104-Topic 5 - Polyprotic acid PDF

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Polyprotic acid...

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Topic 5: Polyprotic Acid Vid A: Finding Ka Polyprotic acids: more than 1 proton

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The protons of polyprotic acids disassociate in water , the disassociation occurs in a stepwise fashion o Acid donates a proton to the water in equilibrium rxn o In 1st disassociation acid donates a proton and the equilibrium rxn has an associated equilibrium constant called Ka1  Ka indicates an acid and the 1 means it is the first proton o The 2nd disassociation is the transfer of 2nd proton into the water with the Ka2 o The 3rd has an equilibrium constant of Ka 3 The Ka values decrease as they disassociate

Stepwise dissociations for H3PO4

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These equilibria all occur in solution simultaneously When H3PO4 dissolves all 4 species are present in the solution. Even though they are present simultaneously the concentrations of the species vary dramatically

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We always have as many Ka values as we do acidic protons

Vid B: Finding Kb Lets Revisit Carbonic Acid

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H2CO3 always makes an acid when dissolved in water CO3 2- always makes a basic solution when dissolved in water Bi-carb acts an acid or a base

Calculating Kb for polyprotic species

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To find the appropriate Kb must look at the association of the base to produce the acid , once you have conjugate pair you know which Ka to use Vid C: Calculating Ph of an acidic species

Acidic polyprotic species 

We have to look for the dominant species

What is the equilibrium expression?

pH of carbonic acid (use ice table)

What about the 2nd dissociation ? 

Only the 1st dissociation determines the Ph, but 2nd dissociation produces the carbonate ion

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This time the initial concentration is not 0 but is the concentration from 1st disassociation Shifts to the right so a little more hydrogen

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Carbonic acid is the dominant species in the solution because Ka1 is small and rxn shifts only slightly to the right This means that concentration of carbonic acid at equilibrium is almost the same as the initial concentration The resulting Ph is entirely from the carbonic acids 1st disassociation this is because the 2nd is even smaller than first and adds very little hydrogen ion Always use pH calcs in 1st dissociation

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Vid D: Calculating Ph of a basic species Acidic polyprotic species  

Diprotic acid means it has 2 acidic protons. we found that the Ph of a polyprotic acid solution is mostly result from 1 st disassociation We will now look at the fully deprotonated conjugate base, which is carbonate, it is called diprotic base  can accept 2 protons

Polyprotic bases exist as salts!  

We can put polyprotic bases into solution by making it into a soluble salt Remember that all sodium salts are soluble

Basic polyprotic species have Kb values   

The value of Kb1 is found in the Ka table for polyprotic acids Kb x Ka =Kw but which Ka do we need from table Looking at the equilibrium eq for Kb1 we see that acid base conjugate pair in Kb1 corresponds to the same 2 species for Ka2

Kb1 is always the largest  

The 1st protonation is always larger than subsequent protonation pH of diprotic base is the result of the 1st protonation of the base

Ph of sodium carbonate

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CO3 2- is the carbonate We can use these values in the 2nd protonation rxn

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Since Kb2 is so much smaller than Kb1 we assume that x is negligible small when we add or subtract from larger # Solving for X which is the carbonic acid solution The hydroxide concentration remains the same, so the Poh and pH come only from 1 st protonation of the diaprotic base

Vid E: calculating Ph of an amphoteric species Polyprotic acids-amphoteric species  

Bicarb can be put directly into a solution in the form of a salt We can dissolve Sodium bi carb in solution and we recognize the bi- carb is the Ph active species

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Is bi-carb an acidic or basic species

Amphoteric species (ampholyte)

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Act as an bronstead acid and lose a proton to water

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Act as bronstead base and accept proton to water

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To find Ph of ampholyte we look at equilbirum expression for each rxn o The value of Ka2 comes directly from Ka table When we compare K values we see that Kb2 is greater than Ka2 for bicarb o This means basic equilibrium shifts further to the right than acidic shifts to the left, o Bicarb solution will be basic

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Calculating Ph of an ampholyte 

Both equilibria contribute to the overall pH

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The Ph of an amphoteric species can be approximated pKa1 is directly related to pKa2, even though an amphoteric species can act as acid and base .pH is based on pKa values

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Notice that pH is basic just as we expected also that pH doesn’t depend on concentration of Bi-carb solution o In general pH of a solution containing only ampohlyte can be approximated as the avg, of the relevant pKa values o The relevant pKa values are those associated with the disassociation and protonation rxns of the ampholyte...