Thermodynamics and Entropy
Grade 12 · Science · Study Guide
Understand the laws governing energy transfer, heat, work, and the directionality of natural processes.
Thermodynamics is the branch of physics that studies energy and its transformations, particularly the relationships between heat, work, and the internal energy of systems. The four laws of thermodynamics describe fundamental constraints on any physical or chemical process. The Zeroth Law defines thermal equilibrium: if two systems are each in thermal equilibrium with a third, they are in equilibrium with each other. The First Law is the law of energy conservation: energy cannot be created or destroyed, only transferred or converted.
The Second Law introduces a crucial concept: entropy (S), a measure of disorder or the number of possible microstates of a system. The Second Law states that in any spontaneous process, the total entropy of the universe increases. This gives time a direction — it explains why heat flows from hot to cold, why gases expand to fill their containers, and why perpetual motion machines are impossible. A process is spontaneous if the Gibbs free energy change (ΔG = ΔH − TΔS) is negative.
The Third Law states that the entropy of a perfect crystalline substance at absolute zero (0 K) is zero. This is a theoretical floor — real substances approach but never quite reach absolute zero. Understanding thermodynamics is critical in engineering, chemistry, and even biology: life itself maintains low entropy locally by increasing entropy in its surroundings, essentially paying an entropy tax to stay organized.
| ΔH | ΔS | Spontaneous? |
|---|---|---|
| Negative (exothermic) | Positive (more disorder) | Always spontaneous |
| Negative (exothermic) | Negative (less disorder) | Spontaneous at low T |
| Positive (endothermic) | Positive (more disorder) | Spontaneous at high T |
| Positive (endothermic) | Negative (less disorder) | Never spontaneous |
Key terms
- Entropy (S)
- A measure of the disorder or randomness of a system; increases in spontaneous processes.
- First Law of Thermodynamics
- Energy is conserved: ΔU = Q − W (change in internal energy = heat added minus work done by system).
- Second Law of Thermodynamics
- The total entropy of an isolated system always increases in any spontaneous process.
- Gibbs Free Energy (G)
- G = H − TS; a negative ΔG indicates a spontaneous process at constant temperature and pressure.
- Enthalpy (H)
- A measure of total heat content of a system at constant pressure; ΔH < 0 for exothermic reactions.
- Thermal equilibrium
- The state where two objects in contact no longer exchange net heat because they are at the same temperature.
- Absolute zero
- 0 Kelvin (−273.15°C); the theoretical minimum temperature where molecular motion stops.
Flashcards
- First Law of Thermodynamics
- Energy is conserved: ΔU = Q − W
- Second Law of Thermodynamics
- Entropy of the universe increases in all spontaneous processes
- Gibbs Free Energy
- ΔG = ΔH − TΔS; spontaneous when ΔG < 0
- Entropy
- Measure of disorder/randomness; symbol S; unit J/K
- Enthalpy
- Total heat content at constant pressure; exothermic = ΔH < 0
- Third Law
- Entropy of a perfect crystal at 0 K equals zero
Practice questions
Which law of thermodynamics states that energy cannot be created or destroyed?
- Zeroth Law
- First Law
- Second Law
- Third Law
Show answer
Answer: First Law
The First Law of Thermodynamics is the law of conservation of energy applied to thermodynamic systems: ΔU = Q − W.
A process with a negative ΔG is non-spontaneous.
Show answer
Answer: False
A negative ΔG means the process is spontaneous (will proceed without continuous energy input). Positive ΔG means non-spontaneous.
The entropy of a perfect crystal at absolute zero is ___ according to the Third Law.
Show answer
Answer: zero
The Third Law provides an absolute reference point for entropy. A perfect crystal at 0 K has perfectly ordered arrangement and therefore zero entropy.
Which combination of ΔH and ΔS always produces a spontaneous reaction regardless of temperature?
- ΔH > 0, ΔS > 0
- ΔH < 0, ΔS < 0
- ΔH < 0, ΔS > 0
- ΔH > 0, ΔS < 0
Show answer
Answer: ΔH < 0, ΔS > 0
When ΔH is negative and ΔS is positive, ΔG = ΔH − TΔS is always negative regardless of temperature, making the process always spontaneous.
In any real process, the total entropy of the universe decreases.
Show answer
Answer: False
The Second Law states the total entropy of the universe increases in all real (irreversible) processes. It never decreases for isolated systems.
Fun facts
- The Second Law explains why you can't unscramble an egg — the increase in entropy is essentially irreversible without a massive energy input.
- Refrigerators don't violate the Second Law — they move heat from cold to hot using work, but the total entropy of the universe still increases.
- Black holes are thought to have enormous entropy, proportional to their surface area, not their volume.
Get a personalized course on Thermodynamics and Entropy built around your child.