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Module 5 Chemistry > Chapter 20 - Acids, Bases and pH > Flashcards

Chapter 20 - Acids, Bases and pH Flashcards

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1
Q

define acids and alkalis using the Arrhenius model

A
  • acids dissociate and release H+ ions in aqueous solution

- alkalis dissociate and release OH- ions in aqueous solution

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2
Q

what is the Bronsted-Lowry definition of a base

A

a base is a proton acceptor

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3
Q

what is the Bronsted-Lowry definition of an acid

A

an acid is a proton donor

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4
Q

define a conjugate acid-base pair and give an example of one

A

“a conjugate acid-base pair contains two species that can be interconverted by the transfer of a proton”

HCl acid with Cl- base

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5
Q

give an example of a reaction with two acid base pairs, identify them

A

HCl + OH- —> H2O + Cl-
HCl and Cl- are an acid-base pair
H2O and OH- are an acid-base pair

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6
Q

define a monobasic acid

A

“monobasic acids are acids where one hydrogen atom can be replaced per molecule”

E.g. HCl

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7
Q

define a dibasic acid

A

“dibasic acids are acids where two hydrogen atoms can be replaced per molecule”

E.g. H2SO4

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8
Q

define a tribasic acid

A

“Tribasic acids are acids where three hydrogen atoms can be replaced per molecule”

E.g. H3PO4

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9
Q

define pH, give the equation

A

-log([H+])

the negative log (base 10) of the hydrogen ion concentration in a solution

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10
Q

what are the things to note about the pH scale e.g. high/low values etc.

A

high value = low H+ conc.
low value = high H+ conc.

its logarithmic so pH 3 is 10x higher H+ conc. than pH 2

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11
Q

what can we say about the H+ ion concentration of a strong acid

A

it is the same as the concentration of the acid because the acid fully dissociates

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12
Q

what happens to the pH of a solution if it is diluted by 10 times

A
  • the pH increases by 1 because the H+ ion concentration decreases by 10 times
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13
Q

what is the difference between a strong acid and a weak acid

A
  • a strong acid completely dissociates/ionises in aqueous solution
  • a weak acid partially dissociates/ionises in aqueous solution
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14
Q

what is Ka

A

a version of Kc to represent the strength of a weak acid/how much it dissociates/where equilibrium lies

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15
Q

how can we calculate Ka for a weak acid HA

A

HA –Reversible–> A- + H+

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

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16
Q

what is the only thing that changes Ka

A
  • it acts in the same way as Kc, the only thing that changes it is temperature
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17
Q

what is pKa

A

like pH,
pKa = -log(Ka)

(base 10)

18
Q

what can we say about the strength of a weak acid compared to its Ka and pKa value

A
  • the stronger the acid, the higher the Ka value, the lower the pKa value
  • the weaker the acid, the lower the Ka value, the higher the pKa value
19
Q

what two factors does the H+ ion concentration depend on in a weak acid

A
  • Ka

- [HA]

20
Q

how can we layout an equilibrium for a weak acid in order to find its Ka

A

HA –reversible–> A- + H+

[HA]start, 0,0 = initial concs
[HA]eqm, [A-], [H+] = eqm concs

[HA} eqm = [HA]start - [H+]eqm

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

21
Q

what is the simplified equation for Ka of a weak acid that allows us to calculate [H+] more easily

A

Ka = [H+]eqm^2 / [HA]start

22
Q

what are the two assumptions we make that allow us to form the simplified equation for Ka of a weak acid

A
  • we can assume that the dissociation of water is negligible and therefore there are no additional H+ ions so [H+] = [A-], allowing us to have x^2 on the top of the equation
  • we can assume that the change in concentration of HA is so small that it is negligible, thus [HA]eqm = [HA]start
23
Q

how can we find [H+] and therefore pH from a Ka and initial conc. of acid

A 
Ka = [H+]eqm^2 / [HA]start
as in the simplified equation
rearrange to
[H+] = sqrt(Ka x [HA]start)
then pH as normal
24
Q

where do the two assumptions we make break down

A
  • we can no longer assume dissociation of water is negligible if, pH > 6, so in very weak or very dilute acids
  • we can no longer assume [HA]start = [HA]eqm if Ka is larger ( Ka > 10^-2) or in very dilute solutions
25
Q

what is a special property of water in terms of acids/bases

A
  • it can act as an acid or a base making it amphotic
26
Q

what is the equation for the dissociation of water

A

H2O –reversible–> OH- + H+

27
Q

what is the Ka equation for water and what can we simplify it to

A

Ka = [OH-][H+] / [H2O]
this can be simplified to
Kw = [OH-][H+]

28
Q

why can this simplification from Ka to Kw be made

A

we can assume that because there is such a large excess of water molecules when it dissociates that the concentration of water is a constant, (1000/18).
this means we can do

Kw = Ka[H2O] = [H+][OH-]

29
Q

what is the value of Kw at 298K and how does this link to pH

A

Kw = 1 x 10^-14
at 298K,
[H+] = [OH-] = sqrt(Kw) = 1 x 10^-7
therefore pH of water is 7

30
Q

when does Kw change

A

when temperature changes, like any other equilibrium constant

31
Q

what is a neutral pH

A

depends on the temperature,
as temperature increases, the pH of neutral decreases, because there are more H+ ions per unit volume because equilibrium moves right
However, there are still equal numbers of H+ and OH- ions so it is still neutral

32
Q

is the dissociation of an acid endothermic or exothermic

A

endothermic, hence why Kw increases when temperature increases

33
Q

what is the equation for pKw

A

pKw = -log(Kw)

base 10

34
Q

why is Kw very important for calculations

A

it acts as an equilibrium constant, if we know [H+] or [OH-] then we can calculate the other

35
Q

explain how to calculate the pH of a strong base, and what to remember

A

1) use concentration of base to find [OH-], watch out for molar ratios!!
2) substitute into Kw to find [H+]
3) use pH formula

NOTE: make sure you use concentration and not moles

36
Q

explain how to find the pH of an acid-base mixture

A

1) calculate number of moles of OH- and H+ from conc’s and vols
2) work out which is in excess, find the amount by which it is in excess
3) using this amount and the total volume, calculate the concentration of H+ or OH-
4) use Kw and [OH-] to find [H+] if its not in excess or just use H+ directly in the pH formula

37
Q

even if the dissociation of the acid does make a difference to the equilibrium concentration of HA should we account for it

A

NO, mark schemes don’t like it

38
Q

what is the name/term for Kw

A

the Ionic product of water

39
Q

what is the ionic equation for a neutralisation, even if it’s a weak acid

A

OH- (aq) + H+ (aq) —> H2O (l)

40
Q

if we have the conc of a weak acid and the conc of the H+ ions, how can we tell if it’s a strong or weak acid

A
  • in a strong acid they would be equal

- in a weak acid they are different

41
Q

what can we say about the concentrations of H+ and OH- ions in pure water at any temperature

A

they will be equal

it is only Kw = 10^-14 at 25 degrees celcius

42
Q

what do we need to watch out for when using conc’s of strong bases and Kw to calc pH

A

some strong bases may release more than one OH- ion per molecule e.g. Ca(OH)2
you would need to double the conc. for use in Kw

Module 5 Chemistry (6 decks)

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