Question

Write the chemical formula for each of the following compounds, and indicate the oxidation state of nitrogen in each: (a) sodium nitrite, (b) ammonia, (c) nitrous oxide, (d) sodium cyanide, (e) nitric acid, (f) nitrogen dioxide, (g) nitrogen, (h) boron nitride.

Short answer

(a) Sodium nitrite: NaNO2, nitrogen oxidation state: \(+3\)\newline (b) Ammonia: NH3, nitrogen oxidation state: \(-3\)\newline (c) Nitrous oxide: N2O, nitrogen oxidation state: \(+1\)\newline (d) Sodium cyanide: NaCN, nitrogen oxidation state: \(-3\)\newline (e) Nitric acid: HNO3, nitrogen oxidation state: \(+5\)\newline (f) Nitrogen dioxide: NO2, nitrogen oxidation state: \(+4\)\newline (g) Nitrogen: N2, nitrogen oxidation state: \(0\)\newline (h) Boron nitride: BN, nitrogen oxidation state: \(-3\)

Step-by-step solution

(a) Sodium Nitrite

Sodium (Na) has an oxidation state of +1, and the nitrite ion (NO2-) has an oxidation state of -1. Since the charges are equal and opposite, they balance each other resulting in the compound sodium nitrite (NaNO2). The oxidation state of nitrogen in sodium nitrite is +3.

(b) Ammonia

Ammonia is a neutral molecule with the chemical formula NH3. Since hydrogen has an oxidation state of +1 and there are 3 hydrogen atoms present, the oxidation state of nitrogen in ammonia equals -3.

(c) Nitrous oxide

Nitrous oxide is also a neutral molecule composed of nitrogen and oxygen atoms with a chemical formula N2O. In this case, the oxidation state of oxygen is -2, so the total oxidation state of the two nitrogen atoms must be +2, resulting in an oxidation state of +1 for each nitrogen atom.

(d) Sodium cyanide

Sodium (Na) has an oxidation state of +1, and the cyanide ion (CN-) has an oxidation state of -1. The compound sodium cyanide has a chemical formula NaCN. In the CN- ion, the oxidation state of carbon is +2, making the oxidation state of nitrogen -3.

(e) Nitric acid

Nitric acid is a compound composed of nitrogen, hydrogen, and oxygen with the chemical formula HNO3. In this case, hydrogen has an oxidation state of +1, and oxygen has an oxidation state of -2. Since there are 3 oxygen atoms, this results in a total oxidation state of -6 for the oxygen atoms. The nitrogen atom must have an oxidation state of +5 to balance the oxidation states in the compound.

(f) Nitrogen dioxide

Nitrogen dioxide is a neutral molecule with the chemical formula NO2. Oxygen has an oxidation state of -2, and since there are 2 oxygen atoms, their total oxidation state is -4. Therefore, the oxidation state of nitrogen in nitrogen dioxide must be +4.

(g) Nitrogen

Elemental nitrogen exists as a diatomic molecule with the chemical formula N2. In its elemental form, the oxidation state of nitrogen is 0 since there are no other elements present.

(h) Boron nitride

Boron nitride is a compound composed of boron (B) and nitrogen (N) atoms with the chemical formula BN. Boron has an oxidation state of +3, so the oxidation state of nitrogen in boron nitride must be -3 to balance the charges in the compound.

Key concepts

Chemical Formula

The chemical formula of a compound is a symbolic representation of its constituent elements and the number of atoms of each element in one molecule of the compound. For instance, the formula for water, H₂O, indicates that each molecule consists of 2 hydrogen (H) atoms and 1 oxygen (O) atom.

Understanding the chemical formula is crucial when determining the oxidation state of an element within a compound. For example, in ammonia (NH₃), the chemical formula immediately tells us that one nitrogen atom is bonded to three hydrogen atoms. This guides us in calculating the oxidation states, considering that hydrogen typically has an oxidation state of +1.

Compound Nomenclature

Compound nomenclature refers to the system of naming chemical compounds based on their composition and structure. Each part of a compound's name provides information about its elements and sometimes even hints at its properties or structure. For instance, the prefix 'nitro-' is associated with nitrogen, and 'sodium' refers to Na, which is the element sodium.

It's also beneficial to recognize polyatomic ions such as nitrite (NO₂⁻), nitrate (NO₃⁻), and cyanide (CN⁻), which appear in names like sodium nitrite (NaNO₂) or sodium cyanide (NaCN). The names reveal the combination of elements and their ratios, serving as a clue for their chemical formulas and influencing how we assess oxidation states.

Balancing Oxidation Numbers

Balancing oxidation numbers is a method used to ensure that the total charge in the compound or molecule is balanced. The rule of thumb is that the sum of oxidation states in a neutral compound must be zero, while in a polyatomic ion, it must equal the ion's charge.

For example, in nitric acid (HNO₃), hydrogen has an oxidation state of +1, and each of the three oxygens has an oxidation state of -2, totaling -6. To balance the compound, nitrogen must have an oxidation state of +5; thus, the sum of oxidation states equals 0, which reflects a neutral compound. Always remembering this principle will guide students in correctly assigning and verifying oxidation states, a fundamental skill in chemistry.