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Acid

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For alternative meanings see acid (disambiguation).

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Acids and Bases:
Acid-base reaction theories
pH
Self-ionization of water
Buffers
Systematic_naming
Redox reactions
Electrochemistry
Strong acids
Weak acids
Weak bases
Strong bases

An acid (represented by the generic formula AH) is typically a water-soluble, sour-tasting chemical compound. In common usage an acid is a species that, when dissolved in water, gives a solution with a pH of less than 7. In general scientific usage an acid is a molecule or ion that is able to give up a proton to a base, or accept an unshared pair of electrons from a base. An acid reacts with a base in a neutralization reaction to form a salt.

Contents
1 Chemical characteristics
2 Characteristics
3 Acids in food
4 Different definitions of acid/base
5 Acid number
6 Neutralization
7 Common acids

7.1 Strong inorganic acids
7.2 Strong organic acids
7.3 Weak inorganic acids
7.4 Weak organic acids

Chemical characteristics

In water the following reaction occurs between an acid (AH) and water:

<math>\mbox{AH} +\mbox{H}_2\mbox{O} \leftrightarrow \mbox{A}^- + \mbox{H}_3\mbox{O}^+<math>

The acidity constant is the equilibrium constant for the reaction of AH with water:

<math>Ka = {[A^-]\cdot[\mbox{H}_3\mbox{O}^+] \over [AH]}<math>

Strong acids have large Ka values (i.e. the reaction equilibrium lies far to the right, lots of H3O+ present; the acid is almost completely dissociated).

Weak acids have small Ka values (i.e. at equilibrium significant amounts of AH and A- exist together in solution; modest levels of H3O+ are present; the acid is only partially dissociated).


Strong acids include the hydrohalic acids - HCl, HBr, and HI - and the oxyacids, which tend to contain central atoms in high oxidation states surrounded by oxygen - including HNO3, H2SO4, HClO4.

Acidic (chemistry), the opposite to basic, reacting with basics to form salts. Acidic (geology), of rock: containing more than 65% of silica.

Characteristics

Acids are generally:

  • Taste: sour when dissolved in water
  • Touch: strong acids have a stinging feeling
  • Reactivity: acids react violently with many metals
  • Electrical conductivity: acids are electrolytes

Acids in food

Different definitions of acid/base

The word acid comes from the Latin acidus meaning sour. Chemically though the term acid has a more specific meaning.

The Swedish chemist Svante Arrhenius defined an acid to be a substance that gave up hydrogen ions (H+) when dissolved in water, while bases are substances that give up hydroxide ions (OH-). Notice that this definition limits acids and bases to substances that can dissolve in water. Later on, Bronsted and Lowry defined an acid to be a proton donor and a base to be a proton acceptor. In this definition, even substances that are insoluble in water can be acids and bases. The most general definition of acids and bases is the Lewis definition. A Lewis acid is an electron acceptor, while a Lewis base is an electron donor. Acid/base systems are different from redox reactions in that there is no change in oxidation state.

The Lewis definition can be explained with molecular orbital theory. In general an acid can receive an electron pair in its lowest unoccupied orbital (LUMO) from the highest occupied orbital (HOMO) of a base. That is, the HOMO from the base and the LUMO from the acid combine to a bonding molecular orbital. Simpler definitions are often sufficient for many situations. An acid is often said to be a proton donor, that is the Brønsted definition above. In this case, the proton (H+) is the actual acid and the acidity of the proton-donating-compound, called an organic acid, is determined by its stability when it donates protons to the solution it is embedded in. So if the organic acid likes letting protons go, it has high acidity because it donates protons with empty molecular orbitals to the solution. This is how organic acids work, here the Brønsted definition is nice for calculations while the Lewis definition is good for understanding.

Acid number

This is used to quantify oxidation. It is the quantity of base, expressed in milligrams of potassium hydroxide, that is required to neutralize the acidic constituents in 1 g of sample.

AN = (Veq-beq)×N×56.1/Woil.

Veq is the amount of titrant (ml) consumed by crude oil sample and 1ml spiking solution at the equivalent point, and beqbeq is the amount of titrant (ml) consumed by 1ml spiking solution at the equivalent point.

The molarity concentration of titrant (N) is calculated as such: N = 1000×WKHP/(204.23×Veq).

In which, WKHP is the amount (g) of KHP in 50ml of KHP standard solution, and Veq is the amount of titrant (ml) consumed by 50ml KHP standard solution at the equivalent point.

Acid number (mgKOH/g oil) for biodiesel is preferred to be lower than 3.

Neutralization

Neutralization is a type of reaction between an acid and a base. The products include salt and water. So, it is also called a water forming reaction <math>acid + base \rarr water + salt <math>
Example: <math>HCl + Na (OH) \rarr H_2O + NaCl<math>

Common acids

Strong inorganic acids

Strong organic acids

  • Trichloracetic acid

Weak inorganic acids

Weak organic acids

bg:Киселина ca:Àcid da:Syre de:Säure et:Hape es:Ácido eo:Acido fr:Acide nl:Zuur ja:酸と塩基 nds:Süür pl:Kwas simple:Acid zh:酸

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