text{KI}\left(aq\right)\to \text{KCl}\left(aq\right)+{\text{I}}_2\left(s\right)$

Start by balancing the number of chlorine, Cl atoms on both sides. This can be done by multiplying potassium chloride, $\text{KCl}$ on the right by 2.

  ${\text{Cl}}_2\left(g\right)+\text{KI}\left(aq\right)\to 2\text{ KCl}\left(aq\right)+{\text{I}}_2\left(s\right)$

Next balance the iodine, I atoms on both sides by multiplying potassium iodide, $\text{KI}$ on the left by 2.

  ${\text{Cl}}_2\left(g\right)+2\text{ KI}\left(aq\right)\to 2\text{ KCl}\left(aq\right)+{\text{I}}_2\left(s\right)$

The number of potassium, K atoms on both sides is equal; hence, the reaction is balanced.