Question

A spontaneous reaction is impossible if

• A. both $\Delta H$ and $\Delta S$ are negative
• B. both $\Delta H$ and $\Delta S$ are positive
• C. $\Delta H$ is negative and $\Delta S$ is positive
• D. $\Delta H$ is positive and $\Delta S$ is negative

Answer 1

Answer: (D)

$\Delta H$ is positive and $\Delta S$ is negative

Answer 2

A spontaneous reaction is impossible if $\Delta H$ is positive and $\Delta S$ is negative as it will lead to positive value of $\Delta$ G. For a reaction to be non spontaneous, $\Delta$ G should be positive . When $\Delta H$ is positive and $\Delta S$ is negative $\Delta G =\Delta H - T \Delta S$ $\Delta G =$ positive $- T \times$ negative $\Delta G =$ positive + positive $\Delta G =$ positive