(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 ?

Explanation

(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.