en>Macrakis: replaced "standard" development with "given"; add ref

2015-01-03T21:01:53Z

replaced "standard" development with "given"; add ref

← Older revision		Revision as of 23:01, 3 January 2015
Line 1:		Line 1:
	~~Got nothing to tell about myself at all~~.~~<br>Feels good~~ to ~~be a member of this site.<br>I just wish I am useful~~ in ~~some way~~ .<br><br>~~My blog post:~~ [http://~~safedietplans~~.~~com~~/~~bmi-chart bmi chart~~]		My name is Dillon from Remscheid Nord studying Computing and Information Science. I did my schooling, secured 89% and hope to find someone with same interests in Target Shooting.<br><br>Feel free to visit my web site - [http://itkt.co/howdoyouremovespraypaint183782 overspray removal austin]

208.45.168.2: /* Methods to perform display gamma correction in computing */

2014-02-04T19:16:08Z

Methods to perform display gamma correction in computing

← Older revision		Revision as of 21:16, 4 February 2014
Line 1:		Line 1:
	~~'''Chemisorption''' is a kind of [[adsorption]] which involves a chemical reaction between the surface and the adsorbate. New chemical bonds are generated~~ at the adsorbant surface. Examples include macroscopic phenomena that can be very obvious, like [[corrosion]], and subtler effects associated with [[heterogeneous catalysis]]. The strong interaction between the [[adsorbate]] and the [[Substrate (chemistry)\|substrate]] [[surface]] creates new types of electronic [[chemical bond\|bonds]].<ref name="oura">{{cite book \|first=K. \|last=Oura \|coauthors=V. ~~G. Lifshits; A. A. Saranin; A. V. Zotov; M. Katayama \|title=Surface Science, An Introduction \|publisher=Springer \|location=Berlin \|year=2003 \|isbn=3-540-00545-5}}~~<~~/ref~~>		Got nothing to tell about myself at all.<br>Feels good to be a member of this site.<br>I just wish I am useful in some way .<br><br>My blog post: [http://safedietplans.com/bmi-chart bmi chart]

	In contrast with chemisorption is [[physisorption]], which leaves the chemical species of the [[adsorbate]] and surface intact. It is conventionally accepted that the energetic threshold separating the [[binding energy]] of "physisorption" from that of "chemisorption" is about 0.5 eV per adsorbed [[Chemical species\|species]].

	~~Due to specificity, the nature of chemisorption can greatly differ, depending on the chemical identity and the surface structure.~~

	~~==Uses==~~
	An important example of chemisorption is in [[heterogeneous catalysis]] which involves molecules reacting with each other via the formation of chemisorbed intermediates. After the chemisorbed species combine (by forming bonds with each other) the product desorbs from the surface.
	[[Image:Hydrogenation on catalyst.png\|thumb\|[[Hydrogenation]] of an [[alkene]] on a solid catalyst entails chemisorption of the molecules of hydrogen and alkene, which form bonds to the surface atoms.]]

	~~==Self-assembled monolayers==~~
	[[Self-assembled monolayer]]s (SAMs) are formed by chemisorbing reactive reagents with metal surfaces. A famous example involves [[thiol]]s (RS-H) absorbing onto the surface of [[gold]]. This process forms strong Au-SR bonds and releases H<sub>2</sub>. The densely packed SR groups protect the surface.

	~~==Gas-surface Chemisorption==~~
	~~===Adsorption Kinetics===~~
	~~As an instance of adsorption, chemisorption follows the adsorption process. The first stage is for the adsorbate particle to come into contact with the surface. The particle needs~~ to be trapped onto the surface by not possessing enough energy to leave the gas-surface [[potential well]]. If it elastically collides with the surface, then it would return to the bulk gas. If it loses enough [[momentum]] through an [[inelastic collision]], then it “sticks” onto the surface, forming a precursor state bonded to the surface by weak forces, similar to physisorption. The particle diffuses on the surface until it finds a deep chemisorption potential well. Then it reacts with the surface or simply desorbs after enough energy and time.<ref name="rettner">{{cite journal \|doi=10.1021/jp9536007 \|first=C.T \|last=Rettner \|coauthors=Auerbach, D.J. \|year=1996 \|title=Chemical Dynamics at the Gas-Surface Interface \|journal=Journal of ~~Physical Chemistry \|volume=100 \|issue=31 \|pages=13021–13033}}</ref>~~

	~~The reaction with the surface is dependent on the chemical species involved~~. ~~Applying [[Gibbs free energy]] equation for reactions:~~

	:<~~math~~>~~\Delta G = \Delta H - T\Delta S</math>~~

	General [[thermodynamics]] states that for spontaneous reactions at constant temperature and pressure, the change in free energy should be negative. Since a free particle is restrained to a surface, and unless the surface atom is highly mobile, entropy is lowered. This means that the [[enthalpy]] term must be negative, implying an [[exothermic reaction]].<ref name="gasser">Gasser, R.P.H.; (1985) ''An introduction to chemisorption and catalysis by metals'', Clarendon Press, Oxford</ref>

	<!-- [[Image:ljgraph.PNG\|thumb\|Figure 1: Graph of energy curves of physisorption (W-O<sub>2</sub>) and chemisorption (W-O). Chemisorption has a deeper energy well. The graphs are shown to either cross above the zero-energy line or below it.]] Deleted file/image-->
	Figure 1 is a graph of physisorption and chemisorption energy curves of [[tungsten]] and [[oxygen]]. Physisorption is given as a [[Lennard-Jones potential]] and chemisorption is given as a [[Morse potential]]. There exists a point of crossover between the physisorption and chemisorption, meaning a point of transfer. It can occur above or below the zero-energy line (with a difference in ~~the Morse potential, a), representing an [[activation energy]] requirement or lack of. Most simple gases on clean metal surfaces lack the activation energy requirement.~~

	~~===Modeling===~~

	~~For experimental setups of chemisorption, the amount of adsorption of a particular system is quantified by a sticking probability value~~.<~~ref name="gasser" /~~>

	However, chemisorption is very difficult to theorize. A multidimensional [[potential energy surface]] (PES) derived from [[Effective medium approximations\|effective medium theory]] is used to describe the effect of the surface on absorption, but only certain parts of it are used depending on what is to be studied. A simple example of a PES, which takes the total of the energy as a function of location:

	:<~~math~~>~~E(\{R_i\}) = E_{el}(\{R_i\}) + V_{\text{ion-ion}}(\{R_i\})</math>~~

	where <math>E_{el}</math> is the [[Energy eigenstates\|energy eigenvalue]] of the [[Schrödinger equation]] for the electronic degrees of freedom and <math>V_{ion-ion}</math> is the ion interactions. This expression is without translational energy, [[rotational energy]], vibrational excitations, and other such considerations.<ref name="norskov">{{cite journal \|doi=10.1088/0034-4885/53/10/001 \|first=J.K. \|last=Norskov \|year=1990 \|title=Chemisorption on metal surfaces \|journal=Reports on Progress in Physics \|volume=53 \|issue=10 \|pages=1253–1295}}</ref>

	~~There exist several models to describe surface reactions~~: ~~the [[Langmuir adsorption model\|Langmuir-Hinschelwood mechanism]] in which both reacting species are adsorbed, and the~~ [~~[Reactions on surfaces\|Eley-Rideal mechanism]] in which one is adsorbed and the other reacts with it.<ref name="gasser" />~~

	~~Real systems have many irregularities, making theoretical calculations more difficult:<ref name="clark">Clark, A.; (1974); ''The Chemisorptive Bond~~: ~~Basic Concepts'', Academic Press, New York and London<~~/~~ref>~~

	* Solid surfaces are not necessarily at equilibrium.
	* They may be perturbed and irregular, defects and such.
	* Distribution of adsorption energies and odd adsorption sites.
	* Bonds formed between the adsorbates.

	Compared to physisorption where adsorbates are simply sitting on the surface, the adsorbates can change the surface, along with its structure. The structure can go through relaxation, where the first few layers change interplanar distances without changing the surface structure, or reconstruction where the surface structure is changed.<ref name="clark" />

	~~<!-- [[Image:stm-OCu.JPG\|thumb\|Figure 2: [[Scanning tunneling microscope\|STM]] image of oxygen adsorbed onto Cu(110)~~. ~~Several defects can be seen, gaps and missing rows. The oxygen is arrayed in the 001 direction.]] Deleted file~~/~~image~~-->
	For example oxygen can form very strong bonds (~4 eV) with metals, such as Cu(110). This comes with the breaking apart of surface bonds in forming surface-adsorbate bonds. A large restructuring occurs by missing row as seen in Figure 2.

	~~===Dissociation Chemisorption===~~

	A particular brand of gas-surface chemisorption is the [[Dissociation (chemistry)\|dissociation]] of [[Diatomic molecule\|diatomic]] gas molecules, such as [[hydrogen]], [[oxygen]], and [[nitrogen]]. One model used to describe the process is precursor-mediation. The absorbed molecule is adsorbed onto a surface into a precursor state. The molecule then diffuses across the surface to the chemisorption sites. They break the molecular bond in favor of new bonds to the surface. The energy to overcome the activation potential of dissociation usually comes from the translational energy and vibrational energy.<ref name="rettner" />

	And example is the hydrogen and [[copper]] system, one that has been studied many times over. It has a large activation energy of .35 - .85 eV. The vibrational excitation of the hydrogen molecule promotes dissociation on low index surfaces of copper.<ref name="rettner" />

	~~==See also==~~
	*[[Adsorption]]
	*[[Physisorption]]

	~~== References ==~~

	~~{{Reflist}}~~

	~~[[Category:Physical chemistry]]~~
	~~[[Category:Catalysis]~~]

en>Glrx: Undid revision 579666985 by 2001:67C:A4:1:DC93:E5E8:19B8:CAFE (talk) The math unduly complicates the idea; slope on log-log sufficient

2013-10-31T22:08:33Z

Undid revision 579666985 by 2001:67C:A4:1:DC93:E5E8:19B8:CAFE (talk) The math unduly complicates the idea; slope on log-log sufficient

Show changes

en>KlappCK: Citation needed.

2012-07-02T18:23:02Z

Citation needed.

New page

We all desire to minimize the fat rapidly. We like to look sleek plus fit for summer activities and social outings. The hardest part of any weight reduction program, nonetheless, is often only getting the motivation to start, and building a strong foundation for success. Because we want to reach a goals, remember that the number one region to begin is the beginning. To lose weight fast plus simple change what we drink, I've seenpeople inside terrible physical shape whom wouldn't drink anythingexcept diet coke. If you want to lose weight but we won't stopdrinking pop then leave this page considering you're wasting yourtime. The right time to do any cardio exercise is initial thing inside the morning before you eat anything. What arises is the fact that considering we haven't consumed anything throughout the evening, as you have been asleep, your body has to use calories that are stored inside the body and burning calories is what you need if you need to lose weight. To see rapid fat reduction simply begin reduce the scale of your food and eat more frequently. And you are able to accomplish this by breaking your daily meals into four smaller food plus 2 snacks. 3 Never eat anything at least 2 hours before bedtime, or eat any food after [http://safedietplansforwomen.com/how-to-lose-weight-fast lose weight fast] 8pm. This keeps the body from storing fat plus calories during a state of rest. Cook brown rice of half a cup for dinner plus entre. You are able to make entre by including some of the leafy green veggies, some carrots and braccia. Vegetables which come below leafy green are beet greens, collards, kale, etc. while those which belong to braccia are cabbage, broccoli, cauliflower. After dinner have an apple and certain dark chocolate of 0.7 ounces. Mike has been composing articles for a couple of years now. His latest passion is on Trout Fishing. He simply created a page in his site on Trophy Trout Fishing Tips. Whenever you receive a minute plus you love to fish yourself come go and visit his website.

Gamma correction - Revision history

en>Macrakis: replaced "standard" development with "given"; add ref

208.45.168.2: /* Methods to perform display gamma correction in computing */

en>Glrx: Undid revision 579666985 by 2001:67C:A4:1:DC93:E5E8:19B8:CAFE (talk) The math unduly complicates the idea; slope on log-log sufficient

en>KlappCK: Citation needed.