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The standard enthalpies of elements in their standard(natural) states are taken as zero. On basis of this statement, we can say that the enthalpy of formation of a compound :

1. Is always negative
2. Is always positive
3. May be positive or negative
4. Is never negative

Correct answer: May be positive or negative

Solution

The enthalpy of formation is the enthalpy change when 1 mole of a compound forms from its elements in their standard states. Because some formations release heat while others absorb it, the value can be either negative or positive.

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