Question

$\Delta H$ is positive and $\Delta S$ is positive then the reaction is:

• A. spontaneous only at low temperature
• B. spontaneous at all temperature
• C. spontaneous only at high temperature
• D. non-spontaneous at high temperature

Answer 1

The correct option is C spontaneous only at high temperature

Sign of $\Delta H$ Sign of $\Delta S$ Comment Example $\Delta H_{298}^o$ $\Delta S_{298}^o$

$-$ $+$ spontaneous $H_2\left(g\right)+C{l}_2\left(g\right)\to 2HCl\left(g\right)$ $-185$ $14.1$

at all temperature $C\left(s\right)+O_2\left(g\right)\to C{O}_2\left(g\right)$ $-394$ $3$

$-$ $-$ spontaneous $H_2\left(g\right)+1/2O_2\to H_{2}O\left(l\right)$ $-44$ $-119$

at low temperature $2S{O}_2\left(g\right)+O_2\left(g\right)\to 2S{O}_3\left(g\right)$ $-198$ $-187$

$+$ $+$ spontaneous $N{H}_{4}Cl\left(s\right)\to N{H}_3\left(g\right)+HCl\left(g\right)$ $176$ $284$

at high temperature $N_2\left(g\right)+O_2\left(g\right)\to 2NO\left(g\right)$ $180$ $25$

$+$ $-$ Non spontaneous $3O_2\to 2O_3$ $286$ $-137$

at all temperature $2H_{2}O\left(l\right)+O_2\left(g\right)\to 2H_2{O}_2\left(l\right)$ $196$ $-126$

The reaction is spontaneous at high temperatures.

Option C is correct.