Acid-Base Equilibria

Question 1

State Arrhenius Theory of acids and bases

Answer 1

An acid is a substance that releases H+ ions when dissolved in water, a base is a substance that releases OH- ions in the presence of water.

Question 2

Limitation of Arrhenius Theory

Answer 2

Applies only to reactions which take place in aqueous medium.

Certain compounds, Ammonia behaves like bases although it does not have a OH- in its structure.

Question 3

Explain what is meant by the terms Bronsted-Lowry acid, Bronsted-Lowry base and conjugate acid-base pair

Answer 3

Bronsted-Lowry acid is a proton, h+, donor

Bronsted-Lowry base is a proton, h+, acceptor

For every Bronsted acid, it has a conjugate base which has one H+ less than the acid.
For every Bronsted base, it has a conjugate acid which has one H+ more than the base.

Question 4

Under which conditions can Bronsted-Lowry reactions occur?

Answer 4

They can occur in aqueous, non-aqueous and gaseous systems as emphasis is on proton transfer.

Question 5

Factor affecting strength of Base

Answer 5

Extent of ionisation in aqueous solution. Strong base completely…

Strong base:
concentration of acid = concentration of OH-

Weak base:
concentration of base /= concentration of OH-

Question 5

Limitation of Bronsted-Lowry reaction

Answer 5

A Bronsted-Lowry acid has to contain an ionisable Hydrogen atom.

Question 6

Factor affecting strength of Acid

Answer 6

Extent of dissociation in aqueous solution. Strong acid completely…

Strong acid:
concentration of acid = concentration of H+/ H3O+

Weak acid:
concentration of acid /= concentration of H+/H30+

Question 7

Auto ionisation of water ionic equation

Answer 7

H2O + H2O -> H3O+ + OH-. Arrow is reversible.

Question 8

Ionic Product of water, Kw value and formula

Answer 8

Kw = (H3O+)(OH-) = 1.0 x 10^-14 mol^2dm^-6 at 25degrees celsius

Question 8

pH scale and pOH scale formula. Formula to find concentration of H+ and OH- in relation

Answer 8

pH = -log(H30+)
pOH = -log(OH-)

(H+) = 10^-pH
(OH-) = 10^-pOH

Question 8

Relationship of pOH and pH

Answer 8

pH + pOH = -log( 1.0 x 10^-14) = 14

Question 8

When is auto-ionisation of water considered negligible and when is it not?

Answer 8

Negligible when concentrations of strong acid and strong base are high.

Not negligible when acids or bases are very diluted;

(H3O+) and (OH-) is lesser or equal to 1 x 10^-7 mol/dm3

This will result in (H3O+) and (OH-) contributed by auto-ionisation of water to be considered significant and hence needed to be included.

Question 9

Formula to find H3O+ concentration using Ka, acid dissociation constant.

Answer 9

(H+) = (Ka x c)^1/2. Square rooted.

Question 9

Limitations of using pH as indicator for strength of acidity or basicity

Answer 9

pH varies with its concentration. pH of a stronger acid need not be lower than pH of a weaker acid if concentrations are different.

To use pH as an indicator of acid strength between 2 acids, concentration of 2 acids must be the same.

Question 10

Definition of a Buffer solution

Answer 10

A buffer solution is a solution in which its pH does not change significantly on the addition of a small amount acid or base.

Question 11

Suitable indicator for titration between:
SA and SB
WA and SB
SA and WB
WA and WB

Answer 11

SA and SB -
Methyl orange (3.1 - 4.4)
Phenolphthalein (8.3 - 10.0)

WA and SB -
Phenolphthalein (8.3 - 10.0)

SA and WB -
Methyl orange (3.1 - 4.4)

WA and WB -
End point cannot be detected accurately by any indicator

Hence, pH>7 use phenolphthalein
pH<7 use methyl orange

Question 12

Which indicator is the best indicator to test for strength of acid and why?

Answer 12

Acid dissociation constant, Ka, as it is a constant at constant temperature and does not vary with concentration. At the same temperature, Ka, of stronger acid is always higher than the Ka of a weaker acid, regardless of their concentrations.

Question 13

Relative strength of conjugate Acid-Base Pair

Answer 13

The more readily an acid donates a proton, the less readily its conjugate base would accept a proton. Vice Versa.

The stronger an acid, the weaker its conjugate base; the stronger a base, the weaker its conjugate acid.