Problem 58 - Introductory Chemistry Atoms First

Question

Which of the following are electron-transfer reactions? (a) \(2 \mathrm{CrO}_{4}^{2-}+2 \mathrm{H}^{+} \rightarrow \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Fe}+\mathrm{NO}_{3}^{-}+4 \mathrm{H}^{+} \rightarrow \mathrm{Fe}^{3+}+\mathrm{NO}+2 \mathrm{H}_{2} \mathrm{O}\) (c) \(2 \mathrm{C}_{2} \mathrm{H}_{6}+7 \mathrm{O}_{2} \rightarrow 4 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O}\) (d) \(2 \mathrm{AgBr} \rightarrow 2 \mathrm{Ag}+\mathrm{Br}_{\dot{2}}\)

Step-by-Step Solution

Answer

The electron-transfer reactions are: (b), (c), and (d).

Step 1: Reaction (a)

For reaction (a): \(2 \mathrm{CrO}_{4}^{2-} + 2 \mathrm{H}^{+} \rightarrow \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} + \mathrm{H}_{2} \mathrm{O}\) Oxidation states for reaction (a): Cr in \(\mathrm{CrO}_{4}^{2-}\): +6 H in \(\mathrm{H}^{+}\): +1 Cr in \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\): +6 H in \(\mathrm{H}_{2} \mathrm{O}\): +1 O in \(\mathrm{H}_{2} \mathrm{O}\): -2 Since there's no change in the oxidation states of all elements involved in this reaction, no electron transfer has occurred. Therefore, reaction (a) is not an electron-transfer reaction.

Step 2: Reaction (b)

For reaction (b): \(\mathrm{Fe} + \mathrm{NO}_{3}^{-} + 4 \mathrm{H}^{+} \rightarrow \mathrm{Fe}^{3+} + \mathrm{NO} + 2 \mathrm{H}_{2} \mathrm{O}\) Oxidation states for reaction (b): Fe: 0 N in \(\mathrm{NO}_{3}^{-}\): +5 H in \(\mathrm{H}^{+}\): +1 Fe in \(\mathrm{Fe}^{3+}\): +3 N in \(\mathrm{NO}\): +2 H in \(\mathrm{H}_{2} \mathrm{O}\): +1 O in \(\mathrm{H}_{2} \mathrm{O}\): -2 We can observe that the oxidation state of Fe changes from 0 to +3, and the oxidation state of N changes from +5 to +2. There is a transfer of electrons in this reaction, making it an electron-transfer (redox) reaction. Therefore, reaction (b) is an electron-transfer reaction.

Step 3: Reaction (c)

For reaction (c): \(2 \mathrm{C}_{2} \mathrm{H}_{6} + 7 \mathrm{O}_{2} \rightarrow 4 \mathrm{CO}_{2} + 6 \mathrm{H}_{2} \mathrm{O}\) Oxidation states for reaction (c): C in \(\mathrm{C}_{2} \mathrm{H}_{6}\): -3 H in \(\mathrm{C}_{2} \mathrm{H}_{6}\): +1 O in \(\mathrm{O}_{2}\): 0 C in \(\mathrm{CO}_{2}\): +4 H in \(\mathrm{H}_{2} \mathrm{O}\): +1 O in \(\mathrm{CO}_{2}\): -2 O in \(\mathrm{H}_{2} \mathrm{O}\): -2 In this reaction, the oxidation state of C changes from -3 to +4, and the oxidation state of O changes from 0 to -2. This indicates a transfer of electrons between the reactants, identifying it as an electron-transfer (redox) reaction. Therefore, reaction (c) is an electron-transfer reaction.

Step 4: Reaction (d)

For reaction (d): \(2 \mathrm{AgBr} \rightarrow 2 \mathrm{Ag} + \mathrm{Br}_{\dot{2}}\) Oxidation states for reaction (d): Ag in \(\mathrm{AgBr}\): +1 Br in \(\mathrm{AgBr}\): -1 Ag: 0 Br in \(\mathrm{Br}_{\dot{2}}\): 0 In this reaction, the oxidation state of Ag changes from +1 to 0, and the oxidation state of Br changes from -1 to 0. This indicates a transfer of electrons between the reactants, classifying this as an electron-transfer (redox) reaction. Therefore, reaction (d) is an electron-transfer reaction. In conclusion, the electron-transfer reactions are: (b), (c), and (d).

Key Concepts

Oxidation State

Understanding oxidation states is crucial for identifying electron transfer in chemical reactions. An oxidation state is a number assigned to an element in a compound, reflecting its degree of oxidation or reduction.
This number helps determine whether an electron transfer has occurred in a reaction. Each element in a compound is assigned an oxidation state based on specific rules:

• Elements in their natural state have an oxidation state of zero.
• For ions, the oxidation state is equal to the charge of the ion.
• In molecules, hydrogen typically has an oxidation state of +1, and oxygen -2.

By comparing the oxidation states of elements in reactants and products, students can identify if electrons have moved, confirming an electron-transfer or redox reaction.

Redox Reactions

Redox reactions are a fundamental type of chemical reaction involving the transfer of electrons between two species. These reactions are characterized by a change in oxidation states of the substances involved.
In a redox reaction, one substance gets oxidized, meaning it loses electrons, while the other gets reduced, gaining electrons. To recognize a redox reaction:

• Check for changes in oxidation states of elements from reactants to products.
• Identify the substance that is oxidized (loses electrons) and the one that is reduced (gains electrons).

These reactions are involved in numerous everyday processes, such as the rusting of iron, combustion, and cellular respiration, making them a significant part of both natural and industrial chemical processes.

Chemical Reactions

Chemical reactions involve the transformation of chemical substances, called reactants, into new substances, known as products. Understanding the types of reactions, like redox reactions, helps chemists to predict how substances will behave and react with one another.
This is particularly useful in fields ranging from materials science to pharmaceuticals. Chemical reactions can be categorized into various types:

• Synthesis reactions, where simpler substances combine to form more complex molecules.
• Decomposition reactions, where compounds break down into simpler substances.
• Single and double displacement reactions, where parts of molecules are exchanged.

Identifying electron-transfer reactions within these categories relies heavily on analyzing changes in oxidation states, as seen in redox reactions.

Oxidation and Reduction

Oxidation and reduction are two half-reactions that occur simultaneously in every redox reaction.
Oxidation refers to the loss of electrons by a molecule, atom, or ion, which increases its oxidation state.
Reduction, conversely, is the gain of electrons, decreasing the oxidation state. The mnemonic **OIL RIG** can help remember:

• Oxidation Is Loss of electrons.
• Reduction Is Gain of electrons.

In practical terms, understanding these processes is key for balancing redox reactions in chemistry. Identifying which species are oxidized and reduced allows for tracking electron flow, crucial for fields such as electrochemistry and metabolic pathways in biology.