Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

OCR A Level Chemistry > Acids, Bases and Buffers > Flashcards

Acids, Bases and Buffers Flashcards

1
Q

What is a Bronsted-Lowry acid?

A

A proton donor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is a Bronsted-Lowry base?

A

A proton acceptor. (Dissociates OH-)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What happens when you put a strong acid into water?

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Why is water both an acid and base?

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What are conjugate acid-base pairs?

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

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

A

HCl, H2S04, H3PO4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is Ka? Give its expression

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

How do you find pH from [H+]

A

pH= -log[H+]

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

How do you find [H+] from pH?

A

[H+]= 10^-pH

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is kw?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What can be Kw used for?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How do you find the pH of strong bases?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is ka in weak acids and explain why.

A

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).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

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

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

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

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is a buffer?

A

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+

17
Q

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

A

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

18
Q

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

A

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

19
Q

How do you make a buffer?

A

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.

20
Q

What is M? eg 0.1M of CH3COOH?

A

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

21
Q

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

A

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)

22
Q

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

A

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.

23
Q

What is a neutralisation curve?

A

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

24
Q

What is the equivalence point on a tritration curve?

A

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.

25
Q

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

A

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.

26
Q

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

A

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.

27
Q

When do indicators change colour?

A

At the equivalence/end point of a tritration.

28
Q

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

A

3.1-4.4. It changes from red to yellow

29
Q

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

A

8.8 - 10.0 It changes from colourless to pink.

30
Q

What is pKa and how can it be used?

A

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

31
Q

How do you find Ka from pKa?

A

Ka= 10^-Pka

32
Q

What does a low ka vs Pka mean?

A

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

OCR A Level Chemistry (61 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions