NEETChemistryThermodynamics (C)
Consider the dimerization of nitrogen dioxide gas into dinitrogen tetroxide gas: 2 NO _ 2( g ) N ₂ O _ 4( g ) Which of the following correctly describes the signs of enthalpy change ( H ) and entropy change ( S ) for this process, and the temperature condition for its spontaneity?
Options
- AH < 0 , S < 0 ; spontaneous at high temperatures
- BH > 0 , S > 0 ; spontaneous at high temperatures
- CH < 0 , S < 0 ; spontaneous at low temperatures
- DH 0 ; spontaneous at all temperatures
Correct answer
C. H < 0 , S < 0 ; spontaneous at low temperatures
Step-by-step solution
In the dimerization process, 2 NO _ 2( g ) N ₂ O _ 4( g ) , a new N-N bond is formed without breaking any existing bonds. Bond formation releases energy, so the process is exothermic ( H The number of gaseous moles decreases from 2 to 1 ( n_g = -1 ), which means the randomness of the system decreases, so S According to the Gibbs free energy equation, G = H - T S . For the reaction to be spontaneous, G must be negative. Since both H and S are negative, G will be negative only when the magnitude of H is greater than