Acids and bases are perhaps among the first names that you hear while entering into the chemistry world. Right from the early years of our life when we were little children we have heard about chemicals and chemical reactions. We know that two chemicals mix to form something you regularly use. Lemons, tamarind, soaps, baking soda and what not, we make use of acids and bases so much in our everyday life.
Let’s have a deeper understanding of the chapter acids and bases through this article.
What are Acids and Bases?
One of the old definitions of acids is that they are sour and change the color of blue litmus to red. Whereas alkali, which is a more general term used for bases, are those which are bitter, slippery to touch, and change the color of the red litmus to blue. But every time it is not possible to taste and categorise chemicals and the change of colour of litmus serves as just a method to identify acidic or alkaline nature. So here is a detailed definition that explains the characteristics, properties and behavior of acids and alkalies.
An acid or alkali reacts with each other to form water and salt. A common behavior of acids and alkalies are that they liberate gas on reaction with metals. There are many theories used to characterise acids and alkalies, but the most accepted ones among them are:
- Arrhenius concept of acids and bases
- Bronsted-Lowry concept
- Lewis acids and bases
The Arrhenius Concept
Arrhenius acids are those acids that 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)
One of the common mistakes in our understanding is that a substance is acidic if it has H atoms in its formulae and basic if it has OH. However, one must refrain from such a generalization, because there are compounds like NH3 and other amines which are basic in nature, and compounds like alcohols which are weakly acidic despite having an OH group.
Bronsted Lowry Concept
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)
Acidic nature of Organic Compounds: The acceptance of H+ ion in organic compounds is owed to effects of other factors like lone pairs, electron cloud shift, the presence of functional groups, etc. Hence, the acidic and basic behavior is not so obvious from the presence of H or OH. The compounds can be termed acidic or basic only by their behavior as explained by the Bronsted Lowry concept.
Lewis Theory of acids and bases
Lewis concept is also very similar to the Bronsted-Lowry concept. The only difference in them is the fact that the transfer of lone-pair of electrons is taken as the criteria here. 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 ranges from 1 to 14. The species of pH neutral species is taken as 7. Below 7, they are acidic and above 7 are considered basic.
Acid-base systems are of great significance especially in the chemical and biological systems. There are several methods of defining acids and bases. These definitions do not contradict each other but they do vary in how inclusive they are. The most common definition of acids and bases are Arrhenius acids and bases, Bronsted Lowry acids and bases, and Lewis acids and bases.
Properties of Acids and Bases
Robert Boyle described the qualities of acids and bases in the year 1661. The characteristics given by him may be used to easily distinguish between the two sets up chemicals 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
The strength of acids and bases depends on their ability to dissociate or break into their ions in water. A strong acid or strong base completely dissociate, while a weak acid or weak base only partially dissociates.
The acid dissociation constant and base dissociation constant indicates the relative strength of an acid or base.
HA+H2O = A-+H3O+, Where HA is the acid and A- is the conjugate base
pka = -log10Ka
The larger the pKa value, the smaller the dissociation of the acid and the weaker the acid. Strong acids have a pKa of less than -2.
Some Quick Facts about Acids and Bases
Any aqueous 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 weak acid or base. How corrosive an acid or base is does not relate to its strength.
The pH scale is a measure of the acidity or basicity or a solution. The scale runs from 0 to 14, with acids having a pH less than 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. This reaction produces salt and water and leaves the solution closer to a neutral pH than before.
One common test of knowing whether a substance is acid or bases is to wet a 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. Whereas, a neutral chemical won’t change the paper’s colour.
Acids and bases react with metals. Acids release hydrogen gas when reacted with metals. At times, hydrogen gas is released when a base reacts with a metal, such as reacting sodium hydroxide and zinc. Another typical reaction between a base and a metal is a double displacement reaction which may produce a precipitate metal hydroxide.
Comparison between Acids and Bases
Characteristics | Acids | Bases |
Reactivity | Accept electron pairs or donate hydrogen ions or protons | Donate electron pairs or donate hydroxide ions or electrons |
pH | Less than 7 | Greater than 7 |
Taste | Sour | Soapy or bitter |
Corrosivity | May be corrosive | May be corrosive |
Touch | Astringent | slippery |
Litmus Paper | Red | Blue |
Conductivity in Solution | Conduct electricity | Conduct Electricity |
Common Examples | Vinegar, lemon juice, sulphuric acid, hydrochloric acid, nitric acid | Bleach, soap, ammonia, sodium hydroxide, detergent |
Conclusion
Gone through the complete article? Then we at Aakash are sure that the topic of Acids and Bases will not bother you anymore. All you need to do is practice all the questions and formulas of Acids & Bases and you will become a master. Furthermore, join our IIT JEE Classroom program and start scoring well. All the best.
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