A cup of coffee is so hot that it cannot be drunk. As a result, an ice cube is put into the coffee. What direction is the energy moving? When would the energy flow stop?

The energy will be flowing from hot coffee to ice cube. The energy flow will stop when both temperatures of coffee and ice cube will become equal.

Explanation:

The second law of thermodynamics deals with the direction in which spontaneous processes go. Many processes run spontaneously in one direction, that is, they are irreversible under certain conditions. Although irreversibility is seen in everyday life, for example, a broken glass does not return to its original state, complete irreversibility is a statistical statement that cannot be seen during the life of the universe. More specifically, an irreversible process is a process that depends on the route. If the process can only go in one direction, the reverse path is fundamentally different and the process can't be reversible. For example, heat always involves the transfer of energy from a higher temperature to a lower temperature. A cold object that comes in contact with a hot object never cools, transfers heat to the hot object, and heats it up.