NEET Chemistry: First law of thermodynamics Practice Questions

Correct answer: C — The system released energy to the surroundings.

Work done *by* the system (W) is negative when the system expands against an external pressure (work is done *on* the system). If the question refers to the work done *by* the system being negative, it implies that the system has released energy to the surroundings (if considering the energy flow into the system), or the net change in internal energy calculation reflects energy leaving the system if we use ΔU = Q - W.

Correct answer: D — Heat and work are both path functions.

Heat (Q) and work (W) are path functions, meaning their values depend on the specific path taken between the initial and final states, whereas internal energy (ΔU) is a state function.

Correct answer: C — No energy exchange

An isolated system cannot exchange energy with its surroundings. Therefore, for an isolated system, ΔU = Q + W = 0 (if no boundary work is considered, or more accurately, changes are balanced internally).

Correct answer: C — 30 J

Using the First Law, ΔU = Q - W. Here, Q = +50 J (absorbed) and W = +20 J (work done by the system). ΔU = 50 J - 20 J = 30 J.

Correct answer: D — Enthalpy (H)

A state function is a property that depends only on the current state of the system, not the path taken. Enthalpy (H) is a state function, whereas heat (q) and work (w) are path functions.

Correct answer: C — No heat is exchanged with the surroundings.

An adiabatic process is defined as a process where no heat is exchanged with the surroundings. Therefore, Q = 0.

Correct answer: D — Positive

When a system expands, it performs work on the surroundings, meaning work done *by* the system is positive. For expansion against external pressure, W = -PΔV (work done *on* the system); thus, work done *by* the system is positive (W₍by) = PΔV).

Correct answer: B — Q is zero

In a process where no heat exchange occurs (Q=0), the First Law simplifies to ΔU = 0 + W, meaning ΔU = W. Since Q is the heat transferred, if no heat is exchanged, Q=0.

Correct answer: B — Internal Energy and Temperature

For a process occurring at constant temperature, the change in internal energy (ΔU) is related to the heat transfer and work done. Since ΔU depends on the heat supplied and the work done, and temperature is constant, the relationship simplifies, linking ΔU directly to the energy exchange during the process.

Correct answer: C — ΔU = 0

In a reversible isothermal process, the change in internal energy (ΔU) is equal to the heat exchanged (Q), as the work done is related by ΔU = Q - W, and for reversible processes, Qᵣₑᵥ = Wᵣₑᵥ. If the process is isothermal, ΔU = Q (since ΔS = Q/T and ΔU = Q - TΔS).

Correct answer: A — W = P(V₂ - V₁)

For a process occurring at constant pressure, the work done by the system is defined as W = PΔV = P(V₂ - V₁).

Correct answer: B — Internal energy

Internal energy (ΔU) is a state function, meaning its change depends only on the initial and final states of the system, not the path taken. Heat and work are path functions.

Correct answer: D — ΔU = Q + W

The First Law of Thermodynamics is stated as the conservation of energy, which is mathematically expressed as the change in internal energy (ΔU) being equal to the heat absorbed (Q) plus the work done on the system (W): ΔU = Q + W (or ΔU = Q - W, depending on the sign convention used for work).

Correct answer: C — First Law of Thermodynamics

The First Law of Thermodynamics states that energy is conserved. It is mathematically expressed as ΔU = Q + W, where ΔU is the change in internal energy, Q is the heat exchanged, and W is the work done by the system.

Correct answer: D — Change in temperature only

If the container is rigid, the volume does not change (ΔV = 0). Since internal energy depends on temperature and state variables, the change in internal energy is primarily related to the change in temperature, assuming the process is also isochoric.