Measuring Rates of Reaction

Fundamental Concepts of Reaction Rates

  • Definition of Rate:

    • A rate is related to speed, defined specifically as the amount of change or occurrence happening per unit time.

  • Notation and Unit Conventions:

    • In reaction rates, a superscript of −1-1 denotes the word "per" (representing division by time).
    • The notation g min−1g\,min^{-1} is equivalent to g/ming/min (grams per minute).
    • The notation cm3 s−1cm^3\,s^{-1} is equivalent to cm3/scm^3/s (cubic centimeters per second).

Methods for Measuring Reaction Rates

  • Primary Approaches:

    • Measuring how quickly the reactants are used up (consumed).
    • Measuring how quickly the products are produced (formed).

  • Chemical Reaction Example:

    • Reaction equation:     Mg+2HCl→MgCl2+H2Mg + 2HCl \rightarrow MgCl_2 + H_2
    • Reactant consumption approach:
    • Measures how quickly 1 g1\,g of magnesium (MgMg) dissolves in hydrochloric acid (HClHCl).
    • Calculation example: 1 g1\,g dissolving in 2 min2\,min yields a rate of:       1 g2 min=0.5 g min−1\frac{1\,g}{2\,min} = 0.5\,g\,min^{-1}
    • Product formation approach:
    • Measures how much hydrogen gas (H2H_2) is made in 1 min1\,min.
    • Example rate measurement: 5.5 cm3 min−15.5\,cm^3\,min^{-1}

Mathematical Formula for Rate

  • General Equation:

    • When calculating rate from elapsed time for a fixed process:     Rate=1time\text{Rate} = \frac{1}{\text{time}}

  • Worked Calculation:

    • If it takes 2 min2\,min for MgMg to dissolve completely:     Rate=12 min=0.5 min−1\text{Rate} = \frac{1}{2\,min} = 0.5\,min^{-1}