Question about thermodynamics The Second Law of thermodynamics allows work to be transformed fully into heat, but forbids heat to be totally converted into work. If heat could be transformed fully into work it would violate the laws of entropy.
What are the laws of entropy that would be violated if heat could be fully transferred into work?
 A: Heat can be totally converted to work in a process. That does not violate the second law. An example is the isothermal expansion of an ideal gas. But it is not possible to have a thermodynamic cycle whose only effect is to completely convert heat to work. That would be a violation of the Kelvin-Plank statement of the second law, which states:
“It is impossible to construct a device which operates on a cycle and produces no other effect than the production of work and the transfer of heat from a single body”.
Hope this helps.
A: Consider a Carnot cycle heat engine between a hot reservoir with temperature $T_H$ and a cold reservoir with temperature $T_C$. The thermal efficiency of the engine is
$$\eta_{\text{Carnot}}=\frac{W}{Q_H}=1-\frac{T_C}{T_L}$$
You are considering the case where the net work output is $W=Q_H$. This implies
$$
1=1-\frac{T_C}{T_L}
$$
or
$$
\frac{T_C}{T_L}=0
$$
This is only possible in the limit of $T_C\to 0$ or $T_H \to \infty$, neither of which can be achieved in real life. The Kelvin-Planck statement, mentioned by Bob D, encapsulates this experimental fact. In theory in the limiting case, a Carnot engine would convert the heat perfectly to work.
