Topic 8 - Acids & Bases Flashcards
Arrhenius’ theory
Acid: substance that ionises in water to produce H+
Alkali: soluble base that ionises in water to produce OH-
All alkalis are bases but not all bases are alkali.
Limitation of Arrhenius’ concept
the rxn between NH3 and HCl gas can’t be explained, as NH3 doesn’t contain OH-
NH3 (g) + HCl (g) -> NH4Cl (s)
Bronsted-Lowry theory
Acid: proton (H+) donor
Base: proton (H+) acceptor
In an aq soln, a proton can be represented as either hydrogen (H+) or hydronium (H3O+)
in what conditions will H3O+ form?
when a water molecule forms a coordinate bond with a proton
common acids are referred to as:
- monoprotic: donates 1 proton
eg. HCl - diprotic: donates 2 protons
eg. H2SO4 - triprotic: donates 3 protons
eg. H3PO4
What can be concluded in a reversible rxn involving an acid/base? Give an example.
- the acid/base is weak as they don’t fully dissociate
e.g. CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)
CH3COOH: BL acid
H3O+: conjugate acid
conjugate
if it’s a backwards rxn instead of forwards, the conjugate base will act as a base
conjugate acid-base pair
- conjugate acids and bases will differ from one another by a single proton
- they are called conjugate acid-base pairs
Amphiprotic species
species that can act as either BL acid or BL base depending on the rxn
e.g. HCO3 - (aq) + H2O (l) CO3 2- (aq) + H3O+ (l)
HCO3 - (aq) + H2O (l) H2CO3 (aq) + OH- (aq)
zwitter ion
acts as an acid in the presence of a strong base by donating a proton, and vice versa for strong acids
eg. H2O
Requirements for BL
Acid: must be able to dissociate and release H+
Base: must be able to accept H+ (have lone e- pair)
Amphiprotic: must possess both a lone e- pair and a H+ ion
difference between amphiprotic and amphoteric
- amphiprotic specifically related to BL theory (where emphasis is on proton transfer)
- amphoteric has a broader meaning, describing a substance that can act as both acid & base even in rxns that don’t involve proton transfer
types of bases
- metal oxides/hydroxides
- ammonia
- soluble carbonates
- hydrogen carbonates
why doesn’t HNO3 release H2 gas?
because of its oxidising properties
ACID + METAL -> ?
Acid + metal -> salt + H2
ACID + BASE -> ?
acid + base -> salt + water
ΔH(neut)
enthalpy change occurring when an acid and base react together to form 1 mol of water
for all strong acids & bases, enthalpy change is very similar: ΔH = -57 kJ/mol
ACID + CARBONATE -> ?
acid + carbonate -> salt + H2O + CO2
acid turns phenolphthalein…
colourless
acid turns methyl orange…
red
acid turns litmus paper…
red
acid tastes…
sour
bases taste…
bitter
bases turn litmus paper…
blue
bases turn methyl orange…
yellow
bases turn phenolphthalein…
pink
uses of titration in acid-base titrations
- to calculate the conc of ethanoic acid in vinegar with a standard soln of aq NaOH, using phenolphthalein
- to calculate the conc of NaOH with a standard soln of HCl, using methyl orange
pH scale
- the negative log of the conc of H3O+ or H2
- expressed in moles/litre
Kw
- the autoionisation constant
- expressed as Kw = [H3O][OH]
- expression doesn’t include H2O as its a pure liquid
ionisation of water
- water is a weak electrolyte
- in pure state it doesn’t ionise much
- contains a few H3O+ and OH- ions
- in pure state, it will undergo self-ionisation
differences between strong and weak acids
- strong acids exist entirely as ions in a solution
- weak acids produce an equilibrium mixture in which an undissociated form dominates
- strong acids are good proton donors
- weak acids are bad proton donors
as their dissociation rxn is completed:
- strong acids’ conjugate bases aren’t ready to accept a proton
- weak acids’ conjugate bases are ready to accept a proton
difference between strong and weak bases
- strong bases are good proton acceptors
- weak bases are poor proton acceptors
- strong bases react to form conjugates that don’t show acidic properties
Distinguishing between strong acids & bases and weak acids & bases
- electrical conductivity: strong acids and bases have higher electrical conductivity
- rate of rxn: strong acids and bases react faster
higher no. of H+/OH- ions = higher rate of rxn - pH: the stronger the acid/base, the more polar the pH
acid deposition
the process by which acid-forming pollutants are deposited on the Earth’s surface
acid rain
- most prevalent form of acid deposition
- refers to solns with pH
Sulphur oxides
- forms from combustion of fossil fuels
S (s) + O2 (g) -> SO2 (g)
SO2 (g) + H2O (l) -> H2SO3 (aq)
H2SO3 is sulphurous acid.
nitrogen oxides
- forms at high temps in IC engines or during lightning
N2 (g) + O2(g) -> 2NO (g)
N2 (g) + 2O2 (g) -> 2NO2 (g)
how does dry acid deposition affect plants?
- blocks their stomata
- without Mg2+, plants can’t synthesise chlorophyll
types of acid deposition
- wet acid deposition
eg. rain, snow, etc falls to the ground as aq ppt - dry acid deposition
eg. acidifying particles, gases, etc fall to ground as dust and smoke; later dissolves in water to form acids
Impact of acid deposition on water
- increased acidity of water bodies, causing harm to aquatic life
Al(OH)3 (s) + 3H+ (aq) -> AL3+ (aq) + 3H2O (l)
Al3+ ions interfere with fish gills and reduce oxygen-carrying ability - acid rain also causes eutrophification
effect of SO2 on metals
- accelerates corrosion
Fe (s) + SO2 (g) + O2 (g) -> FeSO4 (s)
Fe (s) + H2SO4 (aq) -> FeSO4 (aq) + H2 - removes protective Al2O3 coating from Al
Al2O3 + 6HNO3 (aq) -> 2Al(NO3)3 (aq) + 3H2O (l)
Impact of SO2 on plant life
- slower growth/healing
- leaching (washing away important minerals) in soil, such as Mg2+ or Ca2+
- can release Al3+, damaging plant roots
pre-combustion method of reducing SO2 emission
- sulphur present as metal sulphides can be removed by crushing coal and washing with water, so metal sulphides (being high density) will sink and separate from coal
- HDS (hydrodesulphurisation)
HDS (hydrodesulphurisation)
- catalytic removal of sulphur from refined petroleum products
- by reacting it with hydrogen to produce H2S
- then can be captured and converted to pure sulphur
post-combination method of reducing SO2 emissions
flue gas desulphurisation
flue gas desulphurisation
CaO (s) + SO2 (g) -> CaSO3 (s)
CaCO3 (s) + SO2 (g) -> CaSO3 (s) + CO2
group trends of hydrogen halides
- relative acidic strength of hydrogen halides increase down the group
- despite decreasing polarity down the group
- due to decreasing bond strength of hydrogen-halogen bonds as halogen ion increases in size
Lewis acid
lone pair acceptor
Lewis base
lone pair donor
difference between Lewis base and Brønsted-Lowry base
- actually still the same group of compounds
- because by both definitions they are species with a lone pair of e-s
difference between Lewis acid and Brønsted-Lowry acid
- Brønsted-Lowry definition of acids is narrower (only includes H+)
- all Brønsted-Lowry acids are Lewis acids but not vice versa
- Lewis acids include any species able to accept a lone pair of e-s
- this means Lewis acids include molecules with incomplete valence shells (which Brønsted-Lowry doesn’t include)
what is the bond formed from a Lewis acid-base reaction
- covalent coordinate bond
- because both electrons will come from the Lewis base
nucleophile
- likes nucleus
- e-rich species
- donates a lone pair to form a new covalent bond in a rxn
- basically a Lewis base
electrophile
- likes electrons
- electron-deficient species
- accepts a lone pair from another reactant to form a covalent bond
- basically a Lewis acid
autoionisation of water
when 2 H2O molecules turn into a hydronium and hydroxide ion
- the hydronium ion takes the proton from the H of the other H2O molecule
- the hydroxide ion got its proton (H atom) stolen but retains the 2 e-s from the bond
quantity of H3O to OH- in an acidic soln
H3O > OH
quantity of H3O to OH in a basic soln
H3O
quantity of H3O to OH in a neutral soln
H3O = OH
when does ionisation of water become significant
in extremely dilute acids/bases
chemical method of distinguishing between strong and weak acids
- rxn with reactive metal/carbonate
- the stronger acid reacts faster
OR
- rxn with alkali
- stronger acid will induce higher temp change
physical method of distinguishing between strong and weak acid
- conductivity test
- strong acid will conduct more
weak acids in environment
- sulfurous acid, H2SO3
- nitrous acid, HNO2
- carbonic acid, H2CO3
effect of H2SO3 on environment
- leaching of soil
- corrosion of limestone buildings/statues/marble
- harms/kills plants
effect of HNO2 on environment
- leaching of soil
- corrodes marble/limestone buildings/statues
- harms/kills plants
effect of H2CO3 on environment
- acidification of lakes
- corrosion of marble/limestone buildings/marble
Example of weak base
NH3
example of strong base
NaOH
example of strong acid
HCl, H2SO4
weak acid
CH3COOH
differing properties between stronger and weaker acids
- stronger = better conductor
- stronger = more vigorous rxn with carbonates and metals
- stronger = lower pH
explain how water can act as both a BL acid and BL base
- BL base: water can accept a proton to form H3O+
- BL acid: water can donate a proton o form OH-
reagents and conditions to prepare ethanoic acid from ethanol
H2SO4 (sulphuric acid) and Cr2O7 2- (chromium dichromate)
heat under reflux.
why would a very acidic aq solution contain OH- ions?
- in v acidic solns, [H+] increases and [OH-] decreases but there are still some present
[OH-] = Kw / [H+] therefore [OH-] cannot be zero.
what is the effect of increasing temp on the equilibrium constant of water dissociation equation (given that dissociation of water is endothermic)?
- forward rxn favoured as it is endothermic
- Kw increases as [OH-] and [H+] increase
- endothermic favoured as it will use up some of the heat supplied
A piece of metal is added to a strong acid and a weak acid. Describe observations that would allow one to distinguish between them:
Strong acid:
- greater temp change
- faster rate of Hydrogen production
- faster rate of Mg dissolving
Ka
- acid dissociation constant
- larger Ka = stronger acid
Kb
- base dissociation constant
- larger Kb = stronger base
pH
calculated as pH = - log [H+]
pOH
calculated as pOH = - log [OH-]
relationship between pH and pOH
pH + pOH = 14
pKw
- the negative log of Kw
- pKw = 14 if condition is room temp
pH + pOH = pKw
