Question

Determine whether each reaction is a redox reaction. For those reactions that are redox reactions, identify the substance being oxidized and the substance being reduced. (a) \(\mathrm{Zn}(s)+\operatorname{CoCl}_{2}(a q) \rightarrow \mathrm{ZnCl}_{2}(a q)+\operatorname{Co}(s)\) (b) \(\mathrm{HI}(a q)+\mathrm{NaOH}(a q) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{NaI}(a q)\) (c) \(\mathrm{AgNO}_{3}(a q)+\mathrm{NaCl}(a q) \rightarrow \mathrm{AgCl}(s)+\mathrm{NaNO}_{3}(a q)\) (d) \(2 \mathrm{~K}(s)+\mathrm{Br}_{2}(l) \longrightarrow 2 \mathrm{KBr}(s)\)

Short answer

(a) Redox reaction, Zn is oxidized, Co is reduced. (b) Not a redox reaction. (c) Not a redox reaction. (d) Redox reaction, K is oxidized, Br2 is reduced.

Step-by-step solution

Determine the oxidation states

To determine if a reaction is redox, start by assigning oxidation states to each element in the reactants and products. The sum of the oxidation states for a neutral compound should be 0, while for a polyatomic ion should be equal to the charge of the ion.

Analyze changes in oxidation states for reaction (a)

For the reaction \(\mathrm{Zn}(s)+\mathrm{CoCl}_{2}(aq) \rightarrow \mathrm{ZnCl}_{2}(aq)+\mathrm{Co}(s)\), assign oxidation states: Zn is 0 in its elemental form, Co is +2 in \(\mathrm{CoCl}_{2}\), Cl is -1 in \(\mathrm{CoCl}_{2}\) and \(\mathrm{ZnCl}_{2}\), Zn is +2 in \(\mathrm{ZnCl}_{2}\), and Co is 0 in its elemental form. Zn goes from 0 to +2 (oxidation), and Co goes from +2 to 0 (reduction).

Analyze changes in oxidation states for reaction (b)

For the reaction \(\mathrm{HI}(aq)+\mathrm{NaOH}(aq) \longrightarrow \mathrm{H}_{2}\mathrm{O}(l)+\mathrm{NaI}(aq)\), assign oxidation states: H is +1 in \(\mathrm{HI}\) and \(\mathrm{NaOH}\), I is -1 in \(\mathrm{HI}\) and \(\mathrm{NaI}\), Na is +1 in \(\mathrm{NaOH}\) and \(\mathrm{NaI}\), O is -2 in \(\mathrm{NaOH}\) and \(\mathrm{H}_{2}\mathrm{O}\). None of the elements undergo a change in oxidation state, so this is not a redox reaction.

Analyze changes in oxidation states for reaction (c)

For the reaction \(\mathrm{AgNO}_{3}(aq)+\mathrm{NaCl}(aq) \rightarrow \mathrm{AgCl}(s)+\mathrm{NaNO}_{3}(aq)\), assign oxidation states: Ag is +1 in \(\mathrm{AgNO}_{3}\) and AgCl, Na is +1 in \(\mathrm{NaCl}\) and \(\mathrm{NaNO}_{3}\), Cl is -1 in \(\mathrm{NaCl}\) and AgCl, O is -2 and N is +5 in \(\mathrm{AgNO}_{3}\) and \(\mathrm{NaNO}_{3}\). No change in oxidation state; therefore, not a redox reaction.

Analyze changes in oxidation states for reaction (d)

For the reaction \(2\mathrm{K}(s)+\mathrm{Br}_{2}(l) \longrightarrow 2\mathrm{KBr}(s)\), assign oxidation states: K is 0 in its elemental form and +1 in \(\mathrm{KBr}\), Br is 0 in its elemental form and -1 in \(\mathrm{KBr}\). K goes from 0 to +1 (oxidation), and Br goes from 0 to -1 (reduction).

Key concepts

Oxidation State Determination

Understanding the oxidation state of elements is a fundamental aspect of analyzing redox reactions. Oxidation state refers to the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic. To determine whether a chemical reaction is redox or not, we first assign oxidation states to each atom in the reactants and products.

Rules such as oxygen usually having an oxidation state of -2 (except in peroxides or when bonded to fluorine), hydrogen usually being +1 (except when bonded to metals where it can be -1), as well as established oxidation states for metals in their ionic form, help in assigning oxidation states. Additionally, for elements in their pure form, the oxidation state is always zero.

Once the oxidation states have been established, observing any change in these states between reactants and products will indicate if oxidation or reduction has occurred, which are hallmark events of a redox reaction. For instance, Zinc (Zn) in its elemental form has an oxidation state of 0, but in zinc chloride (ZnCl₂), it has an oxidation state of +2, signaling it has undergone oxidation.

Substance Oxidation

Substance oxidation is a process where an element loses electrons, resulting in an increase in its oxidation state. It is important to note that oxidation can also involve the addition of oxygen or the loss of hydrogen.

For example, in the reaction where solid zinc reacts with cobalt(II) chloride, the oxidation state of zinc increases from 0 to +2. This can be interpreted as each zinc atom losing two electrons. Thus, zinc is the substance being oxidized in this reaction.Oxidation is often paired with reduction in redox reactions, making it crucial to identify the substance being oxidized to comprehend the bigger picture of chemical changes taking place.

Substance Reduction

Conversely, substance reduction is characterized by an element gaining electrons, which leads to a decrease in its oxidation state. Reduction can also be defined as the addition of hydrogen or the removal of oxygen from a molecule.

In the aforementioned zinc and cobalt chloride reaction, it is not just zinc that undergoes a chemical change. Cobalt (Co) is reduced as its oxidation state decreases from +2 in cobalt(II) chloride to 0 in metallic cobalt. This implies that cobalt ions gain electrons during the reaction. Identifying the substance that is reduced during a chemical reaction allows students to track the flow of electrons and better understand the driving forces behind the reaction processes.

Chemical Reaction Analysis

Chemical reaction analysis goes beyond simply recognizing reactants and products; it involves a deeper investigation into the actual processes taking place during a reaction. When analyzing a reaction to determine if it is a redox reaction, one must consider both oxidation and reduction events.

Oxidation cannot occur without reduction, as electrons must be conserved. They are simply transferred from one species to another. Redox reactions are therefore a subset of chemical reactions wherein changes in oxidation states indicate electron transfer between reactant species. Analyzing reactions like the neutralization between hydroiodic acid and sodium hydroxide or the precipitation of silver chloride from silver nitrate and sodium chloride reveals that no oxidation or reduction takes place because there are no changes in oxidation states among the atoms involved.Chemical reaction analysis thus requires a comprehensive look into electron transfer, bond changes, as well as the conservation of mass and charge to fully appreciate the complexities of the reactions under scrutiny.