Notes on Rates of Reaction

Rates of Reaction

• Objective Overview:
  • Development of a model to show the effects of concentration and temperature on reaction rate.
  • Analyze data on rates and conditions to graphically describe their relationship mathematically.
  • Assess patterns of reaction rate changes with varying conditions.
  • Make predictions about reaction changes due to concentration and temperature variations.

Expressing Rates of Change

• Definition of Rate:

  • Rate is a measure of change occurring within a specific timeframe.

• Examples of Rates:

  • Unit rates that represent a ratio of two measurements where the second measurement is standardized to one (e.g., miles per hour, calories per serving).

• Practical Example:

  • Monitoring the ripening of bananas by measuring the area of skin changing from yellow to brown over several days can provide a rate for the ripening process.

Calculating Reaction Rates

• Definition of Reaction Rate:

  • The rate at which reactants are converted to products in a chemical reaction.
  • Example equation that describes a reaction: $aA + bB \rightarrow cC$ or simply represents how concentrations change over time.

• Rate Formula:

  • Based on stoichiometry:
    $\text{Rate} = -\frac{1}{a} \frac{d[A]}{dt} = -\frac{1}{b} \frac{d[B]}{dt} = \frac{1}{c} \frac{d[C]}{dt}$
  • Each reactant has a negative sign to ensure that reaction rates remain positive.

• Example Calculation:

  • For the reaction of hydrogen and iodine:
  • Given: $[H2]<em>{t</em>1} = 0.210 \text{ mol/L} \quad [H2]<em>{t</em>2} = 0.185 \text{ mol/L} \quad t<em>1 = 10 s \quad t</em>2 = 20 s$
  • Calculate the average reaction rate during the specified time interval.

Collision Theory

• Overview:
  • Describes the necessary conditions for a chemical reaction to occur.
  • Factors affecting the frequency and effectiveness of collisions between reacting particles:
  • Orientation: Collisions must occur with correct orientation to break bonds.
  • Energy: Sufficient energy is required for possible reactions; some collisions will not lead to reactions due to insufficient energy.

Effect of Concentration on Reaction Rates

• Impact of Concentration:
  • Higher concentration increases the number of particles present, resulting in more frequent collisions, hence higher reaction rates.
  • Visual Example: Comparing a dilute solution versus a concentrated one in the same volume shows more crowded particles in the latter, leading to increased collision frequency.

Effect of Temperature on Reaction Rates

• Kinetic Energy:

  • Higher temperatures result in greater kinetic energy for particles, leading to more frequent and forceful collisions.

• Understanding Rates at Different Temperatures:

  • The fraction of molecules capable of overcoming activation energy increases with temperature.

Effect of Particle Size on Reaction Rates

• Surface Area:

  • An increase in surface area (e.g., smaller chunks versus larger lumps) enhances the number of particles available for reaction and thus increases reaction rates.

• Example:

  • Steel Wool vs. Steel Nail: Steel wool, having a larger surface area, reacts more readily with oxygen than a steel nail, resulting in more vigorous reactions.

Summary of Factors Affecting Reaction Rates

• Key Factors:

  • Concentration of reactants
  • Temperature
  • Particle size
  • Presence of catalysts

• Reaction Rate Formula Insight:

  • The change in concentration of reactants or products per unit time is termed the reaction rate which is influenced by all the factors above.

• Example Question:

  • A reaction $H<em>2 + Cl</em>2 \rightarrow 2HCl$ starts with $[H2] = 0.9 M$ and ends with $[H2] = 0.1 M$ after 4 seconds, what is the reaction rate? Options include:
  • A. 0.1 mol/L.s
  • B. 0.2 mol/L.s
  • C. 0.3 mol/L.s
  • D. 0.4 mol/L.s