Flashcards
21.3 Neutralisation
Flashcards do conjunto (24)
- 1What do you use in acid-base titratiobns to monitor neutralisation reactions accurately?Indicators - results can be analysed to find out some unknown information about the acid or base
pH meter can be used to monitor changes in pH - 2What is the procedure to monitor the pH as an aqueous base added to an acid solution?1) Using a pipette, add a measured volume of acid to a conical flask.
2) Place the electrode of the pH meter in the flask
3) Add the aqueous base to the burette and add to the acid in the conical flask, 1cm3 at a time
4) After each addition, swirl the contents. Record the pH and the total volume of the aqueous base added.
5) Repeat steps 3 and 4 until the pH starts to change more rapidly. Then add the aqueous base dropwise for each reading until the pH changes less rapidly.
6) Now add the aqueous base 1cm3 at a time again until an excess has been addded and the pH has been basic, with little change, for several additions - 3What graph would you plot with the results from the experiment?A graph of pH against total volume of aqueous base added is then plotted
- 4What is an alternative method to measure pH?Alternative automatic method - you could attach the pH meter to a datalogger and use a magnetic stirrer in the flask
The aqueous base would then be added from the burette to the flask slowly, and the pH titration curve could be plotted automatically using the datalogger or appropriate software on a computer - 5Draw basic pH titration curve for addition of a base to an acidEquivalence point - the centre of the vertical section of the pH titration curve
Excess of base - pH increases slowly as basic solution is added
Vertical section - pH increases rapidly on addition of a very small volume of base. Acid and base concentrations similar.
Excess of acid - pH increases slowly as basic solution is added - 6When the base is first added - what is the pH like? What about when you approach vertical section?When the base is first added, the acid is in great excess and the pH increases very slightly
As the vertical section is approached, the pH starts to increase more quickly as the acid is used up more quickly - 7How is the vertical section produced? How many drops are needed for the whole vertical section?Eventually, the pH increases rapidly during addition of a very small volume of base, producing the vertical section. Only drops of solution will be needd for the whole vertical section.
- 8What is the pH like after the vertical section?The pH will rise very slightl as the base is now in great excess
- 9What is the equivalence point of the titration?The equivalence point of the titration is the volume of one solution that exactly reacts with the volume of the other solution.
The solutions have then exactly reacted with one another and the amounts used matching the stoichiometry of the reaction.
The equivalence point is the centre of the vertical section of the pH titration curve. - 10What would the graph for adding an acid to a base look like?A pH titration curve can also be plotted with the acid added from the burette to the base in the flask
The shape is essentially the same, just the other way around, going from high pH to low pH - 11What is the acid-base indicator?Weak acid, HA, that has a distinctively different colour from its conjugate base, A-, for example, for the common indicator methyl orange
- 12When using methyl orange - what colour is the weak acid?The weak acid, HA is red
- 13When using methyl orange - what colour is the conjugate base?The conjugate base, A- is yellow
- 14At the end point of a titration what does the indicator contain and what colour will the indicator be for methyl orange?At the end point of a titration, the indicator contains equal concentrations of HA and A- and the colour will be in between the two extreme colours
For methyl orange, the colour at its end point is orange
HA(aq) ⇌ A-(aq) + H+(aq) - 15Explaining indicator colour changes - Where is the equilibrium position shifted in acidic conditions and where is it shifted in basic conditions?An indicator is a weak acid
The equilibrium position if shifted towards the weak acid in acidic conditions
Towards the conjugate base in basic conditions, changing the colour as it does so - 16Explaining indicator colour changes - In a titration in which a strong base is added to a strong acid what colour is methyl orange initially and why?Red as the presence of the H+ ions forces the equilibrium position well to the left
- 17Explaining indicator colour changes - What happens in the addition of a basic solution containing OH-(aq) ions into acid?OH-(aq) ions react with H+(aq) in the indicator:
H+(aq) + OH-(aq) → H2O(l)
The weak acid, HA, dissociates, shifting the equilibrium position to the right
The colour changes, first to orange at the end point and finally to yellow as the equilibrium position is shifted to the right
HA(aq) ⇌ A-(aq) + H+(aq)
------------->
equilibrium right - 18Explaining indicator colour changes - If methyl orange is added initially to a basic solution and acid is added.H+(aq) ions react with the conjugate base, A-(aq)
The equilibrium position shifts to the left
The colour changes, first to orange at the end point and finally to red when the equilibrium position has shifted to the left
HA(aq) ⇌ A-(aq) + H+(aq) - 19How sensitive is the end point? - Different indicators have different what values?Different indicators have different Ka values and change colour over different pH ranges
- 20How sensitive is the end point? - Write equations to show what happens at the end pointAt the end point:
[HA(aq)] = [A-(aq)] and Ka = [H+(aq)] [A-(aq)] / [HA(aq)]
Ka = [H+(aq)] and pKa = pH
The pH of the end point is the same as the pKa value of HA - 21How sensitive is the end point? - What does the sensitivity depend upon?Indicator itself and eyesight
Most indicators change colour over a range of about two pH units - 22Choosing the indicator - What should you use?In a titration, you must use a indicator that has a colour change which coincides with the vertical section of the pH titration
- 23Choosing the indicator - Ideally the end point and equivalence point would coincide. Why may this not be possible?The end point may give a volume that is slightly different from the equivalence point
But any difference will be very small in order of one or two drops - 24Why is there no indicator suitable for a weak acid-weak base titration?There is no vertical section and even at its steepest, the pH requires several cm3 to pass through a typical pH indicator range of 2 pH units
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- A level Chemistry (Module 5: Physical chemistry and transition elements) FlashCards sobre 21.3 Neutralisation, criado por Urvi B em 22-01-2019.
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- Urvi B
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