Which of the following is an isolated system?

Enthalpy $(H)$ is equal to

$A$ $5 \, L$ cylinder contains $10 \, mol$ of $O_2$ gas at $27 \, ^\circ C$. Due to a leak,the gas escapes into the atmosphere until the pressure inside the cylinder equals the atmospheric pressure $(1.0 \, atm)$. What is the work done by the gas?

Which one of the following is an extensive property?

The total internal energy change for a reversible isothermal cycle is

For the reaction,$A_{(s)} + 3B_{(g)} \to 4C_{(s)} + D_{(l)}$,$\Delta H$ and $\Delta U$ are related as $\dots$