Dissociation vs. Ionization

By: Sheikh Amir Hamza

Subject: Pharmaceutical Chemistry

Roll Number: 256FBAS/BSBT/F12

DissociationIn 1887, Svante August Arrhenius proposed that when a substance dissolves in water, particles of the substance separate from each other and disperse into the solution (called dissociation).

Non-electrolytes disperse electrically neutral particles (therefore don’t conduct electricity)

Electrolytes disperse electrically charged particles called ions (therefore do conduct electricity)

Why do ionic compounds disperse electrically charged particles?

RESULT: The ions at the edges of the ionic crystal are tugged and dislodged until each one is surrounded by water molecules and is floating around as an independent entity

DISSOCIATION – the separation of ions that occurs when an ionic compound dissolves in water

The negative ions are surrounded by the positive end of the polar water molecules.

The positive ions are surrounded by the negative end of the polar water molecules.

Dissociation Equations

There are two requirements of dissociation:

it must be soluble in water and must be ionic

Dissociation Equations:

shows the separation of ions in a chemical equation

H20(l) is not shown because it is a solvent and is NOT consumed nor changed.

It is shown in the aqueous states for the products (aq means “dissolved in water”)

Examples: (you still need to make sure the equations are balanced)

NaCl(s) Na + (aq) + Cl –

(aq) K3PO4(s) 3 K +(aq) +

PO43-

(aq)

Arrhenius acids and bases

Arrhenius extended his dissociation theory to include acids and bases

Bases – ionic compounds containing the hydroxide ion (OH-) that dissolve into cations and OH–

(aq) in water

Ba(OH)2 (s) Ba 2+ (aq) + 2 OH-

(aq)

NOTE: All bases are ionic, so all bases could dissociate if soluble in H2O

Acids – molecular compounds containing hydrogen that yield H+

(aq) ions when they dissolve in water

- Acids behave as molecular substances; CH3COOH (l) – does not conduct electricity or change the color of litmus paper

- As soon as acids are dissolved in water, their acid properties appear; CH3COOH (aq) will conduct electricity and changes litmus paper from blue to red

NOTE: Acids are not ionic, so they DO NOT dissociate. They IONIZE.

Acids Ionize??

Arrhenius came up with the idea of ionization to explain why acids are electrolytic:

IONIZATION – the reaction of neutral molecular compounds forming charged ions

The non-electrolytic molecular compound separates into ions when dissolved in water, becoming electrolytic

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

(aq)

Dissociation vs. Ionization

What is the difference between dissociation and ionization?

Both produce (aq) ions...

Dissociation, however, is the separation of ions that already exist before dissolving in water

M+X-(s) M+

(aq) + X-(aq)

Ionization involves the production of new ions, specifically hydrogen ions

HX0(aq) H+

(aq) + X-(aq)

SUMMARY

Substance Process General Equation

Molecular Disperse as individual, neutral molecules

XY (s/l/g) XY (aq)

Ionic Dissociate into individual ions

MX (s) M+ (aq) + X-

(aq)

Base (ionic hydroxide)

Dissociate into positive ions and hydroxide ions

MOH(s) M+ (aq) +

OH-(aq)

Acid Ionize to form hydrogen ions and anions

HX (s/l/g) H+ (aq) + X-

(aq)