5.1.3 - acids, bases and buffers Flashcards by Maddie C

what is the definition of a Bronsted-lowry acid?

H+ (aq) donor / proton donor

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definition of a strong Bronsted-lowry acid?

proton donor that completely dissociates

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definition of a weak Bronsted-lowry acid?

proton donor that partially dissociates

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that is the acid dissociation constant?

a measure of the extent of dissociation of an acid

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what is the relationship between the extent of dissociation and the Ka (or pKa)

the larger the Ka (or smaller the pKa), the stronger the acid

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definition of a Bronsted-lowry base

H+ (aq) acceptor / proton acceptor

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definition of an alkali?

soluble base that releases oh- ions when dissolved

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definition of a salt?

when the H+ in an acid is replaced by a metal ion

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what is the range of pH s for a strong acid?

pH < 1

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what is the range of pH s for a weak acid?

pH > 1

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give 3 examples of strong acids

HCl hydrochloric acid
H2SO4 sulfuric acid
HNO3 nitric acid

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give three examples of weak acids

ethanoic acid
benzoic acid
phenol

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give an example of a strong alkali

sodium hydroxide NaOH

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give an example of a weak alkali

ammonia, NH3

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define a monobasic acid and give and example of a monobasic acid

one mole of acid dissociates forming one mole of H+
eg. HCl

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define a dibasic acid and give an example of a dibasic acid

one mole of acid dissociates to form 2 moles of H+
eg. H2SO4

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define a tribasic acid and give an example of a tribasic acid

one mole of acid dissociates to form 3 moles of H+
eg. H3PO4

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define a conjugate acid

a species formed when a proton is added to base

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define a conjugate base

a species formed when a proton is removed from an acid

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define a conjugate acid-base pair

2 species that differ by 1 H+

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why is a logarithmic scale used for [H+]

gives more manageable numbers to work with

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what is the difference between successive whole pH numbers

a factor of 10 for [H+]

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how many decimal places should you give a value of pH to?

2 decimal places

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how do you convert [H+] to pH?

pH = -log[H+]

how do you convert from pH to [H+]?

[H+] = 10^(-pH)

for a strong monobasic acid, what is the relationship between [H+] and [acid]?

[H+] = [acid]

for a di/tribasic acid, what is the relationship between [H+] and [acid]?

[H+] = 2 or 3 x [acid] (2x for dibasic. 3x for tribasic)

for a weak acid HA, what is the relationship between [H+] and [HA] and why?

[H+] does not equal [HA], it is much less, as the equilibrium lies to the left.

what are the 2 approximations we use to calculate [H+] for weak acids and what do the mean?

1. [HA] at equilibrium ~ [HA] undissociated i.e. [HA] >>[H+], as only partial dissociation 2. [A-] at equilibrium = [H-] at equilibrium i.e. the amount of H+ ions from the dissociation of water is neglibible.

what is the equation for the acid dissociation constant for a weak acid

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

using the approximations for weak acids, what is the approximation equation for Ka?

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

what is the equation for the % dissociation of a weak acid?

[H+]/[HA] x 100

what are the limitations of using the approximation [HA] at equilibrium = [HA] undissociated?

for "stronger" weak acids there is a greater dissociation of HA and there is a considerable difference between [HA] undissociated and [HA] at equilibrium. the approximation breaks down.

what are the limitations of using the approximation [A-] = [H+]?

Water also partially dissociated into H+ and OH-. if you have a very weak or very dilute solutions of a weak aci, then the dissociation of water into H+ can become significant and [H+] > [A-]. the approximation breaks down.

how can Ka values be made more manageable?

convert them into the logarithmic pKa

how do you convert from Ka to pKa?

pKa = -log(Ka)

how do you convert from pKa to Ka?

Ka = 10^(-pKa)

what external condition affects Ka (and pKa)?

Ka (and pKa) vary with temperature.

what ions does pure water separate into?

H2O can ionise into H+ and OH- ions

what is the Kc expression for the dissociation of water?

Kc = [H+][OH]/[H2O]

how do you rearrange the Kc expression for the dissociation of water into the ionic product of water equation?

Kc [H2O] = [H+][OH-] becomes Kw = [H+][OH-]

to calculate the pH of water, [H+] = .....

[H+]=[OH-]

how can Kw be used to find the pH of strong bases? use NaOH as an example

[NaOH] = [OH-] [H+] = [Kw]/[OH-] solve for pH using [H+] as normal

what is a buffer solution?

a buffer solution minimises the change in pH when a small amount of acid or alkali is added.

hat are the 3 things a buffer solution consists of?

a high concentration of a weak acid, HA a high concentration of its conjugate base, A- a low concentration of H+

when does a buffer solution work best?

when pH = pKa (this happens when [HA] = [A-])

what are the 2 methods of making a buffer?

1. add a salt to an excess of weak acid 2. partially neutralise an excess of weak acid (to make the salt) with a metal/metal oxide/hydroxide/carbonate, but some acid remains

explain what happens when extra acid is added to the buffer.

HA ⇋ H+ + A- H+ added reacts with A- position of equilibrium shifts to the left to make more HA any change in pH has been minimised

explain what happens when extra alkali is added to a buffer.

HA ⇋ H+ + A- OH- added reacts with H+ position of equilibrium shifts to the right to make more H+ any change in pH has been minimised

what is the correct range of Ph for human bloord?

7.35 - 7.45

what is the equation for the formation of the hydrogen - carbonic acid buffer from water and carbon dioxide?

H2O + CO2 ⇋ H2CO3 ⇋ H+ + HCO3-

in terms of buffers in the blood, explain what happens when an acid is added to blood.

H2CO3 ⇋ H+ + HCO3- H+ added H+ reacts with HCO3- position of equilibrium shifts to the left to make more H2CO3 CO2 is breathed out and any change in pH has been minimised

in terms of buffers in the blood, explain what happens when an alkali is added to blood.

H2CO3 ⇋ H+ + HCO3- OH- reacts with H+ position of the equilibrium shift to the right to make more H+ less CO2 breathed out/more CO2 breathed in and any change in pH has been minimised

how do you find the ration of acid : salt in the buffer

[H+]/[Ka] = [HA]/[A-] (acid to salt) or [Ka]/[H+] = [A-]/[HA] (salt to acid)

how does an acid-base indicator work?

usually a weak acid (HA). the acid HA and its conjugate base, A- have different colours. eg: HA ⇋ H+ + A- HA is red, A- is yellow

how does an indicator show you that the end point has been reached? use this reaction as an example: HA ⇋ H+ + A- where HA is red, A- is yellow

when there is an equal concentration of the acid and its conjugate base ([HA] = [A-]) the indicator is at its end point, where pH = pKa in the example, the solution would be orange, as [A-] = [HA] so equal mix of the red and yellow

what is the equivalence point of a titration?

the volume of one solution at which it reacts exactly with a known volume of a solution with a known concentration. on a pH curve, it is the vertical section of the curve.

how do you choose an indicator for a titration?

indicator is chosen so that the pH range of the indicator (or the end point of the indicator) matched the vertical section of the titration curve (the equivalence point.)