CHEM 126 Lecture Notes - Lecture 11: Stoichiometry, Ideal Gas Law, Strong Electrolyte

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violetcamel580

9 Aug 2020

School

Simon Fraser University

Department

Chemistry

Course

CHEM 126

Professor

Jenny Wong

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Document Summary

Electrolysis of pure water requires excess energy in the form of overpotential to overcome various activation barriers. Without the excess energy the electrolysis of pure water occurs very slowly or not at all. This is in part due to the limited self-ionization of water. Pure water has an electrical conductivity about one millionth that of seawater: for example, a solution conducts current when positive and negative ions are randomly dissolved in it. Pure water doesn"t have enough ions but when you add an acid like hcl there is now a presence of a strong electrolyte. After the addition, the solution can conduct electricity very well. However, you can perform an electrolysis even without the acid, but this is a very slow process. Since there are positive and negatives ions and this will cause one to think that the pure water will conduct a current.

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Explain it briefly?

Production from renewable sources Production of H_2 by electrolysis of water is costly and viable only in areas where electricity is cheap or if it is a byproduct of an eco-nomically important process. Environmental pressures are driving technologies to produce H_2 more efficiently from surplus or renewable energy, including solar and biological sources. Electrolysis is used to produce H_2 that is free from contaminants. To drive this reaction, a large overpotential is required to offset the sluggish electrode kinetics, particularly for the production of O_2. The best catalysts are based on platinum, but it is too expensive to justify its use in large-scale plants. As a consequence, electrolysis of water is economical and environmentally benign only if the electrical power stems from cheap, renewable resources or if it is surplus to demand. These conditions are found in countries that have plenty of hydroelectric or nuclear energy. There is also scope for off-shore wind farms that can employ resources far from the electricity grids and are far from the population areas that are otherwise necessary for conventional power generation. Electrolysis is carried out using hundreds of cells arranged in series, each operating at 2 V with iron or nickel electrodes and aqueous NaOH (or an ion-selective membrane) as electro- lyte (Fig. 10.5). Temperatures of 80-85 degree C are used to increase the electrolytic current and to lower the overpotential. The most important electrolytic H_2 production method is the chlor-alkah process (Box 1 1.2), in which H, is produced as a byproduct of NaOH manufacture. In this process the other gaseous product is Cl, which requires a lower overpotential than O_2. As yet, however, only about 0.1 per cent of the global H_2 demand is produced by electrolysis, including that produced in the chlor-alkali process. This percentage could be improved by the development of cheap and efficient clectrocatalysts to reduce the economically wasteful overpotential. Hydrogen can be produced by fermentation, using anaerobic bacteria that use cultivated biomass or biological waste as their energy source (Box 10.1). Research is also under way to establish how best to produce H_2 by exploiting solar energy directly either through physical methods (solar-powered thermolysis or photo)

give chemical equations and synthesis reactions for the TiO2 solar cell

dye + light â dye*

dye* + TiO2 â e- (TiO2) + oxidized dye

oxidized dye + 3/2 I- â dye + Â½ I3 -

Â½ I3 - + e- (counter electrode) â 3/2 I

PROCEDURE

Preparation of TiO2 paste Grind about 0.5 gram of nanocrystalline titanium dioxide (TiO2) in a mortar and pestle with a few drops of very dilute acetic acid. Alternate grinding and addition of a few drops of very dilute acetic acid until you obtain a colloidal suspension with a smooth consistency, somewhat like latex paint. A toothpaste-like consistency is too thick. Also mix in a drop of clear dishwashing detergent as a surfactant. This quantity of TiO2 is enough for several solar cells. For easier distribution, transfer the TiO2 paste to a syringe. Wrap the end of the syringe with parafilm to keep the paste from drying out when not in use. (If the paste dries out, the titanium dioxide will need more water. See previous step.) Preparing the electrode 1. Preheat your hotplate. 2. Obtain two pieces of conducting glass (fluorine doped tin oxide). One will be your anode with the titanium dioxide film and dye, the other the cathode. 3. Identify the conducting side of a tin oxide-coated piece of glass by using a multimeter to measure resistance. The conducting side will have a resistance of 20-30 ohms. Switch the multimeter to the resistance setting (â¦) and test the piece of glass for conductivity. Only one side is conductive and this is the side you must use as a substrate for the titanium dioxide film and dye. 4. With the conducting side up, tape the glass on three sides to the center of a spill tray using one thickness of tape. Wipe off any fingerprints or oils using a tissue wet with ethanol. Opposite sides of tape will serve as a spacer (see below) so the tape should be flat and not wrinkled. The third side of tape gives an uncoated portion where an alligator clip will be connected. Applying the film 5. Add a small amount of titanium dioxide paste and quickly spread by pushing down and across with a microscope slide before the paste dries. The tape serves as a 40-50 micrometer spacer to control the thickness of the titanium dioxide layer if you push down. 6. Wait approximately two minutes for the film to dry. Carefully remove the Scotch tape border, paying attention to not splatter any of the wet paste or scratch the surface. 7. Carefully place the piece of glass on top of your preheated hotplate. Keep it there for 20 minutes. The surface turns brown as the organic solvent and surfactant dries and burns off to produce a white or green sintered titanium dioxide coating. (Note: this requires a plate that gets quite hot.) Allow the glass to slowly cool by turning off the hotplate. The sample will look quite similar before and after heating; you only know it is done if you have observed the darkening stage along the way. Preparing the dye 8. While the film is heating/cooling, begin to prepare the dye. Obtain one (1) raspberry or blackberry and crush it in a beaker. Add 3-4 drops of deionized water and continue to crush the berries, obtaining red juice.

9. Remove the cooled piece of glass (your anode) and place enough berry dye on the titanium dioxide film to completely cover it. Keep it there for 10 minutes. The white TiO2 will change color as the dye is absorbed and complexed to the Ti(IV). 10. Rinse gently with water to remove any berry solids and then with ethanol to remove water from the porous TiO2. The ethanol should have evaporated before the cell is assembled. Preparing the Counter electrode 11. Using a pencil, âcolor inâ the conductive side of the second piece of glass. It will take on a slight gray color. The graphite will function as a catalyst for the redox electrolyte. Make sure that this is done on the conductive side! Wipe off any carbon along the perimeter of three sides of the carbon-coated glass plate using a dry cotton swab. Assembling the cell 12. Place the titanium dioxide/dye electrode face up on the lab bench. Place the counter electrode on top of it, offsetting it so that the film is still covered by the counter electrode, but there is about â â of the glass exposed for connections on both electrodes. Do not rub or slide the plates.

13. Clamp the two electrodes together using two binder clips. They should be placed on the sides adjacent to the electrodes. 14. Add the redox electrolyte by placing 1 drop of a triiodide solution to opposite edges of the plate. Capillary action will cause the KI3 solution to travel between the two plates. (The KI3 electrolyte solution consists of 0.5 M KI and 0.05 M I2 in anhydrous ethylene glycol.) The solution can corrode the alligator clips in the next step so wipe off an excess. Properties of the cell Connect a multimeter using an alligator clip to each plate (the negative electrode is the TiO2 coated glass and the positive electrode is the carbon coated glass). â¢ Test the current and voltage produced by solar illumination, or... â¢ Test the current and voltage produced by illumination from an overhead projector..

CHEM 126 Lecture Notes - Lecture 11: Stoichiometry, Ideal Gas Law, Strong Electrolyte

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