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NEET levelSingle correct optionDifficulty 1 of 3td115

Which of the following has a non-zero standard enthalpy of formation at $298 K$ ?

A $O X_{2} (g)$
B $C$ (diamond)
C $Br X_{2} (l)$
D $C$ (graphite)

Answer: B

By convention the standard enthalpy of formation of an element in its most stable form at

298 K

and

1

bar is zero.

O X_{2} (g)

,

Br X_{2} (l)

and graphite are reference forms; diamond is not, and its

Δ_{f} H^{\circ}

is about

+ 1.9 kJ mo l^{- 1}

.

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