Acids and Bases

By: Tana Smithsakol

Introduction


People discovered vinegar, lemon juice and many other foods that taste sour over thousands of years ago. However it was only until a few hundred years ago when people discovered why these foods taste sour. The term acid came from acere, in Latin meaning "sour".
In the seventeenth century, an amateur Irish chemist Robert Boyle classified different substances as either acids or bases, he called bases alkalies. Boyle labeled these substances according to these characteristics:
  • Acids taste sour, are corrosive to metals, change litmus (a dye extracted from lichens) red, and become less acidic when mixed with bases.
  • Bases feel slippery, change litmus blue, and become less basic when mixed with acids.

In the late 18th century, Svante Arrhenius, a Swedish scientist proposed that water can dissolve many compounds by separating them into their individual ions. Arrhenius proposed that acids are compounds containing hydrogen that dissolves in water and donates one or more hydrogen ions (H+) into the solution. Acids are known as proton donors.
An example of the dissociation of hydrochloric acid (HCl) in water:

HCl(g) -> H+(aq) + Cl-(aq)


Arrhenius proposed that bases are substances that dissolves in water and release hydroxide ions (OH-) into the solution. Bases are also known as proton acceptors.
An example of the dissociation of sodium hydroxide (NaOH) in water:

NaOH -> Na+(aq) + OH-(aq)


The Arrhenius definition supports Boyle's observation that acids are bases are the opposites of each other and they counteract each other. The idea that acids can make bases weaker and vice versa, led to a new term called neutralization.

Neutralization

Acid and Base Neutralization

Acids releases H+ ions and bases releases OH- ions into the solution. If acids and bases were mixed together, the H+ and OH- ions would combine to form H2O, or water.

H+(aq) + OH-(aq) -> H2O

Neutralization of acid and base will always produce water and salt, shown below:
*A salt is any ionic compound that contains any combination between the alkaline metals and halogens.

Acids
+
Bases
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Water
+
Salt
HCl
+
NaOH
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H2O
+
NaCl
HBr
+
KOH
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H2O
+
KBr

In 1923, Danish scientist Johnnnes Bronsted and Englishman Thomas Lowry refined Arrhenius's theory. Under the Bronsted definition of bases is any substance that can accept a hydrogen ion, and acids are referred to as proton donors because H+ ions are simply protons. And therefore, acids and bases are the opposites. The Bronsted-Lowry definition also explains why some substances that do not contain OH- ions can act like bases which accepts a proton.
For example, NaHCO3 or baking soda acts like a base by accepting a proton from an acid.


Acid
+
Base
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Acid
+
Salt
HCl
+
NaHCO3
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H2CO3
+
NaCl

In this example, the carbonic acid (H2CO3) formed undergoes rapid decomposition into water and gaseous carbon dioxide (CO3 + H2 -> CO2 + H2O), and CO2 gas is released.

Ka and Kb Constants


Ka is a constant that has specific values for different acids. It is the measurement that gives values that indicates the strength of certain acids, for a strong acid, it ionizes completely in water. The stronger the acid, the higher the Ka values.

A “strong” acid is an acid that completely ionizes.
E.g. HCl + H2O → H3O + Cl

A “weak” acid is an acid that doesn’t ionize completely.
E.g. HF + H2O ↔ H3O + F

Example, HA(aq) + H2O(l) ↔ H3O(aq) + A–(aq)
Ka = [H3O+][A–] / [HA]


Kb is a constant similar to Ka, Kb is a value for bases. The higher the value of Kb, the strong the base.
Kb is similar to Ka except b stands for base

The general reaction involving a base can be
written as B(aq) + H2O ↔ BH(aq) + OH–(aq)
Thus Kb = [BH+] [OH–] / [B]

Kw and pH Constants


Pure water ionizes very slightly, H2O(l) -> H+(aq) + OH-(aq), this self-ionization of water is an endothermic process. It can be explained using the le Chatlier's principle. The ionization equilibrium is affected by the temperature, if temperature is increased, the rate of ionization of water increases.

Defining Kw
  • Kw is the ionic product of water.
  • Kw = [H+(aq)] [OH-(aq)]
  • At 25 degrees Celsius, Kw is equal to 1.0 x 10^-14 mol/L.
  • Kw is always constant at a constant temperature regardless of the value of pH.

Defining pH
pH is the abbreviation for partial Hydrogen. Defining pH under the Bronsted-Lowry definition, the pH of the acids and bases are related to the concentration of hydrogen ions present. Acids increase the concentration of hydrogen ions, while bases decreases the concentration of hydrogen ions. The acidity of a solution can be measured from its hydrogen ion concentration.
pH is logarithmic, meaning that 1 unit change in pH is a change factor of 10 in the concentration of proton ions, pH is calculated by:
pH = -log[H+(aq)]
The pH scale
ph_scale.JPG

pKw and pOH Constants

Defining pOH
Certain concentration of hydrogen and hydroxide ions has to be present in a solution for it to be acidic or basic. Similar to pH, pOH is the measurement of concentration of OH- ions in a solution. Because all solutions considered acidic or basic contains these ions, the alkalinity of the solution can be measured in pOH.
pOH similar to the pH, is logarithmic, a 1 unit increase in the scale means a change in the factor of 10 in the concentration of OH- ions in the solution, pOH is calculated by:
pOH = -log [OH-(aq)]


Defining pKw

pKw is the sum of the values of pH and pOH of a solution.
pkw can be calculated by: pKw = pH + pOH = 14

*Adding the values of pH and pOH together always yeild a product of 14. For example, if the pH of a solution is 2, the pOH of the solution is 12. Therefore, pH values of 7 is neutral, because pH is 7, pOH must be 14-7=7.