Which of the following substances has a standard heat of formation δh°f of zero? choose one or more: a. pb at 1000°c b. c3h8 (g) at 25.0°c and 1 atm pressure c. solid glucose at room temperature d. n2(g) at 25.0°c and 1 atm pressure

Answer : The correct answer is option D.  N₂(g) at 25.0°C and 1 atm pressure

Explanation :

Standard heat of formation defined as the change in heat when one mole of a compound is formed from its elements in standard state.

Therefore all the substances which are compounds will have some value of heat of formation.

On the other hand, a substance will have zero heat of formation when it is present in elemental form and in its standard state at STP

If we take a look at the given options, we can see that Pb and N₂ exist in elemental forms.

But we have Pb at 1000° C. Melting point of Pb is around 620° C. Above this temperature, Pb will be present in liquid state which is not its standard state.    (standard state of Pb is solid as it is a metal)

Therefore the only substance that is present in elemental form and in its standard state is N₂.

The correct option is N₂ (g) at 25°C and 1 atm

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