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(a) Why cannot a chemical change be norm...

(a) Why cannot a chemical change be normally reversed ?
(b) Why is it always essential to balance a chemical equation ?
(c) Why do diamond and graphite, the two allotropic forms of carbon evolve different amounts of heat on combustion ?
(d) Can rusting of iron take place in distilled water ?

Text Solution

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(a) In a chemical change, the products are quite different from the reactants. Therefore, it cannot be normally reversed.
(b) A chemical equation has to be balanced to meer the requirement of the law of conservation of mass. According to the law, the total mass of the reacting species taking part in the reaction is the same as that of the products formed. Since there is a direct relationship between the mass of the different species and their number, it is always essential to balance a chemical equation.
(c) Because they differ in the arrangement of carbon atoms present and have different shapes. The attractive forces among the atoms in the two cases are not same. That is why they evolve different amount of heat.
`C("diamond") + O_(2)(g) to CO_(2)(g) + 393*5 kj` ltBrgt `C("graphite") + O_(2)(g) to CO_(2)(g) + 395*4 kj`
Please note that diamond and graphite are the two allotropic forms of carbon. For details, consult chapter 4.
(d) No, rusting of iron cannot take place in distilled water because it neither contains dissolved oxygen not carbon dioxide. Both are essential for the rusting of iron.
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Why do diamond and graphite, the two allotropic forms of carbon evolve different amounts of heats on combustion?

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Knowledge Check

  • Heat of reaction is the change in enthalpy or internal energy as represented by a balanced thermochemical equation . The amount of energy released during a chemical change depends upon the state of reactants and products, the conditions of pressure and volume at which reaction is carried out, and temperature. The variation of heat of reaction (DeltaH_(1) or DeltaE) with temperature is given as DeltaH_(2)-DeltaH_(1)=DeltaC_(P)[T_(2)-T_(1)] or DeltaE_(2)-DeltaE_(1)=DeltaC_(v)(T_(2)-T_(1). Standard heat enthalpy of elements in their most stable state is assumed to be zero whereas standard heat enthalpy of compound is referred as heat of formation of that compound at 1 atm pressure and 25^(@)C . Oxidation of N_(2) to N_(2)O,NO,NO_(2) shows absorption of energy whereas heat of combustion of N_(2) is exothermic like other heat of combustion. Heat of combustion of carbon in diamond and amorphous form ar -94.3 and -97.6 kcal//mol . The heat required to convert 6g carbon from diamond to amorphous form is :

    A
    `-1.65kcal`
    B
    `+1.65kcal`
    C
    `-3.3kcal`
    D
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