Concepts and Types of Acid and Base

Edited by Jen Moreau

Acid and Base Concepts

Acid

Word acid is derived from Latin word acidus which means sour.we use acids and bases in our daily routine.We use orange and lemon and they both have sour taste due to the presence of citrus acid in them.

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Types of acids

There are two types of acids Monoprotic acid and polyprotic acids.

Monoprotic Acid

The acid which can donate one proton is called monoprotic acid.

Examples:

HCl, HBr and HCN are monoprotic acids.

Polyprotic Acids

Those acids which can donate two or more than two protons in an aqueous solution is called polyprotic acid.

Examples:

H2SO4 etc

Amphoteric substance

That substance which can both like acid and like the base is called Amphoteric substance. When we treat amphoteric substance with an acid it starts behaving like base while if we treat an amphoteric substance with a base it behaves like an acid.

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Examples:

Water is amphoteric because it can act as an acid and base. Water is the most common example, acting as a base when reacting with an acid such as hydrogen chloride:

H2O + HCl H3O+ + Cl�', and acting as an acid when reacting with a base such as ammonia:

H2O + NH3 NH4+ + OH�'

Strong acid and weak acid

Acid which ionizes completely in water and gives maximum number of H+ ion in aqueous the solution is known as strong acid.

Examples:

HCl, HNO3 are strong acids

Ionization of HCl is 92%

Ionization of HNO3 is 84%

HCl +H2O. = (H+) + (cl-)

It has been proved that in gaseous state HCl ionization is 100%. Weak acid is that substance which ionizes partially in an aqueous solution and gives lower a number of H+ ion in an aqueous solution.

Examples;

CH3COOH and H2CO3

CH3COOH ionization is 13%

H3PO4 ionization is 28%

Strong base and weak base

A base which ionizes completely or up to a large extent is called strong base.Strong base gives maximum number of OH- ions in aqueous solution.

Examples:

NaOH and KOH are strong bases.

A base which ionizes partially in an aqueous solution is called a weak base. Weak base leaves a lower number of OH- ions in an aqueous solution.

Examples:

NH4OH (aq) ---> NH4+ (aq) + OH- (aq)

Ammonium hydroxide is a weak base as it leaves a lower number of OH- ions.


There are several concepts about acids and bases like

  • Arrhenius concept
  • Bronsted-Lowery concept
  • Lewis concept

Arrhenius concept

A scientist Arrhenius explained the phenomena of acids and bases as follows

Arrhenius Acid

Acid is that species which leaves H+ ion in an aqueous solution and increases the concentration of H+ ion in aqueous solution. 1 HCl(aq)[]H+(aq)+CI-(aq) HCl is an acid as it increased the concentration of H+ ion in aqueous solution.

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Arrhenius Base

Base is that species which leaves OH- ion in an aqueous solution and increases the concentration of OH- ion in an aqueous solution. NaoH∆ (Na+). + (OH-) NaOH is a base as it increased the concentration of OH- ion in an aqueous solution.

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Bronsted-Lowery concept

According to Bronsted and Lowery concept

Acid is that substance which donates a proton in aqueous solution while if we treat water with the base. While Base is that substance which accepts a proton and requires a lone pair to make a bond with H+.

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We can use Bronsted-Lowery in gaseous state NH3+HCl=NH4Cl In this reaction, NH3 acts like base as it accepts the proton while Hcl acts like an acid as it donates a proton.

Lewis Acid-Base concept

According to Lewis those species which can accept electron pair is called Acid and those species which can donate electron pair is called Base.

Examples:

Al3+(aq) + 6 H2O(l) <-----> Al(H2O)63+(aq)

Al3+(aq) + 6 H2O(l) <-----> Al(H2O)63+(aq)

In the above example, H3N donated electron pair and acted like a base while H+ accepted electron pair and acted like an acid.

Conjugate Acid &Base

Conjugate Acid

When a a e. which can donate the proton and acts like an acid.This species is called Conjugate Acid.

Examples:

NH3(g)+H2O(l) NH+4(aq)+OH�'(aq)

NH3(g)+H2O(l) NH4+(aq)+OH�'(aq)

We say that NH4+ is the conjugate acid to the base NH3 because NH3 gained a hydrogen ion to form NH4+, the conjugate acid. The conjugate base of an acid is formed when the acid donates a proton. In the equation, OH- is the conjugate base to the acid H2O because H2O donates a hydrogen ion to form OH-, the conjugate base. Note: The stronger the acid or base, the weaker the conjugate. The weaker the acid or base, the stronger the conjugate.

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Conjugate Base

When an acid donates a proton it forms a negatively charged species which can accept proton and acts like a base this base is called conjugate Base.

HNO3 + H2O H3O+ + NO3-.

HNO3 is an acid because it donates a proton to water and its conjugate base is NO3-.An easy way to identify the conjugate base is that it differs from the acid by one proton.

H2O is a base because it accepts a proton from HNO3 and its conjugate acid is H3O+. Again, to identify the conjugate acid (or any conjugate pair) is to see that it differs from the base by one proton.

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PH and POH scales

PH scale is a logarithmic and or orbitary scale used to measure the strength of an acid and base. PH scale was devised by a Danish Biochemist Sorenson in 1909.

PH scale or Hydrogen ion concentration

PH means power of hydrogen ion PH means the potential of hydrogen ions.

The acidity of an aqueous solution depends upon the number of [H+] ions present in it.

PH=-log[H+]

Examples:

pH of water is 1/10 mol/litre

POH scale

The basicity of an aqueous solution depends upon the number of [OH-] present in it.

POH=-log[OH-] Examples:

POH of water is 1/10 mol/litre

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