During which change is heat energy released

During the uncoupling of electron transport chain energy that is generated by the transport of electron along with proton, is released as heat.

Explanation:

The proton motive force generate energy for the phosphorylation of ADP with inorganic phosphate to form ATP an energy rich molecule during the process called oxidative phosphorylation.

There are some chemical agents knowns as uncouplers for examole thermogennin, 2'4-dinitophenyl. They does not interfere with the transport of electrons through various electron carriers which generate energy, but uncouplers blocks the phosphorylation step.

As a result the energy that is released during transport of electron, is liberated as heat.