8.7 pH Curves Flashcards
What is the equivalence point?
The point at which equivalent number of moles of acid and alkali have been added.
What are the difference in shape for the pH curves for a strong and weak acid?
They have the same basic shape.
What are the difference in initial pH for the pH curves for a strong and weak acid?
The initial pH is higher for a weak acid since it only dissociates partially, initial pH for a strong acid is lower because it fully dissociates.
What are the difference in volume of NaOH required for exact reaction for the pH curves for a strong and weak acid?
The volume of NaOH required for each reaction is the same because the same volume of and concentration of acid is used.
What are the differences in the steep parts of the curve for the pH curves for a strong and weak acid?
For a week acid the steep part is more in the alkaline region since the base is stronger than the acid.
The steep part of the curve for a strong acid is equally in the acidic and alkaline regions.
What are the difference in equivalence point for the pH curves for a strong and weak acid?
The pH equivalence point > 7 for a weak acid because the base is stronger than the acid.
The pH equivalence point = 7 for a strong acid because the base and the acid are both strong.
What are the difference in final pH for the pH curves for a strong and weak acid?
The final pH is the same for both because the same alkali was used.
What is important for base tritiations?
They are the same as the acid ones but the other way round.
What is the pH at equivalent point for a strong acid strong base titration?
7
What is the pH at equivalent point for a strong acid weak base titration?
Below 7
What is the pH at equivalent point for a weak acid strong base titration?
Above 7
What is the pH at equivalent point for a weak acid weak base titration?
Depends on the relative strength of the acid and base.
What is important about the shape of the pH curve for a weak acid weak titration?
There is no steep part.
How is the pKa of the acid found on the pH curve for a weak acid strong base titration?
By the Ph at the half equivalence point.
How is the pKb for a weak base found from the pH curve?
For a titration where a strong acid is added to a weak base pKb = pOH at the half equivalence point, since at 25 degrees Celsius pH + pOH = 14
14 - (pH at the half equivalence point) = pKb for the weak base.
What is an indicator?
Weak acids or weak bases which exhibit different colours depending on the pH, the ionised and unionised forms have different colours.
What is the equation of an indicator that is a weak acid?
HIn (aq) H+ (aq) + In- (aq)
Colour 1 colour 2
What happens when more H+ is added to the indicator?
The position of equilibrium will shift to the left to use up as far as possible the H+ that has been added, the indicator will appear as colour 1.
What happens when more alkali is added to the solution?
The indicator changes to colour 2. When we add alkali the OH- from the alkali reacts with the H+ on the right hand side of the equilibrium to produce water. The position of equilibrium thus shifts to the right to replace the H+ as far as possible.
What is the pH range of an indicator?
The pH range over which intermediate colours of the indicator can be seen.
What is the end point of a titration?
The point at which the indicate changes colour.
How should the correct indicator be chosen?
So that the pH range of the indicator occurs entirely within the very steep art of the pH curve.
How should the correct indicator be chosen if the pH curve has not been given?
Use the pH at equivalence point as a guide.
What indicator should be used for a strong base strong acid titration with an equivalence point of pH seven?
Most common indicators used are methyl red.
