Energy Balances on Closed Systems – CHE 252

Thirty practice flashcards covering key principles, equations, and example results from the CHE 252 lecture on energy balances for closed systems.

How is a system classified as open or closed?

By whether or not mass crosses the system boundary during the time interval considered.

What type of process system is inherently closed?

A batch process system.

Which types of systems are considered open?

Semi-batch and continuous process systems.

Write the general conservation balance equation.

Input + Generation − Output − Consumption = Accumulation.

Why do the generation and consumption terms disappear in an energy balance?

Because energy can neither be created nor destroyed.

After eliminating generation and consumption, what does the energy balance reduce to?

Accumulation = Input − Output.

For a closed system, what forms of energy can still cross the boundary?

Heat (Q) and work (W).

Express the first law for a closed system in differential form.

ΔU + ΔEk + ΔEp = Q − W.

Under what conditions can ΔU be taken as zero?

When there are no temperature or phase changes, no reactions, and pressure changes are small.

When is ΔEk equal to zero in an energy balance?

When the system is not accelerating.

When does ΔEp become zero?

When the system’s center of mass does not appreciably rise or fall.

What is an adiabatic process?

A process for which Q = 0 due to perfect insulation or equal temperature between system and surroundings.

Give two mechanical ways work can be done by a closed system.

Movement of a piston and rotation of a shaft through the boundary.

Name a non-mechanical way a closed system can do work.

Passage of an electrical current or radiation across the boundary.

When can W be set to zero in the energy balance?

If there are no moving parts, electrical currents, or radiation at the system boundary.

Convert 2.00 kcal of heat to joules.

2.00 kcal ≈ 8,368 J (8.37 kJ).

In Example 2.1, how much internal energy did the gas gain when heated from 25 °C to 100 °C at constant volume?

Approximately 8,370 J.

During the constant-temperature expansion in Example 2.1, how much work was done by the gas?

100 J.

How much additional heat was required during that constant-temperature expansion?

100 J.

State the simplified energy balance for the rigid-tank vaporization example.

ΔU = Q, because ΔKE = ΔPE = 0 and W = 0.

How much heat is required to vaporize 1 kg of water from 10 °C liquid to 100 °C steam at 1 atm?

2,471 kJ.

What specific volume of steam at 100 °C and 1 atm was used in the vessel example?

1.673 m³ / kg.

Why is W = 0 in the water-vaporization vessel example?

Because the tank boundary is fixed and no shaft, electrical, or other work crosses the boundary.

Write the expression for initial system energy in Equation 2.3.

Ui + Eki + Epi.

Write the expression for final system energy in Equation 2.3.

Uf + Ekf + Epf.

In the first-law notation, what does Q represent?

Heat transferred to the system from its surroundings.

In the first-law notation, what does W represent?

Work done by the system on its surroundings.

How does pressure affect the internal energy of an ideal gas?

Internal energy of an ideal gas is independent of pressure.

How does internal energy depend on pressure for liquids and solids?

It is nearly independent of pressure.

Which learning objective involves knowing when to neglect terms in the energy balance?

Stating the conditions under which each of the five terms in the balance can be neglected.