SECTION 02: CHEMICAL REACTIONS AND ENZYMES AS CATALYSTS

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35 Terms

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🧪 What is ΔG?

  • ΔG = Gibbs Free Energy

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

  • Formula:
    👉 ΔG = ΔH – TΔS

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📘 What do the symbols mean?

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🧭 What does ΔG tell us?

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💡 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

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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).

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🧪 What is ΔG (Gibbs Free Energy)?

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🚀 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

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🔄 What is the Transition State?

  • A high-energy unstable state between reactants and products

  • Reaction must pass through this point

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First Graph (Exergonic Reaction)

  • Reactants have more energy than products

  • ΔG is negative → spontaneous

  • Activation energy still needed to start

  • Releases energy to surroundings

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Second Graph (Endergonic Reaction)

  • Products have more energy than reactants

  • ΔG is positivenot spontaneous

  • Needs continuous energy input

  • Often coupled with exergonic reactions in cells

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🧠 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

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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.

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🔥 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)

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🧪 What do enzymes do?

  • Enzymes are biological catalysts

  • They lower activation energy by:

    • Stabilizing the transition state

    • Using weak interactions at the active site

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📊 What does the image show?

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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.

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🧪 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)

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🔢 Step-by-Step (Image Numbered 1–4):

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📊 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

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🔑 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


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.

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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

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What is ATP?

  • ATP = energy carrier in cells

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

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📈 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

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🚧 Why are reactions slow?

  • The activation energy acts like a barrier

  • Without help, reactions happen too slowly to support life

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🧪 What do catalysts (like enzymes) do?

  • Lower activation energy

  • Speed up reactions without being used up

  • Make reactions happen faster and easier

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🧬 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

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🔑 Lock & Key Model:

  • Enzyme = Lock

  • Substrate = Key

  • Only the right substrate fits perfectly

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🌀 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)

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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).

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What is a catalyst?

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catalyst - 1. Increase rates of reaching chemical equilibrium

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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

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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

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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."