Chapter 21 buffer And Aneutriskisaiton Flashcards
What is a buffer solution (2 marks sample answer EXACT WORDS)
A system that acts to MINIMISE pH changes when SMALL amounts of an acid or a base are added.
/ smalldilution
What components in any buffer solution
1) WEAK ACID
2) ITS CONJUGATE BASE
Why does a buffer need these two components?
They work to minimise ph change, thr weak acid will remove and added tiny amount alkali, and same for the conjugate base will react and remove any small acid
What happens when a component of the buffer is used up?
When buffer used up the System loses its power to buffer
In general does pH change with a buffer soltuon and small amounts?
YES IT CHANGES, buffer soltuij acts ti MINIMISE, not end the pH change completely , so it still, goes up a bit
What TWO WAYS to prepare a buffer
(Say based on ethnaoic acid)
1) using a weak acid and it’s (any ) corresponding salt
2) partial neutralisation of thr weak acid ti get both components
How do you prepare a buffer using 1) weak acid and it’s corresponding (any) salt ?
Remember buffer needs = weak acid + conjugate base
- ethanoic acid is a weak acid that when added to water has a LOW DISSCOATION CONSTANT , so will partially dissociate and most wil, stay as ethnaoic acid = WEAK ACID COMP
- If you use ANY salt , and dissolve in water, it will completely disscoates, giving you metal ion and the weak acid ion WHICH IS THE ACID CONJUGATE
now you have both
How to prepare a buffer using partial neutralisation of the weak acid
Adding soltuij of an alkali like NAOH to excess of weak acids
Weak acid is partially neutralised , forming conjugate base in soltuon , and some weak acid full left behind, and BOTH COMPOENTNS ARE HERE
How to use le chatellier principle to explain what happens when acid and alkali added to a buffer soltuon
Acid
- when acid added gonna shift to the left
- this because aditional acid increases H+ ions conc , so remove this goes left, they react with more conjugate base to reduce
Alkali
- added alkali shift to right
- this because added alkali react with h+, reducing h+ , so to restore h+, gotta shift equilibrium to right
This way the pH change is minimised
When is the buffer solution most effective
So what’s the Ph of system equal to at this point
(IMPORTANT)
Buffer soltuij most effective when concentration of acid and conjugate base THE SAME
At this point, the pH is just = to the PKa if the system, as the others cancel out
What range do buffers tend to work at normally compared to their PKA
+-1 away from their PKa
How ti find pH if a buffer soltuion
IMPORTSNT don’t get mixed up
Not that hard,
1) write down Ka expression for just the weak acid
- aim is to find the unknowns using other disscoation
2) find them and plug in
Might be a case if finding moles, and then finding NEW CONC BY DIVIDING BY WHOLE VOLUMES
rearrange for H+
What about PH for partial neutralisation ?
Remember what’s happening here
1) the alkali reacts with weak acid to produce conjugate base
2) leftover weak acid is next cimoient.
SO FIRST
Find out the moles of how much conjugate base, by looking at moles of ALKALI USED = MOLES OF CONJUGATE BASE
Now 1:1 reaction, moles of alkali used is the same moles the EXCESS acid is used up, so subtract moles of acid by moles of alkali to get remaining moles
And now you have moles, work out concs
Sun it in
Happy days
Why does ph need to be maintained in the body
This because enzymes work at an optimum pH and need to kept in small range or get denatured
WHAT PH does blood plasma, which has buffer soltuon, normally range at
Healthy blood?
7.35-7.45, , and HEALTHY BLOOD AT 7.40
What happens if blood goes below 7.35 or above 7.45
Below 7.35 acidosis so fatigue, above 7.45 alkalosis which is muscle spasms light new dense etc
May seem small bit logarithmic scale so drop by that actually a lotm
What is the buffer system called in blood and give the disscoation
Carbonic acid, hydrogen carbonate buffe
Use le dhatellaier principle to explain how buffer buffers with small amounts of alklai and acid
If acid added
- more h+ ions, so shifts to life, reacts with hydrogen cabrknste to form more csbronsid acid
If alklai added
- more react with h+, reduce, so shift to right, as more h+ is made and carbonic decreased
How to work out ratios ?
Basicallt rearrange but what KEY STEP
Rearrange for ratio calm
But just remember ti divide by 1 after rounding or leave in ratio form
How to use a pH meter really quickly
1) place electrode in beaker etc , record the pH
2) rinse with water
3) di it again
Or use MAGENTIC stirrer and automatic data logger to plot graph
what is the EQUIVALENCE POINT of a graph
WHERE IS IT
Of a trite ruin is where the volume of one solution EXACTLY reacts with the volume of another soltution
It is half way between the VERTICAL SECTION OF THE LINE
What is an acid base indicator made out of and how does it work
Like a buffer, made form a weak acid and it’s conjugate base
- how it works is that the weak acid and conjugate base will have distinctively DIFFERENT colours,
- throughout the titration as these changes so will colour
Okay sk if you know how indicator works, WHAT IS THE END POINT?
The end point of a tirstjon is the point where the CONCENTRARIONS OF CONJUGATE BASE AND ACID ARE EQUAL
And thus the colour shown is BETWEEN THE TWO EDTREMES
So in the case of methyl orange , end point is orange!
Again what difference between end point and equivalence point
Equaivalnende point is half way vertical, pout. Where amount if volume of Ken soltuons reacts exactly with the volume of another
End point is when conc conjugate base and acid are equal in indicator and middle colour displayed
Use le chatellier principle to explain colour changes in an indicator if start off with acid and alklai and vice Verca
1) string acid, causes equkibkrum ti shift ti left, as too many H+ ions, sondusolays red
- as base added, reacts with H+, causes ti go down, so shift to right
- this goes from red through orange AT THE END POINT and then to yellow
2) If start string alklai, initially alkali react with h+ , making it go down, so right and yellow
- adding acid, means goes to left to remove acid ,
- yellow to end point orange to red
What happens at the end point of an indicator in terms of pH and PKa
Write down the disscoation, we know the conc of conjugate base and acid same,
So H+ = Ka
So ph = PKa at the end point
HOW DO YOU KNOW IF AN INDICATOR IS SUITBSLE FOR AN EXPERIMENR
1) draw titration curve
2) if the range of the INDICATOR OPERATES BETWEEN AND INTERSECTS VERTICAL SECTION, THEN IT DAN
If not can’t
How does string acid string base titration curve look like
- ph string acid so starts low
- string base so ends high
- standard titration curve nothing wrong here
How does a WEAK ACID, string base look like
- weak acid so starts a bit higher
- string alkali so finishes high
HOWEVER
- weak acid present and so it basically acts like a BUFFER FOR SMALL INCREASES KF ALKLI
- thus the curve initially is increasing for a bit
- AND THEN ITS BACK TO NORMAL
Remmeber = weak acid = curve changes v acts temporary buffer
Titration string acid vs weak alklai
String acid = low oh
- weak alkli = low oh
Nothing els e fishy going on
Finally weak acid weak alkali
2) how is vertical section looking
3) does any indicator tend to go?
Weak acid = high ph
Weak base = low ph
Weak acid = BUFFER EFFECT FIR S BIT
+ vertical section a bit autistic ( slanted to right)
3) no normally no indicator range falls in it’s ugly vertical section, so no I dictir can be used
How does equbilande and end point. Differ normally
Ideally they are the same, so you have a signal to tell you when the volumes of solution exactly reacts with volume with another
However more time they differ by a few drops, hence we do average e
Remember for curve change questions
- if volume half’s, conc half’s
Normally it will be groan at different area
And if dibasic then need two times more volume etc
Don’t lack kf weak acid use formula , if string acid , if string alkli same , remember di basic tri basic, if buffer use normal
Remember for ionic product qs
Any sort of Oh csn try relate, find oh find Oh and see if basic and say that’s find
