Acids and bases

Question 1

What is a neutralisation reaction?

Answer 1

When an acid reacts with a base

Question 2

Acid + metal —>

Answer 2

salt + hydrogen

Question 3

Acid + metal oxide —>

Answer 3

salt + water

Question 4

Acid + metal hydroxide —>

Answer 4

salt + water

Question 5

Acid + metal carbonate —>

Answer 5

salt + CO2 + water

Question 6

Bronsted Lowry definition of an acid

Answer 6

proton donor

Question 7

Bronsted Lowry definition of a base

Answer 7

proton acceptor

Question 8

Define pH

Answer 8

-log10[H+]

Question 9

How to work out concentration of H+ ions using pH

Answer 9

10^-pH

Question 10

What does a smaller pH indicate about the concentration of H+ ions

Answer 10

The smaller the pH, the greater the concentration of H+ ions

Question 11

What is a strong acid?

Answer 11

A strong acid fully dissociates into its ions

Question 12

Position of the equilibrium of strong acid dissociation

Answer 12

Equilibrium lies entirely to the right hand side

Question 13

What is a weak acid?

Answer 13

A weak acid only slightly dissociates into its ions

Question 14

Position of the equilibrium of weak acid dissociation

Answer 14

Equilibrium lies entirely to the left hand side

Question 15

What is Ka

Answer 15

[products] / [reactants]

Question 16

Ka of a weak acid

Answer 16

[H+]2 / [HA]

Question 17

H+ ions of weak acid

Answer 17

square root of Ka x [HA]

Question 18

pH of a strong acid

Answer 18

-log[H+]

Question 19

Ka conversion into pKa

Answer 19

pKa = -log10Ka

Question 20

pKa conversion into Ka

Answer 20

Ka = 10^- pKa

Question 21

How does a stronger acid affect the value of pKa

Answer 21

The stronger the acid, the lower the pH and the lower the pKa value

Question 22

Position of equilibrium of water

Answer 22

To the left

Question 23

What does the position of the equilibrium of water indicate about [water]

Answer 23

Position of equilibrium is very far to the left so [H2O] is effectively constant

Question 24

What is Kw?

Answer 24

Ionic product of water

Question 25

Value of Kw at 298K

Answer 25

1 x 10^-14

Question 26

Kw expression for water

Answer 26

Kw = [H+][OH-]

Question 27

Why is pure water always neutral

Answer 27

[H+] = [OH-]

Question 28

How to work out [H+] of pure water

Answer 28

[H+]2 = Kw

Question 29

Position of equilibrium of strong bases

Answer 29

Equilibrium lies entirely to the right hand side

Question 30

How to work out [H+] of strong bases

Answer 30

[H+] = Kw / [OH-]

Question 31

What is a buffer?

Answer 31

A solution which can resist changes in pH when a small amount of acid or base is added

Question 32

What is an acidic buffer made of?

Answer 32

A weak acid and a soluble salt of that acid. Maintains a pH below 7

Question 33

What is a basic buffer made of?

Answer 33

A weak base and a soluble salt of that base. Maintains a pH above 7

Question 34

How to work out [H+] of a buffer

Answer 34

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

Question 35

How to work out pH of a buffer made by neutralisation?

Answer 35

ICE box use end moles in equation

Question 36

What is half neutralisation?

Answer 36

Enough base has been added to neutralise exactly half of the acid present

Question 37

At half neutralisation point what does Ka equal?

Answer 37

Ka = [H+]

Question 38

At half neutralisation point what does pKa equal?

Answer 38

pH

Question 39

What is a standard solution?

Answer 39

Exact concentration is known

Question 40

What are the practical steps you need to take to produce a pH titration curve?

Answer 40

Add 25cm3 of acid into a 250ml beaker Measure the initial pH of the acid solution in the beaker using a pH meter and record Add 1cm3 of base from the burette Swirl the mixture Measure the new pH and record Continue adding base until it is in excess Add in smaller intervals when approaching end point

Question 41

Why is it important to calibrate pH meter?

Answer 41

After storage it may not give an accurate reading

Question 42

How to calibrate pH meter?

Answer 42

Place the pH meter in a solution of known pH and then adjust the meter accordingly

Question 43

What is the equivalence point?

Answer 43

When exactly enough acid has been added to neutralise the base

Question 44

What is the end point?

Answer 44

Exact volume of acid or base which needs to be added to cause an indicator to change colour

Question 45

How to choose a suitable indicator?

Answer 45

Changes colour somewhere on the vertical section of the pH titration curve

Question 46

When does phenolphthalein change colour and what colour is phenolphthalein in acid and alkali? (3)

Answer 46

At a pH of about 10 Acid: colourless Alkali: red

Question 47

When does methyl orange change colour and what colour is methyl orange in acid and alkali? (3)

Answer 47

At a pH of about 4.5 Acids: red Alkali: yellow

Question 48

What is the half-neutralisation point?

Answer 48

Enough base has been added to neutralise exactly half of the acid

Question 49

A student has an unknown weak acid of unknown concentration and a bottle of 0.5 mol dm-3 NaOH. Explain, using brief practical details, how you would determine the Ka of the weak acid. [6]

Answer 49

Measure 25 cm3 of acid into a beaker/conical flask using volumetric pipette. Use a pH probe to measure the initial pH Add 1cm3 NaOH from a burette swirl and record the pH Repeat until the NaOH is in excess Plot a graph of pH against vol NaOH added in cm3 Use the vertical section to find the volume of NaOH needed for neutralisation The half neutralisation point is half the volume of neutralisation. Read off the pH at the half neutralisation point. At half neutralisation ka = [H+]/pKa=pH Ka = 10-pH

Question 50

Why is a logarithmic scale, the pH scale, is used as a measure of hydrogen ion concentration? (1)

Answer 50

Concentration of hydrogen ions in aqueous solution covers a very wide range

Question 51

What are the properties of a good indicator? (3)

Answer 51

Sharp colour change - no more than one drop of acid/alkali needed for colour change End point must be the same as the equivalence point, or titration gives wrong answer Distinct colour change so it is obvious when end point has been reached

Question 52

What products are buffers found in? (2)

Answer 52

Shampoos Detergents