There are several ways to define acids and bases.

While these definitions do not contradict each other, they do vary in their inclusiveness.

The most common definitions of acids and bases are Arrhenius acids and bases, Br?nsted-Lowry acids and bases, and Lewis acids and bases.

Acids and bases were also observed, but not formally defined, by Antoine Lavoisier, Humphry Davy, and Justus Liebig.

Svante Arrhenius Acids and Bases Arrhenius' theory of acids and bases dates back to 1884 and is based on his observation that salts such as sodium chloride break down into what he called ions when placed in water.

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Acids produce H+ ions in aqueous solutions Bases produce OH - ions in aqueous solutions Water is required, so only aqueous solutions are allowed Only protic acids are allowed; hydrogen ions are required Only hydroxide bases are allowed Johannes Nicolaus Br?nsted - Thomas Martin Lowry Acids and

The Br?nsted or Br?nsted-Lowry theory of bases describes acid-base reactions as an acid that releases a proton and a base that accepts the proton.

While the definition of an acid is almost identical to the one proposed by Arrhenius (a hydrogen ion is a proton), the definition of a base is much broader.

Acids are proton donors Bases are proton acceptors Aqueous solutions are allowed Bases other than hydroxides are allowed Only protic acids are allowed Gilbert Newton Lewis Acids and Bases The Lewis theory of acids and bases is the least restrictive model.

It does not deal with protons at all, only electron pairs.

Acids are electron pair acceptors Bases are electron pair donors Least Restrictions on Definition of Acids and Bases Properties of Acids and Bases Robert Boyle described the properties of acids and bases in 1661.

These characteristics can be used to easily differentiate between two chemicals without the need for complicated tests: Sour Taste Sour (Don’t!) – The word “acid” comes from the Latin acere, meaning “sour” Acid is corrosive

Acids change litmus (a blue plant dye) from blue to red Their aqueous solutions (water solutions) conduct electricity (are electrolytes) React with bases to form salts and water Reactive metals such as alkali metals, alkaline earth metals, zinc,

Aluminum) reacts to produce hydrogen gas (H 2 ) Common acids Citric acid (from some fruits and vegetables, especially citrus fruits) Ascorbic acid (vitamin C, from some fruits) Vinegar (5% acetic acid) Carbonic acid (carbonated for soft drinks)

) Lactic acid (in buttermilk) The bases taste bitter (don’t taste them!) feel slippery or soapy (don’t touch them!) The bases don’t change the color of the litmus; they can acidify the red color

Litmus turns back to blue Their aqueous (water) solution conducts electricity (is an electrolyte) Reacts with acids to form salts and water ***Same base detergent soap lye (NaOH) Household ammonia (aqueous solution) Strong and weak acids and bases Acids

The strength of bases depends on their ability to dissociate or break down into ions in water.

Strong acids or bases dissociate completely (e.g., HCl or NaOH), while weak acids or bases dissociate only partially (e.g., acetic acid).

The acid dissociation constant and base dissociation constant represent the relative strength of an acid or base.

The acid dissociation constant K a is the equilibrium constant for acid-base dissociation: HA + H 2 O ? A - + H 3 O + where HA is the acid and A - is the yoke base.

K a = [A - ][H 3 O + ] / [HA][H 2 O] This is used to calculate pK a, the logarithmic constant: pk a = - log 10 K a The larger the pK a value, the acidic

The smaller the dissociation, the weaker the acid.

A strong acid has a pK a less than -2.