Y13 Acids, bases and buffers

Question 1

Bronsted-Lowry acid definition

Answer 1

Proton donor

Question 2

Bronsted-Lowry base definition

Answer 2

Proton acceptor

Question 3

Alkali definition

Answer 3

Soluble base which dissolves in solution to release OH- ions

Question 4

Monobasic acid definition

Answer 4

Can release 1 proton per molecule e.g. HNO3

Question 5

Strong acid definition

Answer 5

Proton donor which fully dissociates in aqueous solution

Question 6

Weak acid definition

Answer 6

Proton donor which partially dissociates in aqueous solution

Question 7

What is formed when an acid reacts with a base?

Answer 7

Salt + water

Question 8

What is formed when an acid reacts with a carbonate?

Answer 8

Salt + water + carbon dioxide

Question 9

What is formed when an acid reacts with a metal?

Answer 9

Salt + hydrogen

Question 10

What is formed when an acid reacts with ammonia?

Answer 10

Ammonium salt

Question 11

Which of the 4 acid + x reactions is NOT neutralisation?

Answer 11

Acid + metal = redox because metal is oxidised and H+ ions are reduced

Question 12

Conjugate acid-base pair definition

Answer 12

A pair of 2 species which transform into each other by the gain or loss of a proton

Question 13

Label the acid-base pairs in this reaction: NH4+ + CO3(2-) –> NH3 + HCO3-

Answer 13

NH4+ = CO3(2-) –> NH3 + HCO3-
A1 B2 B1 A2

Question 14

pH definition

Answer 14

-log [H+]

Question 15

How do you calculate [H+] from pH?

Answer 15

[H+] = 10^(-pH)

Question 16

How can you calculate pH when given the acid concentration of a STRONG acid (HA)?

Answer 16

pH = -log [HA]

Question 17

What are the 3 stages to calculating pH after a dilution?

Answer 17

1. Dilution factor: new volume / original volume
2. New concentration: old conc / dilution factor
3. pH = -log [HA]

Question 18

How is a Ka expression written?

Answer 18

Ka = ([H+] x [A-]) / [HA]

Question 19

How is pKa calculated from Ka?

Answer 19

pKa = -log (Ka)

Question 20

How is Ka calculated from pKa?

Answer 20

Ka = 10^(-pKa)

Question 21

What does a smaller pKa value mean?

Answer 21

Stronger weak acid

Question 22

Which acids will act as the conjugate acids in a conjugate-base pair?

Answer 22

The weaker acids

Question 23

What 2 approximations are made when calculating the pH of a weak acid and what is ignored/assumed?

Answer 23

1. [H+]eqm = [A-]eqm Ignored: dissociation of H2O
2. [HA]eqm = [HA]start Assumed: dissociation is very small

Question 24

What is the expression for Kw?

Answer 24

Kw = [H+] [OH-]

Question 25

What is the value of Kw at 25°C?

Answer 25

1.00 x 10^-14

Question 26

What is a neutral solution?

Answer 26

A solution where [H] = [OH-]

Question 27

What is the equivalence point on a pH curve?

Answer 27

The mid point of the vertical section on the curve

Question 28

Where is the equivalence point on the 3 different pH curves (that we need to know)?

Answer 28

1. Strong acid-strong base: at pH 7.00 2. Strong acid-weak base: below pH 7.00 3. Weak acid-strong base: above pH 7.00

Question 29

How must you write pH values?

Answer 29

To 2 decimal places

Question 30

What are the 4 steps to sketching a pH curve?

Answer 30

1. Find the start and end points using calculation 2. Find the volume added (calculating moles of substance in flask, using this value to find volume of substance added in burette) 3. Shape: weak/strong and the mid point 4. Proportion: 2 pH gradual increase, vertical line minimum of 3 pH

Question 31

What is the end point of a reaction?

Answer 31

When there are equal amounts of the weak acid and conjugate base present

Question 32

How is a suitable indicator chosen for a reaction?

Answer 32

pH range of the indicator matches the vertical section of the pH curve

Question 33

Buffer solution definition

Answer 33

Mixture which minimises pH changes by the addition of small amounts of acid or base

Question 34

What are the 2 ways of making a buffer solution?

Answer 34

1. Weak acid with its conjugate base 2. Excess of a weak acid with a base (partial neutralisation)

Question 35

Explain how a buffer solution works

Answer 35

Added acid: [H+] increases which reacts with A- to produce HA which shifts equilibrium to the left hand side Added alkali: [OH-] increases which reacts with H+ to form H2O. Equilibrium shifts right to increase [H+] which was used up when forming H2O (Include equations!)

Question 36

What is the equation for calculating [H+] of a buffer solution?

Answer 36

[H+] = Ka x ([HA] / [A-])

Question 37

How is the pH of a buffer solution made through PARTIAL NEUTRALISATION calculated?

Answer 37

1. Moles of initial HA 2. Moles of base 3. Moles of excess HA (moles of HA - moles of base) 4. [H+] = Ka x ([HA] / [A-]) 5. pH = -log [H+]

Question 38

What pH range does blood plasma need to have?

Answer 38

pH 7.35-7.45