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chemistry > acids and bases and PH > Flashcards

acids and bases and PH Flashcards

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1
Q

what is a bronstead lowry acid

A

a proton donor

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2
Q

what is a bronstead lowry base

A

a proton acceptor

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3
Q

what is a conjugate acid

A

it donates a proton to form the conjugate base

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4
Q

what is a conjugate base

A

it accepts a proton to form the conjugate acid

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5
Q

what is the role of water in acid- base equilibria

A

an acid can only donate a proton if there is a base to accept it . by mixing eg HCl with warer an equilibrium is set up consisiting of two acid - base conjugate pairs and a hydronium ion forms

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6
Q

draw a dot and cross diagram of the hydronium ion

A
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7
Q

how would we use the hydronium ion to show neutralisation

A

H3O+ + OH- = 2H20

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8
Q

what is a polybasic acid

A

monobasic , dibasic and tribasic acids refer to the total number of hydrogen ions in the acid that can be replaced per molecule in an acid - base reaction

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9
Q

how do you calculate the amount of alkali needed for the complete neutralisation of polybasic acids

A

1) decide whether the acid is monobasic , dibasic or tribasic
2) write the equation using as many NAoh units needed to replace the hydrogen atoms in the acid
3) use the balanced equations to deduce the molar ratio of acid to alkali and carry out the calculations

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10
Q

what must we remeber about all bases

A

not all bases are alkalis . a base is only an alkali if it is soluble in water . eg magnesium hydroxide is a base but not an alkali

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11
Q

definition of a monobasic acid

A

one mol of a monobasic acid will release one mol of hydrogen ions in solution

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12
Q

give an example of a dibasic acid

A

carbonic acid H2CO3

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13
Q

give an example of a tribasic acid

A

H3BO3

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14
Q

what is sorensens PH scale

A

converted PH colour values to hydrogen ion concentration values

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15
Q

how do you work out PH using logs

A

PH = - log [H+]

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16
Q

how do you work out PH from hydrogen ion concentration

A

[H+] = 10^ - ph

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17
Q

how many times greater is the [H+] for a PH of 1 than a PH of 2

A

10 times greater

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18
Q

what do we know about [H+] with a small PH value

A

[H+] concentration is high and vice versa

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19
Q

how come we can calculate PH for a strong acid from h+ concentration

A
  • a strong acid completely dissociates in water therefore the conentration of h+ ions is directly proportional to the acid concentration
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20
Q

what are the strong acids

A

H2SO4 , HCl and HNO3

21
Q

how do we represent the dissociation of a weak acid

A

using a reversible arrow
HA(aq) = H+(aq) + A-(aq)

22
Q

which way will the equilibrium lie for a weak acid disociation

A

the equilibrium will lie to the left because they only partially dissociate. a very small concentration of H+ ions and A- ions so Kc<1

23
Q

what is the acid dissociation constant Ka

A

the extent of an acid dissociation is measured by another type of equilibrium constant Ka
- to find the equation for Ka it is the exact same as for Kp or Kc
we use it to calculate PH for a weak acid

24
Q

what does a large Ka value tell us about the degree of dissociation of an acid and therefore its strength

A
  • a large ka value would tell us it is a stronger acid because the concentration of reactants (acid) is lower than the products ( ions) and the further the equilibrum is to the right
25
Q

what does a small value of ka tell us about the strength of the acid and therefore its strength

A

equilibrium lies to the left therefore it is a weaker acid as fewer dissociation

26
Q

what changes the value of ka

A

temperature

27
Q

why do we not use ka for strong acids

A

because the value would be zero for the denominator

28
Q

what is pKa

A

pKa is used to convert ka values into easier values

29
Q

what are the equations for pka

A

Pka = -log(Ka)
Ka= 10^ -pka

30
Q

what is the correlation between the size of pka value and acid strength

A

the greater the pka value , the smaller the ka value and the weaker the acid

31
Q

what happens to pka values with dibasic and tribasic acids

A

the successive dissociations have a larger pka value as ka is lower as the dissociation is less likely to happen as we are removing a H+ from a negative ion

32
Q

what is the simplified expression for Ka for weak acids

A

ka = [ H+]2 / [HA]

33
Q

what is the first approximation that allows this simplified equation

A

HA dissociates to produce equilibrium concentrations of H+ and A- that are equal . there will also be a very small concentration of H+ from the dissociation of water. this is extremely small and can be neglected compared with the much greater H+ concentration from the acid so [H+] = [A-] so [H+] X [A-] = [H+]2

34
Q

what is the second approximation that allows this expression

A

the equilibrium concentration of [HA] is lower than the start concentration of [HA] but only by a very small amount as it is only partial dissociation so we can assume [HA] equilibrium = [HA] start - [H+] so [HA] equilibrium = [HA] start

35
Q

how can you experimentally work out the value of Ka

A

prepare a standard solution of the weak acid of known concentration
and measure the PH of the solution.

36
Q

when do these approximations start to break down

A

the first approximation assumes that the concentration of [H+] from the dissociation of water is negligible but if PH >6 then the dissociation from water will be significant compared to the dissociation of the weak acid therefore this approximation breaks down for very weak acids or very dilute solutions

37
Q

when does the other approximation start to break down

A

the second approximation assumes that the concentration of [H+] is much smaller than the concentration of [HA] at the start . this starts to break down when [H+] becomes significant and [HA] start - [ H+] is significant so this approximation starts to break down for stronger weak acids with Ka > 10 ^ -2 and for very dilute solutions

38
Q

how does water act as both an acid and a base

A

water ionises very slightly setting up an acid - base equilibrium .
H2O + H2O = H3O+ + OH-
acid 1 base 2 acid 2 base 1

39
Q

what is Kw

A

Kw is the ionic product of water . this is the concentration of OH- and H+ multiplied together

40
Q

what happens to Kw with temperature

A

Kw changes with temperature

41
Q

what is the value for Kw at 298K ( 25 degrees)

A

1.00 x 10 ^ -14 mol2dm-6

42
Q

what is an alkali

A

a soluble base ( all alkalis are bases - but not all bases are alkalis ) a base is only alkali if it is soluble in water

43
Q

what is a strong base

A

a strong base is an alkali that completely dissociates in aqueous solution

44
Q

what is another equation for Kw

A

Kw = Kc x [ H2O ] this is because Kc = [H+][OH-] / [H2O] and when we reararrange

45
Q

what is another equation for Kw

A

Kw = [H+]2

46
Q

what happens when [H+]>[OH-]

A

it is an acidic solution

47
Q

what happens when [H+]<[OH-]

A

it is an alkaline solution

48
Q

what happens to PH and the value of Kw at different temperatures

A

as temperature increases , the value of Kw increases and the value of PH decreases. therefore at a higher temperature the water is more acidic . when temperature increases the equilibrium shifts to the right . this means the forward reaction ( the ionisation of water) is endothermic

tip : think global warming , as temperature is increasing the oceans are becoming more acidic

49
Q
A

chemistry (24 decks)

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