Acids and bases

Question 1

A Bronsted-Lowry acid

Answer 1

A proton donor, an acid dissociates and releases H+ into an aqueous soloution

Question 2

A Bronsted-Lowry base

Answer 2

A proton acceptor, alkalis dissociate and release OH- ions into aqueous soloutions

Question 3

Conjugate acid-base pair

Answer 3

A conjugate acid-base can be interconnected by the transfer of a proton, ie HCl or Cl-

Question 4

Monobasic

Answer 4

Contains one hydrogen ion

Question 5

Dibasic

Answer 5

Contains two hydrogen ions

Question 6

Tribasic

Answer 6

Contains three hydrogen ions

Question 7

Half equation of acid and metal

Answer 7

2H+ + Zn –> Zn+2 + H2

Question 8

Calculation changes in pH of the buffer

Answer 8

If a small amount of alkali is added to a buffer then the moles of the buffer acid would reduce by the number of moles of alkali added and the moles of salt would increase by the same amount so a new calculation of pH can be done with the new values. If a small amount of acid is added to a buffer then the moles of the buffer salt would reduce by the number of moles of acid added and the moles of buffer acid would increase by the same amount so a new calculation of pH can be done with the new values.

Question 9

Half equation of acid and carbonate

Answer 9

2H+ + CO3-2 –> H2O + CO2

Question 10

Half equation of acid with metal oxide

Answer 10

2H+ + MgO –> Mg+2 + H2O

Question 11

Half equation of acid and alkali

Answer 11

H+ + OH- –> H2O

Question 12

pH of strong acid

Answer 12

You assume it fully dissociates so if it is a monobasic acid its concentration will equal the concentration of H+

Question 13

Working out pH

Answer 13

-log10[H+]

Question 14

Working out the concentration of H+

Answer 14

10^-pH

Question 15

Ka

Answer 15

[HA]

Question 16

pKa

Answer 16

-log10[ka]

Question 17

Approximations about ka

Answer 17

That HA dissociates to produce an equal concentration of A- and H+ and the H+ from the dissociation of water will be neglected because it is so small. The second assumption is that because the dissociation of HA is so small it can be discounted, so you assume that the concentration of HA is the same at the start as it is at equilibrium

Question 18

What’s the wrong about the first ka approximation

Answer 18

When the pH is very weak i.e. above 6, then the dissociation of water will be significant when compared to the weak acid

Question 19

What’s wrong with the second ka approximation

Answer 19

The dissociation of the weak acid will be significant for stronger weak acids as there will be a real difference between the concentration of HA at the start and at equilibrium

Question 20

The ionic product of water

Answer 20

Kw = [H+] x [OH-]

Question 21

Value of Kw

Answer 21

1 x10^-14

This value is constant at 25 degrees

Question 22

Figuring out the pH of a strong base

Answer 22

For bases we are normally given the concentration of the hydroxide ion, as we assume the base completely dissociates to give hydroxide ions. To work out the pH we need to work out [H+(aq)] by rearranging the kw expression

Question 23

Equation of simplified Ka

Answer 23

[H+ (aq)]2
——————- = Ka
[HA (aq)]initial

Question 24

To find out the pH of a diluted strong acid

Answer 24

[H+] = [H+]old x old volume
—————————-
new volume
pH = – log [H+]

Question 25

What’s a buffer soloution

Answer 25

A Buffer solution is one where the pH does not change significantly if small amounts of acid or alkali are added to it.

Question 26

How to make a buffer soloution

Answer 26

An acidic buffer solution is made from a weak acid and a salt of that weak acid eg ethanoic acid and sodium ethanoate. The salt content can be added in several ways: a salt solution could be added to the acid or some solid salt added. A buffer can also be made by partially neutralising a weak acid with alkali and therefore producing a mixture of salt and acid. A basic buffer solution is made from a weak base and a salt of that weak base eg ammonia and ammonium chloride

Question 27

What happens when acid is added to a buffer

Answer 27

If small amounts of acid is added to the buffer: Then the equilibrium will shift to the left removing nearly all the H+ ions added

Question 28

What happens when an alkali is added to a buffer

Answer 28

If small amounts of alkali is added to the buffer. The OH- ions will react with H+ ions to form water. The Equilibrium will then shift to the right to produce more H+ ions. Overall the concentration of H+ ions and pH remains constant

Question 29

Finding out the pH of a buffer

Answer 29

You use the Ka expression but you ignore the assumptions we made for a weak acid and we assume that the A- concentration is just due to the added salt. When we are adding an alkali to an acid to make the buffer soloution, we calculate the moles of acid and alkali. The moles of alkali will be the moles of the salt and moles of acid takeaway moles of alkali, will be the moles of acid. You then convert them into concentration

Question 30

Buffer soloution in blood

Answer 30

A carbonic acid (H2CO3) – hydrogencarbonate (HCO3-) equilibrium acts as a buffer in the control of blood pH. It maintains the pH between 7.35 - 7.45

Question 31

Indicator used in strong base-strong acid titration

Answer 31

Methyl orange, phenylthalein

Question 32

Indicator used in strong base-weak acid titration

Answer 32

Phenylthalein

Question 33

Indicator used in weak base-strong acid titration

Answer 33

Methyl orange

Question 34

Indicator used in weak base-weak acid titration

Answer 34

None

Question 35

How to pick an indicator

Answer 35

The end-point of a titration is reached when [HIn] = [In-]. To choose a correct indicator for a titration one should pick an indicator whose end-point coincides with the equivalence point for the titration.