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AQA A-Level Physical Chemistry > 1.12 Acids and Bases > Flashcards

1.12 Acids and Bases Flashcards

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1
Q

What is the Brønsted-Lowry classification of an acid?

A

Proton donor

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2
Q

What is the Brønsted-Lowry classification of a base?

A

Proton acceptor

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3
Q

What is a strong or weak acid?

A

Strong acids completely dissociate (ionise)

Weak acids partially dissociate (ionise)

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4
Q

What are some strong and weak acids?

A 
Strong:
Sulfuric acid
Hydrochloric acid
Nitric acid
Phosphoric acid

Weak:
Carboxylic acid

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5
Q

What are some strong and weak bases?

A

Strong:
Hydroxide

Weak:
Ammonia
Amines

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6
Q

What is a conjugate acid/base?

A

Formed from an acid/base

Conjugate base - species formed when an acid lost a proton

Conjugate acid - species formed when a base gained a proton

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7
Q

What word is significant for acids and bases?

A

Amphoteric

A species can act as an acid or a base

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8
Q

What is significant about water?

A

It is amphoteric and self protonating (always contains some H+ ions and OH-)

It is weakly dissociated

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9
Q

What are the types of acids that fully dissociate?

A

Monoprotic - for each acid molecule = 1H+ ion

Diprotic - for each acid molecule = 2H+ ion

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10
Q

How do you know what type of ‘protic’ a base is?

A

Monoprotic - takes only 1H+ ions to neutralise the 1OH-

Diprotic - takes 2H+ ions to neutralise the 2OH-

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11
Q

What is the equation for pH?

A

pH = -log[H+]

pH is always 2 dp!!!!!

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12
Q

What is the equation for H+ concentration?

A

[H+] = 10 ^-pH

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13
Q

What is Kw?

A

Another constant that incorporates Kc and [H2O]

Determined from pure water

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14
Q

What are some Kw equations?

A

Kw = [H+][OH-]
Or
Kw = [H+]^2

So
[H+] = Kw/[OH-]

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15
Q

What is Kw?

A

Kw = 1x10^ -14

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16
Q

What do you have to remember working out pH or H+ conc?

A

If the acid/base is monoprotic or diprotic!

You might need to multiply or divide by 2 or 4 depending on the direction of the dissociation (so use the equation as well - or make one)

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17
Q

How do we work out partial neutralisation calculations?

A

Find the moles of the acid and base
Find which is in excess (consider the ‘protic’ - multiply by 2? Before decision
Which ever is in excess take one from the other
Excess moles / total volume = conc of excess

If acid - find the pH
If base - use Kw to find [H+] and the find the pH

In Q’s look for ‘solution’

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18
Q

How do you decide which is an acid/base and or conjugate?

A

Use the definitions

Look in the correct direction of the reaction for that compound

19
Q

What is significant about weak acids?

A

They only weakly dissociate in aqueous solutions

20
Q

How is a weak acid equation represented?

A

HA ⇌ H+ + A-

H - the hydrogen ion
A - the undissociated negative ion in the weak acid

21
Q

What is Ka?

A

The weak acid dissociation constant

Ka = [H+] [A-]
—————
[HA]

Units = mol dm-3

22
Q

What is the relationship between Ka and equilibrium?

A

The greater value of Ka
The equilibrium is further to the right
The greater the dissociation
The greater the weak acid

Therefore a lower pH (more H+ ions)

23
Q

How do you work out H+ using Ka?

A

[H+] = square root of Ka x [HA]

24
Q

What are the equations to move between Ka and pKa?

A

pKa = -log Ka

Ka = 10 ^-pKa

25
Q

What is the relationship between pKa and equilibrium?

A

The lower the value of pKa
The stronger the acid
Equilibrium lies to the right side

Due to Ka increasing

26
Q

When are indictors used?

A

Indicators need to change colour in the vertical point on the graph

27
Q

What is significant about the vertical point of an acid/base graph?

A

H+ moles = OH- moles

It contains the equivalence point

28
Q

What happens to the graph with a weak acid and a weak base?

A

There is no vertical point of the graph
As the pH change at equivalence takes place gradually

An indicator can’t be used as they need a sudden change

29
Q 
Where is the equivalence point with:
Strong acid & strong base
Weak acid & strong base 
Strong acid & weak base
Weak acid & weak base
A

Strong acid & strong base - pH 7
Weak acid & strong base - above pH 7
Strong acid & weak base - below pH 7
Weak acid & weak base - pH 7

30
Q

What is significant about indicators?

A

They are weak acids - they will weakly dissociate

We need to control the extra [H+] ions being added

31
Q

What do you need to make sure with colour change of indicators?

A

You write two colours as it’s a colour change!

32
Q

What is meant by the strength of an acid?

A

A measure of how readily an acid dissociates

33
Q

What happens if an acid reacts with an acid?

A

The strongest acid is the proton donor (loses a H+)

The weaker acid gains that proton

34
Q

What is half equivalence?

A

To determine the Ka of a weak acid when titrated with a strong base

1) record the volume of OH- required to produce equivalence (neutralisation)
2) repeat but add exactly half the volume of OH- required (half equivalence)
3) use a pH meter (or pH curve) to find the pH of the solution produced in (2) this pH can determine the Ka of the acid

35
Q

What is a buffer solution?

A

It’s pH is resistant to change on the addition of relatively small quantities if an acid or a base, or by dilution

36
Q

What equation do we use to work out Ka or H+ of a buffer solution?

A

Ka = [H+] [A-]
—————
[HA]

[H+] = Ka x [HA]
—————
[A-]

37
Q

What does a buffer solution do?

A

A buffer solution artificially maintains the A- to allow the equilibrium to move to the left or right to maintain the H+

38
Q

What is an acidic buffer made from?

A

pH < 7

Made from a weak acid
And
Na+ or K+ salt eg sodium ethanoate

39
Q

What is an alkaline buffer made from?

A

pH > 7

Made from a weak base eg ammonia
And
It’s chloride eg ammonium chloride

40
Q

What are the two types of buffer calculations?

A

1) weak acid + salt of the weak acid

2) weak acid + strong base

41
Q

How to do the calculation of a buffer of a weak acid and salt of the weak acid?

A

[H+] = Ka x (moles of HA / total volume)

All divided by

(Moles of salt / total volume)

pH = -log[H+]

42
Q

How are the moles of A- worked out in a weak acid + salt of the weak acid calculation?

A

Moles of salt = moles of A-

43
Q

How do you calculate the new pH when you add something else to a buffer?

A

Work out the moles of the H+ or OH- added

Use HA ⇌ H+ + A- to establish the direction of the equilibrium depending on what you add

+ or - the moles being added from the previous moles of HA and A-

Use the new moles to work out [H+] and don’t forget the addition of the volume for total volume

44
Q

How to do the calculation of a buffer of a weak acid and strong base?

A 
Calculate the moles of both
Write HA + OH- ⇌ A- + H2O 
Use this to do equilibrium moles 
To work out HA do HA - OH- moles 
OH goes to 0 and A- and H2O is the initial OH- moles
Sub these moles into:
[H+] = Ka x (moles of HA / total volume) 
All divided by
(Moles of salt / total volume)

pH = -log[H+]

AQA A-Level Physical Chemistry (12 decks)

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