Acid
Acids donate protons or accept electron pairs in chemical reactions.
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An acid is a molecule or ion capable of donating a proton (a Brønsted–Lowry acid) or forming a covalent bond with an electron pair (a Lewis acid). Acids are fundamental to chemistry, with definitions evolving from Arrhenius to Brønsted–Lowry to Lewis theories, and they appear in everyday substances such as vinegar, citrus fruits, and gastric acid.
- definition_categories
- Brønsted–Lowry acids (proton donors) and Lewis acids (electron pair acceptors)
- common_examples
- Hydrochloric acid, acetic acid, sulfuric acid, citric acid
- characteristic_properties
- Sour taste, turns blue litmus red, pH less than 7, reacts with bases and certain metals to form salts
- etymology
- From Latin acidus, meaning 'sour'
- key_theorists
- Svante Arrhenius, Johannes Nicolaus Brønsted, Thomas Martin Lowry, Gilbert N. Lewis
Lore & Background
The first category of acids are Brønsted–Lowry acids, which donate a proton (H+). In aqueous solutions, these form hydronium ions (H3O+) and are known as Arrhenius acids. Brønsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents. A Brønsted–Lowry or Arrhenius acid usually contains a hydrogen atom bonded to a structure that remains energetically favorable after losing H+. The second category are Lewis acids, which form a covalent bond with an electron pair. An example is boron trifluoride (BF3), whose boron atom accepts a lone pair from a base like ammonia. Lewis considered this a generalization of the Brønsted definition. Notably, many Brønsted–Lowry acids (e.g., hydrogen chloride, acetic acid) can also act as Lewis acids under certain conditions, such as in non-aqueous solvents or when interacting with strong bases, by accepting an electron pair to form a coordinate covalent bond. Conversely, many Lewis acids are not Arrhenius or Brønsted–Lowry acids.
Reader's Guide
Modern definitions of acids are concerned with fundamental chemical reactions. The Brønsted–Lowry definition is the most widely used; unless otherwise specified, acid–base reactions involve the transfer of a proton from an acid to a base. Arrhenius acids increase the concentration of hydronium ions when added to water, while Brønsted–Lowry acids can function in nonaqueous solvents or the gas phase. Lewis acids broaden the concept to electron pair acceptance, covering reactions without proton transfer. Most acids encountered in everyday life are aqueous solutions, so the Arrhenius and Brønsted–Lowry definitions are most relevant. In modern terminology, 'acid' implicitly refers to a Brønsted acid, while Lewis acids are explicitly named as such. Strong acids and some concentrated weak acids are corrosive, though exceptions exist such as carboranes and boric acid.
Did You Know?
- The word 'acid' is derived from the Latin acidus, meaning 'sour'.
- Aqueous Arrhenius acids have a pH less than 7 and can turn blue litmus red.
- Lewis acids accept electron pairs; an example is boron trifluoride (BF3).
- Many Brønsted–Lowry acids, such as acetic acid and hydrogen chloride, can also act as Lewis acids under certain conditions.
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Frequently Asked Questions
Who is Acid?
An acid is a molecule or ion that can either hand off a proton to another species (Brønsted–Lowry) or accept a lone pair of electrons to forge a new covalent bond (Lewis). It is one of the most foundational concepts in all of chemistry.
What are Acid's signature traits?
Acids are recognizable by their sour taste, their ability to shift blue litmus paper to red, and a pH reading below 7. They also react with bases and certain metals to produce salts.
What is Acid's origin story?
The word traces back to the Latin *acidus*, meaning sour, and the modern concept evolved through three successive theoretical frameworks proposed by Arrhenius, Brønsted–Lowry, and Lewis. Each model broadened what scientists could classify as an acid.
How does Acid's arc resolve in a reaction?
In a neutralization reaction, an acid donates its proton to a base, and the two combine to form a salt and water. This is the classic "ending" most commonly encountered in introductory chemistry.
Why is Acid so central to the story?
Acids appear everywhere in daily life—vinegar carries acetic acid, citrus fruits hold citric acid, and your stomach relies on hydrochloric acid for digestion. Without the acid concept, understanding reactions, biology, and industrial processes would be nearly impossible.
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