Topic 12 - Acids

Question 1

Define was an acid and base is according to Bronsted-Lowry

Answer 1

Acid is a substance that donates a proton - proton donor (H+) and a base is a substance that accepts a proton - a proton acceptor

Question 2

What are conjugate acid-base pairs?
Use examples…
HCl + H2O –> H3O+ + Cl-
NH3 + H2O —> NH4+ + OH-

Answer 2

Consists of either a base and its conjugate acid or an acid and its conjugate base - when base accepts a proton species formed is the conjugate acid of the base
When acid donates a proton, species formed is the conjugate base of the acid

1) In forward reaction, HCl acts as acid because donating proton to H2O and H2O acts as base as it accepts the proton. In the reverse reaction, H3O+ acts as an acid because it is donating a proton to Cl- and Cl- behaves as a base as accepts the proton. When HCl loses proton it forms base Cl- and H3O+ loses a proton to form base H2O
2) In forward reaction, NH3 acts as base because accepting proton from H2O and H2O acts as acid because donating a proton to NH3. NH4+ acts as acid because donating proton to OH- while OH- acts as base. So contains two acid-base conjugate pairs

Question 3

What are amphoteric substances?

Answer 3

A substance that can act as either a base or an acid e.g. H2O is a base in HCl but acts as an acid when react with NH3

Question 4

Define strong and weak acids

Answer 4

Strong acid is defined as one that is almost completely dissociated in aqueous solution e.g. Dilute solution of HCl virtually all Hydrogen chloride molecules dissociated
Weak acid defined as one tha is only partially dissociated in aqueous solution usually organic acids such as ethanoic acid

Question 5

Define pH

Answer 5

-log([H+])

Question 6

Calculating the pH of a strong acid e.g.
0.001 mol/dm-3 HCl

Calculating hydrogen ion concentration from the pH e.g.
pH = 4.8

Answer 6

-log(0.001) = pH

pH = 3

10^-4.8 = 1.58 X 10^-5 mol/dm -3

Question 7

Determining the hydrogen ion concentration of a weak acid using HA –> H+ + A-

E.g. Calculate [H+] of ethanoic acid of concentration 0.05 mol/dm-3
Value of Ka = 1.74 X 10^-5

Answer 7

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

We assume that the concentration of H+ is = to A-
1.74x10-5 X 0.05 and square root = 9.33 X 10^-4
-log = pH
= 3.03

Question 8

Convert pKa to Ka and vice versa

Answer 8

pKa - log(Ka)

Question 9

Describe dissociation of water giving the constant

Answer 9

H2O ---> H+ + OH-
= [H+][A-] / [HA] 
Which is simplified to
[H+][OH-]
This is Kw 
Value of Kw at 298K = 1.00 X 10^-14

Question 10

Find the pH of aqueous solution of strong bases e.g. Sodium hydroxide of concentration 0.1 moldm-3 has hydroxide ion concentration of 0.1 find pH

Answer 10

If [OH-] = 0.1 and [H+][OH-] = 1 X 10 -14

==> 1x10-14 / 0.1 = 1x10-13
-log(1x10-13)
pH = 13.0

Question 11

Comparing solutions after measuring the pH

1) strong and weak acids
2) strong and weak bases
3) salts

Answer 11

1) measuring pH at same temperature, the higher the pH, the weaker the acid
2) same method used, the higher the value of pH, the stronger the base
3) e.g. PH of aqueous solutions of various salt concentrations
If salt product of strong acid and base then pH 7
If salt product of strong base but weak acid then it is alkaline
If salt product of strong acid but weak base then it is acidic
If salt product of weak acid and base and relative strengths are the same then neutral pH 7

Question 12

Describe effects of dilution on the pH of aqueous solutions of acids

Answer 12

Strong acids: Each acid has been diluted by factor of 10 - pH increases by factor of one unit for each 10-fold decrease in concentration

Weak acid: With weak acids, pH value increases by factor of 0.5 for each 10-fold decrease in concentration

Question 13

Describe result of acid-base titration of…

1) strong acid with strong base
2) weak acid with strong base
3) strong acid with weak base
4) weak acid with weak base

Talk about where equivalence point lies on graphs

Answer 13

1) pH falls slowly and steep drop between 11 to about pH 5 where equivalence point lies around pH 7.
2) pH falls slowly but equivalence point between pH 8 or 9 - buffer zone after equivalence point which resits around further large change in pH
3) pH falls sharply and curve starts to level out, equivalence point between pH 5 and 6
4) No steep section vbut a point of inflection around pH 7

Question 14

Describe what indicator is appropriate for the following titrations

1) strong acid - strong base
2) weak acid - strong base
3) strong acid - weak base
4) weak acid - weak base

Answer 14

1) pH range for phenolphthalein and methyl orange lies within steep section so both indicators with change colour at end point so are both suitable
2) Only pH range for phenolphthalein lies within steep section so methyl orange is not suitable
3) Only pH range for methyl orange lies within steep section
4) No steep section so no indicator is suitable

Question 15

What is a buffer solution?

Answer 15

Solution that minimises the change in pH when a small amount of either acid or base is added

Question 16

Making a buffer solution from a weak acid and its conjugate base

Answer 16

Simplest example of this is ethanoic acid and sodium ethanoate. The salt of the weak acid has to be soluble in water. The acid is partially dissociated whereas the salt is fully dissociated - mixture will produce buffer solution with pH less than 7

Question 17

How to calculate pH of a buffer solution? E.g. Using solution made of mixing equal volumes of 1.00 Moldm-3 ethanoic acid and 1.00 moldm-3 sodium ethanoate
Ka = 1.74 x 10^-5
STATE ASSUMPTION

Answer 17

Assume extent of dissociation of the acid is negligible then the concentration of CH3COOH at equilibrium is 0.5. Also because dissociation of acid is negligible the concentration of ethanoate ions considered to be made up from sodium salt so also 0.5
[H+] = 1.74 x 10^-5 x 0.5 / 0.5 = 1.74 x 10^-5
pH = 4.76

Question 18

Define the Henderson-Hasselbalch equation

Answer 18

Used to calculate pH of a buffer solution

Ka X [HA]/[A-]