First we want to talk about last thing in Polyprotic Acids : Examples : H2SO4, H2CO3, H3PO4

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1 First we want to talk about last thing in Polyprotic Acids : Examples : H2SO4, H2CO3, H3PO4 If I take a solution of polyprotic Acid, what are the changes that happen to it? How they change? How dissociation happen? and so on. - I want to study it on a certain moment or on a certain ph What are the componanets inside it? what is the percentage of it Fraction? As an example : We will take an acid and it s H2CO3 we start first with ( 1 Mole/ 1L ) or 1M in that acid, it will be found in low ph ( because it s an acid solution ) The dissociation will happen as a first step for the first proton and a result for that the concentration and amount for H2CO3 will decrease and another thing will change and increase (H2CO3 will convert to conjugated base HCO3- ) the concentration for HCO3- increase. ( On curve ph change and Fraction ) Fraction : percentage of something to another on certain ph ( like: acid and conjugate base) 1

2 On curve : We started with low ph, because it s an acid and second thing any acid even strong or weak it will start dissociation at ph equals to its pka or ph was higher that pka So From curve the strongest acid in dissociation steps is H2CO3 ( it s the easiest to give proton and first dissociation because Ka high and pka low ) When I compare Pka and ph both have Log And pka low so ph start in low point and increases with us. Then, H2CO3 decrease and HCO3- increase until H2CO3 finished so all will converted to HCO3- at this moment the next step for dissociation start the HCO3- decrease and CO3-2 increase until we reach approximately complete dissociation ( approximately we said because it is a weak step and ph is high there ) 2

3 From curve : On ph = 10 The Fraction of HCO3- and CO3-2? Fraction of HCO3- : around 0.7 Fraction of CO3-2 : 0.25 ** If we don t have a curve we will use the Henderson-Hasselbalch equation ( ph is known also pka known or they will give us Ka and from it we get the pka) ** log is like a Fraction as an example 9 to 1 9 molecules conjugate base and 1 molecule from acid if I want to convert that to a fraction we will have 90% and 10% ( because together they are 10 molecules ) Fraction = No. of mole X 100 % Total amount 3

4 Now we will start with Titration : Titration is an experiment in which measured amounts of base are added to a measured amount of acid while following up changes in ph using a ph meter. In simple words : It s Neutralization ( the acid by base or base by acid ) ** Note: Not always at end point ph = 7 but can be nearly to 7 Simple curve for titration ( not real one ) : - We used CH3COOH and started to add equivalents of OH- ( anything with OH- like: NaOH or KOH which gives OH- ) So, OH- increase and the acid decrease and the Base and Acid react and give Salt and water - Salt increase ( which is conjugate base ) its concentration increases and the Acid decrease. We use ph meter to know the final ph after Titration 4

5 The real curve : - If I take CH3COOH and started to add equivalents of OH- Example : 1) I have 1 mole of CH3COOH, for titration I need 1 mole of OH- 2) If I have polyprotic especially diprotic I will add 2 mole of equivalent ( each molecule of acid we treat with it alone ) ** Notes : - When I neutralize acid the ph increase because it will not stay acid ( it lost its acidic properties ) and notice that on Y-axis while adding OH- - the bases I add are strong, while the acid is weak, why? To complete dissociation ( all amount I add from base will give same amount of OH- if strong, but if it is weak it will give us less amount of OH- so we will not have a good titration. 5

6 From Curve : - First thing ph increase very fast then a small change like a line then at end it will be back to be fast again - Second thing first region its concavity downward then reach to a point where it at center in the image then its concavity upward ( so that point called inflection point and it is in the center of the curve ) If I add one of equivalents of OH- I am here in fact add 0.5 of equivalent of OH- 0.5 equivalent means half of acid react and consumed and the other half still acid so, Concentration of acid = concentration of conjugate base salt And mole same and volume So from Henderson equation ph = Pka + Log conjugate base / Acid and if conjugate base and Acid same Log 1 = zero so ph = pka and concentration equally this point is called inflection point And we still add until we reach to 1 equivalent and the titration will end - The end point which titration end on it equivalent point ** if we compare titration for more than one acid the curve will be at different ph - From this curve : CH3COOH is the strongest because ph is the lowest and pka low. - When I get inflection point and cut it down with y-axis, and from Henderson equation I can get pka and here it s low so strongest acid. 6

7 Some words help you in calculation : if they said we have buffer, or titration happen - In titration we add volume to volume so total volume different and so the concentration change - Second thing I am consuming the acid so its amount will decrease and so I can t say initial concentration is same to final concentration - I have to take out the amount have been titrated and removed from initial concentration [HA] = [HAi]-[Base]/ V Bec, the same quantity will react with base Example: CH3COOH + OH- CH3COO-+ H2O Calculate the relative amounts of acetic acid and acetate ion present and ph values when 1 mol of acetic acid is titrated with sodium hydroxide. 0.1 mol of NaOH is added when I add 0.1 mol OH- is added, 0.1 mol of acetic acid reacts with it to form 0.1 mol of acetate ion, leaving 0.9 mol of acetic acid. The composition is 90% acetic acid and 10% acetate ion. ( لما نضيف 1.0 من القاعده نفس الكمية راح تستهلك من الحمض ف راح يظل من الحمض و بصير ) 1.0 ph = pka + log 0.1/0.9 ph = log 0.1/0.9 ph = ph = 3.81 ( pka here is known ) 7

8 Titration curve of Glycine ( example from the body ) Amino acid : polymers of proteins and glycine is one type of the 20 types of amino acids Amino acid is a carbonic group ( 4 bonds ) H Amino Group C Carboxylic Group R- Group ( in glycine it s the simplest one so here is H ) And the normal situations in our body for amino and carboxyl groups NH3+ and COO- So in this case in titration I treat with both of them as acids ( not one acid and the other is base ) and each one has its own Ka value ( ألن قوتهم كأحماض مختلفة ( ph ** Titration of different groups happened in different - COOH is stronger, it happened at lower ph 8

9 Same idea in curve first start fast then slowly then fast but here 2 curves because different abilities to donate protons. In Titration first ph increase because COOH will reacts with OH- and give us water And we are still adding base until reach to ph which is suitable for pka of the second acid and then the titration will start and same curve and finally get salt and water. ( مالحظة لو كان عندي Amino Acids فيه مجموعتين Carboxyl راح يصير في 3 مراحل و لكن تبقى مجموعات ال Carboxyl أقوى من مجموعات األمين ) 9

10 Now, we well talk about Buffer : Buffers : are solutions that resist abrupt and sudden changes in ph. Resisting sudden changes in ph is very important, to understand that let s take first an example : - If we have an enzyme which give lactic acid in cellular respiration and so we get an acid, the ph will decease in the cell environment and so the enzyme will be inactivate So the cells should have a certain system from Buffer to maintain their range of ph and not happen a lot of changes so they keep working. ** We will start this chapter from chemical Buffer then move to buffer in human body. Examples to understand what is buffer - why do we need buffer : 1- From chemical side : ( to understand why do we need it ) Reactions happen at certain ph, even if is was in test tube, if we change ph, the rate of reaction will change too and the reaction will not be as well as it have to be so I must keep the reaction in the range. 2- From our life : ( to understand what is it ) * If I have a glass of water and put some drops of lemon juice it will be similar to lemon juice even if it is has a light taste and same ideas for Acidic properties this glass of water will change to Acid * But Buffer solution when we add any Acidic solution it will keep its properties ( NO CHANGE) After and before will be same as it is or near to its state. How does the Buffer act? How they maintain the ph? How do they protect us from sudden change? - To answer that : * If we have a solution and it is a buffer and added HCl ( HCl is a strong acid and gives a lot of H+ also affects ph ) but this Buffer in this state will decrease H+ and this amount which decreased will act with the Buffer Components which contain conjugate base and changed this strong acid to weak one, the weak acid not highly association, So even if it gave protons, it will not decrease the ph as the strong one. Another way: * Maybe H+ reacts with OH- which is in the solution and convert to water. 10

11 - Opposite example : * If we take a lot of Base ( like a person by wrong way drink a bleach ) OH- which will get out from the Base will reacts with acid which is in the solution and one of its component is Buffer so we get salt and water and then ph will not change. ** In the curve ph was high and started to decrease by adding drops of acid - This can be a Buffer. ** The other line which is in water ph decrease very fast and just by few drops so - This can t be a Buffer 11

12 ** In this curve the initial and final there are a very fast change but in the middle a slow change ( this region is called Buffering zone or Buffering region because ph changes very minimal ) and in that region the solution can act as buffer when we move away from that region it can t act as a Buffer ( it has a capacity ). How can we prepare a Buffer solution? ** There are Acidic Buffer and Basic or Alkali Buffer. ( ph that they work on will determine Acid or Base and their components). - If we want to prepare a Buffer from Acetic acid, I have to take a weak acid as acetic acid, I cant take a strong acid Then I will take this weak acid and add to it a strong base like KOH Note that : If I take 1 mole from weak acid I add to it less amount of strong base ( 0.5 mole or round it ), and the half amount from acetic acid will react with the 0.5 mole and consume half and there will be another half from acid as acid and the other will be salt So that Salt with the half acid which not consumed is a Buffer solution. 12

13 - Second type: Base ph > 7 I take a weak base and react it with Strong Acid until we get Salt and base ( we do as we said before but here base ) NH4Cl NH4++ Cl- NH3+ H2O NH4++ OH If I add Acid from outside to check the Buffer : H+ will react with OH- and converted to water so no change in ph If we add base : It will react with NH4+ and convert it to weak base ( Ammonia ) so not giving a lot of OH- Good Luck 13