Chemistry · Thermodynamics · NEET
An adiabatic wall does not let heat pass. So the system cannot exchange heat with the surroundings and q = 0. A diathermic wall (also called a thermally conducting wall) lets heat pass freely. So heat can flow until both sides reach the same temperature. In short: adiabatic = heat blocked, diathermic = heat allowed. This matters for NEET because many numerical questions say 'insulated' or 'in contact', and that one word decides whether q is zero.
No, an adiabatic wall does NOT allow heat to pass. Because no heat can enter or leave, the heat exchanged q = 0. This is the key link students miss. Whenever NEET says 'well insulated', 'thermally insulated', or 'adiabatic', you should immediately write q = 0. Then the first law becomes ΔU = q + w = w.
Yes. 'Well insulated' means the walls block heat, which is exactly what an adiabatic wall does. So a well-insulated container is an adiabatic system and q = 0. NEET 2017 used this exact wording: 'A gas is allowed to expand in a well insulated container...' and the trick was to set q = 0, so ΔU = w.
Two different questions. Across an adiabatic wall, the two sides can stay at different temperatures forever, because heat cannot flow to equalise them. Inside the system, the temperature CAN still change if work is done (like adiabatic compression heats a gas, adiabatic expansion cools it). So 'adiabatic' does not mean 'temperature is fixed'; it only means 'no heat crosses the wall'.
Not permanently. Because a diathermic wall conducts heat, heat flows from the hot side to the cold side until both sides reach the same temperature (thermal equilibrium). At that point there is no more heat flow. So a diathermic wall forces both sides to the SAME final temperature, while an adiabatic wall lets them stay different.
An adiabatic wall only stops HEAT. It can still allow work (like a moving piston). An isolated system exchanges NEITHER heat NOR matter NOR (net) energy with the surroundings. So 'adiabatic' is about the wall blocking heat only, while 'isolated' is a stronger condition for the whole system. All isolated systems have adiabatic walls, but not every adiabatic system is isolated (work can still be done).
Because that single word sets q = 0 and simplifies the first law to ΔU = w. If you miss it, you may wrongly assume heat is exchanged and get the wrong ΔU. For example, in adiabatic free expansion (NEET 2020): the wall is adiabatic so q = 0, the gas expands into vacuum so w = 0, therefore ΔU = 0 and for an ideal gas ΔT = 0.
The correct option for free expansion of an ideal gas under adiabatic condition is:
A gas is allowed to expand in a well insulated container against a constant external pressure of 2.5 atm from an initial volume of 2.50 L to a final volume of 4.50 L. The change in internal energy ΔU of the gas in joules will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Adiabatic means no HEAT crosses the wall (q = 0). The temperature can still change if work is done, such as an adiabatic compression heating a gas or an adiabatic expansion cooling it.
Yes. 'Diathermic' and 'thermally conducting' mean the same thing: a wall that lets heat pass through, so both sides can reach the same temperature.
Immediately write q = 0. Then use ΔU = q + w = w. Decide w from the process (for example, w = 0 in free expansion into vacuum).
A diathermic (conducting) wall. It lets heat flow from hot to cold until both sides reach the same temperature. An adiabatic wall never lets them equalise.
No. An adiabatic wall only blocks heat; work can still be done (a piston can move). An isolated system exchanges no heat, no work-driven energy, and no matter with the surroundings.
