Acids, bases and pH

Question 1

What is a Bronsted-Lowry acid

Answer 1

Proton donor

Question 2

What is a Bronsted-Lowry base

Answer 2

Proton acceptor

Question 3

Conjugate acid base pairs

Answer 3

Each pair differs by 1 H

Question 4

Hydronium ions

Answer 4

Acid part of an aqueous acid, H3O+, easier to use H+

Question 5

Monobasic, Dibasic and tribasic acids

Answer 5

No of hydrogen ions in the acid that can be replaced per molecule in an acid base reaction

Question 6

Role of H+ in acid reactions

Answer 6

The most important part in neutralization reactions

Question 7

Redox reactions

Answer 7

Acid + metal => salt + H2

Question 8

Neutralisation reactions

Answer 8

Acid + carbonate => salt + H20 + CO2
Acid + metal oxide => salt + H2O
Acid + alkali => salt + H2O

Question 9

pH scale key points

Answer 9

For any solution at 25C
Any change in pH = 10 fold change in [H+]
Low [H+] value= high pH value
High [H+] value= low pH values
Always to 2 dp

Question 10

Calculating pH of strong acids

Answer 10

HA => H+ + A-
[H+] = [HA]
pH calculations can be done from the conc of strong acids

Question 11

pH changes on dilution

Answer 11

Find conc of diluted acid

Find pH avalues before and after dilution

Question 12

Equation for Ka

Answer 12

Ka= [H+aq] [A-aq]/[HAaq]

Units to cancel out

Question 13

Ka

Answer 13

Standardized at 25C
Larger the constant, the further along the equilibrium is to the right
Greater the dissociation and greater the acid strength
[HA] <=> [H+] + [A-]

Question 14

Function of pKa

Answer 14

Hard to compare numbers with negative indices
Resolve issue with pKa
Larger pKa values = weaker the acid

Question 15

Conversions between pKa and Ka

Answer 15

pKa= -logKa
Ka= 10-pKa

Question 16

Dissociation in weak di and tribasic acids i relation to pKa

Answer 16

For dibasic and tribasic acids, the first dissociation is stronger than the successive dissociations
pKa gets larger

Question 17

Ka equation for weak acids

Answer 17

Ka = [H+aq][A-aq]/[HAaq]

Question 18

Approximations that you are making when calculating Ka of weak acids

Answer 18

Water will also dissociate, provide a negligible source of [H+]
HA dissociates to produce equilibrium concs of H+ and A- that are equal

As the dissociation of weak acids is small, can assume that [HA] > [H+], can neglect any decrease in conc of HA from dissociation

Question 19

Calculating pH of strong acids

Answer 19

pH= -log[H+]
[H+]= 10-pH

Question 20

Calculating pH of weak acids

Answer 20

[H+aq] =√Ka x [HAaq]

Question 21

Determination of Ka

Answer 21

Prepare a standard solution of the weak acid of known conc

Measuring the pH of the standard solution using a pH meter

Question 22

The Kw constant

Answer 22

Kw=[H+aq][OH-aq]

At room temp= 1x10-14 mol2 dm-6

Question 23

Importance of Kw

Answer 23

Sets up the neutral point in the pH scale

Question 24

Calculating the pH of pure water

Answer 24

[H+aq] = √1.00 x 10-14 = 1.00 x 10-7
pH = -log[H+aq]=7

Question 25

Uses of Kw

Answer 25

Find conc of H+ or OH- with Kw=[H+][OH-]

Question 26

pH of solutions of strong bases

Answer 26

Dissociate completely [NaOH]=[OH-]

Use Kw to find [H+]

Question 27

pH solutions of weak bases

Answer 27

Same approach as weak acids