Carbon+Dioxide

The chemical formula is CO 2, composed of two oxygen atoms bonded covalently to the single carbon atom. It exists in the earth's atmosphere as a gas at standard temperatures and pressures. It is a part of the carbon cycle in the process of photosynthesis. In this process plants and algae absorb CO 2, light, and water to produce carbohydrates for themselves as energy and it gives off oxygen as a byproduct. CO 2 is released by the combustion of coal or hydrocarbons, fermentation of liquids, or breathing due to animals and humans. It is also let off in the process of volcanoes, hot springs, and geysers. There is about 390 ppm by volume in the earths atmosphere, and the amounts differ according to the changes of the seasons on earth. Hence the plants die and CO 2 rises and when they bloom there is more oxygen to be released instead of lingering CO 2. CO 2 is a greenhouse gas as it transmits visible light and absorbs infared and near infared light. Also a food additive and used in the candy Pop Rocks. Is exhaled in human respiration.

Structure and bonding
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.

In aqueous solution
Carbon dioxide is soluble in water, in which it reversibly converts to H 2 CO 3 (carbonic acid). The relative concentrations of CO 2, H 2 CO 3 , and the deprotonated forms HCO − 3.

( bicarbonate ) and CO 2− 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 (HCO 3 − ): H 2 CO 3 HCO 3 − + H + //K//a1 = 2.5×10−4 ; p//K//a1 = 3.6 at 25 °C. At high pH, the bicarbonate ion dissociates significantly into the carbonate ion (CO 3 2− ): HCO3− CO32− + H+//K//a2 = 4.69×10−11 ; p//K//a2 = 10.329 Chemical reactions of CO 2 Overall, CO 2 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 CO 2 to give carboxylates: MR + CO 2 → RCO 2 M (where M = Li or MgBr and R = alkyl or aryl ). In metal carbon dioxide complexes, CO 2 serves as a ligand, which can facilitate the conversion of CO 2 to other chemicals. The reduction of CO 2 to CO is ordinarily a difficult and slow reaction: CO 2 + 2 e- + 2H + → CO + H 2 O 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. CO 2 can react with base producing salt and water. The reaction of CO 2 and magnesium produces carbon and magnesium oxide. People always use CaCO 3 and HCl to produce CO 2 in lab.

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