Introduction to Reaction Rate

1. Fundamentals of Reaction Rates

  • Definition: The rate of a chemical reaction is the change in the amount (molar concentration) of a reactant or product per unit of time.

  • Rate Expression: A mathematical representation of concentration change over time. For a reactant A, the rate is given by -\frac{\Delta[A]}{\Delta t}, and for a product B, it is \frac{\Delta[B]}{\Delta t}.

  • Convention: Reaction rates are always expressed as positive quantities. Since reactant concentrations decrease, their change (Delta) is negative; multiplying by $-1$ ensures the rate is positive.

  • Measurement Methods:

    • Gases: Measured by changes in volume or pressure.

    • Colored Substances: Measured via light absorption.

    • Aqueous Electrolytes: Measured via changes in electrical conductivity.

    • Solutions: Measured by changes in molar concentration (M).

2. Types of Reaction Rates

  • Average Rate: The rate calculated over a specific time interval (\Delta t). It typically decreases as the reaction proceeds.

  • Instantaneous Rate: The rate at a specific point in time (t). It is determined by calculating the slope of a line tangent to the concentration-vs-time curve at that specific point.

  • Initial Rate: The instantaneous rate at the moment the reaction commences (t = 0).

3. Stoichiometry and Relative Rates

  • General Relationship: For a reaction represented by aA + bB \rightarrow cC + dD, the relative rates are:
    \text{Rate} = -\frac{1}{a}\frac{\Delta[A]}{\Delta t} = -\frac{1}{b}\frac{\Delta[B]}{\Delta t} = \frac{1}{c}\frac{\Delta[C]}{\Delta t} = \frac{1}{d}\frac{\Delta[D]}{\Delta t}

  • Application: The rate of consumption of a reactant is related to the rate of formation of a product by the ratio of their stoichiometric coefficients.

4. Factors Affecting Reaction Rates

  • Chemical Nature: The intrinsic properties of the substances involved (e.g., Sodium reacts faster with water than Calcium).

  • Physical State and Surface Area:

    • In heterogeneous reactions, the reaction occurs at the interface between phases.

    • Increasing the surface area of a solid (e.g., using powder instead of a large lump) significantly increases the reaction rate.

  • Temperature:

    • Increasing temperature increases the kinetic energy of particles, leading to faster rates.

    • Reaction rates typically double for every 10 \, ^\circ\text{C} increase in temperature.

  • Concentration: Higher concentrations of reactants increase the frequency of molecular collisions, thereby increasing the rate.

  • Catalysis:

    • Catalysts increase the rate of reaction without being consumed.

    • Enzymes are biological catalysts (e.g., used in glucose urinalysis test strips to speed up the oxidation of glucose).

5. Practical Application: Urinalysis Test Strips

  • These utilize kinetic reactions to detect substances like glucose.

  • Glucose is oxidized to produce H2O2, which then reacts with iodide to produce a color change (brown iodine).