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B.1.6—Internal energy

Syllabus
First assessment 2025
Objective
—
Level
HL

Define Internal Energy

Internal energy

The internal energy of a system is the sum of:

  • random molecular kinetic energy; and
  • intermolecular potential energy associated with forces between particles.

Temperature is only one part

For a fixed phase and amount of substance, raising temperature usually increases the particles’ average random kinetic energy. During a phase change, temperature can stay constant while intermolecular potential energy changes.

Do not equate heat with internal energy

Internal energy is a state property of the system. Thermal energy transfer is energy crossing the system boundary because of a temperature difference.

B.1.6 Exam Analysis

Assessment in practice

1–2 marks
How it is assessed

The evidence uses a two-mark comparison of ice and liquid water during coexistence and a multiple-choice phase-change question about internal energy and intermolecular potential energy.

Command terms

Compare / Explain / State

What earns marks

Split internal energy into random molecular kinetic energy plus intermolecular potential energy. During a phase change at constant temperature, compare the kinetic-energy term first; then explain the difference through intermolecular potential energy. For equal-mass water and ice at 0 °C, liquid water has greater internal energy because its intermolecular potential energy is greater while average kinetic energy is the same.

Watch for

Assuming constant temperature means constant internal energy, or claiming that all transferred energy increases molecular kinetic energy during a phase change.

Representative question

Question 1

[Maximum number: 2]

Between 4 minutes and 64 minutes solid ice and liquid water coexist at 0∘C0^{\circ} \mathrm{C}. Compare and contrast, during this time, the internal energy of solid ice to that of an equal mass of liquid water.