There are different definitions of acids and bases, but they generally refer to the same compounds (there are some differences however). Watch the following video.
Arrhenius Acids and Bases
Substance when dissolved in water produces H+, so it must have H in its formula“>Substance when dissolved in water produces H+, so it must have H in its formula Note that in acid base equations H+ and H3O+ are used interchangeably, they mean the same thing
Example: HCl + H2O → H+ + Cl-
H3O+ ions are called “hydronium ions”
HCl is a acid because it produces H+ and has H in its formula
Arrhenius acids contain one or more hydrogen atoms
Substance when dissolved in water produces OH-, so it must have OH in its formula
OH- ions are called “hydroxide ions”
Example: NaOH + H2O → Na+ + OH- : NaOH is a base because it produces OH- and had OH in its formula
This definition had problems. Arrhenius's definition of acids and bases applied to certain chemicals. For example, his definition only worked for acids and bases that were in a water solution. Another issue is that not all bases and acids can be classified under this definition. For example, NH3 is a base. but does not have -OH in its formula. Due to the limitations of this model different models/definitions were produced.
Bronsted-Lowry Acids and Bases
This is the definition we will use most often when we think about acids and bases. Watch the following video from 5:42 to 8:11.
When discussing this model, we need to discuss conjugate acids and bases. This is a way to describe the compounds in an equation as acid and base pairs.
- The acid on the left side of the equation becomes the conjugate base on the right side.
- The base on the left side of the equation becomes the conjugate acid on the right side.
A buffer is used to stabilize the pH of a solution. Buffers are generally a mixture of an acid and its conjugate base, or vise verse. For example, acetic acid and acetate are an example of an acid a conjugate base. Additionally, blood is a solution which must be maintained in a very narrow pH range, or we die, so the following is used: Carbonic acid and bicarbonate. If an acid or base is added to a solution a buffer will release or absorb H+ ions in order to keep the pH in the narrow range. This works until all the buffer is used up. In lakes and streams, it is calcium carbonate that causes alkalinity (causes a basic solution). Calcium acts as a buffer, a substance that serves to resist small changes in acidity or alkalinity in a solution. When acid rain enters lake water, it can be neutralized by the bases present and thus the lake does not become too acidic. All over the world, right now, pollution workers are measuring calcium carbonate in lakes and streams.
Lewis Acids and Bases
- Lewis Acid: Can accept a pair of electrons
- Lewis Base: Donates a pair of electrons.