unit 4 section 4 Acids, Bases and PH

Question 1

what is a Brønsted-lowry acid

Answer 1

they are proton donors. they release hydrogen ions when mixed with water.

Question 2

what is a Brønsted-lowry base

Answer 2

they accept protons. when in a solution they take hydrogen ions from water molecules.

Question 3

what does dissociation mean

Answer 3

it means that they break up into their positive and negative ions.

Question 4

what dictates the amount of dissociation in a reaction between an acid and an alkali

Answer 4

how weak or strong an acid or base is.
- strong acids almost completely dissociate in water
- strong bases ionise almost completely in water
- weak acids dissociate very slightly in water and a small amount of H+ ions are released ( equilibrium lies to the left )
- weak bases only slightly dissociate in water too ( equilibrium lies to the left )

Question 5

what can water dissociate into and show the formula for this reaction and the ionic formula for this reaction

Answer 5

only a very small amount of water can dissociate, they dissociate into hydroxonium ions and hydroxide ion.
H2O + H2O = H3O+ + OH-
the ionic equation:
H2O = H+ + OH-

Question 6

since only a small amount of water dissociates, where does the equilibrium lie and explain why.
(3)

Answer 6

the equilibrium lies way over to the left. this is because there is a huge amount of water compared to the H+ & OH- ions that the amount of water is considered to be constant.

Question 7

what is the equation for the ionic product of water and how is it dervided
(2)

Answer 7

ionic product of water
Kw = [H+] [OH-]

it is dervided by times the kc for water by the concentration of water Kc of water = [H+] [OH-] / [H2O]

kw = [H+] [OH-] / [H2O] X [H2O] = [H+] [OH-]

Kw = [H+] [OH-]

Question 8

what are the conditions that give you the same value of Kw of a aqueous solution, and what is that value
(2)

Answer 8

temp - 298k or 25 degrees
the same value that is given is
1.00x10^-14 mol^2 dm^-6

Question 9

what is the formula for the kw of pure water and explain why there is a difference
(2)

Answer 9

Kw = [H+]^2
this is because pure water there is always 1 H+ ion for each OH- ion.

Question 10

what is the PH scale
(1)

Answer 10

it is the measure of the hydrogen ion concentration of a solution

Question 11

what is the equation for PH
(1)

Answer 11

PH = -log10 [H+]

-the 10 next to the log is the base so it goes at the bottom on the calculator
- [H+] concentration of H+ ions in solution in mol dm^-3

Question 12

how do you calculate the concentration of hydrogen ions (H+) when you are given the PH.
(1)

Answer 12

you use the equation

[H+] = 10^-PH

Question 13

what is a monoprotic acid and give 2 examples
(3)

Answer 13

a monoprotic acid is an acid that for every molecule of the acid, it will release 1 proton when it dissociates. this means that the H+ concentration is the same as the concentration of the acid.
hydrochloric acid (HCL) & nitric acid (HNO3)

Question 14

what is a diprotic acid and give an example of it.
(2)

Answer 14

a diprotic acid is an acid that for every molecule of acid, it will release 2 protons when it dissociates. this means the concentration of the [H+] ions is double the concentration of the acid
- sulfuric acid (H2SO4)

Question 15

why do you have to use the Ka formula instead of the Kw formula when you are dealing with a weaker base or acid
(2)

Answer 15

this is because weak acids and bases only slightly dissociate in water, this means that the concentration of the H+ ions is not the same as the concentration of the reaction.

Question 16

what is the formula for Ka (the acid dissociation constant )
(1)

Answer 16

Ka = [H+] [ A-] / [HA]
this can be simplified to (because we can assume that all the H+ comes from the acid)
Ka = [H+]^2 / [HA]

Question 17

what is pKa and why is it used, also give the formula for pKa
(3)

Answer 17

pKa is calculated from Ka in the same way that PH is calculated from [H+]. they use pKa because the value of Ka varies massively between different acids, this can make the numbers hard to deal with.

pKa = -log10 (ka)

Question 18

what is
a buffer and what are the two types
(1)

Answer 18

a buffer is a solution that resists changes in ph when a small amount of acid or alkali is added to the solution
- acidic buffers & base buffers

Question 19

what is an acidic buffer
(3)

Answer 19

it is a buffer that has a PH lower than 7, they contain a mixture of a weak acid and one of its salt. they can resist change when an acid or a base is added to it.

Question 20

what is a base buffer
(3)

Answer 20

they have a PH that is higher than 7, they contain a mixture of a weak base and 1 of its salts. they can resist change when an acid or a base is added to it.

Question 21

why can buffer resist change even when they are diluted in water
(3)

Answer 21

this is because if a small amount of water is added to the buffer, the water only slightly dissociates. so the extra H+ ions and the OH- ions push the equilibrium the same amount in both directions, therefore the the equilibrium is left unchanged.

Question 22

what are some of the applications of buffers
(3)

Answer 22

• they are used in shampoo, this is to maintain the ph whilst you wash your hair
• blood - this is in order to keep the blood at a PH very near to 7.4
• ensuring all the tissues in the body are kept at the right PH.

Question 23

what are the two ways of making a buffer
(2)

Answer 23

• mixing a weak acid with its salts
• add a small amount of acid to a weak alkali so that only a small amount of the acid is neutralised to make a salt.

Question 24

what does the HA in the acid dissociation constant represent

Answer 24

it represent the acid in the equation
e.g.
HA + OH = A^- + H20

Question 25

what does it mean if you see pka or ka in a question

Answer 25

it means you are dealing with a weak acid and you make need to use the equation for Ka ( the acid dissociation constant)

Question 26

identify two strong bases

Answer 26

NaOH KOH

Question 26

give an example of a weak acid and what weak acids normally are

Answer 26

ethanoic acid - weak acids tend to be carboxylic acids

Question 27

identify a weak base

Answer 27

NH3 ammonia

Question 28

what is the generic formula for an acid reacting with a base

Answer 28

HA + B = BH^+ + A^-

Question 29

where does the equilibrium lie in a reaction with a strong acid and water

Answer 29

it lies far to the right

Question 29

where does the equilibrium lie in a reaction between a weak acid and water

Answer 29

the equilibrium lies far to the left

Question 30

what equation do you use when you are working out the PH of a weak acid that doesn't contain a buffer

Answer 30

ka = [H+]^2 / [HA] we can use this equation as we can assume that all of the H+ ions comes from the acid

Question 31

what equation do you use when you are working out the PH of a weak acid that contains a buffer

Answer 31

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

Question 32

what is a titration used for

Answer 32

to work out the concentration of an acid or base

Question 33

what are the axis on a titration graph

Answer 33

ph - y axis volume of base - x axis

Question 34

how does the titration curve of a strong base strong acid look like

Answer 34

-the graph starts at around ph 1 as there is an excess of a strong acid - ends at ph 13 as there is now an excess of a stong base - the graph has an S shape with a sharp increase in gradient at the middle of the graph

Question 35

how does the titration curve of a strong base and a weak acid look like

Answer 35

- the graph starts at PH 1 as there is an excess of strong acid - the graph ends at a PH of around 9 as there is now an excess of weak base - the graph has an S shape and there is a steep gradient around the middle of the graph

Question 36

how does the titration curve of a weak acid and a strong base look like

Answer 36

- the graph starts at a PH around 5 as there is an excess of weak acid - the graph ends at PH 13 as there is now an excess of strong base - the graph has an S shape and it has a steep gradient at the middle of the graph

Question 37

how does a titration curve between a weak acid and a weak base look like

Answer 37

- the graph starts at a PH around 5 as there is an excess of weak acid - the graph ends at a PH around 9 as there is now an excess of weak base - the graph has an S shape and still has a steep gradient but isn't as steep as the over curves

Question 38

what is the equivalence point or the end point on a titration curve

Answer 38

it is the point where the acid has been fully nuetralised by the base. this is the point on the graph where there is a sharp change in PH ( steep gradient )

Question 39

what is the half nuetralisation point on a titration curve and what can it be used to do

Answer 39

it is the half way point between 0 and the equivalence point on the curve. it can be used to calculate the pKa of a weak acid by taking the PH at this point.

Question 40

what is an indicator used for in a titration

Answer 40

it is used to determine the end point of a reaction

Question 41

what are the two most common indicators and at what PH do they change colour

Answer 41

- methyl orange : PH 3 - 4.5 - phenolpthalein : PH 8.2-4.5

Question 42

how do you decide which indicator to use for a titration when you are given the titration curve

Answer 42

you at equivalence point/ end point of the reaction. this is where the solution is fully neutralised and there is a steep change in PH. you then look for the indicator that will change colour around that point

Question 43

what is the colour change for methyl orange and phenolpthalein

Answer 43

methyl orange - red to yellow phenolpthalein - colourless to pink

Question 44

what indicator should be used in a reaction between a weak acid and a weak base

Answer 44

no indicator can be used as there is no sharp pH change, therefore a pH meter must be used

Question 45

explain what a titration curve of a diprotic acid looks like

Answer 45

for diprotic acids, they neutralise in two steps. this is because the 2 H+ ions leave separately, they do not leave at the same time. this means that there are two equivalence points, one for each proton that is dissociated