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Ionic Product of Water- Ionic Product, Ionization Constant, Degree of Dissociation, Factors Affecting the Degree of Dissociation, Practice Problems, FAQs
The ability to conduct an electrical current is referred to as conductivity. Copper has high conductivity, allowing an electrical current to pass through the wiring and to a light bulb or ceiling fan. Seawater, on the other hand, has a far higher conductivity than pure water. Do you know why?
Water's electrical conductivity is a word that describes how effectively or poorly water conducts electricity. Because, if there are no contaminants, water has a very low electrical conductivity. Ions must be present in water for it to transmit electricity. So, let's discuss how water dissociates into ions and conduct electricity.
Table of contents
- Ionic Product of water
- Ionization constant of water
- Calculation of degree of dissociation (⍺) of water
- Factors Affecting the Degree of Ionization
- Practice Problems
- Frequently Asked Questions (FAQs)
Ionic Product of water
Water undergoes Self-ionization/Autoprotolysis:
H2O(l)⇌H+(aq)+OH-(aq)
As per the law of chemical equilibrium:
Equilibrium constant = 
Here, the concentration of water is constant. Hence, Kc[H2O]gives rise to the new constant
(Kw) known as the ionic product of water or ionization constant of water.
Kw = [H+][OH-]
At 25oC, the ionic product of water, Kw has a value of 10-14M.
For pure water, [H+] = [OH-] = a (suppose)
Kw = aa
10-14= a2
10-7= a
So, [H3+O] = [OH-] = a = 10-7M
Ionization constant of water
H2O(l)⇌H+(aq)+OH-(aq)
Equilibrium constant
The molarity of pure water [H2O]can be calculated as follows:
Calculation of degree of dissociation (⍺) of water
Effect of temperature on Kw (Ionic product of water)
Water dissociation is an endothermic process. With increasing temperature, the ionic product of water ( Kw) increases. This is because as the temperature rises, the degree of ionization of water rises as well.
Whatever is dissolved in water, the ionic product of water remains constant. It will only be affected by temperature because it is an equilibrium constant.
Factors Affecting the Degree of Ionisation
1. Nature of the electrolyte
Weak electrolytes only partially ionized, whereas strong electrolytes entirely ionized.
2. Nature of the solvent
More is the polar solvent, more is the degree of ionization.
3. Temperature
The degree of dissociation depends on the temperature. Higher the temperature, higher is the value of degree of dissociation (α).
4. Dilution
On dilution, the extent of dissociation increases. Hence, the value of α increases.
5. Presence of other solutes
Due to the common-ion effect, when a material is present in a solution, it can alter the dissociation of another chemical if they are of the same species.
Related Video: 41:00 to 53:31 Discussion on Acid-Bases, Ostwald's Dilution Law, Water, pH Scale | JEE
Practice Problems
Q 1. For water at 300C, the Kw has a value of 1.46 × 10-14. What will be H+ concentration at 300C?
Q 2. If the value of Kw of water at 250C is 10-14. The value of Kw of water at 200C is
a. 10-12
b. 10-15
c. 10-13
d. Remains constant
Answer: (B)
Kw Temperature. If the temperature decreases from 250C to 200C, the value of Kw also decreases. The value less than 10-14 is in option (B) i.e.;10-15
Q 3. Assertion: HNO3(aq) and NaOH(aq) are excellent electrical conductors.
Reason: HNO3(aq) and NaOH(aq) are strong electrolytes.
(a) Both, the assertion and the reason are true and the reason is the correct explanation of the assertion.
(b) Both, the assertion and the reason are true but the reason is not the correct explanation of the assertion.
(c) The assertion is true but the reason is false.
(d) Both, the assertion and the reason are false.
Answer: Both HNO3(aq) and NaOH(aq) totally break down into ions in water, indicating that they are powerful electrolytes. Due to the existence of free ions in strong electrolytes, HNO3(aq) and NaOH(aq) in an aqueous medium are also good conductors of electricity.
Q 4. Explain when acid or base is added to water, what changes are expected in the concentration of H+ and OH-?
Answer: When an acid or base is added to pure water, the concentration of the ionic product can be estimated using the ionic product constant equation, Kw=[H+][OH-], where the value of Kw is constant at a certain temperature while the concentration of the H+ and OH- ions is not.
The addition of the acid causes a rise in the concentration of the hydrogen H+ ion or in other words, a reduction in the concentration of the hydroxyl OH- ion,
[H+]>[OH-]; (Acidic solution)
Similarly, the addition of the base increases the concentration of the hydroxyl OH- ion while decreasing the concentration of the hydrogen H+ ion,
[H+]>[OH-]; (Alkaline or basic solution)
Frequently Asked Questions (FAQs)
Q 1. How do you distinguish between dissociation and ionization?
Answer: Ions already present in the solid solution separate out when the ionic molecule is dissolved. This is what dissociation is all about. A neutral or covalent molecule dissociates into ions in solution when it is dissolved. This is the process of ionization.
Q 2. Comment on the relative concentration of hydrogen H+ ion and hydroxyl OH- ion in blood present in the human body?
Answer: The pH of healthy human body is (7.35-3.45). Since, the blood in the human body is somewhat alkaline, the hydroxyl OH- ion concentration should be higher than the hydrogen H+ ion concentration.
Q 3. How can you tell whether something has entirely ionized?
Answer: We can tell if a compound is entirely ionized or not by looking at the degree of ionization value. The ratio of the number of molecules dissociating to the total number of molecules is known as the degree of ionization. If the value for the degree of dissociation is 1, the molecule is totally ionized.
Q 4. If water dissociation would be an exothermic process with rising in temperature, what will be the effect on Kw?
Answer: If Water dissociation would be an exothermic process then, with increasing temperature, the ionic product of water ( Kw) decreases.
Related Topics
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Hydrolysis of salt- the salt of a weak acid and weak base |
Hydrolysis of salt- the salt of a weak acid and strong base |
|
Hydrolysis of salt- the salt of strong acid and weak base |
Acidity and pH scale |
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