What is Nitric Acid?

by Jessica Clifton

The Chemistry and Properties of Nitric Acid

To understand what nitric acid is and how it is commercially used, it’s important to first understand the properties of an acid and what happens when it’s involved in a chemical reaction. We’ll then look at the chemical composition of nitric acid and its common by-products.

All about acid

What makes an acid an acid? Well, an acid is a chemical substance that readily accepts electrons or donates protons (typically hydrogen ions) in a chemical reaction. An acid has a pH below 7 (as opposed to a base substance, which has pH of above 7.)

The pH scale is a logarithmic scale from 0 to 14. The scale indicates whether a solution of a substance either tends to donate hydrogen (proton) ions or accepts hydrogen ions.

In order to calculate the pH of an aqueous (water-based) solution, you need to know the concentration of the hydronium ion in terms of mole per litre. The computation is based on this formula:

pH = – log [H3O+]

What are the factors for the reactivity of acids?

  • Acid pH level – Each step in the scale from neutral pH 7, either lower or higher, is ten times the previous one. For example, a solution of pH6 is ten times more acidic than a solution of pH7. Conversely, a solution with a pH of 8 is ten times more basic (alkaline) than a solution with a neutral pH of 7. Therefore, acids with lower pH level are more reactive or corrosive.
  • Chemical composition – Aside from the pH level, the reactivity of an acid also depends on its chemical composition. Likewise, the composition of the reactant also matters. For example, nitric acid reacts with most metals but it does not react with precious metals like pure gold.
  • Electronegativity – Acid reactivity also has something to do with the electronegativity of a reactant substance, or how strongly atoms attract bonding electrons. Most metals have low electronegativity, which means that they easily give up electrons. 
  • Polarity – Most mineral and inorganic acids do not react and won’t dissolve in oil because oil is a nonpolar substance. This is the reason why some very corrosive acids like hydrofluoric acid are stored in plastic containers. Plastics are oil-based.

What is the chemical formula for nitric acid?

Nitric acid is a relatively simple compound containing only three elements: hydrogen, nitrogen, and oxygen. Chemically, its formula is written as HNO3.

The chemical formula of nitric acid

You can see that nitrogen is at the centre and one oxygen is double-bonded with the nitrogen, while the other two oxygen atoms are single-bonded. They are in what is called a trigonal planar form (a molecular shape in which there are three bonds and no lone pairs around the central atom) while the hydrogen is attached to one of the oxygen atoms.

Each of the three oxygen atoms carries a negative 2/3 charge while the nitrogen atom carries a positive 1 charge. This arrangement is crucial in the reactivity of the compound.

When it disintegrates, nitric acid produces water and nitrogen dioxide. For instance, when nitric acid reacts with copper, the balanced chemical reaction can be written as:

4 HNO3(l) + Cu(s) ==> Cu(NO3)2(s and aq) + 2 NO2(g) + 2 H2O(l)

Nitrogen dioxide is liberated as a noxious brown gas while cupric nitrate is formed as a blue precipitate. During the reaction, the solution will first turn green, then greenish-brown before finally turning blue as it is diluted with water. Just like in other acid-metal reactions, the process is exothermic, releasing excess heat energy.

Is nitric acid a strong acid?

When dissolved in water, strong acids dissociate their ions completely. Nitric acid is a strong acid and it is one of the seven strong acids. All other acids are weak ones. The other strong acids are the following:

  • HCl – hydrochloric acid
  • H2SO4 – sulfuric acid
  • HBr – hydrobromic acid
  • HI – hydroiodic acid
  • HClO4 – perchloric acid
  • HClO3 – chloric acid

Nitric acid is present in small amounts of rain. It is commercially available at 68% concentration. It is most effective in this concentration. It’s important not to confuse acid strength with acid concentration.

Nitric acid is found naturally in rain water
Nitric acid is found naturally in rain water

A concentrated acid is more corrosive compared to diluted acid. But the measure of strength is ionic dissociation in water. Strong acids like nitric acid are more reactive this way.

What has nitric acid in it?

Nitric acid naturally forms in the atmosphere when nitrogen dioxide is dissolved in rain droplets. Lightning facilitates the chemical reaction by splitting the strong molecular bonds of elemental nitrogen gas in the atmosphere, allowing it to combine with oxygen.

The nitric acid in rain forms nitrates in the soil, which are essential for plant growth. The nitrates serve as the basis for amino acids, which in turn become the building blocks of proteins. Nitrogen fixing-bacteria, on the other hand, follow a different pathway in converting the nitrogen in the air into usable forms such as ammonia.

Nitric acid is used in many types of industrial applications that require the production of other nitrogen compounds. Nitric acid is used in the following:

  • Manufacture of fertilisers
  • Production of explosives
  • Nylon precursors
  • Speciality organic compounds
  • Rocket propellant
Nitric acid is used in fertilisers
Nitric acid is used in fertilisers

Nitric acid also has niche uses such as artificially ageing wood. A 10% solution of the acid is used to change the colours of maple and pine into grey-gold.

It is also very useful as a cleaning agent when combined with water and phosphoric acid. The blend is commonly used as a cleaning agent for food and dairy equipment.

Nitric acid is also combined with alcohol such as methanol or ethanol to form a solution known as nital. The solution is used for etching metal.

Is nitric acid an oxidizer?

As a strong acid, nitric acid completely dissociates its ions in water solution into hydronium (H3O+) and nitrate (NO3−) ions. This means that it is a powerful oxidizing agent because it readily accepts electrons during oxidation-reduction reactions.

The positively-charged ion component of the acid removes the electrons from the atoms of a reactant. For instance, when nitric acid reacts with carbon, it oxidises it, forming carbon dioxide. The balanced chemical equation for this reaction is the following:

4HNO3 + C —> 2H2O+ CO2 + 4NO2

Nitric acid reacts well with other non-metallic elements such as sulphur, phosphorous, and iodine. In these reactions, the acid acts as the oxidising agent.

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