Chemical Kinetics and Rate Laws
Chemical kinetics is the study of the rates and mechanisms of chemical reactions. It provides a quantitative understanding of the reaction rate, which is the speed at which a reaction takes place. The rate of a chemical reaction is determined by the concentration of reactants, temperature, pressure, surface area, and the presence of catalysts or inhibitors. The rate law is an equation that relates the reaction rate to the concentrations of the reactants.
Key Concepts:
- Reaction rate
- Rate law
- Rate constant
- Order of reaction
- Half-life
- Catalysts and inhibitors
Explanation:
The reaction rate is the rate at which reactants are consumed or products are formed. It is expressed as the change in concentration per unit time, usually in moles per liter per second (mol/L/s). The rate law is an equation that relates the reaction rate to the concentrations of the reactants. It is determined experimentally by measuring the rate at different concentrations of the reactants. The rate law is usually expressed as:
rate = k [A]^m [B]^n
where k is the rate constant, and m and n are the order of the reaction with respect to A and B, respectively.
The rate constant is a proportionality constant that determines the rate of the reaction at a specific temperature. It varies with temperature, and its units depend on the order of the reaction. The order of the reaction is the exponent of the concentration term in the rate law, and it can be determined experimentally by method of initial rates or graphical methods.
The half-life of a first-order reaction is the time it takes for the concentration of the reactant to decrease by half. It is a characteristic property of the reaction and can be calculated from the rate constant. The half-life of a second-order reaction is the time it takes for the concentration of the reactant to decrease by a factor of 1/square root of 2.
Catalysts and inhibitors are substances that affect the rate of the reaction without being consumed in the reaction. Catalysts increase the rate of the reaction by lowering the activation energy required for the reaction to take place. Inhibitors decrease the rate of the reaction by interfering with the reaction mechanism or by occupying the active sites of the catalysts.
Examples:
- The decomposition of hydrogen peroxide is a first-order reaction with a rate constant of 1.23 x 10^-3 s^-1 at 25°C. Calculate the half-life of the reaction.
Solution: t1/2 = ln2/k = 562 s - The reaction between iodine and propanone in the presence of an acid catalyst is a second-order reaction with a rate constant of 3.68 x 10^-3 L/mol/s at 25°C. Calculate the rate of the reaction when [I2] = 0.020 mol/L and [propanone] = 0.050 mol/L.
Solution: rate = k [I2]^1 [propanone]^1 = 3.68 x 10^-3 x 0.020 x 0.050 = 3.68 x 10^-6 mol/L/s
References:
- Atkins, P., & de Paula, J. (2013). Atkins’ physical chemistry (10th ed.). Oxford: Oxford University Press.
- Chang, R. (2010). Chemistry (10th ed.). New York: McGraw-Hill.
- Levine, I. N. (2017). Physical chemistry (7th ed.). New York: McGraw-Hill.
- Silberberg, M. S. (2015). Chemistry: The molecular nature of matter and change (7th ed.). New York: McGraw-Hill.
