21 - Acids, Bases and Buffers

Question 1

What is a Bronsted-Lowry acid?

Answer 1

A substance that can donate a proton.

Question 2

What is a Bronsted-Lowry base?

Answer 2

A substance that can accept a proton.

Question 3

What is a Lewis acid?

Answer 3

A substance that can accept a lone pair of electrons.

Question 4

What is a Lewis base?

Answer 4

A substance that can donate a lone pair of electrons.

Question 5

Give an example of water acting as a base.

Answer 5

H2O + HCl -> Cl- + H3O+

Question 6

Give an example of water acting as an acid.

Answer 6

H2O + NH3 -> OH- + NH4+

Question 7

What is the formula for the dissociation of water?

Answer 7

H2O(l) <=> OH-(aq) + H+(aq)

Question 8

Is the dissociation of water exothermic or endothermic?

Answer 8

Endothermic

Question 9

What is the formula for the ionic product of water?

Answer 9

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

Question 10

At 298K what does Kw equal?

Answer 10

1.0 x 10^-14 mol^2 dm^-6

Question 11

What is the formula for Kw at 298K?

Answer 11

Kw = [H+(aq)]^2

Question 12

What effect does increasing the temperature have on the dissociation of water?

Answer 12

As the forward reaction is endothermic to oppose the increase in temperature equilibrium shifts right, so more H+ ions are produced, so the water becomes more acidic.

Question 13

What is the pH scale?

Answer 13

A scale that quantifies the concentration of hydrogen ions (H+) in a solution.

Question 14

What is the formula for pH?

Answer 14

pH = -log10[H+]

Question 15

How many significant figures should you quote pH to?

Answer 15

2 decimal places

Question 16

What is a strong acid?

Answer 16

Acids that completely dissociate in water.

Question 17

What is a weak acid?

Answer 17

Acids that partially dissociate in water.

Question 18

What is the formula for the acid dissociation constant?

Answer 18

[H+][A-]
————- = Ka
[HA]

Question 19

What does the Ka tell us about acids?

Answer 19

The larger the value of Ka the further the equilibrium is to the right, so the more the acid is dissociated , therefore the stronger it is.

Question 20

What is the formula for pKa?

Answer 20

pKa = -log10(Ka)

Question 21

What does a small pKa indicate?

Answer 21

The smaller the pKa the greater the Ka, so the stronger the acid.

Question 22

What is an acid-base titration?

Answer 22

When an acid/base of a known concentration is added from a burette into a measured amount of acid/base until an indicator shows the solution is neutralised.

Question 23

What are acid-base titrations used for?

Answer 23

To find the unknown concentration of an acid/base.

Question 24

What is a pH curver?

Answer 24

A curve produced on a graph of pH against volume of acid/base added, from an acid-base reaction.

Question 25

What is a monoprotic acid?

Answer 25

An acid with only one acidic hydrogen.

Question 26

Where is the endpoint pH for a strong acid-strong base titration curve?

Answer 26

pH = 7

Question 27

Where is the endpoint pH for a weak acid-strong base titration curve?

Answer 27

pH > 7

Question 28

Where is the endpoint pH for a strong acid-weak base titration curve?

Answer 28

pH < 7

Question 29

Where is the endpoint pH for a weak acid-weak base titration curve?

Answer 29

pH > 7 or pH < 7

Question 30

What is the equivalence point?

Answer 30

The steep portion of a titration curve. The point where a sufficient amount of acid/base has been added to neutralise the acid.

Question 31

What does the equivalence point indicate in terms of moles?

Answer 31

The number of moles of acid and base are equal.

Question 32

What is the end point?

Answer 32

The point at which the indicator changes colour.

Question 33

What are the properties an indicator must have?

Answer 33

Sharp colour change (one drop changes colour) End point of the titration should be equal to the equivalence point. Distinct colour change.

Question 34

What is the colour change of methyl orange from acid to base?

Answer 34

Red to yellow

Question 35

What is the colour change of bromophenol blue from acid to base?

Answer 35

Yellow to blue

Question 36

What is the colour change of methyl red from acid to base?

Answer 36

Red to yellow

Question 37

What is the colour change of bromothymol blue from acid to base?

Answer 37

Yellow to blue

Question 38

What is the colour change of phenolphthalein from acid to base?

Answer 38

Colourless to red

Question 39

At what pH does methyl orange change colour?

Answer 39

5

Question 40

At what pH does methyl red change colour?

Answer 40

6

Question 41

At what pH does bromophenol blue change colour?

Answer 41

5

Question 42

At what pH does bromothymol blue change colour?

Answer 42

7

Question 43

At what pH does phenolphthalein change colour?

Answer 43

10

Question 44

What indicator is used for a weak acid, weak base titration?

Answer 44

No indicator is suitable, as there is only a small vertical portion, so a pH meter must be used.

Question 45

What is the half-neutralisation point on a titration graph?

Answer 45

The gentle slope part of the curve before you reach the steep incline.

Question 46

What does the half-neutralisation point indicate?

Answer 46

Half of the acid/base has reacted, so the pKa is equal to the pH.

Question 47

What is a buffer?

Answer 47

A solution that can resist changes of pH when small amounts of acid or alkali are added to them.

Question 48

What is an acidic buffer made from?

Answer 48

A mixture of a weak acid and a soluble salt of that acid.

Question 49

Why do acidic buffers resist changes of pH when a little alkali is added?

Answer 49

When a little alkali is added the OH- ions from the alkali will react with the HA to produce H2O and A-, this removes the added OH- so the pH tends to stay similar.

Question 50

Why do acidic buffers resist changes of pH when a little acid is added?

Answer 50

When the acid is added equilibrium shifts to the left to produce HA and the soluble salt of HA is fully ionised, so the number of A- molecules increases, which remove the H+ ions, so the pH stays similar.

Question 51

What is one way to make an acidic buffer?

Answer 51

By neutralising half of a weak acid with an alkali, so the buffers pH = pKa. (Half-neutralisation point).

Question 52

What are basic buffers made of?

Answer 52

Weak base Salt of that base.