Acid-base equilibria

Question 1

What is a Bronsted-Lowry acid?

Answer 1

A proton donor (loses a proton)

Question 2

What is a conjugate acid-base pair?

Answer 2

2 substances that differ from each other by only 1 transferable H+

Question 3

Stronger acid gives you?

Answer 3

Weaker conjugate base

Question 4

What is a Lewis acid?

Answer 4

An electron pair acceptor (electrophile)

Question 5

What is a Bronsted-Lewis acid?

Answer 5

A substance that donates protons and accepts a LP of electrons

Question 6

What is an Arrhenius acid?

Answer 6

A substance that ionises in water to give H+ ions

Question 7

What is a weak acid? Examples?

Is it the same as a low concentrated acid?

Answer 7

An acid that dissociates partially in water to produce H+ ions. E.g. CH3COOH, H2CO3, HCN
No, that’s just means that the acid is diluted.

Question 8

How to find the strength of acid?

Answer 8

measure using the extent to which A/B dissociates in water to give H+/OH- ions :

degree of ionisation = [acid ionised]/[initial acid]
- the fraction of particles of a substance that dissociates into ions, range(SA to WA) : 0 < alpha < 1

Question 9

What is the ionic product of water?

Answer 9

Kw = [H+][OH-] = 1.00 x 10^-14 mol^2dm^-6 at 25 deg
- since water is amphiprotic (can act as proton donor & acceptor) and auto ionisation process is spontaneous & to a v. small extent

Question 10

When temperature increases, what happens to Kw?

Answer 10

Since dissociation of water is endo, forward endo rxn is favoured, dissociate more, causing Kw to increase

Question 11

What is used to measure acidity of solution?

Is it the same as strength of acid?

Answer 11

pH = -lg[H+} ( [H+] = 10^-pH )
- the larger the pH, the smaller the [H+]
No, as pH measures the [H+] in sol. not the extent of acid dissociation

Question 12

What is the relationship between pH and pOH?

Answer 12

pH + pOH = 14 at 25 deg

Question 13

What is an acidic solution?

Answer 13

[H+] > [OH-]

at 25 degree celsius, pH < 7

Question 14

How to calculate strong A/B?

e.g. Calculate the pH of 0.18moldm^-3 of HNO^3 solution.

Answer 14

1. write eqn to show ions of SA/B (full arrow)
2. find [H+] or [OH-} using {SA] or [SB]
3. use pH = -lg{H+] or pH = 14 - (pOH = -lg[OH-})

Question 15

What is the acid dissociation constant?

Answer 15

Ka = [H3O+][A-] / [HA] moldm^-3

Question 16

Larger Ka means?

Answer 16

greater extent of dissociation, smaller pKa, so stronger acid

Question 17

How to find pKa from Ka?

Answer 17

pKa = -lgKa ( Ka = 10^-pKa)

Question 18

Can we use Ka/Kb to compare the strength of 2 A/B of different initial concentration at a fixed temp.?

Answer 18

Yes, as Ka/Kb is an equilibrium constant dependent on temp. and so it is not affected by initial conc. of A/B

Question 19

How to calculate weak A/B?

e.g. Calculate the pH of 0.01moldm^-3 aqueous sol. of CH3COOH (pKa = 4.74 OR Kb = 1.6 x 10^-5)

Answer 19

1. write eqn to show ions (double arrow)
2. find Ka using Ka = 10^-pKa OR Ka = Kw/Kb
3. find [H+} using [H+] = (Ka x {A/B}initial)^1/2
4. find pH using pH = -lg[H+]

Question 20

What is the relationship between Ka and Kb?

Answer 20

Kw = Ka x Kb = 1.00 x 10^-14 mol^2dm^-6 at 25 deg

Question 21

What is the relationship between pKa and pKb?

Answer 21

pKw = pKa + pKb = 14 at 25 deg

Question 22

What is the pH of an aqueous solution of CH3COONa?

Answer 22

Basic.

CH3COO- is a strong conjugate base of a weak acid CH3COOH VS. Na+ which is a weak conj. acid of a strong base NaOH

Hence the anion CH3COO- is a stronger base than water and it reacts with water(anion hydrolysis) to give a basic solution (pH > 7)