Problem 18 - Chemistry The Central Science In Si Units

Question

For each of the following balanced oxidation-reduction reactions, (i) identify the oxidation numbers for all the elements in the reactants and products and (ii) state the total number of electrons transferred in each reaction. (a) \(\mathrm{H}_{2}(g)+\mathrm{F}_{2}(g) \longrightarrow 2 \mathrm{HF}(g)\) $$ \begin{array}{l} \text { (b) } 2 \mathrm{Fe}^{2+}(a q)+\mathrm{H}_{2} \mathrm{O}_{2}(a q)+2 \mathrm{H}^{+}(a q) \\ \longrightarrow 2 \mathrm{Fe}^{3+}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \\ \text { (c) } \mathrm{CH}_{4}(g)+2 \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(l) \end{array} $$

Step-by-Step Solution

Answer

8 electrons are transferred in reaction (c), 2 in (a) and (b).

Step 1: Reaction (a) Determine Oxidation Numbers

For the reaction \( \mathrm{H}_{2}(g)+\mathrm{F}_{2}(g) \longrightarrow 2 \mathrm{HF}(g) \), we assign oxidation numbers as follows:- \( \mathrm{H}_{2} \): Hydrogen has an oxidation number of 0 (elemental state).- \( \mathrm{F}_{2} \): Fluorine has an oxidation number of 0 (elemental state).- \( \mathrm{HF} \): Hydrogen has an oxidation number of +1, and fluorine has an oxidation number of -1.

Step 2: Reaction (a) Count Electrons Transferred

In the reaction, each \( \mathrm{H} \) atom goes from an oxidation state of 0 to +1, losing 1 electron, while each \( \mathrm{F} \) atom goes from an oxidation state of 0 to -1, gaining 1 electron.Therefore, the total number of electrons transferred is 2 (one from each \( \mathrm{H} \) atom to a \( \mathrm{F} \) atom).

Step 3: Reaction (b) Determine Oxidation Numbers

For the reaction \( 2 \mathrm{Fe}^{2+}(aq) + \mathrm{H}_{2}\mathrm{O}_{2}(aq) + 2\mathrm{H}^{+}(aq) \longrightarrow 2 \mathrm{Fe}^{3+}(aq) + \mathrm{H}_{2}\mathrm{O}(l) \):- \( \mathrm{Fe}^{2+} \): Iron has an oxidation number of +2.- \( \mathrm{Fe}^{3+} \): Iron has an oxidation number of +3.- \( \mathrm{H}_{2}\mathrm{O}_{2} \): Each oxygen usually has an oxidation number of -1.- \( \mathrm{H}_{2}\mathrm{O} \): Hydrogen is +1, and oxygen is -2.

Step 4: Reaction (b) Count Electrons Transferred

From \( \mathrm{Fe}^{2+} \) to \( \mathrm{Fe}^{3+} \), 1 electron is lost per iron ion. Since there are 2 \( \mathrm{Fe}^{2+} \) ions, a total of 2 electrons are transferred in this reaction.

Step 5: Reaction (c) Determine Oxidation Numbers

For the reaction \( \mathrm{CH}_{4}(g) + 2 \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) + 2 \mathrm{H}_{2}\mathrm{O}(l) \):- In \( \mathrm{CH}_{4} \): Carbon is -4, hydrogen is +1.- In \( \mathrm{O}_{2} \): Oxygen is 0 (elemental state).- In \( \mathrm{CO}_{2} \): Carbon is +4, oxygen is -2.- In \( \mathrm{H}_{2}\mathrm{O} \): Hydrogen is +1, oxygen is -2.

Step 6: Reaction (c) Count Electrons Transferred

Carbon goes from -4 in \( \mathrm{CH}_{4} \) to +4 in \( \mathrm{CO}_{2} \), losing a total of 8 electrons. Each oxygen in \( \mathrm{O}_{2} \) goes from 0 to -2 (each gaining 2 electrons), totaling 8 electrons gained by all oxygen atoms in the products.

Key Concepts

Oxidation Numbers

Oxidation numbers are essential for understanding oxidation-reduction (redox) reactions. An oxidation number is a theoretical charge that an atom would have if all bonds it forms were ionic. This concept helps chemists identify which atoms are oxidized and which are reduced in a reaction.

To assign oxidation numbers, follow these basic guidelines:

• Elements in their elemental form have an oxidation number of 0, such as Hydrogen in \( \mathrm{H}_{2} \) and Fluorine in \( \mathrm{F}_{2} \).
• For ions, the oxidation number equals the charge of the ion. For instance, the oxidation number of \( \mathrm{Fe}^{2+} \) is +2, and for \( \mathrm{Fe}^{3+} \), it is +3.
• Oxygen usually has an oxidation number of -2, except in peroxides like \( \mathrm{H}_2\mathrm{O}_2 \), where it is -1.
• Hydrogen is typically +1, except when bonded to metals in binary compounds (like metal hydrides), where it is -1.

Identifying these oxidation numbers helps further understand electron transfer and how substances in a reaction change.

Electron Transfer

The heart of an oxidation-reduction reaction is electron transfer. This transfer is what causes changes in oxidation states of the elements involved in a reaction. Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. This interplay is often remembered using the mnemonic "OIL RIG" — "Oxidation Is Loss" and "Reduction Is Gain."

• In the reaction \( \mathrm{H}_{2}(g)+\mathrm{F}_{2}(g) \longrightarrow 2 \mathrm{HF}(g) \), hydrogen is oxidized as it loses electrons transitioning from an oxidation state of 0 to +1.
• On the same note, each fluorine atom is reduced by accepting electrons, going from an oxidation state of 0 to -1.
• This specific reaction results in a total transfer of 2 electrons — one electron per hydrogen transferred to each fluorine atom.

Understanding electron transfer is imperative. It not only aids in comprehending the oxidation-reduction reaction but also helps in predicting the products formed.

Balancing Chemical Equations

Balancing chemical equations can seem daunting, but it's a crucial skill in chemistry to ensure the law of conservation of mass is satisfied. Each reaction must have the same number of each type of atom on both sides of the equation. This step is vital in ensuring that the amount of matter remains constant throughout the process.

For example, take the equation:
\(2 \mathrm{Fe}^{2+}(aq) + \mathrm{H}_{2}\mathrm{O}_{2}(aq) + 2\mathrm{H}^{+}(aq) \longrightarrow 2 \mathrm{Fe}^{3+}(aq) + \mathrm{H}_{2}\mathrm{O}(l)\)
It's balanced because:

• There are equal numbers of iron, hydrogen, and oxygen atoms on both sides.
• Furthermore, charge is balanced; the total charge on the left equals the total charge on the right.

Balancing equations allows chemists to precisely understand the stoichiometry of a reaction, predicting the amounts of reactants needed and products formed. It also assists in calculating the number of electrons transferred, crucial for redox reactions. Balancing helps ensure the reaction is depicted accurately, aligning with basic chemical principles.