Question

Which of the following are redox reactions? For those that are, indicate which element is oxidized and which is reduced. For those that are not, indicate whether they are precipitation or acid-base reactions. (a) \(\mathrm{Cu}(\mathrm{OH})_{2}(s)+2 \mathrm{HNO}_{3}(a q) \longrightarrow\) \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(a q)+2 \mathrm{H}_{2} \mathrm{O}(I)\) (b) \(\mathrm{Fe}_{2} \mathrm{O}_{3}(s)+3 \mathrm{CO}(g) \longrightarrow 2 \mathrm{Fe}(s)+3 \mathrm{CO}_{2}(g)\) (c) \(\mathrm{Sr}\left(\mathrm{NO}_{3}\right)_{2}(a q)+\mathrm{H}_{2} \mathrm{SO}_{4}(a q) \longrightarrow\) (d) \(\mathrm{SrSO}_{4}(\mathrm{~s})+2 \mathrm{HNO}_{3}(a q)\) \(\mathrm{Zn}(s)+10 \mathrm{H}^{+}(a q)+2 \mathrm{NO}_{3}^{-}(a q) \longrightarrow\) \(4 \mathrm{Zn}^{2+}(a q)+\mathrm{N}_{2} \mathrm{O}(g)+5 \mathrm{H}_{2} \mathrm{O}(l)\)

Short answer

(a) Acid-Base reaction. (b) Redox reaction, Fe is reduced and C is oxidized. (c) Precipitation reaction. (d) Redox reaction, Zn is oxidized and N is reduced.

Step-by-step solution

Cu(OH)2(s) + 2 HNO3(aq) → Cu(NO3)2(aq) + 2 H2O(l)

First, we need to find the change in oxidation numbers of the elements in the reaction. We can determine that copper(Cu) in Cu(OH)2 has an oxidation state of +2, hydrogen(H) in HNO3 has an oxidation state of +1, nitrogen (N) in HNO3 has an oxidation state of +5, and oxygen (O) in HNO3 has an oxidation state of -2 (since oxygen is usually -2 in compounds). In the products, H2O has oxidation states of +1 for hydrogen and -2 for oxygen. Now we can see that there is no change in the oxidation state for any element in the reaction, so this is NOT a redox reaction. Since an acidic substance (HNO3) reacts with a basic substance (Cu(OH)2) to form a salt (Cu(NO3)2) and water, this reaction is an acid-base reaction. ##Reaction (b)##

Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3 CO2(g)

Fe has an oxidation state of +3 in Fe2O3, oxygen has an oxidation state of -2, and carbon has an oxidation state of +2 in CO. In the products, Fe has an oxidation state of 0 (since it's a pure element) and carbon has an oxidation state of +4 in CO2. Here, there is a change in oxidation states. Fe goes from +3 to 0 and C goes from +2 to +4. Fe is reduced (gains electrons) and C is oxidized (loses electrons). Therefore, this is a redox reaction (Fe is reduced and C is oxidized). ##Reaction (c)##

Sr(NO3)2(aq) + H2SO4(aq) → SrSO4(s) + 2 HNO3(aq)

The oxidation state of Sr is +2 in Sr(NO3)2, N is +5, O is -2, and H is +1 in H2SO4, and S is +6 in H2SO4. In the products, SrSO4 has Sr with an oxidation state of +2, S with an oxidation state of +6, oxygen with an oxidation state of -2, and HNO3 with H at +1, N at +5, and oxygen at -2. There is no change in the oxidation state for any element in the reaction; hence, it is NOT a redox reaction. The reaction involves an aqueous solution of a soluble salt (Sr(NO3)2) and an aqueous solution of an acid (H2SO4), which form an insoluble solid (SrSO4) and an acid (HNO3). Therefore, this reaction is a precipitation reaction. ##Reaction (d)##

Zn(s) + 10 H+(aq) + 2 NO3−(aq) → Zn2+(aq) + N2O(g) + 5 H2O(l)

The oxidation state of Zn is 0 (since it's a pure element), H is +1 in H+(aq), N is +5 in NO3−, and oxygen is -2 in NO3−. In the products, Zn has an oxidation state of +2, N has an oxidation state of +1 in N2O, and oxygen has an oxidation state of -2 in N2O and H2O. There is a change in the oxidation states. Zn goes from 0 to +2 and N goes from +5 to +1. Zn is oxidized (loses electrons) and N is reduced (gains electrons). Therefore, this is a redox reaction (Zn is oxidized and N is reduced). So, in summary; (a) Acid-Base reaction. (b) Redox reaction, Fe is reduced and C is oxidized. (c) Precipitation reaction. (d) Redox reaction, Zn is oxidized and N is reduced.

Key concepts

Understanding Oxidation States

Oxidation states, also referred to as oxidation numbers, play a crucial role in understanding redox reactions. An element's oxidation state is a theoretical charge it would have if the compound was composed of ions. It helps in determining which atoms have gained or lost electrons, and thus, which ones have been oxidized or reduced in a reaction.

To assign oxidation states, there are a set of rules we can follow:

• The oxidation state of an atom in its elemental form is always zero.
• For a simple (monatomic) ion, the oxidation state is equal to the charge on the ion.
• Oxygen typically has an oxidation state of -2 in compounds, except in peroxides or when bonded to fluorine.
• Hydrogen generally has an oxidation state of +1 when bonded to nonmetals and -1 when bonded to metals.
• The sum of oxidation states in a neutral compound is zero, while in a polyatomic ion, it is equal to the charge of the ion.

Understanding these rules helps analyze whether a chemical reaction is a redox reaction or not by comparing the oxidation states of atoms in reactants and products. If there's a change, it's a redox reaction; if there isn't, the reaction could be an acid-base reaction or a precipitation reaction.

Acid-Base Reactions Demystified

Acid-base reactions are fundamental types of chemical reactions involving the transfer of protons (H+) from an acid to a base. In the context of the given exercise, reaction (a) exemplifies an acid-base reaction, where hydrochloric acid (HNO3) donates a proton to the base, copper(II) hydroxide (Cu(OH)2), resulting in the formation of water and copper nitrate.

An acid is a substance that can donate a proton, while a base is one that can accept a proton. The classic definition by Bronsted and Lowry describes acids and bases in this manner. It is also important to note that acid-base reactions result in the formation of a salt and water, commonly referred to as a neutralization reaction. Predicting the products of such a reaction requires knowledge of how the constituent ions of acids and bases will combine and whether a strong or weak acid or base is involved.

Precipitation Reactions and Solubility

Precipitation reactions occur when two soluble salts are mixed in an aqueous solution and produce an insoluble solid, known as a precipitate. These reactions are a result of the formation of a product that does not dissolve in water. In reaction (c), a precipitation reaction takes place as strontium nitrate reacts with sulfuric acid to yield strontium sulfate - an insoluble compound that precipitates from the solution, and nitric acid.

To predict whether a reaction will result in a precipitate, you must consult the solubility rules for common ions. These rules guide us in identifying the solubility of various compounds in water. For example, sulfates are generally soluble, with exceptions like barium sulfate, lead(II) sulfate, and strontium sulfate. Therefore, knowing that strontium sulfate is an exception to the general rule about sulfate solubility informs us that it will precipitate in reaction (c). Recognizing precipitation reactions is important not only for academic exercises but also for practical applications such as waste treatment and chemical synthesis.