Lecture 10: Electrolytes and Buffers

Question 1

pH of blood

Answer 1

7.35-7.45

Question 2

What is a bronsted- lowry acid?

Answer 2

proton donor

Question 3

What is a bronsted-lowry base?

Answer 3

proton acceptor

Question 4

Strong Bronsted bases….

Answer 4

fully protonate

Question 5

Strong bronsted acids

Answer 5

fully donates protons/ fully dissociates or ionises in aqueous solution

Question 6

The conjugate base of a strong acid is a…

Answer 6

Weak base

Question 7

The conjugate acid of a strong base is a…

Answer 7

weak acid

Question 8

Why do strong acids fully dissociate?

Answer 8

The conjugate base of a strong acid is a weak base. It has a poor tendency to attract protons/ doesn’t hold onto the proton very tightly.

Question 9

What is a weak bronsted acid?

Answer 9

An acid that partially donates protons. remains partially undissociated/unionised in an aqueous solution.

Question 10

The conjugate base of a weak acid is a…..

Answer 10

strong base

Question 11

Why do weak acids partially dissociate/ remain undissociated?

Answer 11

• the conjugate base of a weak acid is a strong base.
• the strong base has a strong tendency to accept a proton.
• the conjugate base + proton reforms the weak acid and so pushes the equilibrium to the left- towards the formation of the weak acid.
• hence why the weak acid remains undissociated.

Question 12

What is a weak bronsted base?

Answer 12

Has a weak tendency to accept protons- only partially protonates/ionises.

Question 13

The conjugate acid of a weak base is a….

Answer 13

strong acid

Question 14

Why do weak Bronsted bases only partially protonate/ionise?

Answer 14

The strong conjugate acid has a strong tendency to lose its proton in solution, so it shifts the position of equilibrium to the left, towards the formation of the weak base. Hence why the weak base only partially ionises.

Question 15

What is an electrolyte?

Answer 15

A compound that is ionised in solution.

Question 16

Electrolytes conduct in solution. Explain

Answer 16

Electrolytes produce ions when dissolved in water and therefore can conduct an electric current through the solution.

Question 17

What is a strong electrolyte?

Answer 17

a compound that completely ionises in aqueous solution.

Question 18

Give examples of strong electrolytes

Answer 18

• strong acid
• strong base
• salt

Question 19

What is a weak electrolyte?

Answer 19

A compound that is incompletely ionised in an aqueous solution

Question 20

Give examples of weak electrolytes.

Answer 20

• weak acids
• weak bases

Question 21

What is a non-electrolyte?

Answer 21

Substances that do not yield ions when dissolved in water and therefore do not conduct an electrical current in solution.

Question 22

Give an example of a non-electrolyte

Answer 22

sucrose

Question 23

A salt can be formed from a weak acid and a strong base. When it dissolves and ionises what happens?

Answer 23

It will accept proton and the pH rises.

Question 24

Give an example of a salt formed from a weak acid and strong base. draw equation for it dissolving in water

Answer 24

• CH3COONa- comes from weak acid CH3COOH and strong base NaOH.
• CH3COONa from a weak acid and strong base dissolves in water and ionises in water. It reacts with water and accepts H+ to form CH3COOH + OH-. The pH increases- basics.

Question 25

A salt can be formed from a strong acid and a weak base. When dissolved in water what does it do?

Answer 25

ionises and releases protons. The pH lowers

Question 26

Give an example of a salt that comes from a strong acid and weak base. Draw equation for it’s ionisation.

Answer 26

• NH4Cl is made from a strong acid and a weak base. When it dissolves and ionises in water it reacts with water. The NH4 + ion releases a proton that is accepted by H20. The pH becomes acidic.

Question 27

What is a buffer?

Answer 27

A solution that minimises change in pH when small amounts of acid/base are added. Buffers are resistant to changes in pH

Question 28

Buffers are usually a mixture of ……………. or ………………

Answer 28

• weak acid and conjugate base/salt
• weak base and conjugate acid/salt

Question 29

Why do strong acids/bases make poor buffers?

Answer 29

They dissociate completely in water- they do not form conjugate acid/base pairs.

Question 30

When the buffer solution is present and small amounts of strong acid/base are added, what do they react with? What is the effect?

Answer 30

they do not react with water molecules in the solution, instead reacts with the weak acid/conjugate base. This results in a little/no change in pH of the solution.

Question 31

Why are acidic buffers used more than basic buffers>

Answer 31

not effected by temperature.

Question 32

Define the common ion effect.

Answer 32

refers to when a given ion is added to a mixture at equilibrium that already contains the given ion. When this happens, the equilibrium shifts away from forming more of that ion.

Question 33

What would happen if more acetate ion was added to this solution, from soluble sodium acetate?

Answer 33

the equilibrium position would shift to the left to form more of the non-dissociated acetic acid and the concentration of hydrogen ions would decrease. Acetate therefore suppresses the dissociation of acetic acid

Question 34

A buffer solution must contain high concentrations of…

Answer 34

the acidic and basic components.
The concentration of added H+/OH- ions should be smaller than the concentrations of buffer components. If not, the buffering effect is lost.

Question 35

In a solution of CH3COOH and it’s salt CH3COONa what is the common ion? How is the common ion fromed?

Answer 35

Acetate ion
ionisation of weak acid: CH3COOH + H2O < –> CH3 COO- + H3O+

complete dissociation of salt: CH3COONa < –> Na+ + CH3COO-

Question 36

What happens if a strong base/ OH- ions are added to the acetic acid/acetate buffer?

Answer 36

• If OH- ions are added, the weak acid reacts to form the acetate ion, shifting position of equilibrium to the right. This increases H+ concentration.

Question 36

Describe what happens if H+ ions/ acid are added to an acetic acid/acetate buffer

Answer 36

• If H+ ions are added, the acetate ion will react to reform the weak acid, shifting the position of equilibrium to the left. This reduces the H+ concentration.

Question 37

The common ion is responsible for….

Answer 37

minimising the change in pH

Question 38

What is the Henderson-Hasselbach equation for acid buffers?

Answer 38

Question 39

The pH of a 0.1M solution of acetic acid (pKa=4.76) is 2.88. What is the pH after enough sodium acetate has been added to make the solution 0.1 M with respect to this salt?
What is the pH of the buffer?

Answer 39

Question 40

Give the buffer equation for basic buffers.

Answer 40

Question 41

What is buffer capacity?

Answer 41

The magnitude of the resistance of a buffer to pH changes.

Question 42

Other names for buffer capacity

Answer 42

buffer efficiency, buffer inde, buffer value

Question 43

What is the equation for buffer capacity? What does delta m/pH mean?

Answer 43

delta m = number of moles of base/acid needed to change the pH of one litre of solution by a pH unit, delta pH

Question 44

higher buffer concentration= higher ………….. ………….. . Why?

Answer 44

buffer capacity
A greater amount of strong acid/base would have to be added to disrupt the equilibrium and change the pH

Question 45

A buffer is more effective when the concentration of the components of the buffer are similar. Explain.

Answer 45

If the concentration of the components is similar, the component concentration ratio doesn’t change significantly when acid or base is added