3.1.12 Acids And Bases

Question 1

Conjugate acid

Answer 1

The species produced when a base gains a proton.

Question 2

Conjugate base

Answer 2

The species produced when an acid loses a proton.

Question 3

Conjugate acid-base pair

Answer 3

Two species related to one another by the presence of a hydrogen ion or its absence

Question 4

pH =

Answer 4

-log10[H^+]

Question 5

Kw=

Answer 5

[H^+][OH^-]
Delivered from equilibrium constant of water dissociation
Varies with temp

Question 6

State the meaning of the term Brønsted–Lowry acid

Answer 6

Proton donor

Question 7

Strong acid + strong base indicator

Answer 7

Methyl orange or phenolphthalein

Question 8

Weak acid + strong base indicator

Answer 8

Phenolphthalein

Question 9

Strong acid + weak base indicator

Answer 9

Methyl orange

Question 10

Suggest why chloroethanoic acid is a stronger acid than ethanoic acid.

Answer 10

Cl more electronegative
Weakens O-H bond

Question 11

State why calibrating a pH meter just before it is used improves the accuracy of the pH measurement

Answer 11

Over time/storage readings less accurate

Question 12

Describe how you would obtain the pH curve for the titration

Answer 12

Measure pH of acid
Add alkali in 2cm^3 portions
Stir
Measure pH after every portion
When pH reaches end point, add acid in 0.2cm^3 portions
Repeat until alkali in excess
Continue with 2cm^3 portions post end point

Question 13

Acid-base equilibria

Answer 13

Involves transfer of protons

Question 14

Why is logarithmic scale used for pH

Answer 14

Concentration of H+ ions in aqueous solution covers very large range

Question 15

Ka =

Answer 15

Dissociation constant of weak acid
[H^+][A^-]/[HA]

Question 16

pKa =

Answer 16

-log [Ka]

Question 17

Buffer solution

Answer 17

Maintains an approximately constant pH dispute dilution or addition of small amounts of acid or base

Question 18

Acidic buffer contains

Answer 18

Weak acid and the salt of that weak acid

Question 19

Basic buffer solution contains

Answer 19

Weak base and the salt of that weak base

Question 20

State how a buffer solution can be made from solutions of potassium hydroxide and ethanoic acid
Give equation
How does buffer resist small additions of acid

Answer 20

Add enough KOH that the acid contains mixture of ethanoic acid and ethanoate ions
Add excess ethanoic acid to KOH and enough KOH so acid is partially neutralised
KOH + CH3COOH —> CH3COOK + H2O
CH3COO^- reacts with acid

Question 21

Define weak acid and weak base

Answer 21

Acid/base that doesn’t fully dissociate in solution

Question 22

Equivalence point

Answer 22

Point in titration at which the amount of titrant added is just enough to completely neutralize the analyte solution

Question 23

What happens at the half neutralisation point on curves

Answer 23

pH=pKa

Question 24

How do buffers work

Answer 24

Resist pH changes upon the addition of an acidic or basic components. It is able to neutralize small amounts of added acid or base to keep pH stable

Question 25

Lewis acid/base

Answer 25

Species that accepts/donates a pair of electrons

Question 26

Describe what happens in Brønsted−Lowry acid−base reaction

Answer 26

When a Brønsted-Lowry acid loses a proton, a conjugate base is formed. A Brønsted-Lowry base gains a proton, a conjugate acid is formed

Question 27

Explain why the end point of a reaction could be difficult to judge accurately using an indicator

Answer 27

Change in pH can be gradual An indicator would change colour over a range of volumes

Question 28

What physical factors affect Kw + how

Answer 28

Temperature If increased, equilibrium moves right Kw increases + pH decreases

Question 29

Why is pure water still neutral when pH doesn’t equal 7

Answer 29

[H^+] = [OH^-]

Question 30

What is titration (acid/base)

Answer 30

The addition of an acid/base of known titration to a base/acid of unknown titration to determine the concentration An indicator is used to show that neutralisation has occurred

Question 31

Titration curve for strong acid + weak base

Question 32

Titration curve for strong acid + weak base

Question 33

Titration curve for weak acid + weak base

Question 34

Titration curve for weak acid + strong base

Question 35

What happens to pH of solution around equivalence point

Answer 35

Large + rapid change in PH (except in weak acid/weak base titration)

Question 36

3 properties of a good indicator

Answer 36

Sharp colour change (one drop) End point the same as equivalence point Distinct colour change

Question 37

What colour is methyl orange in acid and alkali What pH does is change

Answer 37

Acid-red Alkali-yellow pH-4.5

Question 38

What colour is methyl orange in acid and alkali What pH does is change

Answer 38

Acid-colourless Alkali-red pH-9-10

Question 39

Equation for acidic buffer with added acid

Answer 39

A^- + H^+ —> HA

Question 40

Equation for acidic buffer with added alkali

Answer 40

HA + OH^- —> H2O + A^-

Question 41

How can you achieved an acidic buffer solution other than just mixing the constituents

Answer 41

Neutralise half of a weak acid with an alkali Forms weak acid/soluble salt mixture

Question 42

Equation for buffer system which maintains blood pH at 7.4

Answer 42

H^+ + HCO3 ^- <=> CO2 + H2O

Question 43

What products are buffers found in

Answer 43

Shampoos, detergents