Chapter 5 Flashcards by Yashameet brar

What is a reaction mechanism?

The step‑by‑step sequence of elementary reactions that converts reactants to products.

How well did you know this?

Not at all

Perfectly

Define intermediate in kinetics.

A species that is produced in one elementary step and consumed in a later step; does not appear in the overall balanced equation.

How well did you know this?

Not at all

Perfectly

What is the rate-determining step (RDS)?

The slowest elementary step of a mechanism; it controls the overall reaction rate.

How well did you know this?

Not at all

Perfectly

According to collision theory, what is reaction rate proportional to?

The number of effective collisions per second between reactant molecules.

How well did you know this?

Not at all

Perfectly

What are the two requirements for an effective collision?

Correct orientation
Sufficient kinetic energy to reach or exceed the activation energy.

How well did you know this?

Not at all

Perfectly

What is the collision-theory rate equation?

rate = Zf where Z = total collisions/s and f = fraction that are effective.

How well did you know this?

Not at all

Perfectly

What is the Arrhenius equation?

k = Ae^(-Ea/RT)

How well did you know this?

Not at all

Perfectly

Define the variables in the Arrhenius equation.

k = rate constant
A = frequency (attempt) factor
Ea = activation energy
R = ideal gas constant
T = temperature in kelvin.

How well did you know this?

Not at all

Perfectly

How does a lower Ea affect k in the Arrhenius equation?

The exponent becomes less negative, leading to an increase in k.

How well did you know this?

Not at all

Perfectly

What is the effect of increasing temperature on k according to the Arrhenius equation?

The exponent becomes less negative, resulting in an increase in k; roughly, the rate doubles for every 10 °C increase in many biological systems.

How well did you know this?

Not at all

Perfectly

What causes the frequency factor A to increase?

When the concentration of molecules increases, leading to more possible collisions.

How well did you know this?

Not at all

Perfectly

Define transition state theory.

Reactants form a high‑energy activated complex that can proceed to products or revert to reactants; located at the peak of the reaction‑coordinate diagram.

How well did you know this?

Not at all

Perfectly

What is the relative energy of the transition state compared to reactants and products?

The transition state is the highest of all and cannot be isolated.

How well did you know this?

Not at all

Perfectly

What do the signs of the free-energy change of reaction (ΔGrxn) indicate?

Negative = exergonic (energy released)
Positive = endergonic (energy absorbed).

How well did you know this?

Not at all

Perfectly

What is a catalyst?

A substance that increases reaction rate by lowering Ea; not consumed overall and has no effect on ΔGrxn or equilibrium position.

How well did you know this?

Not at all

Perfectly

Differentiate between heterogeneous and homogeneous catalysis.

Heterogeneous catalysis involves a catalyst in a different phase from reactants, while homogeneous catalysis involves the same phase.

How well did you know this?

Not at all

Perfectly

List four main factors affecting reaction rate.

Reactant concentration
Temperature
Medium/solvent
Catalysts.

How well did you know this?

Not at all

Perfectly

What is the ideal solvent for polar reactant collisions?

A polar protic/peptide medium that stabilizes or polarizes bonds, increasing effective collisions.

How well did you know this?

Not at all

Perfectly

Why does the reaction rate drop sharply past the optimal temperature for enzymes?

Protein denaturation destroys the active site.

How well did you know this?

Not at all

Perfectly

Describe the graph shape for enzyme activity vs. temperature.

A bell curve peaking around 37 °C for humans.

How well did you know this?

Not at all

Perfectly

What are the defining features of kinetic vs. thermodynamic products?

Kinetic product: low Ea, forms faster, less stable
Thermodynamic product: higher Ea, slower, more stable (lower ΔG).

How well did you know this?

Not at all

Perfectly

What is the rate (differential) expression for the reaction aA + bB → cC + dD?

rate = -1/a * d[A]/dt = -1/b * d[B]/dt = +1/c * d[C]/dt = +1/d * d[D]/dt.

How well did you know this?

Not at all

Perfectly

What is the general form of the rate law?

rate = k[A]^x[B]^y where x, y are reaction orders determined experimentally.

How well did you know this?

Not at all

Perfectly

True or False: You can assume x and y equal stoichiometric coefficients in rate laws.

False; do not assume unless the mechanism is single-step or RDS involves reactants as written.

How well did you know this?

Not at all

Perfectly

What are the units of k for a zero-order reaction?

M·s⁻¹

What are the units of k for a first-order reaction?

s⁻¹

What are the units of k for a second-order overall reaction?

M⁻¹·s⁻¹

What is the characteristic of a zero-order rate law and its plot?

rate = k; plot [A] vs. t is linear with slope -k.

How can you change the rate of a zero-order reaction?

Only by changing temperature or adding a catalyst (affects k).

What is the first-order rate law and its integrated form?

rate = k[A]; [A]t = [A]0 e^(-kt) (exponential decay).

What is the linear plot for a first-order reaction?

Plot ln[A] vs. t; slope = -k.

What is a classic example of a first-order reaction?

Radioactive decay.

What are the options for a second-order rate law?

rate = k[A]^2 or rate = k[A][B] (sum of exponents = 2).

What is the linear plot for a second-order reaction of a single reactant?

Plot 1/[A] vs. t; slope = +k.

Define mixed-order (broken-order) reaction.

Rate order is fractional or changes during the reaction.

Provide an example of a mixed-order rate law.

rate = k1[C]A^2/(k2 + k3[A]) (appears first-order in A at high [A]).

What is the experimental method to find reaction order with respect to a reactant?

Hold other reactants’ concentrations constant between trials; see how rate changes when [X] changes.

What is the MCAT EXPERTISE mnemonic for rate-law exponents?

'NOT STOICHI – GOTTA TRY' (stoichiometric coefficients ≠ exponents; use trial data).

How are activation energy and Gibbs free energy related?

They are independent; catalysts lower Ea but do not change ΔG.

How does a catalyst affect the forward vs. reverse rate?

Lowers Ea for both directions by the same factor, accelerating approach to equilibrium without shifting Keq.

What is the mnemonic for homogeneous vs. heterogeneous catalysis?

'HOMO = same phase; HETERO = diff’erent' (e.g., solid catalyst in liquid solution).

What is the effective collision mnemonic?

'HIT HARD & RIGHT' – Hard (enough energy) & Right (proper orientation).

How is the Arrhenius equation rearranged for a straight-line plot?

ln k = ln A - (Ea/R)(1/T); slope = -Ea/R.

How do you identify the RDS on an energy diagram?

The step with the highest activation energy peak.

What changes when a catalyst is added according to an energy diagram?

Peaks (activation barriers) lower; reactant & product energies unchanged; ΔGrxn unchanged.

If Ea = 50 kJ/mol and T rises from 300 K to 310 K, does k increase or decrease?

Increase (higher T lowers the exponent’s magnitude).

In the rate-law data table, if doubling [A] while [B] constant triples the rate, what is the order in A?

x ≈ 1.6 → ~1.5 order (fractional/mixed).

What is the half-life expression for a first-order reaction?

t1/2 = ln 2 / k.

What is the half-life expression for a zero-order reaction?

t1/2 = [A]0 / 2k.

What is the half-life expression for a second-order reaction (single reactant)?

t1/2 = 1 / (k[A]0).

What are the linear plots that yield k for 0-, 1-, and 2-order single-reactant reactions?

* 0-order: [A] vs. t (slope = –k) * 1-order: ln[A] vs. t (slope = –k) * 2-order: 1/[A] vs. t (slope = +k).

What is the mnemonic for remembering plots?

'Zero straight, First ln, Second inverse.'

What is the concept of a saturated catalyst (enzyme) in mixed order?

At high [S], reaction is zero-order with respect to substrate (rate ≈ Vmax); at low [S], first-order.

What is the interpretation of a linear ln[A] vs. t plot with slope -0.693 s⁻¹?

k = 0.693 s⁻¹.

How does solvent polarity affect reaction rate?

Polar solvents stabilize polar transition states or ions, potentially increasing rate.

What impact does the medium state (liquid/solid/gas) have on reaction rates?

Reactants in the same phase collide more easily; gas phase often fastest but specific orientation sometimes favors liquid.

What is the definition of activation energy?

Minimum kinetic energy that colliding molecules must possess to form the activated complex/transition state.

What happens to the rate if the temperature is lowered for a first-order reaction?

Rate decreases (Arrhenius dependence).

What happens if all reactant concentrations are doubled in a zero-order reaction?

No change in rate (rate = k).

What happens when a catalyst is added to a second-order reaction?

k increases, leading to an increase in rate.

List four differences between kinetics and thermodynamics.

* Kinetics = rate, depends on path, Ea, affected by catalysts, no info on energy. * Thermodynamics = favorability, ΔG & Keq, independent of path, unaffected by catalysts.

What should you label on a generic exergonic energy diagram?

Reactants, transition state, Ea, products, ΔG.

Give two biological examples of catalysts lowering Ea via different mechanisms.

* Acid–base catalysis (proton transfer) * Covalent catalysis (enzyme forms transient covalent bond).

True or False: Speeding up RDS always shifts equilibrium.

False; rate increases for both forward & reverse equally, Keq unchanged.

What is the mnemonic for Arrhenius variables?

'A‑E‑R‑T plot' – A = frequency factor, E = activation energy, R = gas constant, T = temperature.

Chapter 5 Flashcards

(65 cards)