Question

In each of the following reactions, identify which element is oxidized and which is reduced by assigning oxidation numbers. a. $\mathrm{Zn}(s)+2{\mathrm{HNO}}_3(aq)\to \mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2(aq)+{\mathrm{H}}_2(g)$ b. ${\mathrm{H}}_2(g)+{\mathrm{CuSO}}_4(aq)\to \mathrm{Cu}(s)+{\mathrm{H}}_2{\mathrm{SO}}_4(aq)$ c. ${\mathrm{N}}_2(g)+3{\mathrm{Br}}_2(l)\to 2{\mathrm{NBr}}_3(g)$ d. $2\mathrm{KBr}(aq)+{\mathrm{Cl}}_2(g)\to 2\mathrm{KCl}(aq)+{\mathrm{Br}}_2(l)$

Short answer

a. Zn is oxidized (0 to 2), N is reduced (5 to 0). b. H is oxidized (0 to 1), Cu is reduced (2 to 0). c. N is oxidized (0 to 5), Br is reduced (0 to -1). d. Br is oxidized (-1 to 0), Cl is reduced (0 to -1).

Step-by-step solution

Assign Oxidation Numbers

For each reaction, assign oxidation numbers for each element present in reactants and products. Recall that the oxidation number of an element in its elemental form, like O2 or Zn(s), is always 0. a. $\mathrm{Zn}(0)+2 \mathrm{H}^{+}(1)\mathrm{NO}_{3}^{-}\left(\mathrm{N}(5\text{, O}(-2))\right) \rightarrow \mathrm{Zn}^{2+}(2)\mathrm{[(NO}_{3})^{-}\left(\mathrm{N}(5\text{, O}(-2))\right)]_{2}+\mathrm{H}_{2}(0)$ b. $\mathrm{H}_{2}(0)+\mathrm{Cu}^{2+}(2)\mathrm{SO}_{4}^{2-}\left(\mathrm{S}(6\text{, O}(-2))\right) \rightarrow \mathrm{Cu}(0)+\mathrm{H}^{+}(1)\mathrm{SO}_{4}^{-}\left(\mathrm{S}(6\text{, O}(-2))\right)$ c. ${\mathrm{N}}_2(0)+3{\mathrm{Br}}_2(0)\to 2\mathrm{N}(5){\mathrm{Br}}_3(-1)$ d. $2 \mathrm{K}^{+}(1)\mathrm{Br}_{2}^{-}(-1)+\mathrm{Cl}_{2}(0) \rightarrow 2 \mathrm{K}^{+}(1) \mathrm{Cl}_{2}^{-}(1)+ \mathrm{Br}_{2}(0)$

Identify Oxidation and Reduction Elements

Compare the oxidation numbers of elements present in reactants and products to determine whether they are oxidized or reduced. If an element's oxidation number increases, it is undergoing oxidation; if it decreases, the element is undergoing reduction. a. Zn is oxidized (goes from 0 to 2) and N is reduced (goes from 5 to 0). b. H is oxidized (goes from 0 to 1) and Cu is reduced (goes from 2 to 0). c. N is oxidized (goes from 0 to 5) and Br is reduced (goes from 0 to -1). d. Br is oxidized (goes from -1 to 0) and Cl is reduced (goes from 0 to -1).

Key concepts

Oxidation Numbers

Understanding oxidation numbers is crucial for analyzing redox reactions. In essence, an oxidation number is a bookkeeping tool that allows chemists to keep track of electron transfers in chemical reactions. It represents the charge that an atom would have if all bonds to atoms of different elements were completely ionic. Here are some basic rules that can help determine oxidation numbers:

• Pure elements have an oxidation number of 0.
• For monoatomic ions, the oxidation number is equal to the charge of the ion.
• Oxygen usually has an oxidation number of -2, except in peroxides or when bonded to fluorine.
• Hydrogen's oxidation number is +1 when bonded to nonmetals and -1 when bonded to metals.
• The sum of oxidation numbers in a neutral compound is 0; in a polyatomic ion, it matches the ion's charge.

By comparing the oxidation numbers in the reactants and products, students can identify which elements undergo oxidation or reduction in a chemical reaction.

Element Oxidation

Element oxidation is a process that involves the loss of electrons. When an element's oxidation number increases during a reaction, it is said to be oxidized. This can be thought of as the atom becoming more 'positive' as it loses electrons, which are negatively charged.

Real-World Examples

For example, in reaction (a), zinc (Zn) is oxidized as it goes from an oxidation number of 0 to +2. This means zinc has lost two electrons. In real-world terms, think of oxidation as the process that causes metals to rust or fruit to turn brown; it's all about the loss of electrons and the subsequent increase in oxidation state.

Element Reduction

Conversely, element reduction is the gain of electrons, resulting in a decrease in the oxidation number. This is often described as the element becoming more 'negative.'

Reduction in Everyday Life

For instance, in reaction (b), copper (Cu) undergoes reduction by changing from an oxidation number of +2 to 0, indicating it gains electrons. In the context of everyday experiences, reduction is part of what happens in a battery, where reduction reactions at the cathode allow for the generation of electrical energy through the flow of electrons.