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Unit 10: Acids and Bases—The Voyage of the Proton

Glossary

Acid
A compound that has a proton or protons that can dissociate in water; also, when one molecule has a proton or protons that dissociate more readily than those of another (i.e., it has a higher Ka), the first is said to be the more acidic molecule.
Acid dissociation constant
A form of the equilibrium constant for the dissociation of an acidic molecule into a proton and its conjugate base. It is abbreviated "Ka." The acid dissociation serves as a measure of how acidic the molecule is; the larger the value of Ka, the more acidic the molecule.
Acid dissociation equation
The chemical equation for the separation of an acidic molecule into a proton and its conjugate base. The general form can be written:

HA + H2O equilibrium arrows H3O+ + A-

where HA is the acidic molecule, H3O+ is the stable form of the proton, and A- is the conjugate base of the acidic molecule.
Acidic solutions
Solutions containing a higher concentration of hydronium ion (H3O+) than that found in pure water (i.e., having a pH below 7); also, when one solution has a greater concentration of hydronium than another, it is said to be the more acidic solution.
Arrhenius acid
Any molecule that can dissociate in an aqueous solution to produce a proton (H+).
Arrhenius base
Any molecule that can dissociate in an aqueous solution to produce a hydroxide ion (OH-).
Base
A compound that has the ability to accept a proton or protons from the surrounding solution. When one molecule associates with a proton or protons from the surrounding solution more readily than another, the first is said to be the more basic molecule. A basic compound can also be referred to as "alkaline."
Basic solutions
Solutions containing a lower concentration of hydronium ion than that found in pure water (that is, having a pH above 7). When one solution has a lower concentration of hydronium than another, it is said to be the more basic solution.
Brønsted-Lowry acid
A molecule that can dissociate in an aqueous solution to produce a proton, thus increasing the concentration of hydronium ion in the solution.
Brønsted-Lowry base
A molecule that can pick up a proton from an aqueous solution, thus decreasing the concentration of hydronium ion in the solution.
Buffer system
A weak acid and the salt of its conjugate base, which together can be used to create a buffered solution.
Buffer
An aqueous solution having the property that its pH changes very little upon the addition of an acid or a base. A buffer is formed by mixing together combinations of weak acids and weak bases.
Conjugate acid
The form of a molecule that has a dissociable proton attached to it. Since that proton can dissociate, this molecule is an acid.
Conjugate base
The form of a molecule that has a proton dissociated from it. Since that proton could potentially re-associate with the molecule, it is said to be a base.
Dissociation
The process in which a molecule falls apart into two pieces, commonly used to describe when an acid loses a proton (H+) and becomes its conjugate base.
Equivalence point
The point in a titration when the number of moles of added reactant is exactly equal (or stoichiometrically proportional) to the number of moles of reactant in the sample.
Hydronium ion
The conjugate acid of water. It consists of a water molecule with an extra proton attached and has the formula H3O+.
Hydroxide ion
The conjugate base of water. It consists of a water molecule with one of the protons abstracted and has the formula OH-.
Neutralization reaction
A reaction in which an equal amount of an acid and a base are mixed together, cancelling each other out, and making the solution neutral with a pH of 7.
pH
A measure of the acidity of a solution. It is the negative log (base 10) of the hydronium concentration in molar (-log10 [H3O+]).
pH scale
A logarithmic scale of the acidity of a solution. For aqueous solutions it runs from -1.7 (most acidic) to 15.7 (most basic), though typical values lie between 0 and 14.
pH unit
One unit on the pH scale. A change of one pH unit in an aqueous solution corresponds to one order of magnitude change in the hydronium concentration.
pKa
A measure of the ease with which the proton dissociates from an acidic molecule. It is equal to the negative log (base 10) of the acid dissociation constant (-log10Ka).
Strong acids
Acids that dissociate completely in solution.
Strong bases
Bases that completely dissociate in solution, usually soluble metal hydroxides.
Titration curve
A graph with the pH of an acidic or basic solution (being tested) as the dependent variable versus the amount of added acid or base as the independent variable. The rate of change in pH can be used to determine the equivalence point.
Weak acids
Acids that do not completely dissociate in solution.
Weak bases
Bases that do not completely dissociate in solution.

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