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) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(a q)+2 \mathrm{LiCl}(a q) \longrightarrow\) (a) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(a q)+2 \mathrm{LiCl}(a q)-2 \mathrm{LiNO}_{3}(a q)\) (b) \(2 \mathrm{HBr}(a q)+\mathrm{Ca}(\mathrm{OH})_{2}(a q) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{CaBr}_{2}(a q)\) (c) \(2 \mathrm{Al}(s)+\mathrm{Fe}_{2} \mathrm{O}_{3}(s) \longrightarrow \mathrm{Al}_{2} \mathrm{O}_{3}(s)+2 \mathrm{Fe}(l)\) (d) \(\mathrm{Na}_{2} \mathrm{O}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{NaOH}(a q)\)

Short answer

(a) Cannot be determined, (b) Not a redox reaction, (c) Aluminum is oxidized and Iron is reduced, (d) Not a redox reaction.

Step-by-step solution

Checking for Redox in Reaction (a)

Analyze the oxidation states of the elements in reactants and products. It appears that there is a typo or an error; the product for reaction (a) is not provided, thus precluding the ability to determine whether it is a redox reaction. Assuming the products involve a change in the oxidation state of either Pb or Li, then it would be a redox reaction.

Checking for Redox in Reaction (b)

Examine the oxidation states of the elements in HBr and Ca(OH)2, and compare them with the products H2O and CaBr2. The oxidation states of the hydrogen, bromine, calcium, and oxygen do not change during the reaction. Therefore, reaction (b) is not a redox reaction.

Checking for Redox in Reaction (c)

Observe the oxidation states of aluminum and iron in reactants and products. In the reactants, aluminum has an oxidation state of 0 in the element form and in the products, it has an oxidation state of +3 in Al2O3. Iron has an oxidation state of +3 in Fe2O3 and 0 in the element form in the products. Aluminum is being oxidized (increase in oxidation state) and iron is being reduced (decrease in oxidation state). Therefore, reaction (c) is a redox reaction.

Checking for Redox in Reaction (d)

Determine the oxidation states of the elements in Na2O and H2O, and compare them to the oxidation states in the product NaOH. The oxidation state of sodium goes from +1 in Na2O to +1 in NaOH, and the oxidation states of hydrogen and oxygen stay the same in reactants and products. Because there are no changes in oxidation states, reaction (d) is not a redox reaction.

Key concepts

Oxidation States

Understanding oxidation states is fundamental to identifying redox reactions. Oxidation states, also referred to as oxidation numbers, are a tool for keeping track of electron transfer in redox processes. In simple terms, they indicate the total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom.

To assign oxidation states, we follow a set of rules such as: elements in their elemental form have an oxidation state of 0, the sum of oxidation states in a neutral compound is 0, and the sum in a polyatomic ion is equal to the charge of the ion. For example, in the molecule H2O, hydrogen has an oxidation state of +1 and oxygen has an oxidation state of -2, resulting in a sum of 0, which matches the charge of a neutral molecule.

Identifying changes in oxidation states can be tricky, but it's key to spotting redox reactions. If during a chemical reaction, the oxidation state of an element increases, it's being oxidized; if it decreases, it's being reduced.

Substance Being Oxidized

In any redox reaction, the substance being oxidized is the one that loses electrons. This is seen through an increase in oxidation state. For instance, in the reaction where aluminum reacts with iron(III) oxide (reaction (c) from the exercise), the aluminum starts with an oxidation state of 0 and ends with +3. This change indicates that aluminum atoms have lost electrons, hence they are oxidized.

To further elucidate this during problem-solving, it's crucial to compare the oxidation states before and after the reaction. When solving homework problems or when in doubt, write out the oxidation states explicitly for all species. Don't forget that in a redox reaction, the total number of electrons lost is always equal to the total number of electrons gained by the substance being reduced.

Substance Being Reduced

Conversely, the substance being reduced in a redox reaction is the one gaining electrons, demonstrated by a reduction in oxidation state. Taking the same reaction mentioned earlier, iron in iron(III) oxide goes from an oxidation state of +3 in Fe2O3 to 0 in elemental iron. Since the oxidation state decreases, it means iron is gaining electrons during the reaction; therefore, it's the substance being reduced.

Remember, in a redox reaction, reduction always occurs alongside oxidation—this is a complementary process. Each electron given up by the oxidized substance is accepted by the reduced substance. This is why it's also termed an 'electron transfer' reaction. An effective tool for students to grasp the concept of substances being reduced is the mnemonic 'OIL RIG' – Oxidation Is Loss, Reduction Is Gain – which refers to the transfer of electrons.