Acids, Bases and pH (20)

Question 1

Arrhenius model

Answer 1

This suggests that acids release H+ ions and alkalis release OH- ions when dissociated and this combines in a neutralisation to form water. An alkali is just a soluble base.

Question 2

Bronsted-Lowry acid

Answer 2

This is a proton donor

Question 3

Bronsted-Lowry base

Answer 3

This is a proton acceptor

Question 4

Conjugate acid-base pair

Answer 4

The best example of this is seeing their dissociation of HCl into H+ and Cl-. The conjugate acid base pair are the HCl and Cl-. This shows two species can be interconverted by the transfer of a proton

Question 5

Conjugate acid in example

Answer 5

This would be the HCl because it’s releasing a proton in the forward reaction to form its conjugate base.

Question 6

Conjugate base in example

Answer 6

This would be Cl- because it is accepting a proton in the backward reaction to become it’s conjugate acid.

Question 7

Examples of conjugate acid-base pairs

Answer 7

HCl H+ and Cl-
HNO3 H+ and NO3-
H2SO4 H+ and HSO4-
CH3COOH H+ and CH3COO-

Question 8

Two conjugate acid-base pairs in the same equation

Answer 8

When you combine the dissociation of HCl and the neutralisation equation, there are two conjugate acid-base pairs. HCl and Cl- are one pair and OH- and H2O are the other pair. In the forward reaction the HCl is the acid as it releases a proton and the OH- is base as it accepts a proton. In the reverse reaction, the H2O is acid and the Cl- is base.

Question 9

Hydronium Ion

Answer 9

Normally in an aqueous solution, dissociation is the movement of a proton from an acid to base and doesn’t work unless water is present. But in this case, water is acting as the base and accepting the H+ ion.
Here the first conjugate acid base pair is the HCl and Cl- and the second pair is water and the hydronium ion. The hydronium ion is the active ingredient in any aqueous acid so you could write any neutralisation with H3O+ instead of H+.

Question 10

Monobasic, Dibasic and Tribasic

Answer 10

This refers to the number of hydrogen ions in the acids that will be replaced by a metal ion to form a salt. You can look at the number of hydrogen to get an idea

Question 11

What is HCl

Answer 11

Monobasic - 1 H+

Question 12

What is CH3COOH

Answer 12

Monobasic - 1 H+

Question 13

What is H2CO3 (carbonic acid)

Answer 13

Dibasic 2 H+

Question 14

What is H3BO3 (boric acid)

Answer 14

Tribasic 3 H+

Question 15

What does pH measure?

Answer 15

It measures the concentration of hydrogen ions. 10^-1 (1) and 10^14 (14) are used to show the 14 pH groups
pH less than 7 shows increasing acidity
pH of 7 is neutral
pH more than 7 shows increasing alkalinity

Question 16

How to measure the pH?

Answer 16

You can use a pH indicator which will give you the most accurate measure or you can use universal indicator and pH indicator paper and observe the colour. Red is acid and alkali is purple

Question 17

Mathematical relationship of pH in terms of H+

Answer 17

pH = -log [H+]

Question 18

Mathematical relationship of H+ in terms of pH

Answer 18

[H+] = 10^-pH

Question 19

How many more H+ ions are there in pH 1 than pH 2?

Answer 19

There are 10 times more ions and which means there’s a big difference between each pH.

Question 20

How to find the pH of a strong acid.

Answer 20

A strong acid will fully dissociate and the concentration of H+ ions is the same as the concentration of the acid. Always round the pH to 2 d.p.

Question 21

What is Ka?

Answer 21

This is the equilibrium constant for the dissociation of an acid. It’s written in the same way as Kc.

Question 22

The relationship between pKa and Ka

Answer 22

pKa = -logKa

Question 23

The relationship between Ka and pKa

Answer 23

Ka = 10^-pKa

Question 24

Strong acid pKa and Ka values

Answer 24

The stronger the acid, the larger the Ka values and the smaller the pKa value

Question 25

Weak acid pKa and Ka value

Answer 25

The weaker the acid, the smaller the Ka value and the bigger the pKa value.

Question 26

How to find the pH of a weak acid.

Answer 26

A weak acid will only partially dissociate so won’t have the same concentration of ions as there are in the acid. It depends on the Ka values and equilibrium concentrations.

Question 27

Approximation 1

Answer 27

[ H+] = [ A- ]

This will only work if the pH is less than 6 and is not a very weak acid or dilute solution

Question 28

Approximation 2

Answer 28

[ HA equil ] = [ HA start ] - [ H+ ]
This means that [ HA start] = [ HA equil]
It only works for small Ka values and is not justified for stronger weak acids.

Question 29

Formula for concentration of H+ ions in weak acid

Answer 29

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

Question 30

Ionisation of water

Answer 30

Water molecules can combine together and act as an acid and a base to form a hydronium ion and a hydroxide. This then makes the normal neutralisation equation

Question 31

Kw

Answer 31

This is the ionic product of water. It sets a neutral point on the pH scale. In water it will be 1x10-14
Kw = [ H+ ] [ OH- ]

Question 32

What pH does water have?

Answer 32

7

Question 33

How do you know when a solution is acidic?

Answer 33

The [ H+ ] > [ OH- ]

Question 34

How do you know when a solution is alkali?

Answer 34

The [ H+ ] < [ OH- ]

Question 35

How do you work out the conc of H+ and OH- ions if you know the pH of water?

Answer 35

The powers should add up to -14. The conc of H+ will be the actual pH.

Question 36

Strong alkali

Answer 36

A strong alkali is a soluble base that releases OH- ions. It fully dissociates into a metal ion and an OH- ion.
NaOH -> Na+ and OH-

Question 37

How do you work out the pH of a strong base?

Answer 37

The conc of the base = conc of the OH- ions.

Then do the Kw calculation and find the conc of H+ ions which you can use to work out the pH.