What reactions take place in the Sun?|
How is energy generated?
How is energy transmitted from the Sun to other places in the universe?
Since the total mass of 4 H atoms is slightly more than that of a 4He atom, some mass is converted to energy in the fussion process. The energy is in the form of heat (including infrared radiation), and light (including visible and ultraviolet). Neutrinos are also produced. The loss of a small amount of mass Dm is converted into a large amount of energy DE as described by Einstein's quation
where c = 3e8 m s-1 is the speed of light.
The energy converted from mass in the Sun is mostly in the form of electromagnetic radiation (or light). As such, its transmission requires no medium and the energy from the Sun transmitts to other places in space in the universe by radiation. When the amount of energy generated equals the amount of energy transmitted out of the Sun, its temperature remains the same and such a state is called a star.
The Sun is currently a star, and it is expected to remain a star for a long long time, at least for some millions of years.
What are called supercritical fluids?|
Discuss the supercritical water.
What are some of the properties of supercritical water that are not present in water?
What are some of the applications of supercritical water?
Most substances have 3 states: solid-, liquid-, and gas-states. A generic phase diagram of a substance is shown on the right:
Pressure | | | | . <= | | LIQUID . Critical | S | . point | O | . | L | . | I | . | D | . | |_ . ' | / | . |' VAPOUR | |______________________________ Temperature
In this diagram, the line separating the SOLID and VAPOUR is the vapour pressure of the solid as a function of temperature. The line separating the LIQUID and VAPOUR is the vapour pressure of the LIQUID. The almost vertical line separating the SOLID and the LIQUID shows the pressure and temperature for the transition between SOLID and LIQUID. For water, this line has a negative slope but for most other substances, it has a positive slope,
The intersection point of these lines is the tripple point. At this temperature and pressure, all three phases co-exist.
The temperature above which the gas cannot be liquified is called the critical temperature. The vapour pressure slightly below the critical temperature is called the critical pressure, and this condition is called the critical point.
Thus, the phase above the critical temperature and pressure is generally called the supercritical fluid.
Physical and chemical properties of critical fluid is very different from those of the liquid. For example, the solubilities of other substances is supercritical fluid are very different from those in liquid. Furthermore, the solubilities are pressure dependent, and this phenomenon makes supercritical fluid a potentially very useful phase for separation of mixtures.
For water, the critical temperature is 374oC, and the ritical pressure is 218 atm. Thus, supercritical water is actually superheated steam under high pressure of more than 200 atmosphere. Under these conditions, supercritical water decomposes many substances.
|The following material indicates that you have done further research and learned more about the application of supercritical water. Inclusion of material you find will be rewarded.|
The Chemmatu Engineering Co. has developped a process called Aqua Critox, which oxidizes substances in waste water for water treatment. In this process, wast water is heated to 600oC under a pressure of 275 bar (1 bar ~ 1 atm). Oxygen passes through the critical water and all organic substances in the waste water (compound containing C, H, N, and O) are converted to CO2 H2O and nitrogen gas.
After the treatment, the water is fit to drink.
Please give your reasons for any assumption you have made.
Data: Molecular weight of CO2 = 44.0; Ka1 = 4.2e-7; Ka2 = 5.0e-11 for H2CO3.
Only carbon dioxide is present in a sample of rain water whose pH is 5.5.
What is the solubility of carbon dioxide in rain water?
What are the concentrations of HCO3- and CO32-?
What is acid rain and what are the causes?
The substance H2CO3 is known as carbonic acid and it ionizes as follows:
----------------- = 4.2e-7 - - - - (1)
HCO3- ® H+ + CO32-
----------------- = 5.0e-11 - - - - (2)
The Ka1 and Ka2 values indicte that H2CO3 and HCO3- are very weak acids. Their degrees of ionization are very small. Therefore, it is save to assume
Therefore substituting (3) in (1) gives
[H2CO3] = (3.2e-6)2 / 4.2e-7
= 2.4e-5 M - - - - (4)
The solubility of CO2 is thus 2.4e-5 mol/L or M. The assumption above gives
Substituting (4) and (5) into 2 gives
The value for [HCO3-] is small compared to [H2CO3], but note that
So, the percentage is somewhat significant. However, the assumption is not far off. Taking the anion into consideration will increase the solubility of CO2 by 13% more. The degree of dissociation of HCO3- is very small indeed, however.
Acid rain usually contains SO2, SO3, HCl, and NO2 and these gases react with water to give stronger acid than H2CO3.