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معلومات عن حامض الفسفوريك

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    حسام النجار
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    معلومات عن حامض الفسفوريك

    Prof. Shakhashiri
    www.scifun.org General Chemistry

    PHOSPHORIC ACID, H
    3PO4

    About 10 million tons of phosphoric acid, H3PO4, are produced in this country each year. Most of the
    acid (about 80%) is used in the production of agricultural fertilizers, with the remainder being used for
    detergent additives (about 10%), cleaners, insecticide production, and cattle feed additives. The commercial
    method of preparation is the addition of sulfuric acid to phosphate rock.
    3 H2SO4(R) + Ca3(PO4)2(s) + 6 H2O(R) xv 2 H3PO4(s) + 3 CaSO4@2 H2O(s)
    Pure anhydrous phosphoric acid is a white solid which melts at 42.35EC to form a viscous liquid. In
    aqueous solution, phosphoric acid behaves as a triprotic acid, having three ionizable hydrogen atoms. The
    hydrogen ions are lost sequentially.
    H3PO4(aq) øõ H+(aq) + H2PO4G(aq) Ka1 = 7.5 × 10G3

    H2PO4G(aq) øõ H+(aq) + HPO4
    2
    G(aq) Ka2 = 6.2 × 10G8

    HPO4
    2
    G(aq) øõ H+(aq) + PO4
    3
    G(aq) Ka3 = 1.7 × 10G12

    Phosphoric acid is not a particularly strong acid as indicated by its first dissociation constant. It is a stronger
    acid than acetic acid, but weaker than sulfuric acid and hydrochloric acid. Each successive dissociation step
    occurs with decreasing ease. Thus, the ion H2PO4G is a very weak acid, and HPO4
    2
    G is an extremely weak
    acid.
    Salts of phosphoric acid can be formed by replacing one, two or three of the hydrogen ions. For
    example, NaH2PO4, sodium dihydrogen phosphate, can be formed by reacting one mole of phosphoric acid
    with one mole of sodium hydroxide.
    H3PO4(aq) + NaOH(aq) xv NaH2PO4(aq) + H2O(R)
    [net ionic form: H3PO4(aq) + OHG(aq) xv H2PO4G(aq) + H2O(R)]
    Similarly, Na2HPO4 (disodium hydrogen phosphate) and Na3PO4, (trisodium phosphate) could be formed
    by the reaction of one mole of H3PO4 with two and three moles of NaOH, respectively. (Be sure you are
    able to write net ionic equations for these processes.)

    Salts containing the anion H2PO4G are weakly acidic. The tendency of this ion to dissociate is greater
    than its tendency to hydrolyse, that is, its Ka2, is larger than its Kb.
    H2PO4G(aq) øõ H+(aq) + HPO4
    2
    G(aq) Ka2 = 6.2 × 10G8

    H2PO4G(aq) + H2O(R) øõ H3PO4(aq) + OHG(aq) Kb = Kw/Ka1 = 1.3 × 10G12

    Because H2PO4G is weakly acidic and of low toxicity, it is used as the acid in some baking powders. These
    baking powders contain NaH2PO4 and NaHCO3 (sodium bicarbonate). The leavening action of baking
    powders results from the production of carbon dioxide gas by an acid-base reaction between these two
    ingredients.
    H2PO4G(aq) + HCO3G(aq) xv HPO4
    2
    G(aq) + H2O(R) + CO2(g)
    In the reaction between them, H2PO4G acts as the Brønsted-Lowry acid, HCO3G as the base. A comparison
    of the ionization constants for these two ions reveals that H2PO4G is a stronger acid than HCO3G.
    H2PO4G(aq) øõ H+(aq) + HPO4
    2
    G(aq) Ka2 = 6.2 × 10G8

    HCO3G(aq) øõ H+(aq) + CO3
    2
    G(aq) Ka2 = 4.8 × 10G11

    Salts containing the anion HPO4G are weakly basic. The tendency of this ion to hydrolyse is greater than
    its tendency to dissociate.
    HPO4
    2
    G(aq) øõ H+(aq) + PO4
    3
    G(aq) Ka3 = 1.7 × 10G12

    HPO4
    2
    G(aq) + H2O(R) øõ H2PO4G(aq) + OHG(aq) Kb = Kw/Ka2 = 1.6 × 10G7

    Solutions containing the phosphate ion, PO4
    3
    G, are quite basic. This ion has no acidic hydrogen, and its
    base ionization constant (hydrolysis constant) is relatively large.
    PO4
    3
    G(aq) + H2O(aq) øõ HPO4
    2
    G(aq) + OHG(aq) Kb = 5.9 × 10G3

    As a result, solutions of soluble phosphates tend to have the same slippery, soapy feel as solutions of strong
    bases, such as NaOH or KOH.
    Phosphoric acid is used primarily in the manufacture of fertilizers, detergents, and pharmaceuticals. In
    the steel industry, it is used to clean and rust-proof the product. It is also used as a flavoring agent in
    carbonated beverages (read the ingredients list on a can of Coca-Cola), beer, jams, jellies and cheeses. In
    foods, phosphoric acid provides a tart, acidic flavor. A recent study reported in the journal
    Epidemiology

    (Vol 18, pp 501–506, July 2007), found that drinking two or more cola beverages per day doubled the risk
    of chronic kidney disease. Cola beverages have been associated with kidney changes that promote kidney
    stones, which may be a result of the phosphoric acid in colas.
    In the manufacture of detergents, phosphoric acid is used to produce water softeners. Water softeners
    remove Ca
    2+ and Mg2+ ions from hard water. If not removed, these hard-water ions react with soap and
    form insoluble deposits that cling to laundry and the washing machine. Phosphates produced from
    phosphoric acid are used extensively as water softeners (builders) in detergents. The most widely used
    phosphorus compound in solid detergent mixtures is sodium tripolyphosphate, Na
    5P3O10. As a water
    softener, sodium tripolyphosphate binds to Ca
    2+ and Mg2+, forming soluble chemical species, called
    complexes or chelates. These complexes prevent the Ca
    2+ and Mg2+ from reacting with soap and forming
    deposits.
    Most phosphoric acid is used in the production of fertilizers. Phosphorus is one of the elements essential
    for plant growth. Organic phosphates are the compounds which provide the energy for most of the chemical
    reactions that occur in living cells. Therefore, enriching soils with phosphate fertilizers enhances plant
    growth.
    Increasing the phosphate concentration in surface waters also enhances the growth of aquatic plant life.
    Run-off from fertilized farm lands can stimulate plant growth in lakes and streams. Waste water that
    contains phosphates from detergents can have the same effect. Lakes that are rich in plant nutrients suffer
    from accelerated eutrophication. When the lush aquatic plant growth in a nutrient-rich lake dies, the
    decomposition of the dead plant material consumes dissolved oxygen. This consumption reduces the level
    of dissolved oxygen to a point where it is insufficient to support animal life. To reduce the threat of lake
    eutrophication, many localities have banned the use of phosphates in detergents. In some cases, the
    phosphates have been replaced by carbonates. In others, new detergents have been developed that do not
    react with the Ca
    2+ and Mg2+ ions of hard water.

    Revised: 6 Feb 2008
    ك/ حسام النجار

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  2. [2]
    مهاجر
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    مشرف عــــــــام
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    السلام عليكم ورحمة الله وبركاته

    جزاك الله خير أخي حسام ‏‎ ‎
    ‎ ‎
    تم نقل الموضوع من مشاركتك السابقة وإفراد موضوع خاص به ‎ ‎

    نشكرك بإسم إدارة الملتقى ... والى الأمام

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  3. [3]
    shaker2011
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    كيف نخفف حامض ألفسفوريك بلماء

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  4. [4]
    كلوزة
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    مشكوووووووووووووووووووور جدا بس لو يكون عندك plant desighn الها تبعتلي اياه

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