Lime water gives a milky white precipitate when reacted with carbon dioxide:
Ca(OH)2(aq) + CO2(g) CaCO3(s) + H2O(l)
It is also treated with a lot less care than the caustic alkalis of group 1 metals such as sodium hydroxide (NaOH). Calcium hydroxide like the other hydroxides in its group, is only partially soluble in water. (Solubility of Ca(OH)2 is 1.5 x 10-3 mol/100g of water at 25˚c).
The reaction between calcium hydroxide and acid is an example of an acid – base titration.
Acid + Base Salt + Water
2HCl(aq) + Ca(HO)2(aq) CaCl2(aq) + H2O(l)
This is a neutralisation reaction. I can write an ionic equation for this reaction:
2H+(aq) + 2Cl-(aq) + Ca2+(aq) + 2OH-(aq) Ca2+(aq) + 2Cl-(aq) + 2H2O(l)
By cancelling out the spectator ions we get the following ionic equation:
2H+(aq) + 2OH-(aq) 2H2O(l)
By dividing by 2 we get the simplest ratio:
H+(aq) + 2OH-(aq) H2O(l)
This ionic equation applies to all neutralisation reactions. The H+ ions can from any acid; the OH- ions can come from any alkali. This reaction is exothermic. The enthalpy of neutralisation is approx 58Kjmol-1.
Apparatus
The apparatus for a titration will include:
- Burette
-
25cm3 graduated pipette
- Pipette filler
- Measuring cylinder
- Distilled water
- Indicator (phenolphthalein)
- Hydrochloric acid
-
250cm3 of limewater
In the titration I would expect the titre to be about 25cm3.
25cm3 solution of Ca(OH)2 will contain approx (25 ÷ 250) x 1 = 0.1g
Mr of Ca(OH)2 = 40.1 + 16 + 16 + 1 + 1 = 74.1g
Moles of Ca(OH)2 = mass ÷ Mr = 0.1 ÷ 74.1 = 1.35 x 10-3
From the equation, 2 moles of HCl will react with 1 mole of Ca(OH)2.
Therefore moles of HCl = 2 x (1.35 x 10-3)
= 2.7 x 10-3
Concentration of HCl is 2 mol dm-3
Moles = (volume ÷ 1000) x concentration
Volume = (moles x 1000) ÷ concentration
= (2.7 x 10-3 x 1000) ÷ 2
= 2.7 ÷ 2
= 1.35 cm3
This shows that 1.35cm3 of 2 moles of HCl is needed. However this acid is too concentrated and dangerous to use therefore I will need to dilute it.
Concentration1 x volume1 = concentration2 x volume2
2 x 1 = 0.1 x volume2
(2 x 1) ÷ 0.1 = volume2
20cm3 = volume2
I will measure out 10cm3 of 2 mol dm-3 HCl using a 10cm3 pipette. To this I would add 190cm3 of distilled water will be measures using a 100cm3 measuring cylinder. This will give me 200cm3 of 0.1M of HCl. This acid will be suitable for my titration.
At the end point the phenolphthalein will change colour from pink to colourless. I will do a rough titration and two accurate titrations, both within 0.1 cm3.
The titration curve for this reaction is shown below:
Where S = end point (25cm3 approx)
Safety
- As this experiment requires the use of Hydrochloric acid, care must be taken when handling this acid. As this acid is corrosive, gloves and protective clothing should be worn.
- Phenolphthalein has been known to have dangers. It may act as a skin, eye or respiratory irritant, and animal experiments indicate a theoretical risk that this material may be carcinogenic in humans if ingested at high levels over a long period. Therefore it is wise that gloves should be worn and when it is not in use it should be keep away in storage.
The indicators used in acid-base titrations are weak acids or weak bases which can dissociate in aqueous solution. Imagine a weak acid, HIn.
HIn(aq) H+(aq) + In-(aq)
(colourless) (red)
If HIn and In- are different colours, then HIn will act as an indicator. In a solution with high concentration of H+ (and acid) the equilibrium will be forced to the left. As predicted by Le Chatelier’s principle, almost all the indicator will exist as HIn and the solution will be colourless. In an alkaline solution where the concentration of OH- is high, H+ ions will be removed as water (H2O).
HIn H+(aq) + In-(aq)
H+(aq) + OH-(aq) H2O(l)
Le Chateliers principle predicts that the equilibrium will move to the right and most of the indicator will exist as In-(red).
Structure of Phenolphthalein
Another indicator suitable for strong acid/weak base titration is methyl orange. This indicator is red in acidic solutions, and yellow in alkaline solutions.
Structure of Methyl Orange
Bibliography
- A – Level Chemistry; E.N. Ramsden
- Introduction to chemistry; B EARL and LDR WILFORD
- Chemistry 1
- Chemistry for A-Level
- http://webs.wichita.edu/chemistry/up_web/BLACKBOARD/Chemistry_112/5
