Equilibrium, Acids, Bases & Buffers

Question 0

Equilibrium constant, Kc for the reaction A + 2B –> C + D is

Answer 0

[C][D] / [A][B]^2

Question 1

What effect does an increase in temperature have on Kc for an exothermic reaction?

Answer 1

An increase in temperature shifts the equilibrium in the endothermic direction. This means the number of moles of the reactants increases, and so Kc decreases

Question 2

Kc is unaffected by….

Answer 2

Changes in concentration or pressure, or by the presence of a catalyst

Question 3

Define acid

Answer 3

A species capable of donating a proton

Question 4

Define base

Answer 4

A species capable of accepting a proton

Question 5

Define a weak acid

Answer 5

Partially dissociates in solution

Question 6

Define conjugate acid-base pairs and give an example

Answer 6

A pair of two species that transform into each other by gain or loss of a proton
Eg HNO2 and NO2-

Question 7

Define a strong acid

Answer 7

Fully dissociates in solution

Question 8

Define pH

Answer 8

-log[H+]

Question 9

Give some examples of strong acids

Answer 9

Hydrochloric, nitric, sulphuric, hydrobromic

Question 10

Give some examples of weak acids

Answer 10

Phosphoric, sulfurous, methanoic, ethanoic

Question 11

Define Ka, the acid dissociation constant

Answer 11

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

Question 12

For a weak acid, it can be assumed that…

Answer 12

[H+] = [A-]

Question 13

What does a large Ka value tell us?

Answer 13

A large Ka means the equilibrium is primarily to the right, which indicates a large extent of dissociation, and so a strong acid

Question 14

pKa =

Answer 14

-log[Ka]

Question 15

For a strong acid it can be assumed…

Answer 15

[H+] = [HA]

Question 16

What is Kw, the ionic product of water

Answer 16

[H+][OH-]

Question 17

For a strong base, it can be assumed…

Answer 17

[OH-] = [base]

Question 18

Describe a buffer solution

Answer 18

A system that minimises pH changes on addition of small amounts of an acid or a base

Question 19

A buffer solution can be made up of…

Answer 19

A weak acid and the salt of the weak acid

Eg CH3COOH and CH3COONa

Question 21

Describe how a buffer solution minimises changes in pH on the addition of an acid

Answer 21

HA –> A- + H+
[H+] is increased
The conjugate base, A-, reacts with H+ ions
The equilibrium shifts to the left, removing most of the added [H+] ions

Question 21

pH of a buffer solution =

Answer 21

pKa + log ([A-]/[HA])

Question 22

Describe how a buffer solution minimises changes in pH on the addition of an alkali

Answer 22

HA –> A- + H+
[OH-] is increased
Some of the H+ ions react with the OH- ions
OH- + H+ –> H2O
This causes more of HA to dissociate, shifting the equilibrium to the right, to restore most of the H+ ions that have reacted

Question 23

What is the equivalence point of a titration curve?

Answer 23

The point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution
(The centre of the vertical section of the titration curve)

Question 24

What buffer is used to control blood pH?

Answer 24

Carbonic acid acts as the weak acid

H2CO3 –> HCO3- + H+

Question 25

Define the end point of a titration

Answer 25

The point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator

Question 26

There are no indicators suitable for a titration of a…

Answer 26

Weak acid with a weak base. There is no vertical section to the titration curve, an indicator would change colour gradually over a few cm^3

Question 27

Define standard enthalpy change of neutralisation

Answer 27

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

Question 28

Definition of an alkali

Answer 28

A soluble base that realises OH- ions in solution

Question 29

What acids have a Ka value?

Answer 29

Weak acids

Question 30

What is the percentage molar dissociation of an acid?

Answer 30

[H+] / [HA] x 100