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Basic Properties of Sulphuric Acid
As a substance often encountered in chemistry, you may wonder “what is sulphuric acid?” Well, sulphuric acid, (spelt as sulfuric acid in the US), is a concentrated reactive acid. It either oxidises, dehydrates, or sulfonates many organic materials. It is made by oxidising sulphur dioxide, is one of the most useful inorganic mineral acids, and plays a crucial role in many industrial processes. It is used as a precursor, reagent, and as a catalyst in manufacturing many types of products such as:
- Fertilisers
- Explosives
- Drugs
- Detergents
- Dyes
- Paints
- Plastics
Not only that, but sulphuric acid has many uses. It is the most common type of acid used in batteries, typically in lead-acid batteries for cars and solar power systems. It is also used in purifying metals such as zinc and copper from ores. It is virtually unimaginable for many industries to function without sulphuric acid, which is why so much of the chemical bottling that happens involves solutions that contain this acid.
The global industrial demand is very high globally, and it is with an estimated that 260 million tonnes of sulphuric acid are produced annually, enough even to fill roughly 100,000 Olympic-size swimming pools.
Key Takeaways:
Sulphuric acid (H₂SO₄)is a highly reactive, versatile acid essential to manufacturing processes across many industries, including fertilisers, metals, and batteries
It is created through a four-stage process involving sulphur extraction and chemical reactions
Sulphuric acid has a highly acidic pH of 2.75 under standard conditions, though this varies with concentration and other factors
It is crucial in industries such as metal processing, chemical manufacturing, battery production, and agriculture due to its reactive properties
Chemical Formula of Sulphuric Acid
The equation for sulphuric acid is H₂SO₄.
This compound is formed in four stages, with each represented by various chemical reactions:
- 1st stage – extraction of sulphur – 2PbS(s) + 3O₂(g) → 2PbO(s) + 2SO₂(g)
- 2nd stage – conversion of sulphur to sulphur dioxide – S(s) + O₂(g) → SO₂(g)
- 3rd stage – conversion of sulphur dioxide to sulphur trioxide – 2SO₂(g) + O₂(g) → 2SO₃(g)
- 4th stage – conversion of sulphur trioxide to sulphuric acid – SO₃(g) + H₂O(I) → H2SO₄(I)
Typically, sulphur is extracted from fossil fuels. Unrefined petroleum and natural gas contain sulphur compounds that are removed during the refining process. As shown in the first equation above, it can also be extracted from metal ores as sulphides.
Molten sulphur is then blasted by dry air at 1300 K. At this stage, the sulphur is burnt, converting it to sulphur dioxide.
In the third stage, a catalyst known as vanadium (V) oxide on silica is used to convert sulphur dioxide to sulphur trioxide at 700 K.
Finally, the sulphur trioxide is converted to sulphuric acid by allowing it to react with water at a constant temperature of 400 K.
Sulphuric Acid Structure
Structurally, a molecule of sulphuric acid, which has a molecular weight of 98.079 g/mol, has a central sulphur atom that is surrounded by four atoms of oxygen. Two of the oxygen atoms are double-bonded with the sulphur. The other two atoms are single-bonded with sulphur but have hydrogen attachments.
Because of valence shell electron pair repulsion (VSEPR), a molecular model that predicts the geometry of the atoms, the oxygen atoms spread out in tetrahedral form. This means that each oxygen is bonded at an angle of 109.5 degrees from the other oxygen atoms.
Is Sulphuric Acid a Strong Acid?
Just like other acids, the strength of sulphuric acid is measured by its acid dissociation constant in a solution. This constant, represented by the symbol Ka, does not change regardless of the concentration of the acid. The pH may change based on the concentration but not the Ka.
In fact, sulphuric acid is one of the seven strong acids. The other strong acids are:
- HCl – Hydrochloric acid
- HNO₃ – Nitric acid
- HBr – Hydrobromic acid
- HI – Hydroiodic acid
- HClO₄ – Perchloric acid
- HClO₃ – Chloric acid
The acid dissociation constant is also known as the chemical reaction equilibrium of acid with its base conjugate. This is directly associated with the ratio between the formation of hydronium ions and the molecular acid.
The larger the Ka, the stronger the acid as its ions dissociate more completely in a solution. Sulphuric acid has a Ka value of 1.0 x 102. This places the acid in a higher rank compared to other acids.
Common Products and Compounds Containing Sulphuric Acid
Although sulphuric acid is essential in many manufacturing processes, it is not commonly found in many products as the active constituent. A few products that contain the acid include:
- Battery electrolyte – sulphuric acid is the electrolyte for lead-acid batteries of automobiles and solar power systems
- Household cleaning products – sulphuric acid is good in dissolving protein and grease similar to an alkaline solution
- Food additive – you probably wouldn’t think that a strong and toxic acid is used as a food additive. At low concentrations, however, it can be used as an acidifying agent in some dairy products like cheese, and is approved in Europe as a food additive under number E513
What is the pH Value of Sulfuric Acid?
Under the standard conditions of 1 mMol/L, 25°C, and 1 atmospheric pressure, the pH value of sulphuric acid is 2.75. This makes it highly acidic.
There are some variables, however: Unlike the dissociation constant, the pH value is directly dependent on the concentration of the acid or base in a solution. This also depends on factors like:
- The type of solvent
- Concentration in a solution
- Temperature
- Pressure
What Colour Does Sulphuric Acid Turn Blue Litmus Paper?
Depending on its concentration, sulphuric acid may not change the colour of blue litmus paper, or it may turn it a shade of red.
As litmus paper is available in either red or blue pigmentation, it does not measure the pH but is sensitive enough to react with an acid or a base.
Red litmus paper turns blue if the solution has a pH of 8 or more. Meanwhile, blue litmus paper turns red if the solution has an acid level of between 4 and 5 or lower.
Is Sulphuric Acid an Element?
Sulphuric acid is not an element, but it is a compound that is composed of two hydrogen atoms, one sulphur atom, and four oxygen atoms. When dissolved in water, it dissociates into ions of H₃O+ and HSO₄–.
Is Sulphuric Acid a Solution?
Yes, sulphuric acid is a solution because an acid can only react if it is in a form of solution. A compound of acid can occur in salt, solid forms but it will almost always have impurities. The purest form of sulphuric acid has a purity grade of 98% with pH value of 0.1. It is most stable for storage in this form.
Other concentrations of the acid include:
- 10% – Dilute sulphuric acid for use in labs (pH 1)
- 33.5% – Battery acid for lead-acid batteries (pH 0.5)
- 62.18% – Chamber or fertiliser fertiliser acid (pH about 0.4)
- 77.67% – Tower or Glover acid (pH about 0.25)
Industrial and Laboratory Uses of Sulphuric Acid
Industrially, sulphuric acid is key to processes involving chemical synthesis, fertiliser production, petroleum refining, and metals processing. Here’s a closer look at its applications:
Metal Processing
While many reagents and chemicals are used in metal production, sulphuric acid plays a key role in a specific process for steel known as “steel pickling”. While the metal is in a hot working process, its surface oxidises, which can cause issues with the final product.
To counter this before cold rolling, the metal goes through a “pickling liquor” made of sulphuric acid, and sometimes followed by additional washes in hydrochloric and other acids.
Chemical Manufacturing
It may seem hyperbolic, but sulphuric acid is a vital, and arguably one of the most important, compounds made in chemical manufacturing. Many other chemicals – including hydrochloric acid, nitric acid, synthetic detergents, sulphate salts, and more – rely on sulphuric acid as a base component.
This becomes all the more clear in laboratory settings, where it is vital as a catalyst for other chemical reactions and in titrations.
Additionally, in the pharmaceutical industry, sulphuric acid helps in the production of drugs by acting as a reactant and catalyst in the synthesis of active pharmaceutical ingredients.
Battery Production
As touched on before, sulphuric acid is vital to the manufacturing of lead-acid batteries, as it is used to activate the lead plates inside the casing, and allow for the transfer of energy.
Much like with metal working, it is one of the last parts of production before the charging cycle begins, with an electrolyte solution made of sulphuric acid and distilled water.
Agriculture
One of the largest users of sulphuric acid is in agriculture, specifically through phosphate fertilisers, ammonium phosphates, and calcium dihydrogenphosphate, though there are other applications. It is used when helping sulphur-deficient land recover, and preventing deficiencies in crops.
Conclusion
As one of the UK’s leading producers and suppliers of sulphuric acid, we know how versatile and important it is to so many industries. Whether it is used in large scale industrial processes, or small scale laboratory applications, it serves a wide scale purpose thanks to its highly reactive properties and strength.
