Chemistry

Introduction to Acid and Base Concepts



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Acids can be defined several ways. The most commonly used definition is the Arrhenius definition. An acid is a compound that releases H+ ions in solution.

According to the Arrhenius definition, a base is a compound that releases OH- ions in solution.

When an acid and base combine, they neutralize one another, to produce something neither acid nor base. The net ionic reaction for the neutralization of an acid and base is then: H+ + OH- H2O

In reality, the H+ ions does not actually exist separately, the hydrogen attaches to water to produce a Hydronium ion with the formula H3O+. We frequently just call it a hydrogen ion and write it as H+ anyway.

The molecular equation for the neutralization of Hydrochloric acid and Sodium hydroxide is: HCl + NaOH NaCl + H2O

Some acids release more than one hydrogen ion per molecule of acid. These are polyprotic acids. For example, hydrogen phosphate, when dissolved in water, is phosphoric acid, and can release three protons into the water:
H3PO4 H+ + H2PO4-
H2PO4- H+ + HPO42-
HPO42- H+ + PO43-
One of the most important polyprotic acid is H2SO4 or Sulfuric acid.

Some molecules that do not contain hydrogen still act as acids when added to water. For example, carbon dioxide combines with water to produce carbonic acid (a diprotic acid):
CO2 + H2O H2CO3
H2CO3 H+ + HCO3-
HCO3- H+ + CO32-
These compounds are called Acid Anhydrides. Anhydride means without water, they become acids when water is added to them.
The most important Anhydride among them is Carbonic Acid.

Bases (using the Arrhenius definition) are compounds that release hydroxide (OH-) ions. They fall into three main classes:

1) hydroxides (ex: Sodium Hydroxide): NaOH Na+ + OH-
2) Basic Anhydrides (ex: Calcium Oxide): CaO + H2O Ca2+ + 2OH-
3) Molecular bases (ex: Ammonia): NH3 + H2O NH4+ + OH-

Binary acids are named after the element, with "hydro-" added as a prefx and "acid" as a suffix.
The Oxoacids are the acids involving nonmetal oxides.
- If the ion comes in two forms, the one with more oxygens is an "-ate" ion, and forms an "-ic" acid (SO42- is the sulfate ion and forms sulfuric acid)
- The one with less oxygen is an "-ite" ion and forms an "-ous" acid (SO32- is the sulfite ion and H2SO3 is sulfurous acid)
- If it has many oxygen forms, then the acids are named like Hypochloric HCLO, chlorous HClO2. Chloric HClO3, Perchloric HCLO4.

When an acid is partially neutralized, you can get an acid salt. For example, take the phosphate ion, you can make 4 different compounds using hydrogen and sodium.

Acids can be classified as either strong or weak acids.
- Strong acids disassociate completely
- Weak acids do not disassociate in water. In solution they can be found in both forms, ex. Phosphoric acid you can find H3PO4, H2PO4-, HPO42- and PO43- ions.
- This is because they exist in a dynamic equilibrium

Consider acetic acid. It can exist in two forms, with its hydrogen as acetic acid HCH3CO2 and without as the acetate ion CH3CO2-. There are, then two chemical reactions we can consider, one, acetic acid disassociating into acetate and a hydronium ion: HCH3CO2 + H2O CH3CO2- + H3O+
Each reaction occurs with some probability based on the nature of water and the acetate ion. If the first one always occurs and the second never does, then it will always be acetate (and will be a strong acid). If the first never does and the second always, it will always be acetic acid. If equal probabilities, then half and half, etc.

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