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Carbon dioxide (chemical formula CO2) is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state, as a trace gas at a concentration of 0.039% by volume.
As part of the carbon cycle known as photosynthesis, plants, algae, and cyanobacteria absorb carbon dioxide, light, and water to produce carbohydrate energy for themselves and oxygen as a waste product. But in darkness photosynthesis cannot occur, and during the resultant respiration small amounts of carbon dioxide are produced. Carbon dioxide is also produced by combustion of coal or hydrocarbons, the fermentation of liquids and the breathing of humans and animals. In addition it is emitted from volcanoes, hot springs, geysers and other places where the earth’s crust is thin; and is freed from carbonate rocks by dissolution. CO2 is also found in lakes at depth under the sea, and commingled with oil and gas deposits.
The environmental effects of carbon dioxide are of significant interest. In the earth's atmosphere, it acts as a greenhouse gas which is believed to play a major role in global warming and anthropogenic climate change. Also a major source of ocean acidification is CO2 which dissolves in water forming carbonic acid,which is a weak acid, because CO2 molecule ionization in water is incomplete.
CO2 + H2O H2CO3
The carbon dioxide molecule is linear and centrosymmetric. The two C-O bonds are equivalent and are short (116.3 pm), consistent with double bonding.Since it is centrosymmetric, the molecule has no electrical dipole. Consistent with this fact, only two vibrational bands are observed in the IR spectrum – an antisymmetic stretching mode at 2349 cm−1 and a bending mode near 666 cm−1. There is also a symmetric stretching mode at 1388 cm−1 which is only observed in the Raman spectrum.Carbon dioxide is soluble in water, in which it reversibly converts to H2CO3 (carbonic acid).
The hydration equilibrium constant Kh (at 25 °C) of carbonic acid is [H2CO3]/[CO2] = 1.70×10−3: Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO2 molecules not affecting the pH. It is an amphoteric substance that can act as an acid or as a base, depending on pH of the solution.
The relative concentrations of CO2, H2CO3, and the deprotonated forms HCO−
3 (bicarbonate) and CO2−
3(carbonate) depend on the pH. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2 – 8.5, contain about 120 mg of bicarbonate per liter.
Being diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion (HCO3−):
H2CO3 HCO3− + H+
Ka1 = 2.5×10−4 ; pKa1 = 3.6 at 25 °C.[5]
At high pH, the bicarbonate ion dissociates significantly into the carbonate ion (CO32−):
HCO3− CO32− + H+
Ka2 = 4.69×10−11 ; pKa2 = 10.329
In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.Overall, CO2 is a weak electrophile. Its reaction with basic water illustrates this property, in which case hydroxide is the nucleophile. Other nucleophiles react as well. For example, carbanions as provided by Grignard reagents and organolithium compounds react with CO2 to give carboxylates:
MR + CO2 → RCO2M (where M = Li or MgBr and R = alkyl or aryl).
In metal carbon dioxide complexes, CO2 serves as a ligand, which can facilitate the conversion of CO2 to other chemicals.[6]
The reduction of CO2 to CO is ordinarily a difficult and slow reaction:
CO2 + 2 e- + 2H+ → CO + H2O
The redox potential for this reaction near pH 7 is about −0.53 V vs NHE. The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process.
Carbon dioxide (chemical formula CO2) is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state, as a trace gas at a concentration of 0.039% by volume.
As part of the carbon cycle known as photosynthesis, plants, algae, and cyanobacteria absorb carbon dioxide, light, and water to produce carbohydrate energy for themselves and oxygen as a waste product. But in darkness photosynthesis cannot occur, and during the resultant respiration small amounts of carbon dioxide are produced. Carbon dioxide is also produced by combustion of coal or hydrocarbons, the fermentation of liquids and the breathing of humans and animals. In addition it is emitted from volcanoes, hot springs, geysers and other places where the earth’s crust is thin; and is freed from carbonate rocks by dissolution. CO2 is also found in lakes at depth under the sea, and commingled with oil and gas deposits.
The environmental effects of carbon dioxide are of significant interest. In the earth's atmosphere, it acts as a greenhouse gas which is believed to play a major role in global warming and anthropogenic climate change. Also a major source of ocean acidification is CO2 which dissolves in water forming carbonic acid,which is a weak acid, because CO2 molecule ionization in water is incomplete.
CO2 + H2O H2CO3
The carbon dioxide molecule is linear and centrosymmetric. The two C-O bonds are equivalent and are short (116.3 pm), consistent with double bonding.Since it is centrosymmetric, the molecule has no electrical dipole. Consistent with this fact, only two vibrational bands are observed in the IR spectrum – an antisymmetic stretching mode at 2349 cm−1 and a bending mode near 666 cm−1. There is also a symmetric stretching mode at 1388 cm−1 which is only observed in the Raman spectrum.Carbon dioxide is soluble in water, in which it reversibly converts to H2CO3 (carbonic acid).
The hydration equilibrium constant Kh (at 25 °C) of carbonic acid is [H2CO3]/[CO2] = 1.70×10−3: Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO2 molecules not affecting the pH. It is an amphoteric substance that can act as an acid or as a base, depending on pH of the solution.
The relative concentrations of CO2, H2CO3, and the deprotonated forms HCO−
3 (bicarbonate) and CO2−
3(carbonate) depend on the pH. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2 – 8.5, contain about 120 mg of bicarbonate per liter.
Being diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion (HCO3−):
H2CO3 HCO3− + H+
Ka1 = 2.5×10−4 ; pKa1 = 3.6 at 25 °C.[5]
At high pH, the bicarbonate ion dissociates significantly into the carbonate ion (CO32−):
HCO3− CO32− + H+
Ka2 = 4.69×10−11 ; pKa2 = 10.329
In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.Overall, CO2 is a weak electrophile. Its reaction with basic water illustrates this property, in which case hydroxide is the nucleophile. Other nucleophiles react as well. For example, carbanions as provided by Grignard reagents and organolithium compounds react with CO2 to give carboxylates:
MR + CO2 → RCO2M (where M = Li or MgBr and R = alkyl or aryl).
In metal carbon dioxide complexes, CO2 serves as a ligand, which can facilitate the conversion of CO2 to other chemicals.[6]
The reduction of CO2 to CO is ordinarily a difficult and slow reaction:
CO2 + 2 e- + 2H+ → CO + H2O
The redox potential for this reaction near pH 7 is about −0.53 V vs NHE. The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process.
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