5.1.3 Acids, bases and buffers

Question 1

what is an acid (B-L)?

Answer 1

a proton donor

Question 2

what is a base (B-L)?

Answer 2

a proton acceptor

Question 3

what is a hydronium ion?

Answer 3

H3O+
when a water molecule form a dative bond with a proton
water exists in equilibrium with hydronium ions

Question 4

what is the conjugate base pair of an acid?

Answer 4

an acid is a proton donor
the conjugate base is a product of the acid that needs a proton to become the acid again and so would act as a base

Question 5

what is the conjugate acid pair of a base?

Answer 5

a base is a proton acceptor
the conjugate acid is a product of the base that needs to lose a proton to become the base again and so would act as an acid

Question 6

what is the equation to calculate pH?

Answer 6

pH = - log [H+]

Question 7

what is the equation to calculate [H+]?

Answer 7

[H+] = 10^-pH

Question 8

what is the difference between strong and weak acids?

Answer 8

strong acids fully dissociate into their ions whereas weak acids only partly dissociate into their ions

Question 9

what is the equation of Ka?

Answer 9

Ka = [products] / [reactants]

Question 10

what does the Ka show?

Answer 10

the higher Ka value the stronger the acid

Question 11

what is the dissociation of weak acids?

Answer 11

< 5% dissociation of [H+]

Question 12

what does Ka equal for weak acids?

Answer 12

Ka = [H+]^2 / [HA]

Question 13

how to calculate the value of [H+] from Ka?

Answer 13

[H+] = square root of (Ka x [HA])

Question 14

why is pKa used for weak acids?

Answer 14

for weak acids there is a large range of very small Ka values

Question 15

how to calculate pKa from Ka?

Answer 15

pKa = - log Ka

Question 16

why does Kw exist?

Answer 16

water ionises partially and contains a very low concentration of ions. since very little water ionises, the conc of water molecules is virtually constant compared with the conc of the ions
so: Kc x [H2O] = Kw

Question 17

what is the expression for Kw?

Answer 17

Kw = [H+] [OH-]

Question 18

how to find [H+] from Kw? (strong base)

Answer 18

Kw / [OH-] = [H+]

Question 19

why is the value of Kw temperature dependent?

Answer 19

value of Kc and Kw is temperature dependent as dissociation increases as temperature increases

Question 20

What is a buffer solution?

Answer 20

• a solution that can minimise small changes in pH when small quantities of acid or base are added

Question 21

How does ethanoic acid / sodium ethanoate act as a buffer?

Answer 21

• acid is added: H+ + CH3COO-
• equilibrium shifts to the left
• alkali is added: OH- + H+
• equilibrium shifts to the right as H+ is used up

Question 22

Does the salt in a buffer fully dissociate?

Answer 22

• salt fully dissociates
• weak acid partially dissociates

Question 23

What is the link between Ka and buffers?

Answer 23

Ka = [H+] [salt] / [acid]
[H+] = Ka [acid] / [salt]

Question 24

What is the blood buffer made up of?

Answer 24

H2CO3 <=> HCO3 + H+
H2CO3 <=> H2O + CO2
- keeps the pH of the blood between 7.35 and 7.45
- prevent acidiosis

Question 25

How does the carbonic acid-hydrogencarbonate ion system acts as a buffer?

Answer 25

• acid is added: H+ + HCO3-
• equilibrium will shift to the left
• alkali is added: OH- + H+
• equilibrium will shift to the right

Question 26

What is the end point and equivalence point of a titration?

Answer 26

• end: equal concentrations of weak acid and conjugate base of indicator
• equivalence: volume of one reactant has reacted completely with volume of second solution

Question 27

How to choose correct indicator?

Answer 27

• changes colour in the pH corresponding to the sharp rise in the titration curve
• disqualify one - changes colour before sharp rise

Question 28

Why can indicator not be used for weak acid weak base titrations?

Answer 28

• no sharp pH change at equivalence point
• indicator would change colour gradually

Question 29

Where is the equivalence point on a titration curve?

Answer 29

• centre of the vertical sections of the titration curve
• volume of alkali added (cm3) on x axis
• pH on y axis