Chemical Kinetics

The One Thing

Chemical kinetics explains reaction speed through rate laws, experimentally determined reaction order, molecularity, integrated rate equations and the Arrhenius relationship. Reaction rate is controlled by factors including concentration, temperature, activation energy, reactant nature, surface area and catalysts.

Reactions, Processes and Experiments

Key Terms

• Chemical kinetics: The branch of chemistry that deals with the rate of reactions, reaction mechanisms and factors affecting reaction speed.
• Rate of reaction: The change in concentration of a reactant or product per unit time.
• Average rate: The change in concentration over a finite time interval; for a reactant R, average rate = -Δ[R]/Δt, and for a product P, average rate = Δ[P]/Δt.
• Instantaneous rate: The rate of reaction at a particular instant, obtained from the slope of the concentration-time curve.
• Rate law: An equation showing how reaction rate depends on reactant concentrations, such as rate = k[A]^m[B]^n.
• Rate constant: The proportionality constant k in the rate law; its value depends on temperature and the nature of the reaction.
• Order of reaction: The sum of the powers of concentration terms in the experimentally determined rate law. For rate = k[A]^m[B]^n, order = m + n.
• Zero-order reaction: A reaction whose rate is independent of reactant concentration: rate = k.
• First-order reaction: A reaction whose rate is proportional to the first power of reactant concentration: rate = k[A].
• Second-order reaction: A reaction whose total concentration power in the rate law is two, for example rate = k[A]^2 or rate = k[A][B].
• Molecularity: The number of reacting species participating in a single elementary reaction step.
• Elementary reaction: A reaction occurring in one simple step for which the molecularity can be specified.
• Complex reaction: A reaction that occurs through two or more elementary steps and may involve intermediate species.
• Activation energy: The minimum energy that reacting particles must possess to form the activated complex and undergo reaction.
• Activated complex: A temporary, high-energy arrangement of atoms formed during a reaction.
• Arrhenius equation: The equation k = A e^(-Ea/RT), which relates the rate constant to activation energy and absolute temperature.
• Pre-exponential factor: The factor A in the Arrhenius equation, representing collision frequency and the proper orientation of reacting particles.
• Catalyst: A substance that changes the reaction rate by providing an alternative pathway with lower activation energy and is regenerated at the end.
• Half-life: The time required for the concentration of a reactant to become half of its initial value.
• Rate-determining step: The slowest step in a multistep reaction mechanism, which often strongly influences the observed rate law.
• Surface area: A factor affecting reaction rate, particularly in heterogeneous reactions; greater surface area generally increases the available contact area between reactants.
• Activation energy, Ea: The energy barrier that reacting particles must overcome for reaction to occur.
• Gas constant, R: The constant appearing in the Arrhenius equation and its logarithmic forms.
• Absolute temperature, T: Temperature measured in kelvin for use in the Arrhenius equation.

Easily Confused

• Order and molecularity: Order is determined experimentally from the observed rate law and may be zero, whole-number, fractional or negative; molecularity applies only to an elementary step and is always a positive whole number.
• Overall equation and rate law: The stoichiometric coefficients of the overall chemical equation do not generally give the reaction order unless the reaction is an elementary step.
• Average rate and instantaneous rate: Average rate is measured over a finite interval, whereas instantaneous rate is the rate at a particular instant obtained from a curve’s slope.
• Rate and rate constant: Rate changes with reactant concentration, whereas the rate constant k is the proportionality constant whose value depends on temperature and reaction nature.
• Zero-order and first-order kinetics: In zero-order kinetics, rate = k and [A] versus time is linear; in first-order kinetics, rate = k[A] and log[A] versus time is linear.
• Catalyst and equilibrium change: A catalyst changes the rate by lowering activation energy but does not change the equilibrium constant or the final equilibrium position.
• Activation energy and activated complex: Activation energy is the minimum energy required to react, whereas the activated complex is the temporary high-energy arrangement formed during reaction.
• Rate-determining step and overall reaction: The rate-determining step is the slowest elementary step in a mechanism, not necessarily the overall balanced chemical equation.

What Gets Asked

• Calculate the average or instantaneous rate using concentration changes, time intervals or the slope of a concentration-time curve. The main mark-losing error is using the wrong sign: reactants require a negative sign, while products require a positive sign.
• Determine the overall order from a rate law such as rate = k[A]^m[B]^n. The specific slip is adding stoichiometric coefficients from the overall equation instead of using the experimentally determined powers m and n.
• Distinguish order from molecularity. Order may be zero, fractional or negative, whereas molecularity is defined only for an elementary reaction step and is a positive whole number.
• Apply integrated rate equations and half-life expressions for zero-, first- and second-order reactions. The main error is using the wrong equation or units of k: zero order: mol L^-1 s^-1; first order: s^-1; second order: L mol^-1 s^-1.
• Identify the correct straight-line plot. A first-order reaction gives log[A] versus time with slope -k/2.303, whereas a zero-order reaction gives [A] versus time with slope -k.
• Use the Arrhenius equation or its logarithmic forms to calculate activation energy, rate constants or temperature effects. Common slips include using temperature other than kelvin and omitting the factor 2.303 in logarithmic equations.
• Explain the effect of a catalyst and temperature on reaction rate. A catalyst lowers activation energy for both forward and reverse reactions but does not change the equilibrium constant or overall enthalpy change; increasing temperature generally increases the rate constant because more particles overcome activation energy.