Acids and Bases

Question 1

Define bronsted-Lowry acid

Give an example

Answer 1

Proton donor

HA + H2O → H3O⁺ + A^-

Question 2

Define bronsted-Lowry base

Give an example

Answer 2

Proton acceptor

B + H₂O → BH⁺ + OH^-

Question 3

Define strong acid

Answer 3

Completely dissociates

Question 4

Define weak acid

Answer 4

Slighty dissociates

Question 5

Define strong base

Answer 5

Completely ionises

Question 6

Define weak base

Answer 6

Slightly ionises

Question 7

What happens during acid base equilibria?

Answer 7

Protons are transferred from acid to base

Question 8

Does water fully dissociate or slightly?

Answer 8

Slightly dissociates

Question 9

Where is k_w derived from?

Answer 9

K_c x [H₂O]

Question 10

Ionic product of water

Unit

Answer 10

K_w = [H⁺] [OH^-]

mol² dm^-6

Question 11

Does the value of K_w change as the temp changes?

Answer 11

Yes, it varies w/ temp

Question 12

What is the value of K_w at 298k?

Answer 12

1.00 x 10^-14 mol² dm^-6

Question 13

Why is a solution w/ pH 6 still considered neutral?

Answer 13

[H⁺] = [OH^-]

Question 14

Equation for pH

Answer 14

pH = -log [H+]

Question 15

pH to conc equation

Answer 15

[H⁺] = 10^-pH

Question 16

Give the expression of K_a

Unit

Answer 16

K_a = [H+] [A-] / [HA]

K_a = [H+]² / [HA]

mol dm^-3

Question 17

When is K_a used?

Answer 17

Weak acid

Question 18

How do you work out K_a from pK_a?

Answer 18

K_a = 10^-pK_a

Question 19

How do you work out pK_a from K_a?

Answer 19

pK_a = -log (K_a)

Question 20

Would you expect strong acids to have higher or lower pK_a value than weak acids?

Answer 20

Lower

Question 21

What is the vertical line in a pH curve?

Answer 21

Equivalence point - neutral

Question 22

How do you work out which indicator to use?

Answer 22

The one that changes colour over narrow pH range that lies on vertical part of pH curve

Question 23

Methyl orange

Colour change

pH range

Answer 23

• Red to yellow
• 3.1 - 4.4

Question 24

Phenolphthalein

Colour change

pH range

Answer 24

• Colourless to pink
• 8.3 - 10

Question 25

Which indicator do you use for

Strong acid/strong base titration

Answer 25

• Either indicator
• Rapid pH change over range of both indicator

Question 26

Which indicator do you use for

Strong acid/weak base titration

Answer 26

Methyl orange

Question 27

Which indicator do you use for

Weak acid/strong base titration

Answer 27

Phenolphthalein

Question 28

Which indicator do you use for

Weak acid/weak base titration

Answer 28

• Neither bc no sharp pH change
• Use pH meter

Question 29

Define buffer

Answer 29

Sol that resists changes in pH when small amounts of acid/base are added or when diluted

Question 30

What is an acidic buffer?

Answer 30

• pH less than 7
• Mixing weak acid w/ salt
• Weak acid slightly dissociates, salt fully dissociates

Question 31

What happens when you add acid to an acidic buffer?

Answer 31

• H+ conc inc
• Equil shifts to left
• Oppose inc in H+
• pH doesn’t change

Question 32

What happens when you add base to an acidic buffer?

Answer 32

• OH^- conc inc
• OH^- reacts w/ H⁺ to form water, removing H⁺ from solution
• Equil shifts to right bc acid dissociates to form H⁺
• pH doesn’t change

Question 33

What is a basic buffer?

Answer 33

• pH greater than 7
• Mixing weak base w/ salt
• eg. ammonia + ammonium chloride
• Salt fully dissociates
• Weak base reacts w/ water
• NH₃ + H₂O ⇔ NH₄⁺ + OH^-

Question 34

What happens when you add base to a basic buffer?

Answer 34

• OH^- conc inc
• Sol = more basic
• OH^- reacts w/ NH₄⁺ → NH₃ + H₂O
• Equil shifts to left to remove OH^-

Question 35

What happens when you add acid to a basic buffer?

Answer 35

• H⁺ conc inc
• Sol = more acidic
• H⁺ reacts w/ OH^- → H₂O
• Equil shifts to right to replace in OH^-
• H⁺ reacts w/ NH₃ → NH₄⁺

Question 36

What are buffers used for?

Answer 36

• Shampoo (pH 5.5) - hair becomes rougher if exposed to alkaline
• Biological washing powder - keep pH right level for enzymes to work efficiently