Acids and Bases A2

Question 1

What is a Bronsted-Lowry acid?

Answer 1

A proton/H+ donor

Question 2

What is a Bronsted-Lowry base?

Answer 2

A proton/H+ acceptor

Question 3

What is the acid dissociation constant?

Answer 3

Ka= [H+(aq)][A-(aq)]
[HA(aq)]

Question 4

How does the acid dissociation constant tell us the strength of an acid?

Answer 4

A large Ka value shows that the extent of the dissociation is large so the acid is strong

Question 4

what are the units for the acid dissociation constant?

Answer 4

mol dm-3

Question 5

What is pH defined as?

Answer 5

pH=-log10[H+(aq)]

Question 6

How can H+(aq) be calculated from pH?

Answer 6

H+=10^-pH

Question 7

How do you calculate the pH of a weak acid and why?

Answer 7

Ka = [H+(aq)]^2
[H20(l)]

Due to the negligible amount of ionisation of water the concentration of H+ is approx = to the concentration of OH-

Question 8

How do you calculate pKa and what is it?

Answer 8

pKa = -log10Ka

It is basically the same thing as pH, it’s just another way to find out the strength of an acid/alkali. So a low value of Ka matches to a high value of pKa

Question 9

How do you find the ionic product of water?

Answer 9

Kw= [H+(aq)][OH-(aq)]

Question 10

What are the units for Kw?

Answer 10

mol^2dm-6

Question 11

At 25 degrees Celsius, what is the value of Kw?

Answer 11

Kw = 1.0 x 10^-14 mol^2dm-6

Question 12

In water what are the concentrations of [H+(aq)] and [OH-(aq)] ions?

Answer 12

They are the same: 10^-7 moldm-3

Question 13

What can we refer to pure water as and why?

Answer 13

Kw = [H+(aq)]^2

Since in pure water the conc of H+ ions = OH- ions

Question 14

How to calculate the pH of strong bases/alkalis?

Answer 14

We need to use Kw to get the [H+(aq)] value to put into the pH equation ( Kw = [H+(aq)] [OH-(aq)] )

also, we need to know that the concentration of the base = the concentration of the OH- ions

Question 15

In strong Alkalis/bases why does the conc of base = the conc of OH- ions?

Answer 15

Most strong bases disassociate to produce 1 OH- ion for every base molecule

Question 16

What does a titration show us?

Answer 16

The endpoint of a titration, as well as the change in pH across a titration

Question 17

What is the endpoint of a reaction?

Answer 17

When neutralisation occurs

Question 18

How do you know how to pick a suitable indicator for your titration?

Answer 18

You need to make sure that it changes colour within the end point of your titration

Question 19

What indicator(s) can be used for the titration between a strong acid and alkali?

Answer 19

Phenolphthalein and Methyl Orange

Question 20

What indicator(s) can be used for the titration between a strong acid and a weak alkali?

Answer 20

Methyl Orange

Question 21

What indicator(s) can be used for the titration between a weak acid and a strong alkali?

Answer 21

Phenolphthalein

Question 22

What indicator(s) can be used for the titration between a weak acid and alkali?

Answer 22

Neither Phenolphthalein nor Methyl Orange

Question 23

What is Methyl Orange like at low and high pH values?

Answer 23

RED: Low pH values
YELLOW: High pH values

Question 24

What is Phenolphthalein like at low and high pH values?

Answer 24

COLOURLESS: Low pH values
PINK: High pH values

Question 25

Between what pHs does Methyl Orange change colour?

Answer 25

pH 3 - 4.5

Question 26

Between what pHs does Phenolphthalein change colour?

Answer 26

pH 8.2 - 10

Question 27

What do buffer solutions do?

Answer 27

A buffer solution maintains an approximately constant pH, despite the dilution or addition of small amounts of acid or base

Question 28

What are there in any buffer solution?

Answer 28

2 equilibrium equations; they co-exist together
Acidic buffer solutions contain a weak acid and the salt of that weak acid.
Basic buffer solutions contain a weak base and the salt of that weak base.

Question 29

What happens to salts in equilibrium?

Answer 29

They dissociate fully