Chaoter 21 Flashcards
What is a buffer soltuon exact definition
2 marks
A buffer soltuon sis a soltuon that MINIMISES THE CHANGE IN Ph when only SMALL amounts of acid or base are added
Or diluted a bit
Minuses , small
What is a buffer made out of
A weak acid and it’s CONJUGATE BASE
These exist in an EQUilinrium, designed that if acid is added shifts if alkali added shifts
What happens when a component of the buffer is used up
If a component is used up, then the buffer soltuon will stop working
Oh an in general can yiu say the ph changes when a small amount of acid and alkali added?
YES THE PH VHANGES OFC, BUT JUST THE CHANGE IS MINISIED THATS IN THE DEFINITON
How is first way of making a base
Using an acid and it’s conjugate base to make the components
1) ethanoic acid weak acid so barely dissociates, thus ethnaoic acid is thr source of thr wrka acid
2) if you use a salt of the acid, and then dissocsted that into water, that will produce a lot of Ethanoate ions, which is the cinjugwte base, as the salt dissocsted into ions comoletley
As a result, the salt is the source for the consjugste base, once combine their two compientnd you can get an buffer
2nd way of producing a buffer with ethnaoic acid
If there is a question with this must use the BUFFER KA EQUATIONS NOT KA
BY WAY OF PARTIAL NEUTRIKISARIOM OF WEAK ACID
Int think mix excess weak acid with AN ALKLAI like NaOH
Acid being in excess means ALL THE ALKLAI IS READRED, which produces source of conjugate base
And some of the ethanoic acid is remaining which is source of weak acid
What is basiclaly do equilibrium produced for a buffer
Ed at,y the same as a weak acid
Ha = Hc cA-
What happens when an acid is added to the buffer
And when alklai?
- increases H+ conc
- H+ will react with A-
- EQUILBKRUM will shift to the left by le chatlelkeir
-producing more HA
, H + ions attempted it be restored, Oh is minised
2)
When alkali added, this will REACT with H+ conc, reducing H+ conc
- the HA thus dissocgsed to produce more H+
- EQUILBKRUM shifts to the right
H+ ions restored, pH is minised
Again exact structure for dice ring EQUILBKRUM with buffers and even indicative
1) alkali added, this reavts with H+ conc, reducing the conc
2) the Ha thus dissocsted, producing more H+ conc
3) EQUILBKRUM shifts ti right
H+ conc increased, pH change is minised
Or increase acid increases h+ conc
Reavts with A- to form more HA
Eaukibkrum shifts ti the left,
Reduced H+ conc and so oH change is mimisnied
What is the BEST EFFECTIIVE BUFFER concentrations
What does PKa=
This is achieved when HA conc = A- conc
At this point they cancel in the Ka expression such that Ka = H+
Thus PKa = pH at this point
What does operating pH of a buffer mean and what is the range typically
This is the range in which the buffer MUST BE AT TO WORK
Typically it’s +-1 pH from the standard oH
The standard pH is it’s OPTIMAL pH
This is when HA and A- are equal etc and PKa = ph
HOW TO DO BUFFER QUESTIOSN WNAT EQUATION AND WHY
although it still is weak acid, this time the A- HAS A WHOLE COMPONENT WHICH WE NORMALLY IGNIRE
THUS NEED TO USE FULL KA EQUATION
How to do buffer question where they give you conc and volume of each component, what special thing have to do
Need to find NEW CKNCENTRATIONS OF ESCH COMOONENT
Do thst by doing n = cv ti find moles of each
Then divide the n by TOTAL VOLUME To find nee concs as it’s a MIXTURE
Then use this in the equations
And how ti do PARTIAL NEUTRIKSIAIRON AUESTION
easy, find moles of each first
Now see limiting reagent
Now write down the mole of conjugate base that will be formed due to limiting
Now all the lining moles will BE REACTED, so they go to 0, from the excess sreganer, subtract moles of limiting reagent
These are you final mole values, divide each by TOTAL VOLUME to get concs, out them into Ka equation
Remmeber they Ishtar just give an EQUATUIN like this without mentioning it’s a buffer
Why must pH be controlled in Hama body
This is because we run using enzymes which all follow a narrow lH range
If you exceed or go under this range, then they denature and you can die
What oH should blood plasma normally be at and what range
Healthy blood = 7.4
Range is 7,34 to 7.45
What happens if goes below 7.35 and above 7.45, why is it deep even tho the change isn’t
It’s deep because this is logarithmic scale, so a small change actually represents a huge change of h+ cinc
Anyways go low acidosis high alkalosis smith are dangerous and potential death
Okay so what’s the BUFFER IN BLOOD CALLED AND WHAT COMOENTDNT
How does sdiwiton go alkali acid , ph still minisied using EQUILBKRUM lit ideadnfiek before
Carbonic acid hydrogen cwbdiante ion buffer
Weak acid is carbonic acid and conjugate base it the hydrogen csbrinste
Icncrease acid, h+ increase, reavts with conjugate base, ewuaikoborum shifts to left, h+ decrease oh minised
Additoj Oh# ions alkali, reavts with h+ , reduced the h+ conc, so HA dissocsted moved to right and then the h+ increased and oH minised
How ti find ratios ( to that hard)
Jsutnrearmaged rhe eauation ti give the denim storm and workmout unknowns
IF ONE AT BOTOTM, LEABE IT THERE
How to use a Ph meter
How to do continous monitoring if pH, what equipment to get data easily
Ph probe
Put acid in concixla flask measure
Add one drop at a time and measure pH
You can resend with water esch tube and do it again
Use an AUTOMATIC DATA LOGGER to log the pH , connected it to the pH probe
Foe even more easy, use MAGENTIC stirrer
What is the EQUIVALNCE POINT OF A GRAPH and what does this value mean
This value is directly the vertical value
It’s when the volume of one acid reacts exactly with the volume of another base
You use this value when doing stoichemtry calculations , as it’s the exact
Often we confused this with end point, in reality the end point is very close to the equivalence point pretty kuch
What is an ACID BAS INDUCATOR max out of
What is end point thus
Badicsllt made out of a buffer
Weak acid and conjugate base
The weak acid will have one colour, conjugate base another
The end point is when the concs of the weak acid = the cicn if the cinjugwte base
At this point the colour observed will be half between the extremes!
How does the colour change stuff works with equikbira
Initally, if you have a base or acid present and put idncistor in, it will give a colour based on what.
If it’s acidic, then conc shift to left, but say the EQUILBKRUM lies at the elft ti begin with disoakhign this colour
If alklai EQUILBKRUM starts at the right
Anyways on the additoj of acid , again h+ increase reavts with a- shifts ti left , colour starts ti changing
Adding alklai reavts with h+ reduces it caused it drive shif to to right colour show dot eirght side more
WHEN THE SIFT HAPPENS, THE COLOUR WILL PASS THROIGH THE END POINT THUS IT WILK BRCOME MIXTURE FIRST
anyways that’s why when acid and alkali equivalence point meet normally it’s around end point but off by a bit. Here the equal amount fi acid and base cause equal amounts of eauoabira to be felt too, so we attribute them
In reality there off by maybe 0,5cm2, so we often go for concord and results for this reason
Again what happens at the end point of an I’d castor in terms of itd oh and PKa
Again end point c angel thr ha and a- form ma equation, and thus thr pH = PKa
So why do we choose an idncistor that PH RANGE FALLS IN THE VERTICAL SECTION OF THE GROAHN
This is bevause thr berticla section is when the stoichemtry is equal, so the pH change must fall in this berticla section ti actually mean something ti us
It’d not to say colours wont be seen bevause they will, jt it won’t mean a hot jt.
So yeah must choose inductor that falls in the range
All 4 tritwtion curve shapes
String acid string alkali
Starts low goes high to Alamo curved there
String acid weak alkali
Starts low end like 9 10
Weak acid and string alkali
starts highs like 34, and CURVES A BIT as it TEMPROWTILY ACTD SS A BUFFER , as here there is a weak acid and cinjugwte base , si oh is minised
Then finished high
Finally WEAK ALAKLAI WEAK ACID
starts high, finished low,
Vertical section is VERY SMALL AND NO ISNCISTOR FITS IT
Why does no idncistor fit thr WEAK AKLAI WEAK ACID
Bevause ther vertical section does not 8ntersevt any range of any idncistor so never any idncistor is suitbsle
AND ALSO SEEN IN WAK ACID WEAK ALAKLI THE START
AGAIN DURVED UP A BITA
AGAIN ACTS AS A BUFFER
DONT LACK
So for curve questions how will curve look like if
Conc changes
If dibasic instead of Monobasic
Always write EQUATUIN if scared
But generally if you half conc, then thr volume needed will half too, so equivalence point drops
And if it’s dibasic, you’ll probably need twice the volume , so don’t lack for they
