1.12 Acids and Bases

Question 1

What is the Brønsted-Lowry classification of an acid?

Answer 1

Proton donor

Question 2

What is the Brønsted-Lowry classification of a base?

Answer 2

Proton acceptor

Question 3

What is a strong or weak acid?

Answer 3

Strong acids completely dissociate (ionise)

Weak acids partially dissociate (ionise)

Question 4

What are some strong and weak acids?

Answer 4

Strong:
Sulfuric acid
Hydrochloric acid
Nitric acid
Phosphoric acid

Weak:
Carboxylic acid

Question 5

What are some strong and weak bases?

Answer 5

Strong:
Hydroxide

Weak:
Ammonia
Amines

Question 6

What is a conjugate acid/base?

Answer 6

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

Question 7

What word is significant for acids and bases?

Answer 7

Amphoteric

A species can act as an acid or a base

Question 8

What is significant about water?

Answer 8

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

It is weakly dissociated

Question 9

What are the types of acids that fully dissociate?

Answer 9

Monoprotic - for each acid molecule = 1H+ ion

Diprotic - for each acid molecule = 2H+ ion

Question 10

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

Answer 10

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

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

Question 11

What is the equation for pH?

Answer 11

pH = -log[H+]

pH is always 2 dp!!!!!

Question 12

What is the equation for H+ concentration?

Answer 12

[H+] = 10 ^-pH

Question 13

What is Kw?

Answer 13

Another constant that incorporates Kc and [H2O]

Determined from pure water

Question 14

What are some Kw equations?

Answer 14

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

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

Question 15

What is Kw?

Answer 15

Kw = 1x10^ -14

Question 16

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

Answer 16

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)

Question 17

How do we work out partial neutralisation calculations?

Answer 17

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’

Question 18

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

Answer 18

Use the definitions

Look in the correct direction of the reaction for that compound

Question 19

What is significant about weak acids?

Answer 19

They only weakly dissociate in aqueous solutions

Question 20

How is a weak acid equation represented?

Answer 20

HA ⇌ H+ + A-

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

Question 21

What is Ka?

Answer 21

The weak acid dissociation constant

Ka = [H+] [A-]
—————
[HA]

Units = mol dm-3

Question 22

What is the relationship between Ka and equilibrium?

Answer 22

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)

Question 23

How do you work out H+ using Ka?

Answer 23

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

Question 24

What are the equations to move between Ka and pKa?

Answer 24

pKa = -log Ka

Ka = 10 ^-pKa

Question 25

What is the relationship between pKa and equilibrium?

Answer 25

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

Question 26

When are indictors used?

Answer 26

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

Question 27

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

Answer 27

H+ moles = OH- moles It contains the equivalence point

Question 28

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

Answer 28

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

Question 29

``` Where is the equivalence point with: Strong acid & strong base Weak acid & strong base Strong acid & weak base Weak acid & weak base ```

Answer 29

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

Question 30

What is significant about indicators?

Answer 30

They are weak acids - they will weakly dissociate We need to control the extra [H+] ions being added

Question 31

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

Answer 31

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

Question 32

What is meant by the strength of an acid?

Answer 32

A measure of how readily an acid dissociates

Question 33

What happens if an acid reacts with an acid?

Answer 33

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

Question 34

What is half equivalence?

Answer 34

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

Question 35

What is a buffer solution?

Answer 35

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

Question 36

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

Answer 36

Ka = [H+] [A-] ————— [HA] [H+] = Ka x [HA] ————— [A-]

Question 37

What does a buffer solution do?

Answer 37

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

Question 38

What is an acidic buffer made from?

Answer 38

pH < 7 Made from a weak acid And Na+ or K+ salt eg sodium ethanoate

Question 39

What is an alkaline buffer made from?

Answer 39

pH > 7 Made from a weak base eg ammonia And It’s chloride eg ammonium chloride

Question 40

What are the two types of buffer calculations?

Answer 40

1) weak acid + salt of the weak acid | 2) weak acid + strong base

Question 41

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

Answer 41

[H+] = Ka x (moles of HA / total volume) All divided by (Moles of salt / total volume) pH = -log[H+]

Question 42

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

Answer 42

Moles of salt = moles of A-

Question 43

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

Answer 43

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

Question 44

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

Answer 44

``` 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+]