5-Acid-Base Equilibria

Question 1

What did Arrhenius think about acids and bases?

Answer 1

Acids had common properties because they all produce H+ ions when dissolved in water
HA H+ + A-
B + H2O BH+ + OH-

Question 2

What evidence is there for the existence of the oxonium ion?

Answer 2

The rate at which positive ions carry electrons is consistent with their size
You’d expect the H+ to move much faster

Question 3

What is the problem with Arrhenius’ definition for acids and bases?

Answer 3

Water must be present
Reaction between NH3 and Hal doesn’t need any solvent
By his definition, not an acid-base reaction without water

Question 4

What are the Brønsted-Lowry definitions of acids and bases?

Answer 4

An acid is a proton donor
A base is a proton acceptor
Makes the dissociation of an acid and acid-base reaction

Question 5

What are the conjugate pairs of the dissociation of water?

Answer 5

HA + H2O A- + H3O+
A B B A
HA + A-
H2O + H3O+

Question 6

Does water act as an acid or a base?

Answer 6

Both
When a weak acid dissolves, the acid and H3O+ compete to donate a proton
From electrical conductivity we can tell that H3O+ is a better proton donor than ethanoic acid therefore it is a stronger acid

Question 7

What do strong and weak Brønsted acids show about their conjugate bases?

Answer 7

Strong Brønsted acids have weak conjugate bases

Weak Brønsted acids have strong conjugate bases

Question 8

What is pH defined by?

Answer 8

pH = -log[H+(aq)]

Measured using universal indicator or a pH meter

Question 9

What do strong acids and weak acids indicate about the dissociation of ions?

Answer 9

Strong acids - all molecules dissociate into ions

Weak acids- only some of the acid molecules dissociate into ions

Question 10

What do strong bases and weak bases indicate about the dissociation of ions? Neutral solutions

Answer 10

Strong bases - all molecules dissociate into ions
Weak bases- only some of the acid molecules dissociate into ions
Neutral solutions - the concentrations of hydrogen ions and hydroxide ions are equal

Question 11

How does a 10 fold dilution affect the pH of acids?

Answer 11

Strong acid- increases pH by 1 unit
Weak acid- decrease concentration of hydrogen by factor of 10. Addition of water produces more H+ ions, causing the pH to decrease

Question 12

What is the ionic product of water?

Answer 12

Kw
Equilibrium between water molecules and the ions they dissociate into
[H+][OH-] = 1 x 10^-14 mol2dm-6 (at 25C)

Question 13

How does temperature affect Kw?

Answer 13

As temperature increases, Kw decreases

Question 14

What is Ka?

Answer 14

Direct measure of the strength of an acid

Larger the value of Ka the more dissociated the acid, and the greater its strength as a proton donor

Question 15

What is Ka affected by?

Answer 15

Unaffected by concentration but affected by temperature

Question 16

What is Ka for the reaction:

HA H+ + A-

Answer 16

[H+] [A-]

[HA]

Question 17

How can pKa be worked out?

Answer 17

pKa = -logKa

Stronger acid has a smaller pKa

Question 18

What assumptions are made when calculating the pH of a methanoic acid?

Answer 18

Ignore [H+] due to the dissociation of water [HCOO-] = [H+]

As HCOOH is a weak acid, its concentration remains largely unchanged

Question 19

How do you find the pH of a solution of base?

Answer 19

Find the concentration of OH- ions then use Kw to find [H+]

For a strong base, assume the base is 100% dissociated

Question 20

How do you find the pH of a strong acid?

Answer 20

Assume that the acid is 100% dissociated

[H+] = [HA]

Question 21

How do you find Ka for a weak acid?

weight, volume and pH

Answer 21

Calculate number of moles used
Calculate concentration
Use pH to calculate [H+]
Use Ka equation (assume [H+]=[A-])
Assume no dissociation took place

Question 22

What is the end-point of a titration?

Answer 22

When the appropriate colour changes take place

Question 23

What is the equivalence point of a titration?

Answer 23

Two reactants are mixed in exactly the same proportions indicated by the equation
(Look at the ratio of the acid to base)

Question 24

Why isn’t the equivalence point necessarily 7?

Answer 24

Sometimes a slightly alkaline/acidic product is formed

Question 25

What is the titration curve of a strong acid- strong alkali?

Answer 25

Begins low pH in y axis Initially acts as a buffer solution Rapid change of pH at the equivalence point Finished at high pH

Question 26

What is the titration curve of a strong acid- weak alkali?

Answer 26

Begins low pH in y axis Equivalence point Finishes around pH 9

Question 27

What is the titration curve of a weak acid- strong alkali?

Answer 27

Curve initially where there is buffer action and mid low pH Equivalence point Finishes at high pH

Question 28

What is the titration curve of a weak acid- weak alkali?

Answer 28

Not suitable for a titration as no area of rapid change | Gentle curve

Question 29

What is the titration curve when running a strong acid into a weak base?

Answer 29

Initially there is buffer action

Question 30

What titration curve is produced when an acid is run into an alkali?

Answer 30

Generally produces a similar graph flipped | Except strong acid into weak base

Question 31

What is an indicator and what can the dissociation of ions be represented as for a weak acid?

Answer 31

Generally weak acids or weak bases | HIn (aq) H+ (aq) + In- (aq)

Question 32

How does a weak acid indicator work?

Answer 32

Either weak acid and/or its conjugate base is coloured If pH increases (hydrogen ions are removed) the equilibrium moves to the right to produce more In-, which causes a colour change

Question 33

How is a suitable indicator selected?

Answer 33

pKin should be equal (or as close as possible) to the pH of the equivalence point of the titration Good indicator would show dramatic colour change to easily detect end-point

Question 34

Why is there no suitable indicator for a weak acid-weak base titration?

Answer 34

No vertical portion | No indicator would change dramatically enough so another method would have to be used

Question 35

How can Ka be found for a weak acid from a pH titration?

Answer 35

Weak acid in a conical flask and a standard solution of NaOH in a burette Measure the pH after each addition and plot this on the graph From the graph you can find the equivalence point and the half equivalence point and estimate the pH at this point

Question 36

What is the half-equivalence point?

Answer 36

When half the total volume of NaOH has been added At this point the concentrations of acid molecules and conjugate base ions are equal pKa = pH

Question 37

What is a buffer solution?

Answer 37

Resist a change in pH | Made from a weak acid and its conjugate base

Question 38

How does a buffer solution resist the change in pH?

Answer 38

Check mark scheme

Question 39

How can the proportions needed to make a buffer solution of a certain pH be calculated?

Answer 39

``` pH = pKa - log([A]/[B]) Rearrange the equation to find [A]/[B] Therefore [B] = 1 To make a buffer solution of 100cm3 mol/total mol x 100cm3 ```

Question 40

How does the haemoglobin act as a buffer?

Answer 40

HHb (aq) + O2 (g) H+ (aq) + HbO2- (aq) If pH falls, [H+] increases and the equilibrium adjusts to the left, displacing oxygen from haemoglobin If pH rises, [H+] decreases and the equilibrium adjusts to the right, causing more oxygen to bing to the haemoglobin

Question 41

Why is carbon dioxide a threat to the blood's pH?

Answer 41

Forms carbonic acid which dissociates to produce hydrogen ions Body has backup system to ensure no dramatic change in pH such as with the kidneys

Question 42

What does buffer capacity describe?

Answer 42

The amount of acid or base required to change the pH of the food significantly More protein there is in food the higher its blood capacity

Question 43

What buffer systems do processed foods such as jams contain?

Answer 43

Citric acid and sodium citrate | Helps to maintain the pH within a range where growth of microogansims is very slow of non-existent