Topic 12: Acid-Base Equilibria

Question 1

Define a Bronsted-Lowry acid

Answer 1

A proton donor
e.g. HCl is a Bronsted-Lowry acid, as it donates a proton:
HCl —> H+ + Cl-

Question 2

Which ions make solutions acidic?

Answer 2

Hydroxonium ions
H3O+

Question 3

Define a Bronsted-Lowry base

Answer 3

A proton acceptor
e.g. OH - is a Bronsted-Lowry base, as it accepts a proton:
OH- + H+ —-> H2O

Question 4

What are conjugate acid-base pairs?

Answer 4

A pair of reactants and products that are linked to each other by the transfer of a proton

Question 5

How do you identify acid-base conjugate pairs?

Answer 5

Acid = acid formula
Conjugate acid = gains a proton
Base = base formula
Conjugate base = loses a proton

Question 6

Define the term pH

Answer 6

the acidity of an aqueous solution depends on the number of H+ ions in solution.
It is defined as -log10[H+]

Question 7

pH equation

Answer 7

pH = -log10[H+]

Question 8

Hydrogen ion concentration

Answer 8

[H+] = 10^-pH

Question 9

What is a strong acid?

Answer 9

A strong acid fully dissociates in solution
e.g. H2SO4, HCl, HNO3

Question 10

What is a weak acid?

Answer 10

Partially dissociates in aqueous solution
e.g. CH3CH2COOH, HCN

Question 11

How do you calculate the pH of a strong acid?

Answer 11

-log10[H+]

Question 12

What is Ka, and when is it used?

Answer 12

Ka is the acid dissociation constant, and is used for weak acids.

Question 13

What is the Ka expression?

Answer 13

Ka = [H+] [A-]
————
[HA]

Question 14

What assumptions are made when writing the equilibrium expression for weak acids?

Answer 14

The concentration of hydrogen ions due to the ionisation of water is negligible

Question 15

What do Ka values indicate about dissociation?

Answer 15

The higher the value of Ka the more dissociated the acid and the stronger it is
The lower the value of Ka the weaker the acid

Question 16

Equation for pH of a weak acid

Answer 16

-log10(√(Ka ×[weak acid]) )

Question 17

Define the ionic product of water

Answer 17

An equilibrium exists where some water molecules partially dissociate:
H2O —> OH- + H3O+
H20 —> OH- + H+
Kw = [H+] [OH-]

Question 18

What is the exact value for Kw

Answer 18

1.00 x 10^-14 mol2dm-6

Question 19

Equation for pH of a strong base

Answer 19

14 - (-log10[H+])

Question 20

Define the term ‘pKa’

Answer 20

pKa = -log10 Ka

The lower the value for pKa, the stronger the acid.

The higher the value for pKa, the weaker the acid

Question 21

Define the term ‘pKw’

Answer 21

pKw = -log10 Kw

The Kw value of 1.00 x 10-14 mol2 dm-6 at room temperature gives you a pKw value of 14

Question 22

How do you calculate Ka for a weak acid from pH?

Answer 22

1: balanced dissociation equation for the weak acid.
2: ICE Table for the disassociation of the weak acid.
3: Write the equilibrium expression of Ka for the reaction
4: Using the given pH, determine the concentration of hydronium ions present with the formula:
5: Solving for the concentration of hydronium ions gives the x M in the ICE table. Substitute the hydronium concentration for x in the equilibrium expression.
6: Simplify the expression and algebraically manipulate the problem to solve for Ka.

Question 23

What is the equivalence point?

Answer 23

when you mix the two solutions together in the correct proportions according to the equation and they neutralise.

Question 24

What is the end point?

Answer 24

When the indicator changes colour

Question 25

What does the pH curve look like for a strong acid-strong base titration? (pH/vol. acid)

Answer 25

Curve begins at pH 14 Decreases to pH 12 - very flat Sharp drop to pH 4 Slow decrease to pH 1

Question 26

How do you know the volume needed to neutralise the acid, from the pH graph?

Answer 26

Calculate the volume at the equivalence point (pH 7)

Question 27

Steps for drawing titration curves

Answer 27

1. Calculate start and end pH values of the substance in flask 2. Calculate the equivalence point via a titration calculation 3. Draw the curve following the correct pH points.

Question 28

Where is the equivalence point for a weak acid-strong base titration?

Answer 28

The volume at halfway up the vertical section

Question 29

Where does pH=pKa on the weak acid-strong base curve?

Answer 29

Halfway to the volume at equivalence point

Question 30

What does the pH curve look like for a weak acid-strong base titration? (pH/vol. alkali)

Answer 30

Curve starts at around pH 3 Small curve upwards Flat Curve shoots up sharply to around pH 13 Flattens at top

Question 31

What values do you centre the pH around for a weak/strong titration?

Answer 31

- A weak acid centres around pH, so draw vertical section from pH 7-11 - A weak alkali centres at pH 6, so draw vertical section from pH 4-8

Question 32

What values do you centre the pH around for a strong/strong titration?

Answer 32

It will centre around pH 7 A strong acid in the burette finishes at pH 0-1. A strong alkali in the burette finishes at pH 12-13.

Question 33

What are indicators?

Answer 33

Weak acids or bases which change colour when they lose or gain protons

Question 34

What is the equilibrium for Litmus paper?

Answer 34

HLit ---> Lit- + H+ red blue

Question 35

How do you choose an indicator?

Answer 35

Choose an indicator that has its pH range within the vertical section of the pH curve. The colour change will then happen on addition of a very small volume of solution from the burette.

Question 36

What are the colour changes for Phenolphthalein?

Answer 36

in Acid - colourless in Alkali - pink

Question 37

What are the colour changes for methyl orange?

Answer 37

in Acid - red in Alkali - yellow endpoint - orange

Question 38

Which indicator do you use for a strong acid?

Answer 38

Methyl orange

Question 39

Which indicator do you use for a strong base?

Answer 39

Phenolphthalein

Question 40

What is meant by the term 'Buffer solution'?

Answer 40

A solution that resists changes in pH when small amounts of acid or alkali are added.

Question 41

Devise an experiment to determine the Ka of a weak acid.

Answer 41

1. Titrate the weak acid with the strong base 2. Measure the pH at regular intervals 3. Plot a graph of pH against the volume of strong base 4. Use the graph to find the pH at half equivalence point 5. A half neutralisation pH = pKa Therefore, Ka = 10^pKa

Question 42

How does blood act as a buffer?

Answer 42

1. CO2 dissolves into the blood forming carbonic acid 2. Equilibrium shifts to the right, producing more H+ ions 3. The large reservoir of HCO3 - ions combine with H+ ions to prevent a pH change