Acids, Bases and Buffers

Question 1

What is a Bronsted-Lowry acid?

Answer 1

A proton donor

Question 2

What is a Bronsted-Lowry base?

Answer 2

A proton acceptor. (Dissociates OH-)

Question 3

What happens when you put a strong acid into water?

Answer 3

The acid donates its proton, which forms a covalent bond with the water, producing H30+ and a negative ion.
Eg: HCl + H20 —> H3O+ + Cl-
(water can act as both an acid and base)

Question 4

Why is water both an acid and base?

Answer 4

HCl + H2O —> H3O+ + Cl-
H30+ + Cl- —> HCl + H2O
Water accepts protons from stonger acids (base) and donates protons and donates a proton to the Cl- (acid)
(water can act as both an acid and base)

Question 5

What are conjugate acid-base pairs?

Answer 5

Where both species in a reversible reaction act as both a base and acid.
Eg: CH3COOH (aq)+H2O (l)⇌CH3COO- (aq) +H3O+ (aq)

HCl(aq)+ H2O(l)⇌H3O+(aq) + Cl-(aq)

Question 6

Give an example of a monobasic, dibasic, and tribasic acid.

Answer 6

HCl, H2S04, H3PO4

Question 7

What is Ka? Give its expression

Answer 7

Acid dissociation constant, useful for weak acids as they don’t fully dissociate.
Ka = [H+][A-] / [HA]
HA⇌ H+ + A-
[H+] is the concentration of hydrogen ions, [A-] is the concentration of the conjugate base, [HA] is the concentration of the acid

Question 8

How do you find pH from [H+]

Answer 8

pH= -log[H+]

Question 9

How do you find [H+] from pH?

Answer 9

[H+]= 10^-pH

Question 10

What is kw?

Answer 10

The acid dissociation constant of water multiplied by water concentration = ionic product of water = Kw.
kc = [H+] [OH-] / [H2O]. Because the concentration of water is constant:
kc = [H2O] = [H+] [OH-]
Kw = [H+] [OH-] = 1x10^-14 mol2 dm-6 AT 298K

Question 11

What can be Kw used for?

Answer 11

Any aqueous solution. So the product oh [H+] [OH-] must be 1x10^-14 mol2 d m-6

Question 12

How do you find the pH of strong bases?

Answer 12

[OH-] = base concentration (when monoprotic).
Insert into kw equation and rearrange for [H+].
[H+] = 1x10^-14 / [OH-]
Insert [H+] into pH=-log[H+]

Question 13

What is ka in weak acids and explain why.

Answer 13

ka = [H+]^2 / [HA]
This is because weak acids partially dissociate so the concentration of H+ will be equal to conjugate base concentration (when the acid is all that is in solution).

Question 14

What does a very small ka indicate about ewuilibrium posotion and acid strength.

Answer 14

The equilibrium is far to the left so the acid must be weak due to lack of dissociation into the conjugate base and protons.

Question 15

What are the two assumptions made when using ka to find pH of weak acids?

Answer 15

That the concentration of H+ and conjugate base are the same.
That the concentration of the associated acid stays the same, or barely changes.
(As ka increases, more H+ is dissociated so the second assumption becomes less true)
So, ka is only useful for weak acids.

Question 16

What is a buffer?

Answer 16

A solution that minimises change to pH on the addition of strong acids or alkali. It contain s a mixture of weak acid and its conjugate base.
Eg: aqueous mixture of ethanoic acid and sodium ethanoate
CH3COOH (ethanoic acid) + CH3COO- (sodium ethanoate ion)
CH3COOH <—> CH3C00- + H+

Question 17

Explain what happes when acid is added to a buffer solution.

Answer 17

CH3COOH <—> CH3C00- + H+
The acid dissociates into its conjugate base and H+ ion. The ion is accepted by the conjugate base of the weak acid in the buffer solution. This shifts the equilibrium to the left so more of the weak acid is made. so [H+] doesn’t increase much. Therefore pH doesn’t decrease much.
H+ + CH3COO- —> CH3COOH

Question 18

Explain what happens when alkali is added to a buffer solution.

Answer 18

CH3COOH <—> CH3C00- + H+
OH- addition.
OH- + CH3COOH —> CH3COO- +H2O
OH - + H+ —. H20
Shifts equilibrium to the right.
Lowered [OH-] means pH won’t increase much

Question 19

How do you make a buffer?

Answer 19

1.Mixing together the acid and a salt.
2.Reacting excess acid with a strong base. You make a salt and excess acid in solution together.

Question 20

What is M? eg 0.1M of CH3COOH?

Answer 20

It stands for molarity, meaning in every litre, there are 0.1 moles of a solute.
Eg, in 100cm3 of 0.1M CH3COOH, is 0.01 mol of CH3COOH.
1000cm 3 (1L) = 0.1mol
100cm3 = 0.01 mol
50cm3 = 0.005mol
25cm3 = 0.0025 mol

Question 21

How is the pH of blood controlled? Give the equation.

Answer 21

Via a buffer solution of hydrogen carbonate ions and carbonic acid.
1. CO2 (g) + H2O (l) —> H2CO3 (aq) (carbonic acid)
2. H2CO3 (aq) —-> H+ (aq) + HCO3- (aq)
CO2 (g) + H2O (l) ⇌ H+ (aq) + HCO3- (aq)

Question 22

Why is the ratio of hydrogen carbonate and carbonic acid important?

Answer 22

Because the ratio of H2CO3 and HCO3- control blood pH by acting as a buffer so that ka and therefore acid dissociating doesn’t change too much. So the body systems aren’t affected by additions of acid or alkali to the blood.

Question 23

What is a neutralisation curve?

Answer 23

A graph that shows how pH changes in a tritration/ as base/acid is added to a base/acid.

Question 24

What is the equivalence point on a tritration curve?

Answer 24

The point at which just enough of the solution in the burette is added to completely neutralise the solution in the conical flask. AKA the midpoint of the curve.

Question 25

Why do weak weak titration curves have no end point/neutralisation point?

Answer 25

There is no sudden pH change at the end point. So, there is no suitable indicator. Neither the base or acid dissociate a high enough concentration of H+ and OH- to change the pH drastically.

Question 26

Why do specific indicators need to be used in specific tritrations?

Answer 26

Indicators are all in an equilibrium reaction which is dependent on a specific pH. The conjugate acid and base are 2 different colours. The colour change only occurs at specific concentrations of H+ which is represented by pH.

Question 27

When do indicators change colour?

Answer 27

At the equivalence/end point of a tritration.

Question 28

What pH is methyl orange used for and what is its colour change?

Answer 28

3.1-4.4. It changes from red to yellow

Question 29

What pH is phenolphathalein used in and what is its colour change?

Answer 29

8.8 - 10.0 It changes from colourless to pink.

Question 30

What is pKa and how can it be used?

Answer 30

pKa=-log Ka. It is another method of measuring the strength of an acid. The lower the value the stronger the acid

Question 31

How do you find Ka from pKa?

Answer 31

Ka= 10^-Pka

Question 32

What does a low ka vs Pka mean?

Answer 32

Low ka is a weak acid but a low Pka is a strong acid.