E

Overview

  • Definition of Kinetics: The study of reaction rates and factors influencing them, crucial for drug design and predicting reaction behaviors.

  • Rate of a Reaction: Describes how reactant/product concentration changes over time.

  • Rate Law: Relates reaction rate to reactant concentrations and a rate constant.

What is Kinetics?

  • Studies reactant (A) conversion to product (B).

  • Rate Formula:
    \text{rate} = -\frac{\Delta[A]}{\Delta t} (reactants)
    \text{rate} = +\frac{\Delta[B]}{\Delta t} (products)

  • Concentration Change: $\Delta[A]$ is negative as reactants decrease; $\Delta[B]$ is positive as products increase.

Reaction Example

  • Example:
    \text{CH}4 (g) + 2 \text{O}2 (g) \rightarrow \text{CO}2 (g) + 2 \text{H}2 \text{O} (g)
    Shows reactants decreasing and products increasing.

Rate of Reaction

  • Units: Commonly Molarity per second (M/s).

Monitoring Changes in Concentration

  • Spectrophotometry: Uses Beer’s Law ( A = \rho b c ) to monitor concentration changes over time, where $A$ = Absorbance, $\rho$ = Molar absorptivity, $b$ = Path length, $c$ = Concentration.

Rate Law

  • Definition: Links reaction rate to reactant concentrations raised to powers.

  • General Form:
    \text{Rate} = k [A]^x[B]^y

  • Experimental Determination: Must be determined experimentally for each reaction.

First-Order Reactions

  • General Form:
    \text{Rate} = -\frac{\Delta[A]}{\Delta t} = k[A] (k in $s^{-1}$).

  • Concentration Over Time:
    \text{ln} [A] = \text{ln} [A]0 - kt ($[A]$ at time $t$, $[A]0$ initial concentration).

  • Half-Life ($t{1/2}$): Time for reactant concentration to halve.
    t    {1/2} = \frac{0.693}{k}
    Example: For $k = 5.7 \times 10^{-4} s^{-1}$, $t_{1/2} = 1200 s$ or $20 minutes$.

Half-Life of Antidepressants

  • Various antidepressants have distinct half-lives (e.g., Fluoxetine: 2-3 days, Citalopram: 34 hours).

Factors Influencing Reaction Rates

  • Temperature: Generally, higher temperature increases rates.

  • Concentration: Higher concentration increases rates.

  • Pressure: For gases, increasing pressure can increase rates.

Catalysis

  • Definition of Catalyst: Increases reaction rate without being consumed.

  • Rate Constant Equation:
    k = A \times e^{-\frac{Ea}{RT}} where $Ea$ is activation energy, $R$ is gas constant, $T$ is temperature.

  • Effect of Catalysts: Lowers activation energy ($E_a$).

Rate Law Determination Experiment

  • Example: Allura Red with NaOCl producing a colorless product.

  • Set up: Use a Vernier spectrophotometer to measure absorbance over time, blanked with deionized water.

  • Steps: Measure initial absorbance ($A_0$), add bleach while recording absorbance, and analyze data by plotting $\text{ln}[D]$ versus time (first-order) or $\frac{1}{[D]}$ versus time (second-order).

Varying Temperatures in Kinetics Experiments

  • Cold Trials: Use ice bath (5-10°C).

  • Warm Trials: Heat solution (40-45°C).

Enzyme Catalysis

  • Enzymes form enzyme-substrate complexes to facilitate reactions, releasing products without being consumed.

Activation Energy (Ea)

  • Definition: The minimum energy needed to start a chemical reaction, allowing reactants to pass through a transition state to products.

Rate Enhancements by Enzymes

  • Enzymes significantly enhance reaction rates, often by factors of $10^5$ to $10^{17}$ compared to uncatalyzed reactions (e.g., Cyclophilin, Carbonic anhydrase).