Free Materials And Methods Report Example

Purpose: To learn about the hydrogen and water reaction mechanism by trapping one of the reaction intermediates, hydrogen peroxide, in a hydrogen flame through a quick cooling before it is degraded. Hydrogen peroxide, the reaction intermediate, can be measured with permanganate in acidic solution.

All glassware, stir rod, and test tubes were cleaned and dried before the experiment. Potassium permanganate solution (1.26 10-4 M KMnO4) filled a 20 mL buret. The experimental setup consisted of a buret with a ring stand, and a small beaker was placed below. A solution of sulfuric acid 2.0 M H2SO4 was added to each test tube. Ten drops of KMnO4 was added to this test tube. (This is the colorimetric control). Volumes of KMnO4 were measured and recorded. Additionally, two test tubes containing the same volume of melted melted ice drippings were weighed. Ice drippings were made by holding an ice cube over a hydrogen flame.
The reaction was started by adding KMnO4 solution to the test tube containing the ice shavings. KMnO4 can react with the hydrogen peroxide and this can be observed visually as there is no pink color observed. Further aliquots of KMnO4 was added to the ice dripping mixture to get an endpoint comparable to our colorimetric control. This was repeated to verify the results. The amount of MnO4- which reacted was used to calculate the amount of hydrogen peroxide in solution using the following stoichiometry:
2 MnO4- + 6 H+ (aq) + 5H2O2 → 2Mn2+ (aq) + 8 H2O + 5 O2 (g)(eq1)

All chemical were disposed of in a safe manner and all laboratory safety measures were maintained.

The procedure was followed as outlined in the laboratory notebook.
Results
The volume of the different solutions were recorded within this experiment and recorded in Table 1.

When we added the permanganate solution to the ice-melted with the hydrogen flame we did not see the liquid turning pink like when we measured the same amount on the control indicating that there was a reaction taking place. By understanding the reaction stoichiometry (eq1) we were able to determine the amount of hydrogen peroxide in the solution of the ice melted with the hydrogen flame. The permanganate solution found suitable end-points by comparing color to the control experiment.

Discussion

Water is one of the most important elements for life so understanding how it can be formed is an important scientific question. Formation of water needs an explosive amount of energy to split a oxygen to form water. However, there are several other reactions to get water for example from the decomposition an intermediate peroxide. In order to trap the reaction intermediate we had to bind it to a manganese. Manganese can exist in two states II and VII with an intense purple color the latter forming in permanganate ions. Permanganate is a really good chemical to use in chemical reactions of this sort. It is safe to work with and also can find endpoints and determine them colorimetrically. Within this experiment the permanganate was titrated to measure the amount of a reaction species peroxide. Within this experiment we found that the permanganate will be reduced to a clear solution when peroxide is present. However, once the peroxide has reacted it will show an endpoint and purple or pink will be observed in the solution. This technique has been found to provide an accurate measurement of peroxide in solution.
More recently, it was found that acid takes part in the reaction and has a positive effect on the rates at the initial and autocatalytic phases (Simoyi, 1986). By understanding and identifying the reaction intermediates it has potential for developing better catalysis mechanisms or changing the reactions at either the reactant or product side to make the reactions more efficient. These reactions have value as they have recently been used in fuel cells (Fu ,2010).

Conclusion

Understanding of the reaction mechanism kinetics and thermodynamic gives a good understanding of how certain reactions and reaction intermediates would be favorable or unfavorable. Using good analytical techniques the different intermediates can be determined quantitatively.

References

Simoyi, Reuben H., et al. "Reaction between permanganate ion and hydrogen peroxide: kinetics and mechanism of the initial phase of the reaction." Inorganic Chemistry 25.4 (1986): 538-542.
Huckaba, Charles E., and Frederick G. Keyes. "The accuracy of estimation of hydrogen peroxide by potassium permanganate titration." Journal of the American Chemical Society 70.4 (1948): 1640-1644.
Fu, Lei, et al. "Synthesis of hydrogen peroxide in microbial fuel cell." Journal of chemical technology and biotechnology 85.5 (2010): 715-719.
Additional Questions
Why might the reaction between hydrogen peroxide and MnO4- proceed more slowly towards the end of the reaction?
The reaction might proceed more slowly at the end as there is less H2O2 to react with. Although manganese is in excess it still has to ‘find’ the peroxide.
Calculate the free energy of the reaction: H2 (g) + O2 (g) → H2O2 (g) = -178.0 kJ/mol
and the reaction: H2O2 (g) + H2 (g) → 2 H2O (g)
Adding equation 1 to reverse of equation 2 is 684.8 kJ/mol.
How do the free energy values for these two reactions compare the free energy of the combustion of hydrogen gas?
The free energy of these two values are a lot lower than the combustion of hydrogen making it them less favorable.
Based on your calculations in the previous question and your understanding of reaction mechanisms, draw a qualitative energy diagram of the reaction’s progress