Question

Assign oxidation states to the elements involved in the following reactions. Indicate which are redox reactions and which are not. (a) \(\mathrm{MgCO}_{3}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq}) \longrightarrow\) \(\mathrm{Mg}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{CO}_{2}(\mathrm{g})\) (b) \(\mathrm{Cl}_{2}(\mathrm{aq})+2 \mathrm{Br}^{-}(\mathrm{aq}) \longrightarrow 2 \mathrm{Cl}^{-}(\mathrm{aq})+\mathrm{Br}_{2}(\mathrm{aq})\) (c) \(\mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{NO}_{3}^{-}(\mathrm{aq}) \longrightarrow\) \(\mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(1)+\mathrm{NO}_{2}(\mathrm{g})\) (d) \(2 \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{CrO}_{4}^{2-}(\mathrm{aq}) \longrightarrow \mathrm{Ag}_{2} \mathrm{CrO}_{4}(\mathrm{s})\)

Short answer

Reactions (b) and (c) are redox reactions. Reactions (a) and (d) are not redox reactions.

Step-by-step solution

Assign Oxidation States

Assigning oxidation states to each species in the reactions before and after the reaction. Remember that for a neutral compound, the sum of the oxidation states is zero, and for an ion, the sum of the oxidation states is equal to the charge of the ion. For reaction (a), \( Mg \) has an oxidation state of +2 both before and after the reaction. This is due to its position in the periodic table. \( C \) goes from +4 in \( MgCO_{3} \) to +4 in \( CO_{2} \). \( O \) is in -2 both in \( MgCO_{3} \) and \( CO_{2} \). The \( H \) atoms go from +1 in \( H^{+} \) to +1 in \( H_{2}O \). For reaction (b), \( Cl \) goes from 0 in \( Cl_{2} \) to -1 in \( Cl^{-} \). \( Br \) goes from -1 in \( Br^{-} \) to 0 in \( Br_{2} \). Reaction (c), \( Ag \) goes from 0 in \( Ag \) to +1 in \( Ag^{+} \). \( N \) goes from +5 in \( NO_{3}^{-} \) to +4 in \( NO_{2} \). \( O \) stays at -2 and \( H \) at +1. For reaction (d), \( Ag \) stays at +1, \( Cr \) stays at +6, and \( O \) stays at -2.

Identify Redox Reactions

After assigning oxidation states, look for changes. For reaction (a), no atoms change oxidation state, so it is not a redox reaction. For reaction (b), \( Cl \) goes from 0 to -1 (reduction) and \( Br \) goes from -1 to 0 (oxidation) thus it is a redox reaction. For reaction (c), \( Ag \) goes from 0 to +1 (oxidation) and \( N \) goes from +5 to +4 (reduction), so it also is a redox reaction. For reaction (d), no atoms change oxidation state, so it is not a redox reaction.

Key concepts

Redox Reactions

Understanding redox reactions is crucial when analyzing chemical reactions, as they involve the transfer of electrons between substances. In these reactions, one substance undergoes oxidation, and another undergoes reduction. Oxidation refers to the loss of electrons by a molecule, atom, or ion, while reduction is the gain of electrons. They always occur simultaneously in redox reactions.

• Oxidation and Reduction Together: To identify a redox reaction, watch for changes in oxidation states of the involved elements. For example, as seen in reaction (b) from our exercise, chlorine (\(Cl_2\)) is reduced as its oxidation state changes from 0 to -1, and bromine (\(Br^-\)) is oxidized as its oxidation state goes from -1 to 0.

Not all reactions are redox reactions. Some lack changes in oxidation states, indicating they involve no electron transfer. In our examples, reactions (a) and (d) fall under this category. A deep understanding of redox reactions helps in predicting the direction of chemical reactions and is vital in fields like chemistry, biology, and environmental science.

Oxidation and Reduction

Oxidation and reduction are essential concepts in understanding chemical processes. Recognizing these processes is necessary for grasping how substances interact at a molecular level.

• Oxidation: It involves the loss of electrons. In reaction (c), the silver (\(Ag\)) undergoes oxidation, increasing its oxidation state from 0 to +1.
• Reduction: It entails the gain of electrons. In the same reaction, nitrogen in nitrate (\(NO_3^-\)) is reduced as it goes from +5 in \(NO_3^-\) to +4 in \(NO_2\).

The mnemonic "OIL RIG" may help: "Oxidation Is Loss, Reduction Is Gain." This pattern occurs across many chemical disciplines and is key for reactions like metabolism, corrosion, and combustion.By examining the changes in oxidation states, we can determine which elements in a reaction are oxidized and which are reduced. This understanding is crucial for scientists and engineers in developing efficient and sustainable chemical processes.

Reaction Analysis

Analyzing chemical reactions enables us to comprehend the underlying mechanisms and the nature of elements involved. Reaction analysis involves assigning oxidation states, determining whether a reaction is redox, and predicting the products formed.

• Assigning Oxidation States: This is the first step in any reaction analysis. For instance, in our exercise, each element's oxidation state was assigned to identify changes before and after the reaction occurs.
• Identifying Redox Reactions: After assigning oxidation states, if we notice any changes, we can identify it as a redox reaction. This form of analysis was used to distinguish reactions (b) and (c) as redox reactions, where electron transfer was evident.

Such analysis helps chemists to predict reaction products and understand reaction energetics and kinetics. By practicing these steps, a deeper insight into chemical behavior is gained, which is integral to advances in chemical research and technology.