Overview of Acids and Bases - Chemistry Chapter

What are acids and bases?

Normally it is said that acids are sour in taste and change the colour of blue litmus to red. Whereas alkali, which is a more general term used for bases are bitter, and change the colour of the red litmus to blue. But here is a detailed definition that explains the characteristics, properties and behaviour of acids and alkalies. There are many theories used to characterise acids and alkalies, but the most accepted ones among them are:

1. Arrhenius concept of acids and bases
2. Bronsted-Lowry concept
3. Lewis acids and bases

1. The Arrhenius Concept

- The Arrhenius theory of acids and bases was originally proposed by the Swedish chemist Svante Arrhenius in 1884. He suggested classifying certain compounds as acids or bases based on what kind of ions formed when the compound was added to water.

Arrhenius acids are those acids which increase the concentration of H+ ions in their aqueous state. A substance qualifies as an acid if it ionises in water to furnish H+ ions, thereby increasing the concentration of H+ in the solution.

HCL(aq)  = H+(aq) + Cl – (aq)

In the same way, an alkali ionises to furnish OH-ions in the aqueous solution, thereby increasing the concentration of OH- in the solution.

NaOH(aq)  = Na + (aq) + OH – (aq)

2. Bronsted Lowry Concept

In the early 1900s, an alternate definition for acids and bases was proposed by Johannes Bronsted and Thomas Lowry to account for the fact that ammonia can neutralize the acidity of HCl even if water isn't present.

This concept defines acids or bases on the basis of H+ transfer. According to this theory, acids are those capable of donating an H+ ion and bases are those which are capable of accepting an H+ ion.

NH3(g) + HCL(g)                Nh4 + Cl –(S)

Here HCL donates its H+ ion to NH3. Hence, NH3 acts as a base and HCL acts as an acid. This is a neutralisation reaction between an acid and base to form ammonium chloride salt.

Some significant facts:
Bronsted Lowry concept forms a more general definition than Arrhenius concept to include a broader spectrum of compounds under the name acidic or basic, especially organic compounds.

Amphoteric Substances: These are substances which act as both acids as well as bases according to the situation. Water is a good example for this.

As base: HNO3 (aq) + H2O(1)                  H3O+(aq) + NO3 – (aq)

As acid: NH3 (aq) + H20 (1)                      NH 4 + (aq) + OH – (aq)

In this reaction, nitric acid behaves as an acid because it gives a proton to ammonia. Ammonia behaves as a base because it accepts the proton from nitric acid.

3. Lewis Theory of acids and bases

It is also very similar to the Bronsted-Lowry concept. The only difference is that the transfer of lone-pair of electrons is taken as the criteria here. The Lewis theory of acids and bases states that acids act as electron pair acceptors and bases act as electron pair donors. This definition doesn't mention anything about the hydrogen atom at all, unlike the other definitions. It only talks about the transfer of electron pairs.

The Lewis acids are those which can accept a lone pair of electrons. Compounds like BF3 and AICI3 are Lewis acids and acts as catalysts in many organic reactions. Lewis bases are those which can donate a lone pair of electrons. Ammonia and its derivatives are common examples for Lewis bases.

* PH scale

The PH Scale stands for Hydrogen Potenz scale and is a quantitative method to compare the relative acidic strength of species. It is the negative logarithm of H + ion concentration.

Ph = – log10 (H+)

The pH scale is often said to range from 0 to 14, and most solutions do fall within this range, although it’s possible to get a pH below 0 or above 14. Anything below 7.0 is acidic, and anything above 7.0 is alkaline, or basic.

The pH inside human cells (6.8) and the pH of blood (7.4) are both very close to neutral. Extreme pH values, either above or below 7.0, are usually considered unfavourable for life.

* Properties of Acids and Bases

Characteristics of acid and bases given by Robert Boyle in the year 1661 may be used to easily distinguish between these two without performing complicated tests.

* Acids
- Taste sour
- Acids are corrosive
- It changes litmus from blue to red
- Acids are electrolytes
- They react with bases to form salts and water
- They evolve hydrogen gas upon reaction with an active metal such as alkali metals, alkaline earth metals, zinc, and aluminium
* Bases
- Tastes bitter
- Feel slippery or soapy
- They don’t change the colour of the litmus, they can turn red (acidified) litmus back to blue
- Acids are electrolytes
- React with acids to form salts and water

* Examples of Common acids

- Citric acid
- Ascorbic acid
- Vinegar
- Carbonic acid
- Lactic acid
- Examples of common bases

* Detergents
- Soaps
- Lye (NaOH)
- Household ammonia
- Strong and Weak acids and bases

Some Quick Facts about Acids and Bases

- Any aqueous (water-based) liquid can be classified as an acid, base, or neutral. Oils and other non-aqueous liquids are not acids or bases.
- There are different definitions of acids and bases, but acids can accept an electron pair or donate a hydrogen ion or a proton in a chemical reaction, while bases can donate an electron pair or accept hydrogen or a proton.

- Acids and bases are characterized as strong or weak. A strong acid or strong base completely dissociates into its ions in water. If the compound does not completely dissociate, it's a weak acid or base. How corrosive an acid or a base is does not relate to its strength.

- The pH scale is a measure of the acidity or alkalinity (basicity) or a solution. The scale runs from 0 to 14, with acids having a pH less than 7, 7 being neutral, and bases having a pH higher than 7.

- Acids and bases react with each other in what is called a neutralization reaction. The reaction produces salt and water and leaves the solution closer to a neutral pH than before.

- One common test of whether an unknown is an acid or a base is to wet litmus paper with it. Litmus paper is a paper treated with an extract from a certain lichen that changes colour according to pH. Acids turn litmus paper red, while bases turn litmus paper blue. A neutral chemical won't change the paper's colour.

- Because they separate into ions in water, both acids and bases conduct electricity.

- Acids and bases are important in the human body. For example, the stomach secretes hydrochloric acid, HCl, to digest food. The pancreas secretes a fluid rich in the base bicarbonate to neutralize stomach acid before it reaches the small intestine.

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