s&r topic 8

Question 1

define bronsted-lowry acid

Answer 1

a species with a tendency to donate a proton

anything with a hydrogen atom

eg. alcohol, HCl, phenol

Question 2

define bronsted-lowry base

Answer 2

a species with a tendency to accept a proton ]

usually contain a lone pair or pi-system

eg OH-, CN-, NH3, alkenes, carbonyl groups

Question 3

describe the premise of conjugate acids and bases

Answer 3

every acid has a conjugate base
every base has a conjugate acid

conjugate means it has been derived from the dissociation/protonation of the original acid/base

Question 4

give the equilibrium chemical equation of base B with acid HA

Answer 4

B: + HA ⇄ BH⁺ + A⁻

acid HA loses a proton and forms conjugate base A⁻

base B becomes protonated to form conjugate acid BH⁺

Question 5

give the equilibrium equation of the self-ionisation of water

Answer 5

H₂O: + H-O-H ⇄ H₃O⁺ + OH⁻

Question 6

what is the pH scale a measure of

describe it in terms of the concentrations of the substances present

Answer 6

a measure of the concentration of H₃O⁺, the tendency of an acid to transfer a proton to water

pH 0 → [water] = [H₃O⁺]
pH 7 → neutral, just water
pH 14 → [water] = [OH⁻]

Question 7

what is the acid dissociation constant

Answer 7

Ka

a quantitive measure of the strength of an acid in a solution

Question 8

how do you calculate Ka

give the equation for an acid in water

Answer 8

Ka is the equilibrium concentrations of the products divided by the equilibrium concentrations of the reactants

for acid in water:

Ka = [H₃O⁺][A⁻] / [HA]

Question 9

how do work out pKa from Ka and vice versa

Answer 9

pKa = -log Ka

Ka = 10^-pKa

Question 10

what is the value of pKa and the position of equilibrium for these values of Ka:

• Ka&raquo_space;> 1
• Ka «< 1

Answer 10

• pKa is negative, equilibrium lies to the right, products are favoured
• pKa is positive, equilibrium lies to the left, reactants are favoured

Question 11

how do the values of Ka and pKa determine the strength of the acid

Answer 11

larger Ka value → stronger acid

lower pKa value → stronger acid

Question 12

what is the case at the pKa value

Answer 12

the concentration of the acid and the conjugate base are the same

Question 13

what does a strong acid mean for the strength of the conjugate base?

what does a strong base mean for the strength of the conjugate acid?

Answer 13

strong acid = more stable/weaker conjugate base

strong base = more stable/weaker conjugate acid

Question 14

name 3 factors affecting acid strength

Answer 14

• strength of HA bond
• the solvent in which the acid is dissolved
• stability of conjugate base

Question 15

name 4 factors affecting conjugate base strength

Answer 15

• electronegativity
• delocalisation
• hybridisation
• substituents for resonance of inductive effects