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Give the appropriate reason for the foll...

Give the appropriate reason for the followings:
a. It is a preferable to determine a change in enthalpy than change in internal energy.
b. It is necessary to define the 'standard state.
c. It is necessary to specify the phases of the reactant and products in a thermochemical equation.

Text Solution

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a. This is because of the reason that most of the possesses are carried in open containers i.e. at constant pressure. Since the volume can change, therefore the changes in heat energy are enthalpy changes `(DeltaH)` and not internal energy change `(DeltaE)`. Therefore, it is preferable to determine a change in enthalpy than change in internal energy.
b. Yes, standard state, specifies `P` and `T` condition. If it not mentioned than temperature and `P` conditions are not known.
c. Physical states of all species is important. and must be specified in a thermochemical equations as `DeltaH` depends on the phase of the substances. For two states of water in following two equations value of `Delta_(r)H` is different
`2H_(2)(g)+O_(2)(g) rarr 2H_(2)O(g), DeltaH=-483.7 kJ`
`2H_(2)(g)+O_(2)(g) rarr 2H_(2)O(l), Delta_(r)H=-571.1 kJ`
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  • Assertion : K_(p) can be less than, greater than or equal to K_(c) Reason : Relation between K_(p)andK_(c) depends on the change in number of moles of gaseous reactants and products (Deltan) .

    A
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    B
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    C
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    D
    If both assertion and reason are false.

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