1.12 Acids and Bases Part 2

Question 1

What happens in an acid-base titration

Answer 1

A solution of base (an alkali) of know conc is added from a burette to a measured amount of acid.
An indicator is used to find the end point
From the volume of base required, the conc of acid can be calculated

Question 2

What is a titration curve

Answer 2

-A plot of pH against volume of base added
-Ph is measured using a pH meter
-The shape of the curve depends on the strength of the acid and base used

Question 3

What is the equivalence point

Answer 3

The equivelence point of a titration is the point at which the acid and base are in the correct molar ratio

Question 4

What are important features of a pH curve

Answer 4

The following depend on strength of acid and base used:
-Initial pH
-Volume of base required for equivalence
-Ph at the equivalence point
-Ph range of the vertical portion of the curve
-The final pH

Question 5

What is an indicator for an acid-base titration

Answer 5

weak acid
Different indicators change colour at different pH values beacuse they have different Ka Values

Question 6

What is the end point of a titration

Answer 6

-The end point of a titration is the point at which the indicator changes colour
-The equivalence point of a titration is the point at which the acid and base are in the correct molar ratio
-For a successful titration, these two points must correspond, so that the indicator changes sharply at the euivelence point

Question 7

How do you choose the indicator used in a titration

Answer 7

-Choice of indicator depends on the strengths of the acid and base used in the titration
-The pH range of the indicator must correspond to the region of rapid pH change.
This is the vertical portion of the pH curve

Question 8

How do you calculate Ka from half equivalence

Answer 8

At half equivalence, HA = A-
Half of the acid HA has been neutralised and turned into A-
Ka = [H+]

Question 9

What is a buffer

Answer 9

A solution that maintains an approximatley constant pH on the addition of small amounts of acid or base

Question 10

What are the applications of buffers

Answer 10

-Shampoos

-Biological washing powder

Question 11

What is an acidic buffer solution

Answer 11

An acidic buffer solution is a mixture of a weak acid, HA and one of its salts, A-.
It maintains a pH below 7

Question 12

How can acidic buffer be made

Answer 12

-Add together a weak acid and one of its salts. HA + A-
-Partly neutalise a weak acid to get some HA and some A-

Question 13

What is the action of a buffer when H+ is added

Answer 13

H+ + A- -> HA
A- reacts with added H+ to create HA.
Equilibrium shifts to the LHS

Question 14

What is the action of a buffer when alkali, OH- ions are added

Answer 14

OH- + H+ -> H2O
OH- added reacts with H+ to create H2O.
As a result, equilibrium shifts to the RHS

Question 14

What is a basic buffer solution

Answer 14

A mixture of a weak base, B andone of its salts, BH+. It maintains a pH above 7

Question 15

How can a basic buffer be made

Answer 15

-Add together a weak base, B and some of its salt, BH+
-Partly neutralise some weak base, B, to leave some B and some of its salt, BH+

Question 16

How do you calculate the pH of an acidic buffer solution

Answer 16

-In an acidic buffer solution [HA] and [A-] are much greater
-Large quantities of HA and A- present
-Unlike a weak acid alone [H+} does not equal [A-]
-the pH of an acidic buffer soltion depends on the Ka of the acid and the ratio of [HA] to [A-]

Question 16

Whats the equation to calcualate the pH of an acidic buffer solution

Answer 16

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

Question 17

What is the buffer range

Answer 17

The range of pH values which a buffer solution using a particular weak acid can have. This is normally pKa ± 1

Question 18

How do you prepare an acidic buffer solution

Answer 18

Adding sodium hydroxide solution to excess weak acid
HA + NaOH -> NaA + H2O
If HA is in excess, HA and A- remain

Question 19

What is enthalpy of neutralisation

Answer 19

the energy change that accompanies the neutralisation of an aquous acid by an aqueous base to give one mole of water under standard conditions
-If the temp increases during a reaction ΔH is negative
-If temp decreases during a reaction ΔH is positive