Chemistry 04 | Thermodynamics | JEE Main Practice
A reaction may be spontaneous at one temperature but not at another. In JEE Chemistry, you can often work out what happens without calculating a numerical value. Start by checking the signs of enthalpy change ((\Delta H)) and entropy change ((\Delta S)).
In this I WIN Learning lesson, the question is: Which combination of (\Delta H) and (\Delta S) makes a reaction spontaneous at all temperatures?
The formula to remember
Gibbs free energy connects enthalpy, entropy, and temperature:
[
\Delta G=\Delta H-T\Delta S
]
For a reaction to be spontaneous under the stated conditions, (\Delta G<0). Temperature (T) is measured in kelvin, so it is positive.
Work through the signs
Suppose (\Delta H<0) and (\Delta S>0). The enthalpy term is negative. Because (T) and (\Delta S) are positive, the term (-T\Delta S) is negative too.
Adding two negative terms keeps (\Delta G) negative at every positive temperature. That is why a reaction with negative (\Delta H) and positive (\Delta S) is spontaneous at all temperatures.
Other sign combinations can give different results. When both (\Delta H) and (\Delta S) are positive, spontaneity depends on temperature. The same is true when both are negative. If (\Delta H) is positive and (\Delta S) is negative, (\Delta G) remains positive.
A quick way to remember it
For an all-temperature spontaneity question, look for the combination where both parts of (\Delta G=\Delta H-T\Delta S) are negative:
Negative enthalpy change + positive entropy change = negative Gibbs free energy at every positive temperature.
Watch Chemistry 04: Spontaneity to follow the question as it is explained in the lesson.
Learn JEE Chemistry with I WIN Learning
I WIN Learning offers online JEE Main preparation for Class 11 and Class 12 students. The program includes live online classes, recorded lessons, weekly practice tests, learning portal access, expert doubt clarification, and weekly progress reports. A free trial is available.
Fee: ₹1,500 + GST per month
Call: 94919 53078
WhatsApp: 9133348691
Website: https://i-win.in
