Acid

For alternative meanings see acid (disambiguation).

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Acids and Bases:
Acid-base reaction theories
pH
Self-ionization of water
Buffer solutions
Systematic naming
Redox reactions
Electrochemistry
Strong acids
Weak acids
Strong bases
Weak bases

An acid (often represented by the generic formula AH) is typically a water-soluble, sour-tasting chemical compound. In common usage an acid is any substance 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 (H⁺ ion) 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 1.1 Number of acid dissociations 2 Characteristics 3 Different definitions of acid/base 4 Acid number 5 Neutralization 6 Naming acids 7 Common acids 7.1 Strong inorganic acids 7.2 Medium to weak inorganic acids 7.3 Weak organic acids 8 Acids in food 9 Sources

Chemical characteristics

In water the following reaction occurs between an acid (AH) and water, which acts as a base:

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

The acidity constant (or acid dissociation constant) is the equilibrium constant for the reaction of AH with water:

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

Strong acids have large K_a values (i.e. the reaction equilibrium lies far to the right, lots of H₃O⁺ present; the acid is almost completely dissociated). For example, the K_a value for hydrochloric acid (HCl) is 10⁷.

Weak acids have small K_a 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). For example, the K_a value for acetic acid is 1.8 x 10^-5.

Strong acids include the hydrohalic acids - HCl, HBr, and HI. (However, hydrofluoric acid, HF, is relatively weak.) Oxyacids, which tend to contain central atoms in high oxidation states surrounded by oxygen, are also quite strong and include HNO₃, H₂SO₄, HClO₄. Most organic acids are weak acids.

A few clarifications:

• The terms "hydrogen ion" and "proton" are used interchangebly; both refer to H⁺.
• In chemical equations H⁺ is often written, although in water it will actually be H₃O⁺.
• The strength of an acid is measured by its K_a value. pH measures how many hydrogen ions are present, which depends on both the type of acid (or base) and how much is there.
• Acid strength is also defined by pK_a= - log(K_a).

Number of acid dissociations

Some acid molecules are able give up more than one H⁺ ion (proton). Those acids which can give up only one H⁺ ion per molecule are called monoprotic acids, those acid molecules that can give up two H⁺ ions are diprotic acids, those that can give up three are triprotic acids, etc. A monoprotic acid can undergo one dissociation (sometimes called ionization) as follows and simply has one acid dissociation constant as shown above:

AH + H₂O → A^- + H₃O⁺         K_a

A diprotic acid (here symbolized by AH₂) can undergo one or two dissociations depending on the conditions (namely pH). Each dissociation has its own dissociation constant, K_a1 and K_a2.

AH₂ + H₂O → AH^- + H₃O⁺       K_a1

AH^- + H₂O → A^-2 + H₃O⁺        K_a2

The first dissociation constant is typically greater than the second; i. e. K_a1 > K_a2 . For example, sulfuric acid (H₂SO4) can give up one H⁺ to form the singly charged bisulfate anion (HSO4^-), for which K_a1 is very large; then it can give up a second H⁺ to form the doubly charged sulfate anion (SO4^-2) where the K_a2 is intermediate strength. The large K_a1 for the first dissociation makes sulfuric a strong acid. Similarly, the weak unstable carbonic acid (H₂CO₃) can lose one H⁺ to form a singly charged bicarbonate anion (HCO₃^-) and lose a second to form a doubly charged carbonate anion (CO₃^-2). Both K_a values are small, but K_a1 > K_a2 .

Analogously, a triprotic acid (AH₃) can undergo one, two, or three dissociations and has three dissociation constants, where K_a1 > K_a2 > K_a3 .

AH₃ + H₂O → AH₂^- + H₃O⁺        K_a1

AH₂^- + H₂O → AH^-2 + H₃O⁺       K_a2

AH^-2 + H₂O → A^-3 + H₃O⁺         K_a3

An inorganic example of a triprotic acid is orthophosphoric acid (H₃PO₄), usually just called phosphoric acid. All three of its H atoms can be successively lost as H⁺ (or H₃O⁺ in water) to yield H₂PO₄^-, then HPO₄^-2, and finally PO₄^-3 , the triply charged orthophosphate ion, usually just called phosphate. An organic example of a triprotic acid is citric acid, which can successively lose three H⁺ ions to finally form the triply charged citrate ion. Even though the positions of the H atoms on the original molecule may be equivalent, the successive K_a values will differ since it is energetically less favorable to lose a positive H⁺ if the ion is more negatively charged.

Characteristics

Acids:

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

Acids are very dangerous. They are extremely reactive with metals and strong acids can give very serious burns just by touching them for an instant. If contact with an acid should occur seek medical attention immediately.

Different definitions of acid/base

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

The Swedish chemist Svante Arrhenius defined an acid to be a substance that gives up hydrogen ions (H⁺) when dissolved in water (the product of the solution, H₂O + H⁺, is called a hydronium ion, H₃O⁺), while bases are substances that give up hydroxide ions (OH^-). This definition limits acids and bases to substances that can dissolve in water. Later on, Brønsted 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, given by the American chemist Gilbert N. Lewis. Lewis theory defines a "Lewis acid" as an electron-pair acceptor and a "Lewis base" as an electron-pair donor. It can include acids that do not contain any hydrogen atoms, such as iron(III) chloride. Acid/base systems are different from redox reactions in that there is no change in oxidation state. The Lewis definition can also 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.

The Brønsted-Lowry definition, where an acid is treated as a proton donor, is sufficient for many situations. In this case, the proton (H⁺) is the actual acid and the acidity of the proton-donating-compound, such as 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 such as carboxylic 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 the amount of acid present, for example in a sample of biodiesel. 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 = (V_eq-b_eq)×N×56.1/W_oil.

V_eq is the amount of titrant (ml) consumed by the crude oil sample and 1ml spiking solution at the equivalent point, and b_eqbeq 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×W_KHP/(204.23×V_eq).

In which, W_KHP 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 a salt and water. So, it is also called a water forming reaction <math>acid + base \rarr water + salt <math>
Example: <math>HCl + NaOH \rarr H_2O + NaCl<math>

This type of reaction forms the basis of titration methods for analysing acids, where a pH indicator shows the point of neutralization.

Naming acids

Acids are named according to the ending of their anion. That ionic ending is dropped and replaced with a new suffix according to the table below. For example, HCl has chloride as its anion, so the -ide suffix makes it takes the form hydrochloric acid.

Anion Ending	Acid Prefex	Acid Suffix
ate		ic acid
ite		ous acid
ide	Hydro	ic acid

Common acids

Strong inorganic acids

• Hydrobromic acid
• Hydrochloric acid
• Hydroiodic acid
• Nitric acid
• Sulfuric acid
• Chloric acid
• Perchloric acid

Medium to weak inorganic acids

• Boric acid
• Carbonic acid
• Phosphoric acid
• Pyrophosphoric acid

Weak organic acids

• Acetic acid
• Benzoic acid
• Butyric acid
• Citric acid
• Formic acid
• Lactic acid
• Malic acid
• Methanethiol
• Propionic acid
• Pyruvic acid
• Valeric acid

Acids in food

• acetic acid or ethanoic acid: (E260) found in vinegar and tomato sauce
• adipic acid: (E355)
• alginic acid: (E400)
• benzoic acid: (E210)
• boric acid: (E284)
• ascorbic acid (vitamin C): (E300) found in fruits
• citric acid: (E330) found in citrus fruits
• carbonic acid: (E290) found in carbonated soft drinks
• carminic acid: (E120)
• cyclamic acid: (E952)
• erythorbic acid: (E315)
• erythorbin acid: (E317)
• formic acid: (E236)
• fumaric acid: (E297)
• gluconic acid: (E574)
• glutamic acid: (E620)
• guanylic acid: (E626)
• hydrochloric acid: (E507)
• inosinic acid: (E630)
• lactic acid: (E270) found in dairy products such as yoghurt and sour milk, also is reactant of cellular fermentation, the reason muscles burn
• malic acid: (E296)
• metatartaric acid: (E353)
• methanethiol: found in cheese and some other fermented foods.
• nicotinic acid: (E375)
• oxalic acid: found in spinach and rhubarb
• pectic acid: found in fruits and some vegetables
• phosphoric acid: (E338)
• propionic acid: (E280)
• sorbic acid: (E200) found in foods and drinks
• stearic acid: (E570), a type of fatty acid.
• succinic acid: (E363)
• sulfuric acid: (E513)
• tannic acid: found in tea
• tartaric acid: (E334) found in grapes

Sources

• Listing of strengths of common acids and bases (http://www.csudh.edu/oliver/chemdata/data-ka.htm)
• Zumdahl, Chemistry, 4th Edition.bg:Киселина

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