Operational Definition
If we recall, operational definitions are based on what we can actually observe in the laboratory.
Acids
- Corrosive ('burns' your skin)
- Sour taste (e.g. lemons, vinegar)
- Contains hydrogen ions (H+) when dissolved in water
- Has a pH less than 7
- Turns blue litmus paper to a red color
- Reacts with bases to form salt and water
- Reacts with metals to form hydrogen gas
- Reacts with carbonates to form carbon dioxide, water and a salt
Examples
- Hydrochloric acid (HCl) in gastric juice
- Sulphuric acid (H2SO4)
- Nitric acid (HNO3)
- Carbonic acid in softdrink (H2CO3)
- Uric acid in urine
- Ascorbic acid (Vitamin C) in fruit
- Citric acid in oranges and lemons
- Acetic acid in vinegar
- Tannic acid (in tea and wine)
- Tartaric acid (in grapes)
Bases
- Corrosive ('burns' your skin)
- Soapy feel
- Has a pH more than 7
- Turns red litmus paper to a blue color
- Many alkalis (soluble bases) contain hydroxyl ions (OH-)
- Reacts with acids to form salt and water
Examples:
- Sodium hydroxide (NaOH) or caustic soda
- Calcium hydroxide ( Ca(OH)2 ) or limewater
- Ammonium hydroxide (NH4OH) or ammonia water
- Magnesium hydroxide ( Mg(OH)2 ) or milk of magnesia
- Many bleaches, soaps, toothpastes and cleaning agents
Conceptual Definition
It gives the meaning of a word in terms of the theories of a specific discipline.
There are three major conceptual definitions of acids and bases. Let’s site first the other definitions given by different scientists.
Robert Boyle
Characterized acids and alkalies (bases) as the following:
Acids: · Sour taste · Corrosive · Change litmus (dye extracted from lichens) from blue to red · Become less acidic when combined with alkalies. Alkalies (Bases): · Feel slippery · Change litmus from red to blue · Become less alkaline when combined with acids. |
Antoine Lavoisier
· Believed that all acids contained oxygen after studying several acids e.g. H2SO4 - sulfuric acid, HNO3 - nitric acid |
Humphry Davy |
· Questioned Lavoisier's theory, noting that hydrochloric acid (HCl) did not contain oxygen yet is an acid. · Soon thereafter, several more acids without oxygen were found. e.g. HBr, HF, HI |
Justig Liebig
· Suggested that acids contain one or more hydrogen atoms which can be replaced by metal atoms to produce salts. e.g. HSCN is an acid because the H atom can be replace by a metal to form a salt, such as NaSCN. |
The three major conceptual definitions of acids and bases are as follows:
Svante Arrhenius
· In 1884, Arrhenius proposed that salts dissociate when they dissolve in water to give charged particles which he called ions. · In 1887, Arrhenius extended this his idea by defining acids and bases as the following: · Arrhenius acid - Any substance that ionizes when it dissolves in water to give the H+ ion. e.g.  · Arrhenius base - Any substance that ionizes when it dissolves in water to give the OH- ion. e.g.  |
Arrhenius's theory helped to explain why acids have similar characteristics, since they all give H+ ions when they dissolve in water. It also explained why acids are neutralized by bases and why bases are neutralized by acids; the H+ ions from acids combine with the OH- ions from bases to form water:
Though the Arrhenius theory helped to explain more about acids and bases, there were still several drawbacks to this theory.
· The theory can only classify substances when they are dissolved in water since the definitions are based upon the dissociation of compounds in water.
· It does not explain why some compounds containing hydrogen such as HCl dissolve in water to give acidic solutions and why others such as CH4 do not.
· The theory can only classify substances as bases if they contain the OH- ion and cannot explain why some compounds that don't contain the OH- such as Na2CO3 have base-like characteristics.
To extend the Arrhenius theory a little further, consider the formation of water from the combination of an H+ ion and an OH- ion:
To extend the Arrhenius theory a little further, consider the formation of water from the combination of an H+ ion and an OH- ion:
This reaction is actually reversible, represented by the forward/backward arrow in the following reaction:
Based on the fact that the above reaction is reversible, we can conclude the following operational definitions for acids and bases.
· Acid - Any substance that increases the concentration of the H+ ion when it dissolves in water.
· Base - Any substance that increase the concentration of the OH- ion when it dissolves in water.
NOTE: A common shorthand notation for the concentration of a substance is placing it in brackets:
NOTE: A common shorthand notation for the concentration of a substance is placing it in brackets:
[H+] = concentration of the H+ ion
Now, substances not containing H+ or OH- ions can be classified as acids or bases if they alter the [H+] or [OH-] when they dissolve in water.
e.g. CaO cannot dissociate to give OH- but it does increase [OH-] when it dissolves in water.
Johannes Brønsted and Thomas Lowry
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| Johannes Bronsted |
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| Thomas Lowry |
In 1923, Johannes Brønsted and Thomas Lowry separately proposed a new set of definitions for acids and bases which are known as either Brønsted acids and bases or Brønsted-Lowry acids and bases. · Brønsted Acid - Any substance that can donate a proton, H+ ion to a base. aka: hydrogen-ion donors or proton donors · Brønsted Base - Any substance that can accept a proton, H+ ion from an acid. aka: hydrogen-ion acceptor or proton acceptor  In the above reaction, the H from HCl is donated to H2O which accepts the H to form H3O+, leaving a Cl- ion. The dissociation of water can be represented as follows:  | |
The Brønsted-Lowry model of acids and bases brings rise to the concept of conjugate acid-base pairs. The part of the acid remaining when an acid donates a H+ ion is called the conjugate base. The acid formed when a base accepts a H+ ion is called the conjugate acid. For the generic acid HA:
For the generic base A-:
More examples of conjugate acid-base pairs:
In the following reactions, it is shown how H2PO4- and H2O can act as both acids and bases. Such compounds are said to be amphoteric.
· Strong acids have weak conjugate bases.
· Strong bases have weak conjugate acids.
Advantages to the Brønsted-Lowry model of acids and bases
· Acids and bases can now be ions or neutral molecules.
· Acids and bases can now be any molecule with at least one pair of non-bonding electrons.
· It explains the role of water in acid-base reactions; H2O accepts H+ ions from acids to form H3O+ ions.
· It can be applied to solutions with solvents other than water and even in reactions that occur in the gas or solid phases.
· It relates acids and bases to each other with conjugate acid-base pairs and can explain their relative strengths.
· It can explain the relative strengths of pairs of acids or pairs of bases.
· It can explain the leveling effect of water.
1923 - G.N. Lewis
Proposed another method of defining acids and bases. · Lewis acid - Any substance that can accept a pair of non-bonding electrons. aka: electron-pair acceptor · Lewis base - Any substance that can donate a pair of non-bonding electrons. aka: electron-pair donor |
In the following example, the Al3+ ion acts as an acid, accepting electron pairs from water which acts as a base, an electron-pair donor. The two combine to form Al(H2O)63+, an acid-base complex or a complex ion.
Here are some quizzes for you to try out!
