12P- Acids And Bases Flashcards
Definition of an acid
Proton donator
Definition of a base
Proton acceptor
What happens at an acid base equilibrium
The transfer of protons
Why is the pH scale logarithmic
pH scale measures H+ ion conc
In an aqueous solution H+ conc can cover a large range
Equation to calculate pH
-log[H+]
Equation to calculate H+ conc
[H+]= 1x 10^-pH
How many decimal places is pH given to
2 dp ALWAYS
How to calculate pH of a strong acid when given its conc
Monoprotic acid, same [H+] in moles/conc
Diprotic acid, double [H+] in moles/conc
Sub this value into pH= -log[H+]
What happens when water is ionised
Disassociates slightly
H2O(l) <—> H+(aq) + OH-(aq)
How is Kw derived and what the equation
From equilibrium constant Kc
Kc= [H+][OH-]/ [H2O]
Kc[H2O]= [H+][OH-]
H2O is much larger than the conc of any of the ions, assume it’s value is constant, make LHS equal to Kw
Kw= [H+][OH-]
What effects the value of Kw
Temperature
How to is Kw to calculate the pH of a strong base from its conc
Value of Kw at specific temp should be given
Sub [OH-] into Kw eq, to find [H+]
Sub [H+] into pH eq
Definition of weak acids and weak bases
They disassociate slightly in aqueous solutions
What’s Ka and form an equation
Disassociation constant for a weak acid
HA(aq)—> H+(aq) + A-(aq)
Ka= [H+][A-]/[HA]
Equation for pKa
pKa= -log10(Ka)
What’s the value of Kw at room temp
1x10^-14 mol^2 dm^-6
Kw eq for pure water
[H+] = [OH-]
Kw= [H+]^2
Strong acid x strong base
Strong acid x weak base
Weak acid x strong base
Weak acid x weak base
Graphs
What’s the equivalence point
Just enough base/acid is added to neutralise the acid/base
Part of the curve where the pH just changes
What’s the end point
Volume of base/acid causing the indicator to change colour
If the correct indicator is chosen the end point will be the same as the equivalence point
How to choose the correct indicator
The pH change values for the indicator need to lie within the range of pH values where the graph is completely vertical
What’s half neutralisation point
If base/acid is added there will only be a tiny change in pH
Between 0 and the equivalence point in the x axis, halfway point where the line is almost horizontal
[HA] = [A-]
Ka= [H+]
pKa = pH
Acidic buffers and how they resist changes in H+ and OH-
(eq of buffer sol)
Weak acid and salt
HA <—> H+ + A-
A weak acid and strong base can be used, as pH = pKa
Add alkali (acid component)
HA + OH- —> H2O + A-
Add acid (salt component)
A- + H+ —> HA
What’s a basic buffer and how does it resist changes to H+ and OH-
Buffer sol
Weak base and salt
NH4CL<—> NH4+ + Cl-
Add alkali (salt component)
NH4+ + OH- —> NH3 + H2O
Add acid (base component)
H+ + NH3 —> NH4+
