"Hot!"
Toma put down the test tube.
Rei said quietly. "Exothermic reaction."
"Why does it get hot?" Kana asked.
"Difference in bond energy," Rei explained. "Products are more stable, so excess energy is released as heat."
"Excess?"
"Yes. Energy released when forming product bonds is greater than energy needed to break reactant bonds."
Toma calculated. "The difference becomes heat."
"Correct. This is enthalpy change, ΔH."
Kana wrote in her notebook. "ΔH < 0 for exothermic?"
"Yes. Energy exits the system."
Toma took out another test tube. "This one's cold."
"Endothermic reaction," Rei answered. "Absorbs energy. ΔH > 0."
"Why does it proceed? Wouldn't endothermic be disadvantageous?"
"Enthalpy alone doesn't decide," Rei continued. "Need to consider entropy too."
"Entropy?"
"Measure of disorder. Nature favors increasing disorder."
Kana was confused. "So what determines the reaction?"
"Free energy, ΔG," Rei wrote on the whiteboard. "ΔG = ΔH - TΔS"
"T?"
"Absolute temperature. S is entropy."
Toma understood. "If ΔG is negative, reaction proceeds."
"Correct. Overall evaluation of enthalpy and entropy."
Kana asked. "What about reactions in living organisms?"
"Most are endothermic," Rei answered. "But coupled with ATP hydrolysis."
"Coupled?"
"Combining exothermic and endothermic reactions. Making overall ΔG negative."
Toma gave an example. "Glucose phosphorylation?"
"Yes. Alone, ΔG is positive. But combined with ATP breakdown, becomes negative."
Kana made notes. "That's why ATP is needed."
"As energy currency, drives unfavorable reactions."
Rei continued. "Temperature is also important. ΔG = ΔH - TΔS, when T is large, entropy term matters."
"At high temperature?"
"Entropy increase becomes favorable. Reactions may proceed more easily."
Toma drew an enzyme diagram. "How do enzymes relate?"
"Lower activation energy," Rei explained. "Don't change ΔG, but increase reaction rate."
"Lower the mountain?" Kana metaphorized.
"Good analogy. Mountain height decreases, but elevation difference between peak and base stays same."
Toma understood. "So equilibrium doesn't change."
"Right. Just reaches equilibrium faster."
Kana looked outside. "Why is body temperature constant?"
"Utilizing heat from exothermic reactions," Rei answered. "Metabolic byproduct."
"Not waste."
"Rather necessary. Enzymes are temperature-sensitive, must maintain constant."
Toma summarized. "The story told by reaction heat?"
Rei smiled. "Energy flow. Where it comes from, where it goes."
"Conserved?"
"First law of thermodynamics. Energy is conserved. Only form changes."
Kana murmured. "But entropy increases."
"Second law of thermodynamics," Rei nodded. "Source of irreversibility."
"Arrow of time."
"Yes. From past to future, one-way."
The three fell silent. The test tube's heat speaks of universal laws. Energy changes form, entropy increases. That creates time's flow.