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For the reaction: \( 2 H I \rightarrow H_{2}+I_{2} \). the expression \( \frac{-1}{2} \frac{d(H I)}{d t} \) represents:

1. the rate of formation of \( H I \)
2. the rate of disappearance of \( H I \)
3. the instantaneous rate of the reaction
4. the average rate of reaction

Correct answer: the rate of disappearance of \( H I \)

Solution

In a reaction rate expression, a negative sign is used for reactants because their concentration decreases with time. The factor of 1/2 comes from the stoichiometric coefficient in 2 HI → H2 + I2, so this term gives the rate at which HI disappears.

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