In the previous section, we saw the factors which affect equilibrium. In this section, we will see ionic equilibrium
• We have seen a large number of chemical equations showing reversible reactions
For example: H2(g) + I2(g) ⇌ 2HI(g)
• In some cases, 'ions' will be present in the place of 'reactants' or 'products'
For example: Fe3+(aq) + SCN-(aq) ⇌ [Fe(SCN)]2+(aq)
• This equation indicates that:
Fe3+, SCN- and [Fe(SCN)]2+ are present in an aqueous solution
♦ Fe3+ and SCN- continuously combine together to form [Fe(SCN)]2+ ion
✰ This is the forward reaction
♦ [Fe(SCN)]2+ continuously dissociates into Fe3+ and SCN-
✰ This is the backward reaction
• At equilibrium, the rate of forward and backward reactions become equal
• Before learning about ionic equilibrium, we must first learn about electrolytes. It can be written in 8 steps:
1. Pure water does not conduct electricity. But we do not have pure water in nature
• So all water that we see in our day to day life, will conduct electricity
2. Two opposite cases:
(i) If we dissolve sugar into pure water, that sugar solution will not conduct electricity
(ii) If we dissolve common salt (NaCl) in pure water, that salt solution will conduct electricity
3. The English scientist Michael Faraday did extensive research and discovered the reason for such opposing behaviour
• The following steps from (4) to (8) help us to get a clear idea about his findings
(4) Michael Faraday classified all substances into two categories
(i) The first category of substances are able to conduct electricity when dissolved in water
♦ These substances were called electrolytes
(ii) The other category are not able to conduct electricity when dissolved in water
♦ These substances were called non-electrolytes
5. Electrolytes were further classified into strong electrolytes and weak electrolytes
This is shown in fig.7.7 below:
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| Fig.7.7 |
6. When strong electrolytes are dissolved in water, they are ionized completely
• Let us see an example:
• When some NaCl is dissolved in water, all the NaCl molecules separates into Na+ and Cl- ions
♦ We will not find any NaCl molecule in the solution
7. When weak electrolytes are dissolved in water, they are not ionized completely
• Let us see an example:
• When some CH3COOH (acetic acid) is dissolved in water, all the CH3COOH molecules do not separate into CH3COO- and H+ ions
♦ The solution will contain three items:
✰ CH3COOH molecules
✰ CH3COO- ions
✰ H+ ions
8. Extent of ionization:
♦ Almost 100 % of the added NaCl will separate into ions
♦ Only 5% of the added CH3COOH will separate into ions
Now we can define ionic equilibrium. It can be written in 3 steps:
1. In the case of weak electrolytes, molecules as well as ions will be present in the solution
2. An equilibrium will be established between those ions and molecules
For example: CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq)
3. This type of equilibrium involving ions in aqueous solution is called ionic equilibrium
• Note the double arrow in the above step 2
♦ It indicates that, both forward and backward reactions occur simultaneously
✰ The acetic acid molecules continuously separate into ions
✰ The ions continuously combine to give back the acetic acid molecules
♦ When the rates of both the reactions are equal, equilibrium is attained
• There is no need for a double arrow in the case of NaCl
♦ Because, the Na+and Cl- ions do not combine to give back NaCl molecules
♦ We can write: NaCl(s) → Na+(aq) + Cl-(aq)
• Let us recall some simple facts about acids, bases and salts that we saw in our earlier classes:
Acids
♦ Acids turn blue litmus paper into red
♦ Acids liberate H2 when they react with metals
♦ Acids have a sour taste
♦ Examples of acids: HCl (Hydrochloric acid), CH3COOH (acetic acid)
Bases
♦ Bases turn red litmus paper into blue
♦ Bases feel soapy
♦ Bases have a bitter taste
♦ Bases that dissolve in water are called alkalies
♦ Examples of bases: NaOH (Sodium hydroxide), KOH (Potassium hydroxide)
Salts
♦ Salts are formed when acids and bases are mixed together in right proportions
♦ Example: HCl + NaOH → NaCl + H2O
• NaCl (sodium chloride) is common salt
• Some basics about NaCl solution can be written in 3 steps:
1. In the solid state, NaCl exists as Na+ and Cl- ions
♦ But those ions are not separated from each other
♦ They are held closely together by the electrostatic forces of attraction
2. When NaCl is dissolved in water, NaCl separates into Na+ and Cl- ions
• But why don't they attract and become NaCl again?
• The answer is that, water has a special property (called high dielectric constant), which can reduce the force of attraction between Na+ and Cl-
• We will see details about dielectric constant in higher classes
3. So the Na+ and Cl- are separated from each other in the solution
• Now recall what we learnt in enthalpy of solution (Fig.6.15 in section 6.10 of the previous chapter)
• We saw that:
♦ Each Na+ will be surrounded by H2O molecules
✰ Thus we obtain Na+(aq)
♦ Each Cl- will also be surrounded by H2O molecules
✰ Thus we obtain Cl-(aq)
• Now we can compare NaCl and CH3COOH. The comparison can be written 6 in steps:
1. The original samples:
• In the original sample of NaCl, there are separate Na+ and Cl- ions (held together by strong electrosattic forces)
• In the original sample of CH3COOH, there are no separate ions. Only CH3COOH molecules
2. Separation into ions
• When NaCl is added to water, all NaCl molecules get completely separated into Na+ and Cl- ions
• When CH3COOH is added to water, only a small number of molecules get separated into CH3COO- and H+ ions
3. Recombination
• In the case of NaCl, the Na+ and Cl- ions cannot combine again to give back NaCl because, water has a high dielectric constant
• In the case of CH3COOH, inspite of the high dielectric constant of water, some CH3COO- and H+ ions can combine again to give back CH3COOH. We will see the reason in later sections
4. Hydration by water
• In the case of NaCl, the Na+ and Cl- ions get hydrated by water. Thus they become Na+(aq) and Cl-(aq)
• In the case of CH3COOH also, the CH3COO- and H+ ions get hydrated by water. Thus they become CH3COO-(aq) and H+(aq)
5. Dissociation and Ionization
• If the original sample exists as separate ions (as in the case of NaCl), the process of 'separation into ions' is called dissociation
• If the original sample exists as separate molecules (as in the case of CH3COOH), the process of 'separation into ions' is called ionization
◼ In this present chapter, both the words 'dissociation' and 'ionization' will mean the same thing: The separation of original molecules into ions
6. So now a question arises:
◼ NaCl undergoes complete ionization when dissolved in water. But CH3COOH undergoes only partial ionization. Why is there such a difference?
• The answer can be given based on bond strength. We will see the details in later chapters
• At present, all we need to know is this:
When partial ionization takes place, an ionic equilibrium will be established between the ions and the original molecules
Arrhenius concept of Acids and Bases
• First we will see how the Arrhenius concept is applied to acids. It can be written in 5 steps:
1. According to Arrhenius theory, acids can be represented as HX
• This is because, H (hydrogen atom) will be present in all acids
2. So the ionization of an acid can be represented as:
HX(aq) → H+(aq) + X-(aq)
3. We know that, the H atom has only one electron and one proton
• So H+ ion will be having only one proton
• That means, an H+ ion consists of only the nucleus with the one proton
• We know that, nucleus is very small when compared to the total size of the atom
♦ So the H+ ion will be very small in size
• The +ve charge will be concentrated in a very small space
♦ So the charge density is high
4. This makes the H+ ion very reactive
• It attaches itself to the lone pairs of electrons (see fig.4.104 of section 4.17) in the H2O molecule
◼ The H2O molecule thus becomes H3O+ ion
• This can be represented as: H2O(l) + H+ → H3O+(aq)
5. Adding equations in (2) and (4), we get:
HX(aq) + H2O(l) + H+(aq) → H+(aq) + X-(aq) + H3O+(aq)
• Cancelling H+ on either sides, we get:
HX(aq) + H2O(l) → H3O+(aq) + X-(aq)
◼ This simply means that:
When the acid HX is dissolved in water, H3O+(aq) and X-(aq) ions are produced
• H3O+ is known as the hydrated proton. It is also known as hydronium ion
• Next we will see how the concept is applied to bases. It can be written in 2 steps:
1. According to Arrhenius theory, bases can be represented as XOH
• This is because, OH will be present in all bases
2. So the ionization of a base can be represented as:
XOH(aq) → X+(aq) + OH-(aq)
◼ This simply means that:
When the base XOH is dissolved in water, X+ and OH- ions are produced
• OH- ion is called hydroxyl ion
◼ In general, we can write:
• According to Arrhenius theory:
♦ Acids are substances that dissociates in water to give H3O+(aq) ions
♦ Bases are substances that produce OH-(aq) ions
The Arrhenius theory suffers some limitations. They can be written in 2 steps:
1. The theory is applicable only to aqueous solutions
2. Substances like ammonia do not possess the hydroxyl group. But they exhibit basic nature. Arrhenius theory cannot explain the basic nature of such substances
In the next section, we will see Brönsted-Lowry Acids and Bases
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