Question

Assign oxidation numbers to the atoms in each substance. a) \(\mathrm{PCl}_{5}\) b) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{Se}\) c) Ag d) \(\mathrm{Li}_{2} \mathrm{O}_{2}\)

Short answer

a) P = +5, Cl = -1; b) N = -3, H = +1, Se = -2; c) Ag = 0; d) Li = +1, O = -1

Step-by-step solution

Understand the Rules for Oxidation Numbers

To assign oxidation numbers, remember: - The oxidation number of an element in its natural form is zero. - The oxidation number of a monoatomic ion is equal to its charge. - Oxygen usually has an oxidation number of -2, and hydrogen is typically +1. - The sum of oxidation numbers in a neutral compound is zero.

Assign Oxidation Numbers in PCl5

Phosphorus chloride (PCl_5) is a neutral compound. Chlorine generally has an oxidation number of -1. Let the oxidation number of phosphorus be x. Thus, the equation for oxidation numbers is: \[x + 5(-1) = 0\]Solving gives x = +5. Hence, the oxidation numbers are P = +5 and Cl = -1 each.

Assign Oxidation Numbers in (NH4)2Se

In ammonium (NH_4^+), the oxidation number of hydrogen is +1, while nitrogen is typically -3 to counterbalance:\[ N + 4(+1) = +1 \]So, N = -3. In selenide (Se^{2-}), selenium's oxidation number is -2. Calculating for the entire compound, (NH_4)_2 Se, the hydrogen and nitrogen add to zero:\[-3 + 8(+1) = 0\]while selenium remains -2.

Assign Oxidation Numbers in Ag

Pure elements like silver ( Ag ) have an oxidation number of zero. Therefore, the oxidation number of silver here is 0 .

Assign Oxidation Numbers in Li2O2

Lithium peroxide (Li_2O_2) has lithium generally at +1. Oxygen in peroxides has an oxidation number of -1. Therefore:\[ 2(+1) + 2(-1) = 0 \]Thus the oxidation numbers are Li = +1 and O = -1 each.

Key concepts

PCl5 oxidation numbers

The compound phosphorus pentachloride (\(\text{PCl}_5\)) consists of phosphorus bonded to five chlorine atoms. As a neutral compound, the oxidation numbers must sum to zero. In \(\text{PCl}_5\), we start by assigning chlorine its usual oxidation number of -1, as it is more electronegative than phosphorus.
To find the oxidation number of phosphorus, we let it be \(x\). Thus, we have the equation: \[x + 5(-1) = 0\]Solving for \(x\), we find \(x = +5\).
This means in \(\text{PCl}_5\):

• Phosphorus (P) has an oxidation number of +5.
• Each Chlorine (Cl) has an oxidation number of -1.

(NH4)2Se oxidation numbers

In the compound \((\text{NH}_4)_2\text{Se}\), we deal with two parts: the ammonium ion (\(\text{NH}_4^+\)) and the selenide ion (\(\text{Se}^{2-}\)).
For ammonium, hydrogen usually has an oxidation number of +1. We assume nitrogen generally has -3 to balance the hydrogens: \[\text{N} - 3 = (1)(+4) = +4 \rightarrow \text{N} = -3\]For selenide, selenium has an oxidation number of -2, consistent with its role as an anion in such compounds. Combining two ammonium ions (\(\text{NH}_4^+\)) with selenide leads to a neutral compound. The nitrogen and hydrogen balance to zero (\(-3 + 8(+1) = 0\)).
Hence in \((\text{NH}_4)_2\text{Se}\):

• Nitrogen (N) is -3.
• Hydrogen (H) is +1.
• Selenium (Se) is -2.

Ag oxidation numbers

Silver (Ag) is often utilized in its elemental form, particularly in metals like coins and jewelry. In its natural, pure form, any element has an oxidation number of zero. This is because it is not combined with any other element.
Thus, when asked to determine the oxidation number of elemental silver (Ag), remember:

• Silver (Ag) in its pure form has an oxidation number of 0.

This holds true unless silver is in a compound where it could potentially have different oxidation numbers.

Li2O2 peroxide oxidation

Lithium peroxide (\(\text{Li}_2\text{O}_2\)) highlights a unique case because it includes a peroxide ion.
In this compound, lithium is typically assigned an oxidation number of +1, as it belongs to the alkali metals group. In peroxides, oxygen has an unusual oxidation number of -1. This differs from its usual -2 in most oxides. To check the compound's neutrality:\[2(+1) + 2(-1) = 0\]Thus in \(\text{Li}_2\text{O}_2\):

• Lithium (Li) has an oxidation number of +1.
• Oxygen (O) has an oxidation number of -1.

Being aware of peroxides' peculiar oxidation states is crucial in such calculations.