Ch#8-Acids, Bases and Salts Chemistry Notes FSc 2020-21 New Course kpk.
Long Questions Chemistry 11th Notes for kpk
Table of Contents
Q.2) What are conjugate acids and bases? Give the conjugate bases of the following acids; HClO4, HCN, H2CO3, NH4+.
Answer:
Conjugate Acid
A species formed from a base by gaining a proton is called the conjugate acid of that base. For example, ammonia (NH3) is a base that gains proton from water and forms conjugate acid, NH4+.
NH3 + H2O ⇋ NH4+ + OH-
Conjugate Base
A species formed from an acid by the loss of a proton is called the conjugate base of that acid. For example, HCl is an acid which losses a proton and forms a conjugate base, Cl–.
HCl + H2O ⇋ Cl– + H3O+
Conjugate Base of HClO4
HClO4 losses proton (H+) and forms conjugate base ClO4–.
HClO4 + H2O ⇋ ClO4– + H3O+
Conjugate Base of HCN
HCN losses proton (H+) and forms conjugate base CN– (Cyanide ion).
HCN + H2O ⇋ CN– + H3O+
Conjugate Base of H2CO3
H2CO3 losses proton (H+) and forms conjugate base HCO3-1.
H2CO3 + H2O ⇋ HCO3-1 + H3O+
Conjugate Base of NH4+
NH4+ losses proton and forms conjugate base NH3 (ammonia).
NH4+ + H2O ⇋ NH3 + H3O+
Q.3) Classify as acids and bases giving reasons; BF3, NH3, NH4+, Ag+, CaO, KCN, H2S, SO42-, Na+, Cl–.
Answer:
BF3:
BF3 is an acid according to Lewis concept. Central atom boron in BF3 has an incomplete octet (six electrons in valence shell) and hence can accept a pair of electron.
NH3:
NH3 is a base according to Lewis concept. Central atom nitrogen in NH3 has a lone pair on it which it can donate.
NH4+:
NH4+ is an acid according to Bronsted-Lowery concept because it can give proton (H+) to any other substance.
Ag+:
Ag+ is an acid according to Lewis concept because it is electron deficient and hence can accept a pair of electron.
CaO:
CaO is a base according to Lewis concept because it can donate electron pair due to the presence of two electron pairs on oxygen atom in the structure.
KCN:
KCN is a basic salt because it is a result of the reaction between a strong base KOH and weak acid HCN. KCN upon hydrolysis in water gives basic solution. That is why we can consider KCN as a base.
H2S:
H2S is an acid according to Lowery-Bronsted concept because it can give proton (H+) to other substances.
SO42-:
SO42- is a base according to Lewis concept because it can give electron pair due to the presence of negative charge on the ion. It’s also Lowery-Bronsted base because it can accept proton (H+) to form HSO4-1.
Na+:
Na+ is a Lewis acid because it is electron deficient which means it can be electron acceptor.
Cl–:
Cl– is a Lewis base because it has electrons in excess which makes it electron donor.
Q.4) Classify the following as Lewis acid or Lewis Base;
CO2, H2O, SO2, I–, NH3, OH–, BCl3
Answer:
Lewis Acids:
CO2, H2O, SO2, BCl3
Lewis Bases:
NH3, OH–, H2O, I–, SO2
Q.5) What is the pH of 0.0001M Ca(OH)2 solution.
Answer:
[Ca(OH)2] = 0.0001 M
[OH–] = 2 x 0.0001 = 0.0002 M
Using the formula,
pOH = -log[OH–] = -log(0.0002) = 3.7
As we know that,
pH + pOH = 14
pH = 14 – pOH = 14 – 3.7 = 10.3
Q.6) What is [H+] and [OH–] ions concentration of a solution which has a pH= 4.87.
Answer:
pH of solution = 4.87
[H+] = ?
[OH–] = ?
As we know that,
pH = -log[H+]
Converting from logarithmic to exponential form,
[H+] = 10-pH
Putting the value of [H+],
[H+] = 10-4.87 = 1.35 x 10-5
Now for [OH-], using the relation
[H+][OH–] = 10-14
[OH–] = 10-14/[H+]
Putting the value of [H+] in the above equation,
[OH–] = 10-14/(1.35 x 10-5) = 7.40 x 10-10
Explain how a buffer solution resists changes in pH when a small amount of an acid or a base is added?
Answer:
The resistance of Buffer Against the Change in Its pH
“Buffer solution is a solution which resists the changes in its pH when small amount of acid or base is added to it”.
Both acidic buffers and basic buffers resist this change. Buffer action of acidic and basic buffers is explained in the following examples.
Buffer Action of Acidic Buffer
Consider an acidic buffer solution consisting of acetic acid and sodium acetate. When a strong base such as NaOH is added, the added OH– ions are removed by the reaction with acetic acid molecules.
CH3COOH + OH- → CH3COO- + H2O
When a strong acid is added, H+ ions of the acid react with acetate ions of the buffer.
H+ + CH3COO- → CH3COOH
Both of these reactions go to completion. Hence the added OH- and H+ ions are removed and the pH of the buffer solution remains constant. In the second reaction acetic acid is formed but being a weak acid it does not affect the pH as much.
Buffer Action of Basic Buffer
Consider a basic buffer solution consisting of weak base ammonium hydroxide and its salt with strong acid NH4Cl. When a strong acid HCl is added to this basic buffer, the added hydrogen ions are removed by reaction with NH4OH.
H+ + NH4OH → NH4+ + H2O
When the base is added, the OH- ions of the base react with ammonium ions of the buffer solution and are removed.
NH4+ + OH- → NH4OH
In this reaction, NH4OH is formed but as it is weak acid it does not affect the pH of the solution significantly.
Q.8) Explain why?
a. A solution of Na2CO3 is alkaline.
b. A solution of FeSO4 is acidic.
c. A solution of NaCl is neutral.
Answer:
a)
Na2CO3 is a salt of a weak acid (H2CO3) and a strong base (NaOH). The solution of Na2CO3 in water is alkaline due to the hydrolysis of carbonate ions. Hydrolysis of CO3-2 ions yields a weak acid H2CO3 and a strong base OH-. Due to the dominant basic character of OH–, the solution is alkaline in nature.
b)
FeSO4 is a salt of strong acid H2SO4 with weak base Fe(OH)2. The solution of FeSO4 is acidic in nature due to the hydrolysis of SO4-2 ions. Hydrolysis of SO4-2 ions yields a strong acid H2SO4 and a weak base Fe(OH)2. Due to the dominant acidic character of H+ ions, the solution is acidic in nature.
c)
NaCl is a salt of strong acid (HCl) and strong base (NaOH). It ionizes to form Na+ and Cl–. These ions upon interaction with water produce both H3O+ and OH– ions and the overall reaction is the reaction between these ions. The solution is neutral due to the formation of water as a product.
Q.10 i) a. What is the Bronsted- Lowery definition of (i) an acid (ii) a base.
Answer:
An Acid:
An acid is a specie (molecule or ion) which donates or tends to donate a proton. For example, HCl.
A Base:
A base is a substance which accepts or tends to accept a proton. For example, NH3.
Q.10 i) b. Write equations and indicate the conjugate acid-base pairs for the following; (i) Acetic acid and water (ii) Ammonia and Hydrochloric acid.
Answer:
Q.10 ii) a. What are Buffer solutions? How do they act? Derive an expression relating the pH of a buffer with the concentration of its components.
Answer:
Buffer Solution:
A solution, which resists changes in its pH when a small amount of a strong acid or a base is added to it, is called a buffer solution.
Buffer Action:
Buffer Action of Acidic Buffer
Consider an acidic buffer solution consisting of acetic acid and sodium acetate. When a strong base such as NaOH is added, the added OH– ions are removed by the reaction with acetic acid molecules.
CH3COOH + OH– → CH3COO– + H2O
When a strong acid is added, H+ ions of the acid react with acetate ions of the buffer.
H+ + CH3COO– → CH3COOH
Both of these reactions go to completion. Hence the added OH– and H+ ions are removed and the pH of the buffer solution remains constant. In the second reaction acetic acid is formed but being a weak acid it does not affect the pH as much.
Buffer Action of Basic Buffer
Consider a basic buffer solution consisting of weak bas ammonium hydroxide and its salt with strong acid NH4Cl. When a strong acid HCl is added to this basic buffer, the added hydrogen ions are removed by reaction with NH4OH.
H+ + NH4OH → NH4+ + H2O
When base is added, the OH– ions of the base react with ammonium ions of the buffer solution and are removed.
NH4+ + OH– → NH4OH
Derivation of pH expression:
The pH of a buffer solution is determined mainly by two factors:
(a) The value of Ka of the weak acid or Kb of the weak base
(b) The ratio of the concentrations of the weak acid and its conjugate base
The equation for the ionization of a weak acid is:
HA ⇌ H+ + A–
Ionization constant Ka of the weak acid is;
Read more: Chemistry Chapter 7 Chemical Equilibrium Class 11 Notes
Q.10 iii) b. A buffer solution contains 1 mole dm-3 each of acetic acid and sodium acetate. Find the pH of the buffer solution. pKa of acetic acid is 4.74.
Answer:
Conc. of acetic acid = [CH3COOH] = 1 mol dm-3
Conc. of sodium acetate = [CH3COONa] = 1 mol dm-3
pKa of acetic acid = 4.74
pH=?
The formula to calculate the pH of buffer solution is as follows,
Q.10 iii) Calculate the pOH of a buffer solution containing 0.04 M NH4Cl and 0.02 M NH4OH. Kb for NH4OH is 1.8 x 10-5.
Answer:
Conc. of NH4Cl=0.04 M
Conc. of NH4OH=0.02 M
Kb for NH4OH=1.8 x 10-5
pOH=?
pKb = -logKb = -log(1.8 x 10-5) = 4.74
The formula to calculate pH of buffer solution is as follows,
Chemistry Class 11 Notes Short Questions
Q.9 i) Buffer solution resists changes in pH.
Answer:
Buffer solution resists the changes in its pH when a small amount of a strong acid or base is added to it. The buffer solution can not exactly maintain the pH at a constant level but it can minimize the effect of strong acid or base on pH change. H+ ions added from a strong acid or OH- ions added from a strong base react with the ions present in the buffer solution. The resulting product of these reactions is either water or weak acid or weak base. Weak acid or base has little effect on pH and hence the pH remains nearly constant.
Example:
Consider an acidic buffer solution consisting of acetic acid and sodium acetate. When a strong base such as NaOH is added, the added OH– ions are removed by the reaction with acetic acid molecules.
CH3COOH + OH– → CH3COO– + H2O
When a strong acid is added, H+ ions of the acid react with acetate ions of the buffer.
H+ + CH3COO– → CH3COOH
Both of these reactions go to completion. Hence the added OH– and H+ ions are removed and the pH of the buffer solution remains constant. In the second reaction acetic acid is formed but being a weak acid it does not affect the pH as much.
Q.9 ii) NH3 is a base according to Lewis concept, comment.
Answer:
NH3 has an electron pair on it which it can donate to any other specie and hence it acts as a Lewis base.
Example:
For example, NH3 donates its electron pair to hydrogen ion (H+) and forms ammonium ion (NH4+) as an additional product.
Q.9 iii) Water can act either as a weak acid or as a weak base. Give reasons.
Answer:
Water is an amphoteric compound because it can act as acid by donating proton (H+) and also as base by accepting proton (H+). When water reacts with acids like HCl, it behaves as weak base. While upon treatment with bases like NH3, it acts as a weak acid.
HCl + H2O → H3O+ + Cl–
weak base
NH3 + H2O → NH4+ + OH–
weak acid
Q.9 iv) If the dissociation constant (Ka) of a substance increases, the value of the dissociation constant of its conjugate base (Kb) decreases. Why?
Answer:
Dissociation constant (Ka) of a substance is the measure of its acidic strength. Greater the value of Ka, greater will be its acidic strength and it will dissociate more completely. Strong acid (greater Ka value) wants to remain in the dissociated form and hence its conjugate base is stable. Due to its stability, the conjugate base doesn’t have tendency to accept proton (H+) and hence it is weak acid. So, with the increase in Ka of a substance, the value of Kb of its conjugate base decreases.
HCl + H2O → H3O+ + Cl–
acid conjugate base
Q.9 v) The sum of pKa and pKb is always equal to 14.
Answer:
If pKa of an acid increases, the pKb of its conjugate base decreases. The overall sum of pKa and pKb always remains equal to 14.
Derivation:
Consider a weak acid HA is ionized in water and it gives a conjugate base A–.
HA + H2O → H3O+ + A–
acid conjugate base
Because the concentration of hydronium ions H3O+ actually represents the concentration of H+ ions, Ka for acid HA is written as,
Q.9 vi) A strong acid has always a weak conjugate base and vice versa.
Answer:
A strong acid tends to dissociate completely and the conjugate base formed by its ionization is stable. The conjugate base does not tend to accept proton as much and acts as a weak base. On the other hand, weak acid dissociates up to little extent and the conjugate base formed is not stable. The conjugate base of weak acid tends to accept proton and acts as a strong base.
Q.9 vii) Justify your answer with equations that CH3COONa gives a basic solution while NH4CI an acidic solution with water.
Answer:
CH3COONa is a salt of weak acid (CH3COOH) and strong base (NaOH), so it gives basic solution in water. Sodium acetate CH3COOH on hydrolysis gives a strong base OH– and a weak acid CH3COOH. Due to dominant basic character of OH–, the solution is basic in nature.
CH3COONa → CH3COO– + Na+
CH3COO– + H2O → CH3COOH + OH-
NH4Cl is a salt of strong acid HCl and weak base NH4OH, so it gives basic solution. Ammonium chloride (NH4Cl) upon hydrolysis gives a weak base NH3 and a strong acid H3O+. Due to dominant acidic character of H3O+, the solution is acidic in nature.
NH4Cl → NH4+ + Cl–
NH4+ + H2O → NH3 + H3O+
Q.9 viii) Why do you call AICI3 and BF3 as lewis acids and Cl– and NH3 as lewis bases.
Answer:
AlCl3 and BF3 both have incomplete octet of their central atoms Al and B respectively. Due to deficiency of electrons in their valence shells, AlCl3 and BF3 can accept electrons and hence they act as Lewis bases.
On the other hand, Cl– has electrons in excess which makes it electron donor and hence a Lewis base. NH3 has a lone pair of electrons on its central atom nitrogen which it can donate to others and hence acts as a Lewis base.
