Thermodynamics, Kinetics, and Gas Laws

Hund’s rule

“Empty Seats First!”

Think of electrons on a bus—they don't like sitting together unless they have to!

Molar Enthalpy (ΔH)

“One Mole, One Goal—Track the Heat Toll!”

"Molar enthalpy counts the heat when a mole makes its move—whether melting, boiling, or bonding."

Energy in Bond Breaking and Forming

"Chemical changes trade energy—breaking costs, forming pays back."

“Break Takes, Form Frees!”

Exothermic Reactions — Bond Energies

If bond breaking energy is less than bond forming energy than the reaction is exothermic

Surrounding temperature rises due to excess energy being released as heat

Endothermic Reactions — Bond Energies

Endothermic reactions are energy thirsty—they sip heat from the surroundings to stay balanced.

heat energy is pulled in → surroundings cool down

Enthalpy Change (ΔH)

Represents the heat energy absorbed or released by a reaction at constant pressure

Positive vs. Negative ΔH

• Positive ΔH → Endothermic (absorbs heat from surroundings)

• Negative ΔH → Exothermic (releases heat to surroundings)

Change in Total Energy and Enthalpy

"Enthalpy Equals Energy—Especially in Easy Pressure."

Highlights that under easy (constant) pressure, enthalpy change mirrors energy change.

Hess’ Law

"Hess doesn't stress—he just adds the steps!"

With Hess’ Law, you just sum up the enthalpy changes of the individual steps to get the total.

Manipulating Reactions for Hess’ Law

"Flip the sign, scale the line." → If you reverse a reaction, flip the sign of ΔH. If you scale it, stretch ΔH the same way.

Hydrogen Bonding

Hydrogen bonding is a strong intermolecular force that occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like N, O, or F, is attracted to another electronegative atom nearby.

🌊 It’s responsible for many unusual properties of water, like high boiling point and surface tension.

Why do structures with greater intermolecular forces have higher boiling points?

Because stronger intermolecular forces require more energy to overcome, meaning more heat is needed for molecules to separate and enter the gas phase—resulting in a higher boiling point.

Types of Forces:

• Dispersion (London)

• Dipole-dipole

• Hydrogen bonding

• Ion-dipole

🌡 Boiling point increases with strength of intermolecular attraction.

What type of enzyme is a kinase?

Kinases are transferases that move phosphate groups

What is the equation for Michelis Menten and what does it represent? (Michaelis Mentens the speed)

it's about reaction velocity depending on substrate availability.

v = Vmax [S] / Km + [S]

What does the k_cat/K_m ratio represent in enzyme kinetics? (“Efficiency = Speed ÷ Affinity Cost”)

Reflects how efficiently an enzyme converts substrate to product at low substrate concentrations

What does k_cat measure in enzyme kinetics?

called the turnover number

• k_cat is the rate constant for the catalytic step of an enzyme reaction

• A higher k_cat means the enzyme is more efficient at converting substrate to product

What does K_m indicate in enzyme kinetics?

• K_m is the substrate concentration at which the enzyme operates at half its maximum speed

• It reflects the affinity of the enzyme for its substrate

What does a high and low K_m mean?

• Low K_m → high substrate affinity (enzyme binds substrate tightly)

• High K_m → low substrate affinity (enzyme requires more substrate to be effective)

What is the induced-fit model in enzyme catalysis?

• The induced-fit model proposes that an enzyme’s active site is flexible, not a rigid lock

• Upon substrate binding, the enzyme undergoes a conformational change that aligns catalytic groups for reaction

What is catalysis?

A process where a catalyst speeds up a chemical reaction by lowering activation energy

What does traditional Michaelis–Menten kinetics describe about initial velocity (V₀) and substrate concentration ([S])?

• More Substrate, More Speed—Then a Plateau Indeed”

• The initial velocity (V₀) of an enzyme-catalyzed reaction shows a hyperbolic dependence on substrate concentration ([S])

• As [S] increases, V₀ rises rapidly at first, then levels off as the enzyme becomes saturated

How does an uncompetitive inhibitor affect K_m and V_max in enzyme kinetics?

“U-turn for Both: Km & Vmax Down”

What is an uncompetitive inhibitor and how does it affect enzyme kinetics?

• An uncompetitive inhibitor binds only to the enzyme-substrate complex (ES)

• It forms an inactive enzyme-substrate-inhibitor (ESI) complex

• This prevents the enzyme from converting substrate into product

What are the key components of a Lineweaver–Burk plot in enzyme kinetics?

• Intercepts:

  • Y-intercept: 1/V_max

  • X-intercept: −1/K_m