3. EEE-Acids, Bases And Buffers

Question 0

(Brønstead-Lowry) Base

Answer 0

A proton acceptor

Question 1

(Brønstead-Lowry) Acid

Answer 1

A proton donor

Question 2

Conjugate acid-base pair

Answer 2

A pair of two species which transform into each other by the gain or loss of a proton

Question 3

Strong acid

Answer 3

An acid which completely dissociates in solution

Question 4

Weak acid

Answer 4

An acid which partially dissociates in solution and exists in equilibrium

Question 5

Acid dissociation constant, Ka

Answer 5

Shows the extent of acid dissociation

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

Question 6

pKa=

Answer 6

-log(Ka)

Question 7

pH=

Answer 7

-log(H+)

Question 8

Ionic product of water, Kw

Answer 8

=[H+][OH-]

=1*10^-14 (on data sheet)

Question 9

Ka for weak acids

Answer 9

=[H+]^2

[HA]

Question 10

Buffer solution

Answer 10

A system that minimises pH changes in addition of a small amount of an acid or base

Can be made from a weak acid and a salt of the weak acid (a salt of its conjugate base)
eg CH3COOH/CH3COONa

Question 11

Explain the role of the conjugate acid-base pair in the buffer solution in the control of pH

/explain how a buffer works

Answer 11

The buffer solution contains large reservoirs of the weak acid (HA) and its conjugate base (A-)
HA—->A-+H+

Acid added: increasing the concentration of H+
A- reacts with the (extra) H+ (forming HA)
The equilibrium shifts to the left to minimise the change

Alkali added: increasing the concentration of OH-
H+ reacts with the OH- (forming water) removing them from the equilibrium
[H+] decreases
HA dissociates more to produce more H+ ions and minimise the change
Equilibrium shifts to the right

Question 12

How do you choose the acid to use in a buffer solution?

Answer 12

We want [A-] to be as close as possible to [HA]
So choose the acid with a pKa as close as possible to the desired pH

Desired pH of solution=pKa of acid

Question 13

Explain the role of carbonic acid-hydrogencarbonate as a buffer in the control of blood pH

Answer 13

Same as other buffers but using specific names:

HA= carbonic acid
A-=hydrogencarbonate ions
Equation: H2CO3 —> H+ + HCO3-

The human blood plasma needs to have a pH between 7.35 and 7.45

Most materials released into the blood are acidic and are removed by the hydrogencarbonate ions by being converted into carbonic acid

The carbonic acid is then converted into aqueous carbon dioxide by the action of an enzyme

In the lungs the dissolved carbon dioxide is converted into gas and exhaled

Question 14

Enthalpy change of neutralisation

Answer 14

The energy change that accompanies the neutralisation of aqueous acid by an aqueous base (alkali?) to form one mole of H2O(l) under standard conditions

(Weak acids are less exothermic)