Problem 35 - Chemistry: Principles and Reactions

Question

Which of the following species will react with \(1 \mathrm{M} \mathrm{HNO}_{3}\) ? (a) \(\mathrm{I}^{-}\) (b) Fe (c) \(\mathrm{Ag}\) (d) \(\mathrm{Pb}\)

Step-by-Step Solution

Answer

(a) I⁻, (b) Fe, (c) Ag, (d) Pb. Answer: (b) Fe and (d) Pb.

Step 1: Write the half-reactions for the given species

The half-reactions for the given species are: (a) \(\mathrm{I}^{-}(aq) \rightarrow \mathrm{I}_{2}(s) + 2 e^{-}\) (b) \(\mathrm{Fe}(s) \rightarrow \mathrm{Fe}^{2+}(aq) + 2 e^{-}\) (c) \(\mathrm{Ag}(s) \rightarrow \mathrm{Ag}^{+}(aq) + e^{-}\) (d) \(\mathrm{Pb}(s) \rightarrow \mathrm{Pb}^{2+}(aq) + 2 e^{-}\)

Step 2: Determine the standard reduction potentials for each species

Using the standard reduction potential table, we can find the values for the given species as follows: (a) \(\mathrm{I}^{-}(aq) \rightarrow \mathrm{I}_{2}(s) + 2 e^{-}\), E° = +0.54 V (b) \(\mathrm{Fe}(s) \rightarrow \mathrm{Fe}^{2+}(aq) + 2 e^{-}\), E° = -0.44 V (c) \(\mathrm{Ag}(s) \rightarrow \mathrm{Ag}^{+}(aq) + e^{-}\), E° = +0.80 V (d) \(\mathrm{Pb}(s) \rightarrow \mathrm{Pb}^{2+}(aq) + 2 e^{-}\), E° = -0.13 V For \(\mathrm{HNO}_3\), the standard reduction potential is: \(\mathrm{NO_3^{-}}(aq) + 4 \mathrm{H}^{+}(aq) + 3 e^{-} \rightarrow \mathrm{NO}(g) + 2 \mathrm{H_2}O(l)\), E° = +0.96 V

Step 3: Compare the standard reduction potentials with that of \(\mathrm{HNO}_3\)

To determine which species will react with \(\mathrm{HNO}_3\), we must compare the standard reduction potentials of the given species to that of \(\mathrm{HNO}_3\). A reaction is favorable if the standard reduction potential of the given species is lower than the standard reduction potential of \(\mathrm{HNO}_3\): (a) E°(+0.54 V) > E°(\(\mathrm{HNO}_3\)) (b) E°(-0.44 V) < E°(\(\mathrm{HNO}_3\)) (c) E°(+0.80 V) > E°(\(\mathrm{HNO}_3\)) (d) E°(-0.13 V) < E°(\(\mathrm{HNO}_3\))

Step 4: Identify which species will react with \(\mathrm{HNO}_3\)

According to the comparison in Step 3, it can be seen that species (b) Fe and (d) \(\mathrm{Pb}\) have lower standard reduction potentials than \(\mathrm{HNO}_3\). Therefore, these species will react with \(1 \mathrm{M} \mathrm{HNO}_{3}\): (b) Fe (d) \(\mathrm{Pb}\)

Key Concepts

Electrochemistry

Electrochemistry is a branch of chemistry that studies the movement of electrons and the conversion of energy between electrical form and chemical form. This field is essential for understanding how batteries work, the principles behind corrosion, and the mechanisms of certain types of sensors. At the heart of electrochemistry is the concept of redox or reduction-oxidation reactions, where one species gains electrons (reduction) and another species loses electrons (oxidation).

One fundamental aspect of electrochemistry is the standard reduction potential, which is a measure of the tendency of a chemical species to acquire electrons and be reduced. Each half-reaction in an electrochemical cell has a standard reduction potential, which can be found in tables and is measured in volts. The standard reduction potential reflects a species' affinity for electrons; a positive value indicates a greater willingness to be reduced. In the exercise, comparing standard reduction potentials provided insights into which substances would react with 1M HNO3 based on their respective tendencies to gain or lose electrons.

Redox Reactions

Understanding Redox Reactions

Redox reactions are a type of chemical reaction involving the transfer of electrons between two substances. These reactions are composed of two half-reactions: one for oxidation and one for reduction. The substance losing electrons is being oxidized and is known as the reducing agent, while the substance gaining electrons is being reduced and is the oxidizing agent.

Applying Redox Reactions to the Exercise

In the given exercise, several metals and iodide ions are assessed to see if they will react with 1M HNO3 via redox reactions. To determine the likelihood of a redox reaction occurring, one must compare the standard reduction potentials. A useful rule of thumb is that the substance with the lower (more negative) standard reduction potential acts as the reducing agent and is oxidized, which is consistent with the findings in the exercise solution, where Fe and Pb, with negative standard reduction potentials, are predicted to react with HNO3.

Chemical Reactivity

Factors Affecting Chemical Reactivity

Chemical reactivity refers to the speed at which a chemical substance undergoes a reaction. Reactivity is influenced by several factors, including the nature of the reactants, temperature, concentration, and, as in this case, the standard reduction potentials. A species with a higher standard reduction potential is typically less reactive in terms of losing electrons or being oxidized. Conversely, elements with lower reduction potentials tend to be more reactive, as they are more inclined to lose electrons.

Relation of Reactivity to the Exercise

The solution to the exercise takes into account the chemical reactivity of each species in relation to nitric acid (HNO3), which has a high standard reduction potential. Substances with lower standard reduction potentials (Fe and Pb) are predicted to react with HNO3 because they are more likely to be oxidized (lose electrons). This aligns with the overall concept that metals with lower reduction potentials are generally more reactive in redox reactions, especially with an oxidizing agent like HNO3.