acids etc Flashcards
situations in hwich approximations for weak acids break down
dissasociation of water being negligible …thus H+=A-…..(if ph is less than 6 for water the dissasociation is significant comapred w dissociation of weak acid . approximation also break down ffor very weak acids or very
dilute solutions)
that the conc of acid is much greater than conc of H+ at equillibrium… thus HA (eqm_ = HA( start) ….. (approximation only holds for small ka values and breaks down if H+ is significant or if there is a huge diff between HA(start) -H+ (Eqm). Approximation NOT justified for STRONGER weak acids where ka is larger than 10^-2 moldm3 )
what are buffer solutions made of
weak acid and salt of weak acid
OR
weak base and salt of weak base
how is salt of weak acid made
and salt of weak base
and another way buffer can be made
weak acid + strong base
weak base + strong acid
partially neutralsiing weak acid with alkali thus producing mixture of salt and acid
pka of buffer solution
same as pka of HA
note about the concentrations in buffer solutions !
we can enter mthe moles of acid and salt straight into the equation as they both have the same final volume
mol/vol= concentration.
mol(1)/vol (divided by) mol (2)/vol = mol(1) / mol (2)
note
when making buffer solutions from partial neutralisation , some acid is left over unreacted !
how else can buffer solutions be made
preparation by partial neutralisation of weak acid (prepared by adding aqeous solution of alkali to excess of weak acid , weak acid partially neutralised by alkali to form conjugate base . some acid left unreacted ; mixture contains mixture of salt, weak acid and unreacted weak acid . THERE IS NO ALKALI LEFT! AND THE AMOUNT OF MOELS OF SALT= MOLES OF WEAK ACID THAT HAS REACTED
How does the body get rid of build of of h2co3
body contains more acidic materials than alkaline, conjugate base HCO3- converts this into h2co3 which converted into co2 to be exhaled out
green box: using a ph meter
ph meter= electrode in solution connected to meter that displays ph reading. indicator paper usually matched from colour charts to nearest whole number so ph meter able to give more accurate measurements of ph during titration
procedure: monitoring ph as aqeous base added to acid solution
1. use pipette to add acid to conical flask
2. palce electrode of ph meter into flask
3. add aqeous base –:> burette + acid – >conical flask 1 cm3 at a time
4. swirl contentsa after each addition. record ph and total vol of base added
5. repeat 3 and 4 till ph starts changing rapidly, tehn add aq base dropwise for each reading until less rapid ph change
6. + aqeuous base 1cm3 at a time until excess added and ph has been basic with little change
.plot graph f ph agaisnt vol of aq base added
a;ternative: attach ph meter to dataloger and magnetic stirrer in flask ( more accurate)
an indicator is a …
weak acid - eq shifted twoards weak acid in acidic conditions and weak base in basic conditions
eg mthyl orange ; red on the left acidic part , on addition of base (OH-) , colour changes to yellwo by shfiting right
at the endpoint…
HA= A - thus ka= H+ thus pka= ph
methstacid
phenowab
red acid–> yellow alkali
colourless acid–> pink alkali
development of differentm odels overtime to explain acid base behaviour
early: acids and a;la;oes. acids= sour. alkalisa re from plant ashes
1754: base reacts with acid to give solid form
1832: acids contain hydrogen that can be replaced by metal
1884: arhenius model; acids form H+ ions, abses form OH- ions
1923: bronsted lowry model: acid base reactions transfer protons, acid donates proton, base accepts proton
