SECTION 02: CHEMICAL REACTIONS AND ENZYMES AS CATALYSTS

🧪 What is ΔG?

• ΔG = Gibbs Free Energy

• Tells us if a reaction can happen on its own (spontaneous)

• Formula:
  👉 ΔG = ΔH – TΔS

📘 What do the symbols mean?

🧭 What does ΔG tell us?

💡 Study Tips:

• Free energy = energy available to do work

• Enthalpy = heat change

• Entropy = disorder

• Gibbs Free Energy brings all 3 together to predict if a reaction can happen

✨ Summary:

ΔG helps predict whether a reaction is spontaneous, non-spontaneous, or at equilibrium, based on changes in heat (ΔH), disorder (ΔS), and temperature (T).

🧪 What is ΔG (Gibbs Free Energy)?

🚀 What is Activation Energy?

• The initial energy needed to start a reaction

• Required to reach the transition state

• 🔺 Shown as the height from reactants to the peak in the graph

🔄 What is the Transition State?

• A high-energy unstable state between reactants and products

• Reaction must pass through this point

First Graph (Exergonic Reaction)

• Reactants have more energy than products

• ΔG is negative → spontaneous

• Activation energy still needed to start

• Releases energy to surroundings

Second Graph (Endergonic Reaction)

• 

• Products have more energy than reactants

• ΔG is positive → not spontaneous

• Needs continuous energy input

• Often coupled with exergonic reactions in cells

🧠 BONUS TIPS:

• Exergonic = energy exits system → spontaneous

• Endergonic = energy enters system → non-spontaneous

• Don't confuse with exothermic/endothermic, which refer to heat (ΔH), not free energy

✨ Summary:

ΔG shows whether a reaction happens on its own. Exergonic = spontaneous, Endergonic = needs help. Reactions still require activation energy to reach the transition state, even if spontaneous.

🔥 What is a catalyst?

• A substance that speeds up a reaction

• It lowers the activation energy needed to start the reaction

• ❗ It does not change ΔG (reaction spontaneity stays the same)

🧪 What do enzymes do?

• Enzymes are biological catalysts

• They lower activation energy by:

  • Stabilizing the transition state

  • Using weak interactions at the active site

📊 What does the image show?

✅ Key Points:

• Catalysts speed up reactions, but don’t change their final outcome

• Enzymes work by making it easier to reach the transition state

• ΔG stays the same with or without a catalyst

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✨ Summary:

Catalysts (like enzymes) make reactions faster by lowering activation energy, not by changing the spontaneity (ΔG). This allows reactions to happen more easily, especially in biological systems.

🧪 Key Terms:

• E = Enzyme

• S = Substrate

• ES = Enzyme-Substrate Complex

• EP = Enzyme-Product Complex

• P = Product

• k₁, k₋₁, k₂ = Rate constants (speed of forward/reverse steps)

🔢 Step-by-Step (Image Numbered 1–4):

📊 Image Highlights:

• Solid curve = enzyme-catalyzed reaction

• Dotted curve = uncatalyzed (no enzyme) = higher peak = more energy needed

• Transition state (ES‡) is the highest point = energy "hill"

• Enzyme lowers the height of the hill = faster, easier reaction

🔑 Important Notes:

• Enzymes don’t change ΔG (overall energy difference)

• They only lower activation energy

• Each rate constant (k₁, k₋₁, k₂) controls how fast each step happens

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✨ Summary:

Enzymes make reactions faster by lowering activation energy. The reaction has multiple steps: binding, catalysis, product formation, and release. Enzymes do not change the final energy output (ΔG), just how easy it is to get there.

⚡ Why do reactions need energy?

• Even spontaneous (exergonic) reactions need an initial push of energy → this is called activation energy (Ea)

• On early Earth, energy came from lightning, heat, sunlight

• Today, we get energy from food, but it must be converted into something usable—like ATP

⚙ What is ATP?

• ATP = energy carrier in cells

• When ATP is broken, it releases energy that fuels reactions in the body

📈 What is the transition state?

• A temporary high-energy point during a reaction

• Old bonds are breaking, new ones are forming

• It only lasts a fraction of a second

🚧 Why are reactions slow?

• The activation energy acts like a barrier

• Without help, reactions happen too slowly to support life

🧪 What do catalysts (like enzymes) do?

• Lower activation energy

• Speed up reactions without being used up

• Make reactions happen faster and easier

🧬 How do enzymes work?

• Enzymes = biological catalysts

• Each enzyme has an active site shaped for its specific substrate

• When the enzyme binds the substrate:

  • It brings atoms close together

  • Stabilizes the transition state

  • Helps break old bonds and form new ones

  • Product is released → enzyme is reused

🔑 Lock & Key Model:

• Enzyme = Lock

• Substrate = Key

• Only the right substrate fits perfectly

🌀 Induced Fit Model:

• The enzyme slightly changes shape when the substrate binds

• This tightens the fit and stretches the substrate’s bonds

• Makes it easier to break and form bonds (lowering activation energy even more)

✅ Summary:

Enzymes speed up reactions by lowering activation energy. They bind specific substrates at the active site, stabilize the transition state, and help the reaction happen faster. Two models explain this: Lock & Key (perfect fit) and Induced Fit (flexible fit).

❓ What is a catalyst?

catalyst - 1. Increase rates of reaching chemical equilibrium

catalyst - 2. Selectively increase certain products

✔ True

• Catalysts (especially enzymes) can be very specific

• They can favor certain reaction pathways, making more of a desired product

catalyst - 3. Decompose undesirable products

✔ True

• Catalysts can help break down harmful or unwanted compounds

• Example: Catalase breaks down hydrogen peroxide (a toxic by-product in cells) into water and oxygen

✨ Summary:

Catalysts are powerful tools—they speed up reactions, can favor useful products, and even help break down unwanted substances. That’s why the correct answer is "All of the above."