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Chemistry HL > Acids and Bases > Flashcards

Acids and Bases Flashcards

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1
Q

Bronstead Lowry Acid Def.

A

acids are proton donors

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2
Q

Bronstead Lowry Base Def.

A

bases are proton acceptors

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3
Q

Amphoteric Def.

A

act as both an acid and a base

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4
Q

Amphiprotic Def.

A

acts as both a proton donor and a proton acceptor

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5
Q

Conjugate acid and base

A

when an acid donates a proton, what remains is the conjugate base
when a base accepts a proton, what remains is the conjugate acid
(i.e. they differ by one proton - H+)

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6
Q

Condition for a species to act as a base

A

because a proton does not have any electrons of its own, it is always dependent upon some other species to provide the necessary bonding pair. For a species to be able to act as a base, it must have an unshared pair of electrons that it can supply to a proton

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7
Q

Alkalis

A

these are soluble bases, when they dissolve in water they all form hydroxide ions

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8
Q

Indicator colours

A

litmus - acid = red, alkali = blue
methyl orange - acid = red, alkali = orange
phenolphthalein - acid = colourless, alkali = pink

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9
Q

salt

A

refers to the compound formed when the hydrogen of an acid is replaced by a metal or another positive ion

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10
Q

Acid + base reaction

A

Acid + base –> salt + water

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11
Q

Enthalpy of neutralisation

A

the enthalpy change that occurs when an acid and base react together to form one mole of water
for reactions between strong acids and strong bases, the enthalpy is very similar = -57kJ mol
as net reaction is the same

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12
Q

Acid + metal reaction

A

acid + metal –> salt + hydrogen

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13
Q

Acid + carbonate reaction

A

acid + carbonate –> salt + water + carbon dioxide

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14
Q

pH scale

A

it is a logarithmic expression of the concentration of hydrogen ions
so pH = -log (concentration of H+)
usually positive and have no units
a change in one unit of pH represents a 10-fold change in hydrogen ion concentration

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15
Q

Kw

A

ionic product constant of water
Kw= (conc. H+) (conc. OH-)
this is because in water conc H+ = conc OH-
at 298K Kw=1X10-14

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16
Q

Dilute acids

A

show the typical properties of acids
e.g.
turn blue litmus red
react with metal to produce H2 gas
react with carbonates to release CO2 gas
they are good conductors of electricity

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17
Q

Concentrated acids

A

DO NOT show the typical properties of acids

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18
Q

Changing concentrated acids to dilute acids

A

when added to water the concentrated acid molecules donate a H+ to a water molecule. This forms hydronium ions
it is this hydronium ion that gives the acid its properties
c1V1= c2V2

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19
Q

Strong acids

A

strong acids ionize almost completely
i.e. they transfer all their protons to water
Ka is large (lies to right)
good conductors of electricity
fast rate of reaction
strong acids have low pH
e.g. HCl, HNO3, H2SO4

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20
Q

Weak acids

A

weak acids will transfer a few protons to water
i.e. they only partially dissociate in water
Ka is small (lies to left)
poor conductors of electricity
slow rate of reaction
larger pH
e.g. CH3COOH, H2CO3, H3PO4

21
Q

Strong base

A

completely dissociate in water
good conductors of electricity
very high pH
e.g. LiOH, NaOH, KOH, Ba(OH)2

22
Q

Weak base

A

only partially dissociate in water
poor conductor of electricity
lower pH
e.g. NH3, CH3CH2NH2

23
Q

Acid rain

A

refers to solutions with a pH below 5.6, which therefore contains additional acids (oxides of sulfur and nitrogen)

24
Q

Acid deposition

A

includes all processes by which acidic components as precipitates or gases leave the atmosphere
wet acid deposition: rain, snow, sleet etc.
dry acid deposition: acidifying particles, gases

25
Q

Sulfur oxides

A

SO2 produced from the burning of fossil fuels
S+O2 –> SO2
It dissolves in water to form sulfurous acid
H2O + SO2 –> H2SO3
It can then oxidise to sulfur trioxide
2SO2 + O2 –> 2SO3
H2O + SO3 –> H2SO4

26
Q

Nitrogen oxides

A

NO is mainly produced from internal combustion engines
N2 + O2 –> 2NO
2NO + O2 –> 2NO2
or
N2 + 2O2 –> 2NO2
it then dissolves in water to form nitrous acid and nitric acid
H2O + 2NO2 –> HNO2 + HNO3
alternatively it can be oxidised to nitric acid
2H2O + 4NO2 + O2 –> 4HNO3

27
Q

Impacts of acid deposition

A
  1. impact on materials - both sulfur dioxide and sulfuric acid react with marble and limestone forming calcium salt, these reactions lead to the erosion of structures
  2. impact on plant life - direct cause of slower growth, injury or death of plants, leaching (where important minerals in soil wash away before absorbed by plants). Dry deposition can also block plant pores needed for gas exchange
  3. Impact on water - causes ‘dead’ lakes
  4. Impact on human health - particulates in air
28
Q

Methods to reduce SO2 emissions

A

hydrodesulfurization (precombustion) - catalytic process that removes sulfur from refined petroleum by reacting it with hydrogen to form hydrogen sulfide
flue-gas desulfurization (postcombustion) - removes up to 90% of SO2 from flue gas in the smoke stacks of power stations before it is released

29
Q

Methods to reduce NOx emissions

A

catalytic converters in cars - converts toxic emissions to relatively harmless products
lower temperature combustion - NO formation is reduced at lower temperatures

30
Q

Lews Acid Def.

A

is an electron pair acceptor (electrophile - electron-deficient species that accepts lone pair from another species to form a new covalent bond) *dative

31
Q

Lewis Base def.

A

is an electron pair donor (nucleophile - electron rich species that donates a lone pair to form a new covalent bond) *dative

32
Q

Lewis and Bronstead Lowry Acids

A

all Bronstead Lowry acids are Lewis acids, but not all Lewis acids are Bronstead Lowry acids

33
Q

Kw and temperature

A

Kw is temperature dependent
the dissociation of water is endothermic
therefore, an increase in temperature will shift the equilibrium to the right (increase Kw)
a decrease in temperature will shift the equilibrium to the left (decrease Kw)

34
Q

Kw relationships with Ka and Kb

A

Kw = Ka x Kb
pKw = pKa + pKb
pKw = pH + pOH

35
Q

Acidity constants

A

measure the extent of reactions of acids in water (therefore measure acid strength)
the larger the Ka the stronger the acid
the stronger the acid the smaller the pKa

36
Q

Steps in calculating pH of weak acids

A
  1. write hydrolysis equation
  2. write the Ka expression
  3. make assumptions (concentration of products are equal as 1:1 mole ratio, and concentration of reactant is the same as initial concentration as very few molecules are ionised)
  4. rearrange the Ka expression and solve for concentration of H+
  5. calculate pH
37
Q

Steps in calculating pH of weak bases

A
  1. write hydrolysis equation
  2. write the Kb expression
  3. make assumptions (concentration of products are equal as 1:1 mole ratio, and concentration of reactant is the same as initial concentration as very few molecules are ionised)
  4. rearrange the Kb expression and solve for concentration of OH-
  5. calculate pH
38
Q

Conjugate acid-base pairs

A

two species that differ by just one proton (acid is the species with the additional proton)
the stronger the acid the weaker the conjugate base (and vice versa)
stronger the base the weaker conjugate acid (and vice versa)
strong acids produce neutral anions
strong bases produce neutral cations

39
Q

buffer solution

A

an aqueous solution consisting of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes to pH when small amounts of acid or base are added

40
Q

Preparation of buffer solutions

A

a) mixing a weak acid and its conjugate base (or vice versa) there will be more of the weaker component (whichever has a larger volume, the pH of buffer will favour)
b) partially neutralising a weak acid with a strong base so that some weak acid remains and the rest has been converted into its conjugate base

41
Q

Equivalence point

A

is where the acid and base have neutralised each other (n(acid)=n(base))

42
Q

Indicators

A

are very weak acids and have a different colour to their conjugate base

43
Q

Buffering

A

is the flattish part of the titration curve where the pH hardly changes due to the presence of a weak acid and its conjugate base (can be estimated as pH = pKa +/- 1)
where pKa=pH at half equivalence point

44
Q

Strong acid - strong base pH curve

A

start at low pH end at high pH
equivalence point at 7

45
Q

strong acid - weak base pH curve

A

equivalence point < 7
has buffer region

46
Q

weak acid - strong base pH curve

A

equivalence point > 7
has buffer region

47
Q

weak acid - weak base pH curve

A

equivalence point = 7
no buffer region
curve is squished line
indicator does not work for this reaction and equivalence point must be determined with a conductivity test

48
Q

Choosing an indicator

A

must:
- change colour at pH value close to equivalence point (+/- pKa at equivalence)
- not itself interfere with the acid - base reaction being studied

Chemistry HL (18 decks)

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