5.1.3 Acids, bases and buffers

Question 1

Define a Bronsted Lowry acid and base

Answer 1

acid = proton donor
base - proton acceptor

Question 2

What is an acid-base pair?

Answer 2

2 species that can be interconverted by transfer of a proton

Question 3

What is the hydronium ion? When is it formed?

Answer 3

H3O+

acid + water -> hydronium ion and other ion

Question 4

Define monobasic, dibasic and tribasic acids

Answer 4

monobasic: each acid molecule can donate 1 proton in solution
dibasic: each acid molecule can donate 2 protons in solution
tribasic: each acid molecule can donate 3 protons in solution

Question 5

acid + metal

Answer 5

salt + hydrogen

Question 6

acid + alkali

Answer 6

salt + water

Question 7

acid + carbonate

Answer 7

salt + water + CO2

Question 8

How do you calculate pH?

Answer 8

-log[H+]

Question 9

How do you calculate the pH of strong acids?

Answer 9

concentration of H+ ions in a monoprotic strong acid will be the same as the concentration of the acid

Question 10

How do you calculate H+ ion concentration?

Answer 10

10^-pH

Question 11

What is Ka?

Answer 11

[H+][A-]/ [HA]

Question 12

The larger the Ka the…..

Answer 12

stronger the acid

Question 13

The larger the pKa the…

Answer 13

weaker the acid

Question 14

How do you calculate pKa?

Answer 14

-logKa

Question 15

How do you calculate the pH of a weak acid? What two approximations can be made?

Answer 15

When weak acid dissociates, [H+] and [A-] are made in equal quantities. You assume [H+] = [A-] (not 100% accurate due to water dissociating, but acid dissociating has a much greater effect)

Very small levels of dissociation mean [HA] eqm = [HA] start

Ka x [HA] = [H+]^2
Thenn square root, etc.

Question 16

What are the limitations of weak acid approximations?

Answer 16

1. Water dissociation becomes more significant for very weak acids or very dilute solutions
2. Breaks down for stronger weak acids

Question 17

What is Kw? What is it at 25 degrees celsius? What is pH of water at 25 degrees celsius?

Answer 17

[H+][OH-]
1x10^-14

7 since [H+] = [OH-]

Question 18

How does an increase in temp affect Kw?

Answer 18

would push the equilibrium to the right giving a bigger concentration of H+ and a lower pH

Question 19

how do you calculate the pH of strong bases?

Answer 19

[strong base] = [OH-]

then calculate pH using Kw

Question 20

Define a buffer

Answer 20

system that minimises pH changes when small amounts of acids or bases are added. contains an excess of weak acid and a conjugate base

Question 21

How do buffers work?

Answer 21

• need to have an equal mix of acid and base to remove excess H+ or OH-
  -the ratio of [HA]/[A-] stays the same
• weak acid removes added alkali since eqm shifts to right
• conjugate base removes added acid since eqm shifts to left

Question 22

What are the two ways to make buffers?

Answer 22

1. Add conjugate base salt to acid
   Salt completely dissolves and the anion ‘mops up’ hydrogen ions
2. Partial neutralisation
   - add aq alkali (strong base) to excess of weak acid. Results in mixture of unreacted acid and salt, which dissociates.

Question 23

How do you calculate the pH of a buffer?

Answer 23

[H+] = Ka x [HA]/[A-]

ideal buffer: pKa = pH

Question 24

How is pH of blood plasma controlled? What if pH is too low or high?

Answer 24

H2CO3/HCO3-

equilibrium can shift depending on added acid or alkali

if H2CO3 builds up, then can be converted to CO2 and breathing rate increases

If 7.35 = ACIDOSIS. If 7.45 = ALKALOSIS.

Question 25

glycolic acid, HOCH2COOH, is a weak monobasic acid used in some skincare products. A buffer solution is prepared by adding glycolic acid to potassium hydroxide, KOH.Why is this a buffer?

Answer 25

Acid is in excess and is partially neutralised, and the conjugate base (glycolate) is produced

Question 26

Dilute sulfuric acid is added to a colourless solution of Ba(OH)2. Write the ionic equation for this reaction, including state symbols.

Answer 26

2OH- (aq) + 2H+ (aq) + SO42- (aq) + Ba2+ (aq)-> BaSO4 (s) + 2H2O (l)

Question 27

Describe the shape of a pH titration curve, for an addition of a base to an acid.

Answer 27

• base first added = acid in excess so pH rises very slowly
• pH starts to increase more quickly as the acid is used up more quickly
• vertical section is where pH increases more rapidly, with only a few drops of base
• pH then rises very slightly as base is in great excess

Question 28

Note down the definition of equivalence point.

Answer 28

volume of one solution that EXACTLY reacts with a volume of another solution - the centre of the vertical section of the pH titration curve

Question 29

What is an acid-base indicator?

Answer 29

a weak acid, HA, that has a distinctively different colour from its conjugate base, A-

Question 30

What is the end point?

Answer 30

when an indicator contains equal concentrations of HA and A- and the colour will be in between the two extremes.

Question 31

What happens if methyl orange is added initially to a basic solution and then acid is added?

Answer 31

H+ ions react with the conjugate base, A-.
The equilibrium position shifts to the left
The colour changes, first to orange at the end point and finally to red when the equilibrium position has shifted to the left.

Question 32

The pH of the end point is the same as……

Answer 32

the pKa value of HA

Question 33

How do you choose an indicator?

Answer 33

the pH range for the indicators colour change must coincide with the vertical section of its pH titration curve

Question 34

What titration doesn’t have a possible indicator?

Answer 34

weak acid - weak base ( no vertical section)