# Nature of Acids and Bases

## There are three common definitions for acids and bases: the Arrhenius definition, the Bronsted-Lowry definition, and the Lewis definition.

#### Key Points

• Common examples of acids include acetic acid (in vinegar), sulfuric acid (used in car batteries), and tartaric acid (used in baking).  Examples of common bases are sodium hydroxide and ammonia.

• Bases can be thought of as the chemical opposite of acids. A reaction between an acid and base is called neutralization.

• The strength of an acid refers to its ability or tendency to lose a proton; a strong acid is one that completely dissociates in water.

• A base is a substance that can accept hydrogen ions (protons) or, more generally, donate a pair of valence electrons.

#### Terms

• Any electrophylic compound that can accept a pair of electrons and form a coordinate covalent bond.

• Any nucleophylic compound that can donate a pair of electrons and form a coordinate covalent bond.

• Any of the electrons in the outermost shell of an atom; capable of forming bonds with other atoms.

#### Examples

• Common examples of acids include acetic acid (in vinegar), sulfuric acid (used in car batteries), and tartaric acid (used in baking). Examples of common bases are sodium hydroxide and ammonia.

#### Figures

1. ##### Metal and acid reaction

Zinc reacting with hydrogen chloride

2. ##### Lewis acids and bases

Diagram of Lewis acids and bases

3. ##### Acids + Bases Made Easy! Part 1 - What the Heck is an Acid or Base?

Ever wondered what the heck an Acid or Base actually is? Were you ever super confused in high school or college chemistry?In this video I introduce to you guys what the heck an Acid and Base really is forgetting the Lewis or Bronstead/Lowry definitions and then we'll go more in depth in parts 2,3, and 4.

Figure 3

## Acids

Acids have long been recognized as a distinctive class of compounds whose aqueous solutions exhibit the following properties:

• A characteristic sour taste
• Changes the color of litmus from blue to red
• Reacts with certain metals (Figure 1) to produce gaseous H2
• Reacts with bases to form a salt and water

Aqueous acids have a pH under 7, with acidity increasing the lower the pH. Common examples of acids include acetic acid (in vinegar), sulfuric acid (used in car batteries), and tartaric acid (used in baking). As these three examples show, acids can be solutions, liquids, or solids.

There are three common definitions for acids:

• Arrhenius: acids are substances that increase the concentration of hydronium ions (H3O+) in solution.
• Brønsted-Lowry: an expansion of the Arrhenius definition, an acid is a substance that can act as a proton donor.
• Lewis: Lewis acids are electron-pair acceptors. Examples of Lewis acids can be seen here (Figure 2).

Most acids encountered in everyday life are aqueous solutions, or can be dissolved in water, and the first two definitions are most relevant. The reason why pHs of acids are less than 7 is that the concentration of hydronium ions is greater than 10−7 moles per liter.  Since pH is defined as the negative logarithm of the concentration of hydronium ions, acids thus have pHs of less than 7. By the Brønsted-Lowry definition, any compound that can easily be deprotonated can also be considered an acid. Examples include alcohols and amines that contain O-H or N-H fragments.

Hydronium ions are acids according to all three definitions.  Interestingly, although alcohols and amines can be Brønsted-Lowry acids as mentioned above, they can also function as Lewis bases due to the lone pairs of electrons on their oxygen and nitrogen atoms.

The strength of an acid refers to its ability or tendency to lose a proton.  A strong acid is one that completely dissociates in water; in other words, one mole of the strong acid HA dissolves in water, yielding one mole of H+ and one mole of the conjugate base, A, and none of the protonated acid HA. In contrast, a weak acid only partially dissociates, and at equilibrium both the acid and the conjugate base are in solution. Examples of strong acids are hydrochloric acid (HCl), hydroiodic acid (HI), hydrobromic acid (HBr), perchloric acid (HClO4), nitric acid (HNO3), and sulfuric acid (H2SO4).  Each of these essentially ionizes 100% in water.  Two key factors that contribute to the ease of deprotonation are the polarity of the H—A bond and the size of atom A, which determines the strength of the H—A bond. Acid strengths are also often discussed in terms of the stability of the conjugate base.  Stronger acids have a larger Ka and a more negative pKa than weaker acids.

## Bases

A base is a substance that can accept hydrogen ions (protons) or, more generally, donate a pair of valence electrons.  A soluble base is referred to as an alkali if it contains and releases hydroxide ions (OH) quantitatively.  Again, we have three common definitions:

• Arrhenius: a base is a hydroxide anion, which is strictly applicable only to alkali.
• Brønsted-Lowry: a base is a proton (hydrogen ion) acceptor.
• Lewis: a base is an electron pair donor, examples can be seen here (Figure 2).

In water, bases give solutions with a hydrogen ion activity lower than that of pure water, i.e., a pH higher than 7.0 at standard conditions. A strong base is a basic chemical compound that is able to deprotonate very weak acids in an acid-base reaction. Common examples of strong bases are the hydroxides of alkali metals and alkaline earth metals like NaOH and Ca(OH)2. Very strong bases are even able to deprotonate very weakly acidic C–H groups in the absence of water.  Acids with a pKa of more than about 13 are considered very weak, and their conjugate bases are strong bases.

Acids and bases react with one another to yield two products: water and an ionic compound known as a salt.  This reaction is called neutralization.  Bases and acids are seen as opposites because acids increase the hydronium ion (H3O+) concentration in water, whereas bases reduce this concentration.  Bases and acids are typically found in aqueous solution forms. Aqueous solutions of bases react with aqueous solutions of acids to produce water and salts in aqueous solutions in which the salts separate into their component ions. If the aqueous solution is a saturated solution with respect to a given salt solute, any additional such salt present in the solution will result in formation of a precipitate of the salt.

#### Key Term Glossary

acid
an electron pair acceptor; generally capable of donating hydrogen ions
##### Appears in these related concepts:
acidity
The quality or state of being acid.
##### Appears in these related concepts:
activity
In chemical thermodynamics, activity (symbol a) is a measure of the “effective concentration” of a species in a mixture, meaning that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.
##### Appears in these related concepts:
alcohol
class of organic compounds containing a hydroxyl functional group
##### Appears in these related concepts:
alkali metal
Any of the soft, light, reactive metals of Group 1 of the periodic table; lithium, sodium, potassium, rubidium, cesium, and francium.
##### Appears in these related concepts:
alkaline
Having a pH greater than 7.
##### Appears in these related concepts:
Alkaline earth metals
A group of chemical elements in the periodic table with very similar properties: they are all shiny, silvery-white, somewhat reactive metals at standard temperature and pressure and readily lose their two outermost electrons to form cations with charge +2.
##### Appears in these related concepts:
amine
organic compounds or functional group that contains a basic nitrogen atom with a lone pair
##### Appears in these related concepts:
anion
A negatively charged ion, as opposed to a cation
##### Appears in these related concepts:
aqueous
Consisting mostly of water.
##### Appears in these related concepts:
atom
the smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons
##### Appears in these related concepts:
base
A proton acceptor, or an electron pair donor.
##### Appears in these related concepts:
battery
a device that produces electricity by a chemical reaction between two substances
##### Appears in these related concepts:
bond
a link or force between neighboring atoms in a molecule
##### Appears in these related concepts:
compound
A substance made from any combination elements.
##### Appears in these related concepts:
concentration
the proportion of a substance in a mixture
##### Appears in these related concepts:
conjugate base
The species that is created after the donation of a proton.
##### Appears in these related concepts:
deprotonate
to remove one or more protons from a molecule
##### Appears in these related concepts:
deprotonation
The removal of a proton (hydrogen ion) from a molecule to form a conjugate base
##### Appears in these related concepts:
electron
The subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor constitutes electricity.
##### Appears in these related concepts:
equilibrium
the state of a reaction in which the rates of the forward and reverse reactions are the same
##### Appears in these related concepts:
group
A vertical column in the periodic table, which signifies the number of valence shell electrons in an element's atom.
##### Appears in these related concepts:
hydronium
The hydrated hydrogen ion, $H_3O^+$.
##### Appears in these related concepts:
hydroxide
An univalent anion (OH-1) based on the hydroxyl functional group.
##### Appears in these related concepts:
ion
An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
##### Appears in these related concepts:
ionic
of, relating to, or containing ions
##### Appears in these related concepts:
ionic compound
named by its cation followed by its anion
##### Appears in these related concepts:
Lewis acid
Any electrophylic compound that can accept a pair of electrons and form a coordinate covalent bond.
##### Appears in these related concepts:
Lewis base
Any nucleophylic compound that can donate a pair of electrons and form a coordinate covalent bond.
##### Appears in these related concepts:
liquid
A substance that flows and keeps no definite shape, such as water. A substance whose molecules, while not tending to separate from one another like those of a gas, readily change their relative position, and which therefore retains no definite shape, except that determined by the containing receptacle; an inelastic fluid.
##### Appears in these related concepts:
liter
a non-SI metric system unit of volume equal to 1 cubic decimeter (dm^3), 1,000 cubic centimeters (cm^3) or 1/1,000 cubic meter (m^3)
##### Appears in these related concepts:
logarithm
For a number $x$, the power to which a given base number must be raised in order to obtain x. Written logbx. For example, log216 = 4 because 24 = 16.
##### Appears in these related concepts:
lone pair
a valence set of two electrons that exist without bonding or sharing with other atoms
##### Appears in these related concepts:
metal
Any of a number of chemical elements in the periodic table that form a metallic bond with other metal atoms; generally shiny, somewhat malleable and hard, often a conductor of heat and electricity
##### Appears in these related concepts:
mole
In the International System of Units, the base unit of the amount of substance; the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. Symbol: mol. The number of atoms in a mole is known as Avogadro’s number.
##### Appears in these related concepts:
neutralization
A chemical reaction in which an acid and a base react to form a salt.
##### Appears in these related concepts:
nitrogen
a chemical element (symbol N) with an atomic number of 7 and atomic weight of 14.0067
##### Appears in these related concepts:
oxygen
a chemical element (symbol O) with an atomic number of 8 and relative atomic mass of 15.9994
##### Appears in these related concepts:
pH
the negative of the logarithm to base 10 of the concentration of hydrogen ions, measured in moles per liter; a measure of acidity or alkalinity of a substance, which takes numerical values from 0 (maximum acidity) through 7 (neutral) to 14 (maximum alkalinity)
##### Appears in these related concepts:
pKa
A quantitative measure of the strength of an acid in solution; a weak acid has a pKa value in the approximate range −2 to 12 in water and a strong acid has a pKa value of less than about −2.
##### Appears in these related concepts:
polarity
The dipole-dipole intermolecular forces between the slightly positively-charged end of one molecule to the negative end of another or the same molecule.
##### Appears in these related concepts:
Polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment.
##### Appears in these related concepts:
precipitate
a solid that exits the liquid phase of a solution
##### Appears in these related concepts:
product
a chemical substance formed as a result of a chemical reaction
##### Appears in these related concepts:
proton
A positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an element; the nucleus of the most common isotope of hydrogen; composed of two up quarks and a down quark
##### Appears in these related concepts:
reduce
to add electrons/hydrogen or to remove oxygen
##### Appears in these related concepts:
salt
An ionic compound that is composed of cations and anions The product is electrically neutral. The two ions are held together by ionic bonds, not covalent bonds.
##### Appears in these related concepts:
saturated
containing all the solute that can normally be dissolved at a given temperature
##### Appears in these related concepts:
saturated solution
one in which the solvent can dissolve no more of a specific solute at a particular temperature
##### Appears in these related concepts:
solid
A substance in the fundamental state of matter that retains its size and shape without need of a container (as opposed to a liquid or gas).
##### Appears in these related concepts:
solute
Any substance that is dissolved in a liquid solvent to create a solution.
##### Appears in these related concepts:
solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
standard
something used as a measure for comparative evaluations
##### Appears in these related concepts:
Standard conditions
In chemistry, IUPAC established standard temperature and pressure (informally abbreviated as STP) as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm, 1 bar),[1] An unofficial, but commonly used, standard is standard ambient temperature and pressure (SATP) as a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm). The STP and the SATP should not be confused with the standard state commonly used in thermodynamic evaluations of the Gibbs free energy of a reaction.
##### Appears in these related concepts:
strong acid
A strong acid is one that completely ionizes (dissociates) in water; in other words, one mole of a strong acid HA dissolves in water yielding one mole of H+ and one mole of the conjugate base, A−.
##### Appears in these related concepts:
strong base
A strong base is a basic chemical compound that is able to deprotonate very weak acids in an acid-base reaction. Common examples of strong bases are the hydroxides of alkali metals and alkaline earth metals, such as NaOH and Ca(OH)2. Very strong bases are even able to deprotonate very weakly acidic C–H groups in the absence of water.
##### Appears in these related concepts:
substance
Physical matter; material.
##### Appears in these related concepts:
valence
the combining capacity of an atom, radical or functional group determined by the number of electrons that it will lose, gain, or share when it combines with other atoms, etc.
##### Appears in these related concepts:
valence electron
Any of the electrons in the outermost shell of an atom; capable of forming bonds with other atoms.
##### Appears in these related concepts:
valence electrons
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are their "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds.
##### Appears in these related concepts:
weak acid
one that dissociates incompletely, releasing only some of its hydrogen atoms into solution