Chemical kinetics is the study of rates of chemical processes or reactions. The rate of chemical reaction is generally determined by the change in concentration of a chemical specie with respect to time change. Some reaction may take seconds to be completed while some could take a longer time.
In this experiment, Cr(vi) is used to oxidize isopropanol, Cr(vi) is an exceptional oxidizing agent due to its transition metal nature of its parent element which enables it to possess variable oxidation state. Cr(vi) used in the experiment is obtained by the dissociation of K2Cr2O7in water to form HCrO4–. As Cr(vi) oxidizes the isopropanol, it will be itself reduced to Cr(iii) but the oxidation is more visible due to the high oxidizing power of the Cr(vi). The rate of a reaction is also influenced by factors such as concentration. Temperature, surface area, catalyst, pressure etc.
The oxidation of alcohols by Cr(vi) is a reaction that has been widely used to study rate of chemical reaction.Secondary alcohols undergo oxidation to form ketones. The equation and rate law for this reaction are given below;
3C3H7OH + 2HCrO4– + 8H+ → 3CH3COCH3 + 2Cr3+ + 8H2O
The rate law can be reduced to the form; r = kp[HCrO4–]x
where kp= k[H+][C3H7OH]y
when the concentration of C3H7OH and H+ is constant.
The order of the reaction can be determined from the rate plots. A zero order reaction gives a straight line graph of concentration against time. A straight line graph of logarithm of concentration against time indicates a first order reaction. And a second order reaction gives a straight line graph of inverse of concentration against time.
Reagents and Apparatus
- 4M HCl
- 0.02M Na2S2O3.5H2O
- 3% aqueous KI
- Isopropanol solution
- Starch solution
- Distilled water
- Two Burette (one for acid and the other for base)
- Retort stand
- Graduated cylinder
- 10 mL volumetric flask
- 10 mL Pipette
- Two conical flasks (250 mL)
- Weigh 0.6g of K2Cr2O7 and dissolve in water. Pour 500 mL of 4M HCl into the solution and mix well.
- Fill the acid burette with the HCrO4– solution and the base burette with Na2S2O3 solution. Measure out 10 mL of HCrO4– solution into a clean conical flask and add 4 mL of 3% KI. The solution colour changes to reddish-brown.
- Titrate against the Na2S2O3 solution until colour changes to green using starch near the end point. Continue the titration until a pale blue-green end point is reached.
- Measure out 180 mL of the acidic solution into a 250 mL flask, add 2 mL of isopropanol into the solution and note the time.
- Measure out 10 mL of the solution, add 4 mL of 3% KI and repeat step 3. Repeat this step at 10 mins interval for 80 mins.
- Repeat steps 4 and 5 for 3 mL and 4mL of isopropanol and record the volume of Na2S2O3 used in each time interval
Results and Calculations
- Calculate the concentration of HCrO4– at different time intervals for 2 mL, 3 mL and 4 mL isopropanol using the formula below:
- Plot a graph of [ HCrO4–], log[ HCrO4–] and 1/[ HCrO4–]against time for 2 mL, 3 mL and 4 mL isopropanol.
- 3. Determine the order of the reaction and calculate Kp.
Discussion and Conclusion
From the graphs and calculation, you should be able to determine the order of the reaction and rate constant.
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Credits: Nurudeen Yusuf (B.Sc.). Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria.