Problem 52 - Chemistry The Central Science

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 neutralization reactions.\begin{equation} \begin{array}{c}{\text { (a) } \mathrm{P}_{4}(s)+10 \mathrm{HClO}(a q)+6 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow} \\\ \quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad{\quad 4 \mathrm{H}_{3} \mathrm{PO}_{4}(a q)+10 \mathrm{HCl}(a q)}\end{array} \end{equation} \begin{equation} \begin{array}{l}\quad\quad\quad\quad\quad\quad{\text { (b) } \mathrm{Br}_{2}(l)+2 \mathrm{K}(s) \longrightarrow 2 \mathrm{KBr}(s)} \\\\\quad\quad\quad\quad\quad\quad {\text { (c) } \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow 3 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{CO}_{2}(g)}\end{array} \end{equation} \begin{equation} \begin{array}{r}\quad\quad\quad\quad{\text { (d) } \mathrm{ZnCl}_{2}(a q)+2 \mathrm{NaOH}(a q) \longrightarrow \mathrm{Zn}(\mathrm{OH})_{2}(s)+} \\ {2 \mathrm{NaCl}(a q)}\end{array} \end{equation}

Step-by-Step Solution

Answer

(a) Redox reaction: P is oxidized and Cl is reduced. (b) Redox reaction: K is oxidized and Br is reduced. (c) Redox reaction: C is oxidized and O is reduced. (d) Precipitation reaction (not a redox reaction).

Step 1: Reaction (a)

First, assign oxidation numbers to each element before and after the reaction. P: 0 -> +5 (in H₃PO₄) H: +1 (+1 in HClO, H₂O, H₃PO₄, and HCl) Cl: +1 (in HClO) -> -1 (in HCl) O: -2 (in HClO, H₂O, and H₃PO₄) Element P increased its oxidation number from 0 to +5, so it is oxidized. Element Cl decreased its oxidation number from +1 to -1, so it is reduced. Therefore, this is a redox reaction.

Step 2: Reaction (b)

Assign oxidation numbers to each element before and after the reaction. Br: 0 -> -1 (in KBr) K: 0 -> +1 (in KBr) Element Br decreased its oxidation number from 0 to -1, so it is reduced. Element K increased its oxidation number from 0 to +1, so it is oxidized. This is a redox reaction.

Step 3: Reaction (c)

Assign oxidation numbers to each element before and after the reaction. C: -3 (in CH₃CH₂OH) -> +4 (in CO₂) H: +1 (in CH₃CH₂OH and H₂O) O: 0 (in O₂) -> -2 (in H₂O and CO₂) Element C increased its oxidation number from -3 to +4, so it is oxidized. Element O decreased its oxidation number from 0 to -2, so it is reduced. This is a redox reaction.

Step 4: Reaction (d)

Assign oxidation numbers to each element before and after the reaction. Zn: +2 (in ZnCl₂ and Zn(OH)₂) Cl: -1 (in ZnCl₂ and NaCl) Na: +1 (in NaOH and NaCl) O: -2 (in NaOH and Zn(OH)₂) H: +1 (in NaOH and Zn(OH)₂) No elements have their oxidation numbers changed in this reaction. Thus, it is not a redox reaction. Since Zn(OH)₂ is a solid in the products, this is a precipitation reaction.

Key Concepts

Oxidation Numbers

Understanding oxidation numbers is crucial in identifying and balancing redox reactions. Oxidation numbers, also referred to as oxidation states, represent the hypothetical charge that an atom would have if the compound was composed of ions. They are assigned to atoms according to a set of rules:

• The oxidation number of a free element is zero.
• For monoatomic ions, the oxidation number is equal to the charge of the ion.
• Hydrogen generally has an oxidation number of +1, while oxygen typically has an oxidation number of -2 (with some exceptions).
• The oxidation numbers in a neutral molecule must sum up to zero, and in an ion, to the charge of the ion.

The changes in oxidation numbers allow us to determine which elements are oxidized and which are reduced in a reaction.

Chemical Oxidation

In the context of redox reactions, chemical oxidation refers to the process where an atom or molecule loses electrons, resulting in an increase in its oxidation number. It often involves the addition of oxygen or the removal of hydrogen. When we say an element is oxidized, as seen in the provided exercise, phosphorus (P) was oxidized as its oxidation number increased from 0 to +5. This increase signifies that P lost electrons during the reaction.

Chemical Reduction

Opposite to oxidation, chemical reduction involves the gain of electrons by an atom or molecule, which is evidenced by a decrease in the oxidation number. A common sign of reduction is the addition of hydrogen or the removal of oxygen from a substance. For example, in the provided exercises, chlorine (Cl) is reduced as its oxidation number decreases from +1 to -1, indicating that Cl gained electrons.

Precipitation Reaction

Precipitation reactions are chemical reactions that produce a solid, known as a precipitate, when two soluble substances react in solution. For example, reaction (d) from the exercise is a precipitation reaction where an insoluble solid, zinc hydroxide (Zn(OH)₂), forms when zinc chloride (ZnCl₂) reacts with sodium hydroxide (NaOH). Precipitation reactions are essential in various applications, such as water treatment, where they remove unwanted ions.

Neutralization Reaction

Neutralization is a type of chemical reaction in which an acid and a base react to form a salt and water, often with the release of heat. In the context of redox reactions, neutralization reactions do not involve a change in oxidation numbers of the elements, meaning no element is oxidized or reduced. Even though the exercise provided does not include a neutralization reaction, it's beneficial to recognize them as they differ from redox reactions and are important in many industrial and biological processes.