Title: EXPERIMENTATION B: Preparation and analysis of Potassium Dioxalatocuprate(II) K 2[Cu(C 2O4 )2 ].2H2O
Aim: To prepare and analyse the formation of potassium Dioxalatocuprate(II) (K 2[Cu(C2O4)2] 2H2O)
This practical session will help you gain an understanding of stoichiometric relationships between reactants and products of chemical reactions. This includes an understanding of concepts such as limiting reagents and yields. MATERIALS AND EQUIPMENT USED DURING THE EXPERIMENT:
Mass Balance
250 mL beakers
Tongs
Thermometer
Ice Bath
Buchner Funnel
Filter Flask
Chloroform
Distilled Water
Ice
Sulphuric Acid
Potassium Permanganate
Test tubes
Glass Wool
Conical flasks & Volumetric Flasks
Source of heat
Starch
Dropper
Potassium thiocyanate
0.05M Sodium thiosulphate
Iodine
Hotplates
Glass funnel
(1) Preparation Procedures: 3
About 1.50g of copper sulphate is dissolved in approximately 15 cm of water. o
The solution is heated to a temperature of about 90 C.
Likewise a solution of potassium oxalate (4.5g of oxalate in 25cm of water) is heated to
3
o
90 C.
The solution of copper sulphate is added rapidly to the solution of potassium oxalate along with vigorous stirring of the mixture. o
The resulting solution is cooled to 10 C in an ice bath.
The product is filtered by suction.
The crystals are then dried and the weight is recorded.
Mass of copper sulphate used: 1.5045g
Mass of potassium oxalate used:4.5047g
Weight(g) 94.023 92.775 1.248
Crystals + petridish Empty petridish Crystals
QUESTIONS (a) Write down the equation of the reaction. Reagents
Products
A
B
C
D
1CuSO45H2O(aq)+2K2C2O4H2O(aq) 1K2[Cu(C2O4)2]2H2O(s)+ 5H2O + 1K2SO4(aq) (b) Which is the limiting reagent? Give reason(s).
CuSO4 5H2O: K 2C2O4 H2O
1
2
Moles of A = 1.5045/249.6 = 0.006028 Moles of B = 4.5047/184.2 = 0.02445
E
If ever all the 0.006028 moles of CuSO45H2O were to be used in the reaction, it would require (2 x 0.006028) = 0.0120 moles of K 2C2O4H2O for the reaction to go to completion. There are0.02445 moles of K 2C2O4H2O available which is more than the required 0.0120 moles. If all of the 0.02447 moles of K 2C2O4H2O were to be used in the reaction it would require ½ x 0.02445 = 0.01223 moles of CuSO45H2O for the reaction to go to completion. There are only 0.006028 moles of CuSO45H2O available which is less than the required 0.01223 moles.
By definition a limiting reagent is the reactant that will be completely used up during a chemical reaction. There will be some moles of the other reactant in excess after the reaction has gone to completion. The limiting reagent is CuSO4 5H2O. All of the 0.006028 moles of CuSO4 5H2O will be used up when this reaction goes to completion.
(C) Calculate the theoretical yield of the reaction.
Theoretical Yield = (moles of C) x (Mr of C) = (0.006028) x (353.7) = 2.132g (D) Express your yield as a percentage of the theoretical yield.
% Yield =
actual yield,g theoretical yield,g 1.248
=
=
2.132
x 100%
x 100%
58.5 %
(e)Draw the structure of the resulting compound
(2) Quantitative Analysis (a) Determination of the oxalate content PROCEDURES:
3
250 cm of approximately 0.02M KMnO 4 is prepared from a provided reagent bottle. About 0.8g of potassium permanganate is weighed and is transferred into a conical flask.
250 cm3 of water is added and the solution is heated to boil for about 15 min.
The solution is allowed to cool down to room temperature.
The solution is filtered through a glass funnel containing a plug of purified glass wool into a beaker.
3
The resulting Solution is standardised with 25 cm of 0.02M sodium oxalate in the presence of 25 cm3 of dilute sulphuric acid.
Mass of potassium permanganate used=0.8226g
3
About 0.0005 mole of the complex is weighed into a conical flask and 50 cm of dilute sulphuric acid is added.
The mixture is heated until boiling and is titrated with the prepared potassium permanganate solution until a faint pink colour persists.
It is to be noted that during the initial stages of the titration, the solution is cloudy due to the undissolved complex. This clears during the titration.
The experiment is repeated at least two times.
The solutions are retained for copper determination.
Mass of complex used: 0.176g, 0.177g (b) Determination of the copper content
NOTE: To ensure that the solution obtained above contains no excess permanganate, it is boiled until the purple colour is discharged. PROCEDURES:
To the cooled solution, 1.5g of potassium iodide is added and is titrated with 0.05M sodium thiosulphate. When the colour of the resulting suspension turns pale yellow, a few drops of starch is added and the titration is continued until the blue colour begins to fade. 5 cm3 of 10% potassium thiocyanate is added and the contents are swirled. The titration continues until a white suspension is obtained.
Questions (a) What is a primary standard and what are the requirements of primary standard substances? Give some examples of primary standard substances. A primary standard in chemistry is a reliable, readily quantified substance. A primary standard is a reagent that is extremely pure, stable, has no waters of hydration, and has a high molecular weight.
Requirements of primary standard substances 1. High purity 2. Stability (low reactivity) 3. Low hygroscopicity and efflorescence 4. High solubility (if used in titration) 5. High equivalent weight 6. Non-toxicity 7. Ready and cheap availability 8. Eco-friendliness
Examples of primary standard: 1. Benzoic acid, ethanolic sodium and potassium hydroxide 2. Potassium bromate (KBrO3) 3.
Potassium hydrogen phthalate
4.
Sodium carbonate
5.
Sodium chloride
(b) Is potassium permanganate a primary standard? Give reasons. The definition of a secondary standard is that it is one that has been titrated against a primary standard. Therefore potassium permanganate is a secondary standard by definition.
Reasons: Potassium permanganate cannot be considered and used as a primary standard due to its reaction with contaminants in distilled water.
It must be standardized against a primary standard reducing agent directly prior to its use as an oxidizing agent.
The issue with Potassium Permanganate is that it has a high oxidising power( so may cause self reduction),therefore a specified amount cannot be weigh out and a specific concentration cannot be determined.
An approximation needs to be done, and a titration against a sample that will not change as easily or quickly is performed. The sample that is titrated against will be chosen for its stability, and will be the primary standard. The potassium permanganate would be the secondary.
(c) What is a secondary standard? A secondary standard is a substance which may be used for standardization and whose concentration has been found by comparison against a primary standard. This is a solution in which the concentration of dissolved solute has not been determin ed from the weight of the compound dissolved but by the reaction of a volume of the solution against a measured volume of a primary standard solution (d) In the titration, why was sulphuric acid preferred to hydrochloric acid? + H2SO4 being diprotic provides two H ions which makes the reaction proceed faster. +
-
H2SO4 → 2H + SO4 -
If hydrochloric acid is used, Cl is oxidized by potassium permanganate. Since nitric acid itself is oxidizing agent, its use is also inappropriate. 22 MnO4 + Cl2 → 2 MnO4 + 2 Cl o
The reaction is heated because if the temperature is too low (below 55 C), the interaction between the oxalate and the potassium permanganate will move too o slowly as to be used as a practical lab experiment. Above 60 C, oxalate acid begins to decompose, so it’s important to stay in this range. (e) Suggest a method to determine the potassium content of your compound. The potassium content can be evaluated by utilising standard oxalic acid solution. When a sulphuric acid solution of a persulphate is treated with excess of standard oxalic acid solution in the presence of a little silver sulphate as catalyst, the following reaction occurs: H2S2O8 + H2C2O4 2H2SO4 + 2CO2 -
-
The excess of oxalic acid is titrated with standard permanganate solution. An approximately 0.05 M solution of oxalic acid is prepared by dissolving about 6.4 g of the compound and making up to 1000 mL in a graduated flask. Standardise the solution with standard (0.1N) potassium permanganate standard solution. 0.3-0.4 g potassium persulphate is transferred into a 500 mL conical flask, to it is added 50 mL of 0.05M oxalic acid, followed by 0.2 g of silver sulphate dissolved in 20 mL of 10 per cent sulphuric acid. The mixture is heated in a water bath for about 15 to 20 minutes until no more carbon dioxide is evolved. The solution is diluted to about 100 mL with water at about 40 °C, The excess of oxalic acid is titrated with standard 0.1N potassium permanganate.
(3) Qualitative Analysis Procedures:
A solution of the product is prepared in distilled water and a few drops of potassium iodide are added.
The presence for iodide is tested by adding a little chloroform and shaking the tube.
The dilute sulphuric is acidified and shake the tube.
Compare the above with the reaction of a dilute solution of copper sulphate. Account for the difference in terms of the structure of the complex ion.
2 KI(aq) + Cl2(aq) → 2 KCl + I2(aq) 2Cu2+ + 4I- → 2CuI + I2
RESULTS: (1) Preparation
Crystals + petri dish Empty petridish Crystals
Weight(g) 94.023 92.775 1.248
(2)Quantitative analysis (a) Determination of the oxalate ion
Mass of potassium permanganate used=0.8226g Standardising of the solution
3
Final reading/cm 3 Initial reading/cm 3 Volume of KMnO4 used/cm
st
2 19.50 9.80 9.70
st
2 43.90 22.00 21.90
1 9.80 0.00 9.80
nd
Mass of complex used=0.176g, 0.177g
3
Final reading/cm 3 Initial reading/cm 3 Volume of KMnO4 used/cm
1 22.00 0.00 22.00
nd
(b) Determination of copper content st
3
Final reading/cm 3 Initial reading/cm 3 Volume of sodium thiosulphate used/cm
1 12.70 0.00 12.70
nd
2 25.40 12.70 12.70
Calculations: -
2-
Equation of reaction of MnO 4 with C2O4 .
In acid solution, permanganate, MnO 4- [Mn (VII)] is reduced to Mn(II). The half reaction is: MnO4- + 8H+ + 5e- → C2O42- →
Mn2+ + 4 H2O
2CO2 + 2e-
Eo = 1.51 V
Eo = -0.49 V
MnO4- + 5C2O42- + 16 H+ → 2Mn2+ + 10CO2 + 8H2O, E ° = 1.02 V
-
Number of moles of MnO 4 used. = 0.8226/158 =5.2063 * 10
-03
2-
Number of moles of C 2O4 in 1.342g of complex.
2 moles of MnO4- reacts with 5 moles of C2O425.2063 x 10 -03 moles MnO4- → 0.01302 moles of C 2O42-
PRECAUTIONS
Eye Contact: o
Stinging, tearing, redness and pain
Use of gloves and masks are necessary
When manipulating chloroform, the use of appropriate gloves, masks and goggles are necessary to avoid the following potential health problems. Inhalation:
Chloroform acts as a relatively potent anesthetic. Irritates respiratory tract and causes central nervous system effects, including headache, drowsiness, and dizziness. Exposure to higher concentrations may result in unconsciousness and even death. May cause liver injury and blood disorders. Prolonged exposure may lead to death due to irregular heart beat and kidney and liver disorders. Ingestion:
Causes severe burning in mouth and throat, pain in the chest and vomiting. Large quantities may cause symptoms similar to inhalation. Skin Contact:
Causes skin irritation resulting in redness and pain. Removes natural oils. May be absorbed through skin. Eye Contact:
Vapours causes pain and irritation to eyes. Splashes may cause severe irritation and po ssible eye damage. Chronic Exposure:
Prolonged or repeated exposure to vapours may cause damage to the nervous system, the heart and the liver and kidneys. Contact with liquid has defatting effect and may cause chronic irritation of skin with cracking and drying, and corresponding dermatitis. Chloroform is a suspected human carcinogen. Aggravation of Pre-existing Conditions:
Persons with pre-existing skin disorders or eye problems, or impaired liver, kidney or respiratory function may be more susceptible to the effects of the substance.
