CHEM08004: Chemical Analysis and Evaluation
Mass balance, Mixing, Absorbance, Temperature, Weight, Product Absorbance, Equilibrium, Acid, Alkali, Salt, pH, Total moles, Concentration, Potassium Permanganate, Ethanoic acid, Sodium Hydroxide, Sodium Chloride
By the use of mass balances scientists can: Calculate the amounts of raw materials needed for a process, estimate the quantity of product that can be made, test for errors such as leaks or catalysts that do not work properly and allows us to be aware of the full process. Mass balances can also tell us what’s in an unknown stream coming from a process. (CAE Mass Balances CMc Week 1, CAE Lecture notes, University of the west of Scotland, 2013/2014)
In science the capacity for work is called Energy. Energy can be found in a number of forms which can be altered from one form to another but these changes are restricted by a fundamental law of Physics, The Conservation of Energy Principle. This can be stated as:
“Energy can neither be created nor destroyed”
“The total energy of an isolated system remains constant” (http://hyperphysics.phy-astr.gsu.edu/hbase/conser.html#coneng, 18/03/2014, 18:35)
In a mass balance: 𝐼𝑁𝑃𝑈𝑇+𝐺𝐸𝑁𝐸𝑅𝐴𝑇𝐸𝐷−𝑂𝑈𝑇𝑃𝑈𝑇−𝐶𝑂𝑁𝑆𝑈𝑀𝐸𝐷=𝐴𝐶𝐶𝑈𝑀𝑈𝐿𝐴𝑇𝐸𝐷
* Input is the reactants enter into a system
* Generated is the amount created by the reaction
* Output is the amount that leaves the system
* Consumed is the amount of reactants used in the reaction
* Accumulated is the amount that is not released from the system
The aim of this experiment is to prove the principles of energy and mass balance in a system that has two inputs and one output.
o 250cm3 cylinder
o deionised water
o temperature probe
o potassium permanganate solution
o 0.1M Ethanoic acid containing 0.1M sodium chloride
o 0.1M sodium hydroxide
o pH meter
First, using a measuring cylinder, 250cm3 of deionised water (sample 1) was measured out into one of the input beakers and the beaker and its contents was weighed then the temperature (T2)was taken.
Next, a separate 200cm3 sample of deionised water was heated to a temperature of 67°C (T2) and added to the second input beaker and the beaker and its contents was weighed, just before mixing with sample 1 the temperature of sample 2 was taken.
Sample 3 was the mix of the contents of both of the input samples, temperature was immediately taken and both input vessels were weighed which was used to calculate the individual masses of the samples mixed.
Firstly, from the Potassium Permanganate solution (A) that was provided, 60cm3 was diluted to 260cm3 this was named solution (B)
Next the absorbance of both solutions (A+B) was measured individually using a spectrophotometer in 1cm cells at 540nm
After that 150cm3 of solution A was added to one of the input vessels, then 200cm3 of solution B was placed into the other input vessel.
Lastly the two solutions were mixed and the absorbance (A3) was noted.
The pre-prepared solutions of 0.1M ethanoic acid which contains 0.1M sodium chloride and another solution of 0.1M sodium hydroxide solution were collected.
Firstly, 200cm3 of the solution of the ethanoic acid and sodium chloride was measured out and the pH was recorded with the pH meter.
Next, a separate beaker was filled with 100cm3 of sodium hydroxide solution and made up to a total volume of 250cm3 by adding 150cm3 of water and the pH was noted.
Lastly the two solutions were mixed and the pH noted of the final mixture.
𝑚1𝐶(𝑇3−𝑇1)= −(𝑚2𝐶(𝑇3−𝑇2)) 248.3×4.18(𝑇3−21)= −(257.28×4.18×(𝑇3−67)) 1037.894(𝑇3−21)= −(1075.4304(𝑇3−67)) 1037.894𝑇3−21795.744= −1075.4304𝑇3+72053.8368 1075.4304𝑇3 + 1037.894𝑇3=72053.8368+21795.744 2113.3244𝑇3=93849.5808 𝑇3= 93849.58082113.3244 𝑇3= 44.4°𝐶
𝐴3=(𝑉1𝐴1+𝑉2𝐴2)(𝑉1+ 𝑉2) 𝐴3=(150×0.92+200×0.271)(150+200) 𝐴3=(138+54.2)(350) 𝐴3=(192.2)(350) 𝐴3=0.549
𝐶𝐻3𝐶𝑂𝑂𝐻+𝑁𝑎𝑂𝐻 → 𝐶𝐻3𝐶𝑂𝑂𝑁𝑎+ 𝐻2𝑂
Moles of acid:
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑎𝑐𝑖𝑑 (𝑖𝑛𝑖𝑡𝑖𝑎𝑙)=𝑀𝑉 =0.1×0.2 =0.02 𝑚𝑜𝑙𝑒𝑠
Moles of alkali:
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑎𝑙𝑘𝑎𝑙𝑖 (𝑖𝑛𝑖𝑡𝑖𝑎𝑙)= 𝑀𝑉 =0.1×0.1 =0.01 𝑚𝑜𝑙𝑒𝑠 1:1→1:1
Therefore 0.01 moles of alkali →0.01 moles of salt
Excess of acid = 0.01M
Calculation of pH of acid:
𝑝𝐻= 12𝑝𝐾𝑎− 12log(𝑎𝑐𝑖𝑑) 𝑝𝐻= 124.74− 12log(0.1) 𝑝𝐻=2.37−(−0.5) 𝑝𝐻= 2.87
Initial concentration of alkali:
𝑀1𝑉1= 𝑀2𝑉2 0.1×0.1= 𝑀2×0.25 𝑀2= 0.010.25 𝑀2=0.04𝑀
pH of Alkali:
𝑝𝐻=14+ log[𝑂𝐻−] 𝑝𝐻=14+ log[0.04] 𝑝𝐻=14+(−1.4) 𝑝𝐻=12.6
pH of Salt:
𝑝𝐻=𝑝𝐾𝑎+ log[𝑆𝐴𝐿𝑇][𝐴𝐶𝐼𝐷] 𝑝𝐻=4.74+ log(0.010.45)(0.010.45) 𝑝𝐻=4.74+ log(1) 𝑝𝐻=4.74
Total input of moles = 0.02 + 0.01= 0.03
[𝑁𝑎+]= 0.030.45 [𝑁𝑎+]= 0.006𝑀
Heat loss to environment to reduce the heat lost to the environment the beakers could be replaced with a polystyrene cup and lid. This could also prevent heat loss to the colder beaker that the mixture is poured into.
The final value of absorbance measured is 0.558 and the calculated value is 0.549 which is very close to the predicted value, this small error could be due to improper calibration of the spectrophotometer or using a cuvette which is not properly clean, both of these things could play a part in a higher than expected absorbance value. Another more probable reason is improper dilutions of the stock solution.
To experimentally test the result of the sodium ion concentration, in the product solution, a calibration curve of sodium should be constructed by gas chromatography or atomic flame spectroscopy then a sample of the product ran and the concentration can be read from the graph.
To conclude there are some things that could be changed to give better results however some of the results were very close to what was predicted. In part 1 the temperature recorded was very different to the one predicted whereas in Part 2 the absorbance value was only off by 0.009 which is fairly close
Analytical Chemistry, D. Kealey & P.J. Hanes, 2002, Oxford, BIOS Scientific Publishers Limited
Skoog and West’s Fundamentals of Analytical Chemistry, F. James Holler, Stanley R. Crouch, 2004, international edition, USA, cengage
CAE Mass Balances CMc Week 1, CAE Lecture notes, University of the west of Scotland, 2013/2014