"Why won't it proceed!"
Toma shook the test tube.
"It is proceeding," Rei said calmly. "Just slowly."
"How slow?" Kana asked.
"At this rate, several hours."
Toma put down the test tube with dissatisfaction. "But reactants remain."
"At equilibrium, reactants and products coexist."
Kana wrote in her notebook. "Doesn't complete?"
"Depends on equilibrium constant. If K is large, nearly completes."
Rei wrote an equation on the whiteboard. "K = [products]/[reactants]"
"What's K for this reaction?" Toma asked.
"About 1. So becomes half and half."
"Then no more progress?"
"If we change conditions, equilibrium shifts."
Kana showed interest. "How?"
"Le Chatelier's principle," Rei explained. "When external force is applied to equilibrium system, equilibrium shifts to counteract it."
"Force?"
"Changes in concentration, temperature, pressure."
Toma took notes. "If we remove products?"
"Equilibrium shifts right. More products form."
"If we add reactants?"
"Also shifts right."
Kana asked. "What about temperature?"
"For exothermic reaction, lowering temperature shifts equilibrium right."
Rei drew a diagram. "But rate also decreases. Trade-off."
Toma pondered. "Why is rate slow?"
"Either activation energy is high or concentration is low."
"This reaction?"
"Activation energy is high. Needs catalyst."
Kana asked. "Adding catalyst changes equilibrium too?"
"No. Catalyst doesn't change equilibrium. Only increases rate."
"Why?"
"Accelerates both forward and reverse reactions. Just faster to reach equilibrium."
Toma understood. "Equilibrium position is determined by thermodynamics."
"Yes. Rate is kinetics problem."
Rei organized. "Thermodynamics is 'how far it proceeds,' kinetics is 'how fast it proceeds'."
Kana added to her notes. "Separate problems..."
"But both important."
Toma asked another question. "For reactions with multiple steps?"
"Slowest step determines overall rate. Rate-limiting step."
"Bottleneck?"
"Yes. No matter how fast others are, can't exceed rate-limiting step."
Kana requested example. "What about in vivo?"
Rei answered. "Control points in metabolic pathways. Regulating there controls whole."
"Efficient," Toma said.
"Life targets rate-limiting step for control."
Kana pondered. "Many reasons reactions don't proceed..."
"Equilibrium, activation energy, concentration, temperature, presence of catalyst..."
Toma stared at test tube. "Where is this reaction now?"
"Reached equilibrium. Won't proceed further."
"Then give up?"
Rei smiled. "Let's change conditions."
Rei prepared new test tube. "Precipitate the product."
When he added reagent, white precipitate appeared.
"Disappeared from solution!" Kana was surprised.
"Equilibrium shifts. More product forms."
Toma understood. "Le Chatelier's principle."
Indeed, solution color began changing.
"It's proceeding!"
Rei explained. "Equilibrium is dynamic. Always adjustable."
Kana closed her notebook. "Days when reactions don't proceed also have meaning."
"Opportunity to deepen understanding," Rei said.
Toma laughed. "Failure is learning chance."
"Science's essence," Rei nodded.
The three watched reaction proceeding in test tube. Slowly but surely.
"Understanding equilibrium lets us control it," Kana murmured.
Rei smiled. "That's chemistry's power."