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A-level Chemistry > Topic 12: Acid-Base Equilibria > Flashcards

Topic 12: Acid-Base Equilibria Flashcards

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1
Q

Define a Bronsted-Lowry acid

A

A proton donor
e.g. HCl is a Bronsted-Lowry acid, as it donates a proton:
HCl —> H+ + Cl-

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2
Q

Which ions make solutions acidic?

A

Hydroxonium ions
H3O+

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3
Q

Define a Bronsted-Lowry base

A

A proton acceptor
e.g. OH - is a Bronsted-Lowry base, as it accepts a proton:
OH- + H+ —-> H2O

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4
Q

What are conjugate acid-base pairs?

A

A pair of reactants and products that are linked to each other by the transfer of a proton

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5
Q

How do you identify acid-base conjugate pairs?

A

Acid = acid formula
Conjugate acid = gains a proton
Base = base formula
Conjugate base = loses a proton

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6
Q

Define the term pH

A

the acidity of an aqueous solution depends on the number of H+ ions in solution.
It is defined as -log10[H+]

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7
Q

pH equation

A

pH = -log10[H+]

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8
Q

Hydrogen ion concentration

A

[H+] = 10^-pH

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9
Q

What is a strong acid?

A

A strong acid fully dissociates in solution
e.g. H2SO4, HCl, HNO3

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10
Q

What is a weak acid?

A

Partially dissociates in aqueous solution
e.g. CH3CH2COOH, HCN

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11
Q

How do you calculate the pH of a strong acid?

A

-log10[H+]

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12
Q

What is Ka, and when is it used?

A

Ka is the acid dissociation constant, and is used for weak acids.

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13
Q

What is the Ka expression?

A

Ka = [H+] [A-]
————
[HA]

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14
Q

What assumptions are made when writing the equilibrium expression for weak acids?

A

The concentration of hydrogen ions due to the ionisation of water is negligible

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15
Q

What do Ka values indicate about dissociation?

A

The higher the value of Ka the more dissociated the acid and the stronger it is
The lower the value of Ka the weaker the acid

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16
Q

Equation for pH of a weak acid

A

-log10(√(Ka ×[weak acid]) )

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17
Q

Define the ionic product of water

A

An equilibrium exists where some water molecules partially dissociate:
H2O —> OH- + H3O+
H20 —> OH- + H+
Kw = [H+] [OH-]

18
Q

What is the exact value for Kw

A

1.00 x 10^-14 mol2dm-6

19
Q

Equation for pH of a strong base

A

14 - (-log10[H+])

20
Q

Define the term ‘pKa’

A

pKa = -log10 Ka

The lower the value for pKa, the stronger the acid.

The higher the value for pKa, the weaker the acid

21
Q

Define the term ‘pKw’

A

pKw = -log10 Kw

The Kw value of 1.00 x 10-14 mol2 dm-6 at room temperature gives you a pKw value of 14

22
Q

How do you calculate Ka for a weak acid from pH?

A

1: balanced dissociation equation for the weak acid.
2: ICE Table for the disassociation of the weak acid.
3: Write the equilibrium expression of Ka for the reaction
4: Using the given pH, determine the concentration of hydronium ions present with the formula:
5: Solving for the concentration of hydronium ions gives the x M in the ICE table. Substitute the hydronium concentration for x in the equilibrium expression.
6: Simplify the expression and algebraically manipulate the problem to solve for Ka.

23
Q

What is the equivalence point?

A

when you mix the two solutions together in the correct proportions according to the equation and they neutralise.

24
Q

What is the end point?

A

When the indicator changes colour

25
Q

What does the pH curve look like for a strong acid-strong base titration?
(pH/vol. acid)

A

Curve begins at pH 14
Decreases to pH 12 - very flat
Sharp drop to pH 4
Slow decrease to pH 1

26
Q

How do you know the volume needed to neutralise the acid, from the pH graph?

A

Calculate the volume at the equivalence point (pH 7)

27
Q

Steps for drawing titration curves

A
  1. Calculate start and end pH values of the substance in flask
  2. Calculate the equivalence point via a titration calculation
  3. Draw the curve following the correct pH points.
28
Q

Where is the equivalence point for a weak acid-strong base titration?

A

The volume at halfway up the vertical section

29
Q

Where does pH=pKa on the weak acid-strong base curve?

A

Halfway to the volume at equivalence point

30
Q

What does the pH curve look like for a weak acid-strong base titration?
(pH/vol. alkali)

A

Curve starts at around pH 3
Small curve upwards
Flat
Curve shoots up sharply to around pH 13
Flattens at top

31
Q

What values do you centre the pH around for a weak/strong titration?

A
  • A weak acid centres around pH, so draw vertical section from pH 7-11
  • A weak alkali centres at pH 6, so draw vertical section from pH 4-8
32
Q

What values do you centre the pH around for a strong/strong titration?

A

It will centre around pH 7

A strong acid in the burette finishes at pH 0-1.

A strong alkali in the burette finishes at pH 12-13.

33
Q

What are indicators?

A

Weak acids or bases which change colour when they lose or gain protons

34
Q

What is the equilibrium for Litmus paper?

A

HLit —> Lit- + H+
red blue

35
Q

How do you choose an indicator?

A

Choose an indicator that has its pH range within the vertical section of the pH curve.
The colour change will then happen on addition of a very small volume of solution from the burette.

36
Q

What are the colour changes for Phenolphthalein?

A

in Acid - colourless
in Alkali - pink

37
Q

What are the colour changes for methyl orange?

A

in Acid - red
in Alkali - yellow
endpoint - orange

38
Q

Which indicator do you use for a strong acid?

A

Methyl orange

39
Q

Which indicator do you use for a strong base?

A

Phenolphthalein

40
Q

What is meant by the term ‘Buffer solution’?

A

A solution that resists changes in pH when small amounts of acid or alkali are added.

41
Q

Devise an experiment to determine the Ka of a weak acid.

A
  1. Titrate the weak acid with the strong base
  2. Measure the pH at regular intervals
  3. Plot a graph of pH against the volume of strong base
  4. Use the graph to find the pH at half equivalence point
  5. A half neutralisation
    pH = pKa
    Therefore, Ka = 10^pKa
42
Q

How does blood act as a buffer?

A
  1. CO2 dissolves into the blood forming carbonic acid
  2. Equilibrium shifts to the right, producing more H+ ions
  3. The large reservoir of HCO3 - ions combine with H+ ions to prevent a pH change

A-level Chemistry (26 decks)

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