Chapter 21 buffer And Aneutriskisaiton

Question 1

What is a buffer solution (2 marks sample answer EXACT WORDS)

Answer 1

A system that acts to MINIMISE pH changes when SMALL amounts of an acid or a base are added.

/ smalldilution

Question 2

What components in any buffer solution

Answer 2

1) WEAK ACID
2) ITS CONJUGATE BASE

Question 3

Why does a buffer need these two components?

Answer 3

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

Question 4

What happens when a component of the buffer is used up?

Answer 4

When buffer used up the System loses its power to buffer

Question 5

In general does pH change with a buffer soltuon and small amounts?

Answer 5

YES IT CHANGES, buffer soltuij acts ti MINIMISE, not end the pH change completely , so it still, goes up a bit

Question 6

What TWO WAYS to prepare a buffer
(Say based on ethnaoic acid)

Answer 6

1) using a weak acid and it’s (any ) corresponding salt
2) partial neutralisation of thr weak acid ti get both components

Question 7

How do you prepare a buffer using 1) weak acid and it’s corresponding (any) salt ?

Answer 7

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

Question 8

How to prepare a buffer using partial neutralisation of the weak acid

Answer 8

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

Question 9

How to use le chatellier principle to explain what happens when acid and alkali added to a buffer soltuon

Answer 9

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

Question 10

When is the buffer solution most effective

So what’s the Ph of system equal to at this point
(IMPORTANT)

Answer 10

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

Question 11

What range do buffers tend to work at normally compared to their PKA

Answer 11

+-1 away from their PKa

Question 12

How ti find pH if a buffer soltuion

IMPORTSNT don’t get mixed up

Answer 12

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+

Question 13

What about PH for partial neutralisation ?

Answer 13

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

Question 14

Why does ph need to be maintained in the body

Answer 14

This because enzymes work at an optimum pH and need to kept in small range or get denatured

Question 15

WHAT PH does blood plasma, which has buffer soltuon, normally range at

Healthy blood?

Answer 15

7.35-7.45, , and HEALTHY BLOOD AT 7.40

Question 16

What happens if blood goes below 7.35 or above 7.45

Answer 16

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

Question 17

What is the buffer system called in blood and give the disscoation

Answer 17

Carbonic acid, hydrogen carbonate buffe

Question 18

Use le dhatellaier principle to explain how buffer buffers with small amounts of alklai and acid

Answer 18

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

Question 19

How to work out ratios ?
Basicallt rearrange but what KEY STEP

Answer 19

Rearrange for ratio calm

But just remember ti divide by 1 after rounding or leave in ratio form

Question 20

How to use a pH meter really quickly

Answer 20

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

Question 21

what is the EQUIVALENCE POINT of a graph

WHERE IS IT

Answer 21

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

Question 22

What is an acid base indicator made out of and how does it work

Answer 22

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

Question 23

Okay sk if you know how indicator works, WHAT IS THE END POINT?

Answer 23

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!

Question 24

Again what difference between end point and equivalence point

Answer 24

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

Question 25

Use le chatellier principle to explain colour changes in an indicator if start off with acid and alklai and vice Verca

Answer 25

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

Question 26

What happens at the end point of an indicator in terms of pH and PKa

Answer 26

Write down the disscoation, we know the conc of conjugate base and acid same,

So H+ = Ka

So ph = PKa at the end point

Question 27

HOW DO YOU KNOW IF AN INDICATOR IS SUITBSLE FOR AN EXPERIMENR

Answer 27

1) draw titration curve
2) if the range of the INDICATOR OPERATES BETWEEN AND INTERSECTS VERTICAL SECTION, THEN IT DAN

If not can’t

Question 28

How does string acid string base titration curve look like

Answer 28

• ph string acid so starts low
• string base so ends high
• standard titration curve nothing wrong here

Question 29

How does a WEAK ACID, string base look like

Answer 29

• 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

Question 30

Titration string acid vs weak alklai

Answer 30

String acid = low oh
- weak alkli = low oh

Nothing els e fishy going on

Question 31

Finally weak acid weak alkali
2) how is vertical section looking
3) does any indicator tend to go?

Answer 31

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

Question 32

How does equbilande and end point. Differ normally

Answer 32

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

Question 33

Remember for curve change questions
- if volume half’s, conc half’s

Answer 33

Normally it will be groan at different area
And if dibasic then need two times more volume etc

Question 34

Don’t lack kf weak acid use formula , if string acid , if string alkli same , remember di basic tri basic, if buffer use normal

Question 35

Remember for ionic product qs

Answer 35

Any sort of Oh csn try relate, find oh find Oh and see if basic and say that’s find