What are the Laws of Thermodynamics?
The four laws of thermodynamics are fundamental principles that govern energy transfer, heat flow, and entropy in physical and chemical systems. They explain why certain processes spontaneously occur while others don't, and form the foundation of chemistry, physics, and engineering.
Law 0: systems in thermal contact reach equilibrium. Law 1: energy is conserved (ΔU = q + w). Law 2: entropy of isolated systems increases (ΔS_univ ≥ 0). Law 3: absolute zero (0 K) is unattainable. Together, they govern all transformations.
- 1↓Law 0: Thermal EquilibriumTwo systems at different temperatures exchange heat until they reach the same temperature.
- 2↓Law 1: Conservation of EnergyEnergy cannot be created or destroyed; ΔU = q + w (change in internal energy = heat + work).
- 3↓Law 2: Entropy IncreasesEntropy of an isolated system always increases; spontaneous processes have ΔS_univ > 0.
- 4Law 3: Absolute Zero UnattainableThe entropy of a perfect crystal at absolute zero (0 K) is zero; this temperature is unreachable.
Step-by-step worked examples
A hot cup of tea cools down on a table. Which law does this obey?
Tea starts hot (high energy), table is cool (low energy). Heat flows from tea → surroundings (Law 1: ΔU_tea < 0). The system + surroundings reaches thermal equilibrium (Law 0). Entropy increases: disorder of energy spreads (Law 2: ΔS_univ > 0). Spontaneous at room temperature (ΔG < 0).
Why is it impossible to cool a room by leaving a refrigerator door open?
A refrigerator takes heat from inside, uses energy (work), and releases heat outside. If the door is open, heat escapes the system. The work input exceeds the cooling (Law 1: work > heat removed from room). The room warms because: heat from outside + heat from fridge motor > cooling effect. This violates the desired outcome but not the laws.
Can we reach absolute zero (0 K)?
Law 3 (Third Law) states that absolute zero is unattainable. Reason: at 0 K, all motion stops; entropy = 0 (perfect order). To reach 0 K requires infinite energy removal (impossible). Closest labs have reached ~10⁻¹² K using laser cooling. But 0 K itself remains theoretically unreachable.
Flashcards
Quick quiz
Q1.In a calorimeter, a reaction releases 50 J of heat. The surroundings absorb this heat. What is ΔU?
Q2.A spontaneous process has ΔS_univ equal to…
Q3.Why does ice melt at room temperature?
Q4.Which law explains why energy cannot be destroyed in a reaction?
The full card deck, worked steps and AI-tutor support for “What are the Laws of Thermodynamics?” are in Notek — study by hand before your exam.
Common mistakes
Confusing heat with temperature. — Correct: Temperature is average kinetic energy. Heat is energy transfer due to temperature difference.
Thinking the Second Law means disorder always increases in a system. — Correct: The Second Law applies to isolated systems. A system can decrease in entropy if surroundings entropy increases more.
Assuming all endothermic reactions are nonspontaneous. — Correct: An endothermic reaction is spontaneous if ΔS_univ > 0 (entropy gain compensates).
Believing we can reach absolute zero. — Correct: The Third Law states absolute zero is unattainable. Labs have approached it but never reached it.
FAQ
What is internal energy (U)?
The sum of all kinetic and potential energies in a system. ΔU = q + w (heat + work).
How do the four laws relate to each other?
Law 0 defines temperature and equilibrium. Law 1 conserves energy. Law 2 determines spontaneity via entropy. Law 3 sets the entropy baseline at 0 K.
Can a system decrease in entropy?
Yes—but only if the surroundings increase in entropy MORE. ΔS_system can be negative if ΔS_surr is more positive (ΔS_univ > 0).
Why do chemical reactions tend to go forward and not reverse?
Because the Second Law favors increased entropy. Forward reactions typically increase total disorder (ΔS_univ > 0), making them spontaneous.