An Introduction to Kinetics (3) (1)

Title: An Introduction to Kinetics: Measuring Rates of Chemical Reactions

Purposes

• Evaluate and optimize experimental parameters for measuring reaction rates.

• Design an experimental approach for measuring reaction rates.

• Conduct and analyze kinetic experiments.

• Understand the effects of changing reactant concentration on reaction rates.

Background

• Mixing reactants: produces products if the reaction is thermodynamically favorable.

  • Rapid reactions: e.g., combustion and precipitation reactions.

  • Slow reactions: may take generations to complete.

• Experimental variables: Chemists can control factors to alter reaction rates.

  • Speeding up reactions enhances production efficiency.

• Kinetics involves monitoring an observed variable (reactant/product concentration) against time (independent variable).

• Initial reaction slope: steep at the start, gradually lessens as reactants disappear.

  

Reaction Overview

• Analyzing the reaction of crystal violet with hydroxide ions:

  • Reaction: Crystal violet (blue/violet) + OH- → Colorless product

• Spectroscopic monitoring: Using a UV/Vis spectrophotometer to track absorbance at 590 nm.

  • High absorbance indicates reactant presence, low absorbance indicates product.

  

Initial Observations

• Monitor reaction rates and half-lives.

• Determine the order of reaction with respect to crystal violet and hydroxide.

Safety Precautions

• Sodium hydroxide is a strong skin irritant.

• Crystal violet is carcinogenic and stains skin/clothing.

• Required: Gloves and goggles during the experiment.

Procedure

Part A: Kinetic Measurements

• Total reaction volume: 1.00 mL.

• Initial concentration selections: 1x10⁻⁵ M or 5x10⁻⁶ M for crystal violet.

• Sodium hydroxide concentration: at least 5- fold greater than crystal violet.

  • Select two hydroxide concentrations (0.050 M to 0.100 M).

• Use equation CiVi = CfVf for volume calculations.

Reaction Setup:

1. Prepare cuvette with distilled water to blank spectrophotometer.

2. Add distilled water and sodium hydroxide into cuvette.

3. Quickly add crystal violet solution, mix and start data collection (300 seconds).

Part B: Qualitative Data Analysis

• Draw tangent lines at 0s and 150s on graphs; estimate slopes of curves at these times.

  • Determine if rates appear faster, slower, or the same.

• Calculate half-lives for each reaction and analyze dependence on hydroxide & crystal violet concentrations.

Part C: Quantitative Analysis of Kinetic Data

Differential Rate Laws and Reaction Order

• Rate measurement involves both reactant disappearance and product appearance:

  • Define the rate: rate = -1/a (Δ[reactant]/ Δtime) = 1/b (Δ[product]/ Δtime)

• Reaction rate is proportional to reactant concentration:

  • Rate law: rate = k[reactant].

• Reaction orders:

  • The exponent denotes order; first order (1), second order (2), etc.

• For multi-reactant systems:

  • Rate = k[A]ⁿ[B]ⁿ...

Rate Law Determination

• Collect data to determine orders and rate constants.

Integrated Rate Laws

• Monitor how concentration changes over time.

• Integrated rate laws: Depend on reaction order.

  • First-order: [A] = [A]₀ e⁻ᵏᵗ, ln[A] = ln[A]₀ -kt

  • Second-order: 1/[A] = 1/[A]₀ + kt

Reaction Half-Life

• Defined as time for the concentration to halve: t₁/₂ relates to integrated rate law.

  • For first-order reactions: t₁/₂ = ln2/k.

Experimental Analysis

1. Calculate the concentration of crystal violet at time points using Beer-Lambert Law:

   • C = A/Ɛl.

2. Create three graphs for high NaOH concentration:

   • [crystal violet] vs. time (zeroth order), ln[crystal violet] vs. time (first order), 1/[crystal violet] vs. time (second order).

3. Use regression analysis for linearity assessment.

4. Report slopes, determine k’ values for both high and low NaOH concentration experiments.