The second law of thermodynamics decides, whether a given process allowed by first law, will, actually take place or not. For example,

1. Transfer of heat- According to the second law of thermodynamics, dS (entropy) ≥  0, i.e., the system cannot move towards the direction of decreasing entropy/probability. Hence, heat cannot flow by itself from a hotter body to a colder body.

2. Expansion of a gas- According to the second law of thermodynamics, dS  ≥ 0. As entropy cannot decrease, therefore the probability that all the molecules, on their own, go back into one half of the vessel is practically zero. Therefor, this process (of expansion of gas) is irreversible.

3. Diffusion of Gases- According to the second law of thermodynamics, dS ≥ 0. As entropy cannot decrease, therefore process of diffusion of gases is irreversible.

4. A refrigerator or a heat pump is also based on the second law of thermodynamics. Energy in the from of electric energy has to be supplied to enable it to pump heat from inside at lower temperature to the surroundings at higher temperature.