module 6 Flashcards
acids definition
compounds which form hydrogen ions in aqueous solution
eg. HNO3, HCl
bases definition
compounds which form hydroxide ions in aqueous solutions
eg. NaOH, KOH
alkalis
water soluble bases
properties of acids
- pH < 7
- sour
- corrosive
- conductive in solution
- turn blue litmus red
properties of bases
- pH > 7
- caustic
- conductive in solution
- bitter
- turn red litmus blue
acid + base
- neutralisation
- salt + water
(check solubility)
acid + carbonate
salt + carbon dioxide + water
acid + active metal
salt + hydrogen gas
indicators
substances which change colour based on the pH of environment
antoine lavoiser’s theory
- definition
- advantage
- disadvantage
definition: acids were substances which contained oxygen
advantage: worked in many cases
disadvantage: many oxygen containing substances were basic such as CaO
- no proper definition for a base
humphry davy theory
- definition
- advantage
- disadvantage
definition: acids were substances which contained replaceable hydrogen
advantage:
disadvantage:
- no proper definition for a base
- some acidic substances did not contain hydrogen eg. CO2
arrhenius’s theory
- definition
- advantage
- disadvantage
defintion:
- acids were substances which, in aqueous solution, ionised to form H+ ions
- bases were substances which, in aqueous solution, dissociate to form OH- ions
advantages:
- explained acid-base neutralisation reactions
- works for many acids and bases
disadvantages:
- does not recognise the role of the solvent in determining the relative strength or weakness of an acid
- cannot explain:
- the basic nature of carbonates,
acidic or basic salt
- neutralisation not in aqueous
form
advantages to arrhenius
- explained acid-base neutralisation reactions
- works for many acids and bases
disadvantages to arrhenius
- does not recognise the role of the solvent in determining the relative strength or weakness of an acid
- cannot explain:
- the basic nature of carbonates,
acidic or basic salt
- neutralisation not in aqueous
form
bronsted lowry theory
- definition
- advantages
- disadvantages
definition:
- acids are defined as substances which tend to donate protons (H+)
- bases are substances which tend to accept protons (H+)
advantages:
- explains the behaviour for acids and bases and their role in non-aqueous environments
- considers the role of the solvent in determining the strength or weakness of an acid
- explains the acidic and basic behaviour of ions and gases
- explains existence of non-hydroxide bases (NH3)
- shows the amphiprotic nature of substances depending on the environment
disadvantages:
- cannot explain amphoteric substances
- cannot explain acids and bases which do not donate or accept protons
advantages to bronsted lowry
- explains the behaviour for acids and bases and their role in non-aqueous environments
- considers the role of the solvent in determining the strength or weakness of an acid
- explains the acidic and basic behaviour of ions and gases
- explains existence of non-hydroxide bases (NH3)
- shows the amphiprotic nature of substances depending on the environment
disadvantages to bronsted lowry
- cannot explain amphoteric substances
- cannot explain acids and bases which do not donate or accept protons
amphoteric
substance which can act as either an acid or a base
amphiprotic
substance which can act as either an acid or a base specifically due to its ability to either donate or accept a proton in different chemical environments
eg. HCO3-
protic substance
has the ability to act as a proton donor
- mono: 1 available eg. HCl
- di: 2 available eg. H2SO4
- tri: 3 protons available eg. H3PO4
why is the proticity important
- affects the final concentration of hydronium ions in aqueous solutions –> final pH
arrows for first second third ionisation
- strong acid: first arrow normal rest bidirectional
- weak acid: all bidirectional
degree of ionisation
[H3O+]eq / [HA] initial
X 100%
[A-] eq / [HA] initial X 100%
unidirectional arrow
reaction goes to completion
degree of ionisation of strong acid in water
100%
- completely ionises to form hydronium ions
degree of ionisation of weak acids in water
< 100%
- does not completely ionise in an aqeous solution to form hydronium ions
an acids strength is defined by
its ability to donate a proton
acid –> conjugate
base
base –> conjugate
acid
strength of conjugate base of weak acid
weak
strength of conjugate acid of weak base
weak
strength of conjugate base of strong acid
extremely week/almost neutral
strength of conjugate base of strong acid
extremely week/almost neutral
strength of conjugate base of extremely weak/almost neutral acid
strong
strength of conjugate acid of extremely weak/almost neutral base
strong
how to demonstrate amphiproticity of a substance
react the substance with both a strong acid and a strong base
- show that it can be a bronsted lowry base/acid
which types of neutralisation reactions go to completion and how to prove
strong acid and/or strong base
LCP:
- weak acid in water forms equilibrium
- when hydroxide is added –> consumes free H3O+
- reduces [H3O+] –» system shifts right –> increase [H3O+]
- more NaOH is added –> more [H3O+] is consumed and equilibrium continues to shift right
- all CH3COOH is consumed
hence a weak monoprotic acid requires the same to completely neutralise a strong base as does a strong monoprotic acid
Bronsted Lowry theory:
- acetic acid is weak base in relation to water but strong base in relation to NaOH –> NaOH is strong base in water and acetic acid is a stronger acid in water
- acetic acid will ionise to completion in th epreence of NaOH –> neutralisation complete
degree of ionisation definition
ratio of ionised acid molecules to the initial number of acid molecules
substances acidity
dependent on the concentration of hydronium ions in aqueous solutions
pH scale
convenient way to compare acidities of different substances
pH
-log10[H3O+]
- decimal places = number of sig figs in [H3O+]
decimal places of pH
decimal places = number of sig figs in [H3O+]
factors that affect and acids pH
- concentration
- more concentrated (higher [H3O+]) lower pH - strength
- strong acids tend to have lower pH than weaker acids - proticity (strong acids)
- stronger acids with higher proticity will have lower pH - degree of ionisation (weak acids)
- weak acids with a higher degree of ionisation will tend to have lower pH
concentration of acids affecting pH
more concentrated means higher [H3O+] means lower pH
strength of acids affecting pH
the stronger the acid the lower the pH
- 1.0M HCl completely ionises to form [H3O+] = 1.0M
- Acetic acid does not completely ionise (weak acid) [H3O] < 1.0M
proticity affecting pH
is it strong or weak acids
strong
- acids with higher proticity tend to have lower pH
- HCl produces 1.0M [H3O+]
- H2SO4 produces [H3O+] > 1.0M as it has two stages of ionisation
degree of ionisation affecting pH
is it strong or weak acids
weak
- acids with higher degree of ionisation will have a lower pH
- ignore proticity as the subsequent ionisations will have a small k value
which has higher degree of ionisation
- acetic acid (CH3COOH)
citric acid (C6H8O7)
citric acid
pOH
-log10[OH-]
pH + pOH
14
Kw
[H3O+][OH-] = 1.0 x 10^-14
acidic [H3O+] vs [OH-]
pH
[H3O+] > [OH-]
pH < 7
pOH >7
basic [H3O+] vs [OH-]
[H3O+] = [OH-]
pH = 7
pOH = 7
neutral [H3O+] vs [OH-]
[H3O+] < [OH-]
pH > 7
POH < 7
salt produced from strong acid strong base
mostly neutral as its conjugates are extremely weak (dont tend to accept protons in solution)
salt produced from strong acid weak base
mostly acidic salts
- conjugate acid tends to donate protons
- conjugate base is extremely weak (doesn’t accept protons)
salt produced from weak acid and strong base
mostly basic salts
- conjugate base is weak so tend to donate protons
- conjugate acid is extremely weak so barely accepts protons
salt produced from weak acid and weak base
mostly neutral - depends on the nature of reactants and conjugates
levelling of solvent
strength of strong acid is levelled (limited) based of the solvents ability to accept protons
strength of strong base is levelled by ability of solvent to donate protons
titration
technique which the unknown concentration of a solution is determined by measuring volumes of solutions involved in a reaction
- uses neutralisation rwactions to determine the concentration of an acid or base in a titration
what type of reaction does titration use
neutralisation
equivalence point
the point at which equal moles of H3O+ (aq) and OH-(aq) have been reacted together –_> reaction is complete
difference between equivalence point and end point
equivalence –> trying to measure
end point –> what we observe
endpoint
point at which a sustained colour change is achieved using an appropriate indicator
titration error
difference between equivalence point and end point
titre
minimum volume required to reach the endpoint of the neutralisation reaction in titration
titrant
the solution which is used to determine the concentration of the unknown solution
analyte
the solution whose concentration is to be determined
standard solution
a solution with known concentration
steps of titration
- selection of primary standard
- preparation of standard solution
- selection of appropriate indicator
- rinsing of glassware
- performing titration
- calculation
primary standard + properties
substance of sufficiently high purity and stability such that the standard solution can be prepared with a known concentration
- high purity
- easy to store
- inexpensive
- high molecular weight
- known chemical formula
eg. hydrated oxalic acid (strong bases suitable) , anhydrous sodium carbonate , sodium hydrogen carbonate (strong acids suitable)
how to prepare standard solution
- dry solid powdered primary standard in drying oven/desiccator
- weight approximate amount required for beaker
- dissolve the measured mass using demineralised water
- carefully transfer solution to volumetric flask, previously cleaned with demineralised water
- rinse beaker three times –> transfer each rinsing to volumetric flask
- dilute solution by filling up to point where the meniscus on the volumetric flask just touches the gradation line on the neck (hold at eye level to avoid parallax error)
- stopper the flask
- invert 20 times to homogenise the solution holding the stopper tight with thumb
indicator chosen for titration
must change colour over a pH range that closely responds to the pH of the salt produced
- methyl orange (acidic) 3.1 - 4.4
- bromothymol blue (neutral) 6.0 - 7.6
- phenolphthalein (basic) 8.3 - 10.0
rinsing glassware
pipettes and burettes rinsed with the solution they are to contain
- so concentration can be known accurately
- demin water changes the concentration
volumetric flask and conical flask should be rinsed thoroughly with water –> number of moles of sbtsance they contain can accurately be known
volumetric flask + what it should be rinsed with
demin water
- must contain accurate number of moles of solute
conical flask + what it should be rinsed with
demin water
- accurate number of moles of aliquot
pipette + what it should be rinsed with
solution
- must not dilute the solution it is to deliver
- must faithfully represent concentration
burette
solution
- dont dilute the concentration it is to deliver
- must faithfully represent the concentration
performing titration steps
transfer 25ml of standard solution from volumetric flask to conical flask using pipette and bulb filter
(rinsing with demin, the standard solution twice)
- using pipette transfer 25mL of solution into conical flask
- rinse down sides with conical flask to ensure no drops of solution are lost
add few drops of appropriate indicator in the conical flask
fill and position burette appropriately
- rinse with demin water three times
- fill with solution until it is just below the 0mL mark
- read initial value by 2dp to avoid parallax error
perform titration
(rough) then actual
back titration
used to find the concentration of a substance when the direct titration is impossible or impractical
- titrating a solid (antacid tablets, mineral rock)
- when acid/base is insoluble (carbonate)
- when acid/base is a gas
- direct titration results in a weak - acid-weak base titration with poor indicator colour change
- direct titration is too slow of a reaction
neautralisation in real life
- antacid tablest in stomach to stop acid reflux
