Exercise 4: Chemical Reactions. 1. ... What is the equilibrium constant for the
formation of silicon dioxide at ... What is the form of the rate law based on this
data?
Exercise 4: Chemical Reactions 1. Nanostructures for microelectronics applications are often produced on Si substrates. To do this, we need to remove the native oxide from the Si. Use the Richardson-Ellingham diagram to answer the following questions. a) Samples are first dipped in an etching solution to remove the oxide at room temperature. What is the equilibrium constant for the formation of silicon dioxide at room temperature? b) After this initial step the samples must be heated in a chamber containing hydrogen to remove the remaining oxygen. We heat the samples to 900 oC. How pure must the hydrogen be for this to be effective (i.e. what should the H2/H20 ratio be?) c) We would like to be able to store the samples after they have been de-oxidized. The very best glove boxes available have oxygen pressure around 10-9 mbar. Is it possible to store the samples at room temperature? 2. Palladium nanoparticles are used as catalysts for the hydrolysis of water. The overall reaction is: Pa + 2H2O → 2H2 + O2 + Pa a) In order to determine the rate law, the isolation method was used. First, a large amount of palladium catalyst was used together with a low concentration of water. The concentration of water was then measured vs. time, and it was determined that that ln[H20] varied linearly with time. i.e. ln H2 O where m and b are constants (slope and intercept). Next, a small concentration of palladium was used together with a high concentration of water. In this case, the concentration of palladium was measured vs. time, and in this case it was found that the ratio 1 Pa was found to vary linearly with time.
Pa
What is the form of the rate law based on this data? b) For the above reaction, a mechanism is proposed: Pa + H2O → PaO + H2 PaO + H2O → PaH2 + O2 PaH2 → Pa + H2
k1 k2 k3
What form of the rate law does this mechanism give? State any assumptions. Does this mechanism give the correct form of the rate law (i.e. does it match the experimental rate law in b)? 3. It is possible to grow InP nanoparticles using Cu nanoparticles. It is believed that Cu might act as a catalyst for the formation of InP. We have observed that InP nanowires grow at the following rates for the following temperatures: Temperature (oC) 325 350 375
Rate (nm/s) 4,57 6,73 11,78
It is reported in other publications that InP layers grow with the following rates (for certain conditions): Temperature (oC) 490 500 525
Rate (nm/s) 3,43 7,23 16,7
Assume the growth is limited by the chemical reaction between the precursors to form InP (so the growth rate is determined by the chemical reaction rate). Calculate the approximate activation energies for formation of InP with and without Cu. From this data, can we conclude that Cu acts as a catalyst for the formation of InP nanowires? 4. What are the dimensions of the equilibrium constant? 5. Under what conditions is the equilibrium constant independent of the total pressure? 6. What is the dimensionality of the rate constant for a reaction that has an overall order n = 0.5?
