Aqueous sulfuric acid is a common mixture of sulfuric acid and water, used in chemical processes, batteries, and as a dehydrating agent. It is created through the lead chamber or contact processes and is a raw material for fertilizers and chemical intermediates. Lead-acid batteries use sulfuric acid to generate electricity and can be recharged by reversing the chemical reaction.
Aqueous sulfuric acid is any mixture of sulfuric acid and water and is the most common way the acid is sold. The Latin word for water is aqua, which is where the term watery comes from. Sulfuric acid, or H2SO4, is widely used in chemical processes, for batteries and as a dehydrating agent to remove water from other materials.
When sulfuric acid is mixed with water, the hydrogen atoms become ions, which means they separate from the acid molecule, develop a small electric charge and are available to react with other molecules. Because there are two hydrogen atoms in each molecule, sulfuric acid is called dibasic acid. The ionization process requires the presence of water, so most chemical processes use aqueous sulfuric acid.
There is such an affinity, or strong attraction, of sulfuric acid for water, that the strong acid can be used to dry other chemicals. This is known as dehydration, which comes from hydro, the Greek word for water. As the acid absorbs water, it becomes an aqueous solution of weaker sulfuric acid and must eventually be replaced with fresh acid or regenerated into strong acid. Adding sulfuric acid to water creates a large amount of heat, which must be controlled to prevent the mixture from boiling.
There are two main commercial ways of creating sulfuric acid, the lead chamber and the contact processes. Invented in the 1700s, the lead chamber process uses a reaction of sulfur and potassium nitrate, or saltpeter, with steam in a lead-lined reactor to form sulfur trioxide. Sulfur dioxide can be dissolved in water to form aqueous sulfuric acid, typically with concentrations of about 70% acid.
In 1830, the contact process was invented to create a more concentrated acid. This process uses a metal catalyst that is expensive to produce, but can create acid concentrations as high as 98%. This is called concentrated acid and is generally the preferred production method in the 20th and 21st centuries.
Aqueous sulfuric acid is a common raw material for the production of fertilizers, other chemical intermediates and lead-acid vehicle batteries. From the early 20th century, batteries made from lead plates suspended in a sulfuric acid solution were a common way to produce electricity for vehicles, ships and aircraft. Lead reacts with the acidic solution, forming molecular ions which create an electric current.
When the battery generates energy, lead and acid react to form lead sulfate, which is a white powder that accumulates in the bottom of the battery. These batteries are rechargeable which means that an electric current sent through the battery will reverse this reaction and allow the lead to settle back onto the plates. As charging proceeds, the sulfate ions will form sulfuric acid, which will create a more concentrated mixture in the battery and provide more electricity when needed.
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