Thermodynamics

Grade 11 · Science · Study Guide

Explore the laws governing energy transfer, heat, work, and why some reactions happen spontaneously while others require an energy push.

Thermodynamics is the study of energy and its transformations. It answers fundamental questions: where does energy go in a reaction? Why do some processes occur spontaneously while others need input? The field is built on three laws that govern the universe's energy budget.

The First Law of Thermodynamics is the law of energy conservation: energy cannot be created or destroyed, only converted from one form to another. Enthalpy (H) measures heat flow at constant pressure — the conditions of most lab reactions. An exothermic reaction releases heat (ΔH < 0). An endothermic reaction absorbs heat (ΔH > 0). Hess's Law allows calculation of ΔH for a reaction by summing enthalpies of individual steps.

The Second Law states that the total entropy of an isolated system always increases in a spontaneous process. Entropy (S) is a measure of disorder or the number of possible microstates. Gases have higher entropy than liquids, which have higher entropy than solids. The Second Law explains why heat flows from hot to cold and why perpetual motion machines are impossible.

Gibbs Free Energy (G) combines enthalpy and entropy into one criterion for spontaneity: ΔG = ΔH − TΔS. If ΔG < 0, the process is spontaneous. If ΔG > 0, it is nonspontaneous. If ΔG = 0, the system is at equilibrium. Temperature plays a critical role — at high temperatures TΔS dominates; at low temperatures ΔH dominates.

The Third Law states that the entropy of a perfect crystalline substance at absolute zero (0 K) is exactly zero, providing a reference point for all entropy calculations.

Key terms

Enthalpy (H)

Heat content of a system at constant pressure; ΔH < 0 for exothermic, ΔH > 0 for endothermic.

Entropy (S)

Measure of disorder or randomness; increases when states become more dispersed.

Gibbs Free Energy (G)

ΔG = ΔH − TΔS; determines spontaneity: ΔG < 0 = spontaneous, ΔG > 0 = nonspontaneous.

First Law of Thermodynamics

Energy is conserved; the energy of the universe is constant.

Second Law of Thermodynamics

The total entropy of an isolated system always increases in a spontaneous process.

Exothermic

Reaction that releases heat to surroundings (ΔH < 0); products have lower enthalpy than reactants.

Endothermic

Reaction that absorbs heat from surroundings (ΔH > 0); products have higher enthalpy than reactants.

Hess's Law

Enthalpy change of a reaction is the same regardless of pathway; ΔH values can be summed algebraically.

Practice questions

1. A reaction has ΔH = −50 kJ and ΔS = +100 J/K. Is this reaction spontaneous at 298 K?

   1. No, ΔG > 0
   2. Yes, ΔG < 0 at all temperatures
   3. Only at high temperatures
   4. Only at low temperatures
   Show answer

   With ΔH < 0 and ΔS > 0, ΔG = ΔH − TΔS is always negative — spontaneous at any temperature.

2. Which process has the greatest increase in entropy?

   1. Water freezing
   2. Ice melting
   3. Water boiling
   4. Steam condensing
   Show answer

   Liquid → gas transition produces the largest entropy increase; gas molecules have far more possible arrangements.

3. A reaction that releases heat to surroundings and has a negative ΔH is called ______.

   Show answer

   Exothermic reactions release energy as heat (ΔH < 0); products have lower enthalpy than reactants.

4. Hess's Law is based on which fundamental principle?

   1. Conservation of mass
   2. Conservation of energy
   3. Conservation of momentum
   4. Le Chatelier's principle
   Show answer

   Enthalpy is a state function — total ΔH depends only on initial and final states, not the pathway.

5. At what condition is ΔG = 0?

   1. When ΔH = 0
   2. When ΔS = 0
   3. When the system is at equilibrium
   4. When temperature is 0 K
   Show answer

   ΔG = 0 means no driving force to change — forward and reverse processes are equally favored (equilibrium).

Fun facts

• Clausius summarized all thermodynamics in two sentences: 'The energy of the universe is constant. The entropy of the universe tends to a maximum.' These two lines govern every physical and chemical process.
• Your body is a thermodynamic engine — cellular respiration releases about 2,800 kJ/mol of glucose, but your body is only ~40% efficient at converting that to useful work. The rest is released as body heat.
• Gibbs free energy explains why ice melts at exactly 0°C: at that temperature ΔG = 0 for the solid-liquid transition — the two phases are in perfect equilibrium.

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