Reaction kinetics is the study of how fast chemical reactions happen and what controls their speed. It matters because reaction rate affects industrial production, medicine, food spoilage, combustion, and environmental chemistry. A reaction that is favorable by energy can still be very slow if particles do not collide in the right way.
Kinetics helps chemists predict and control reaction speed by changing conditions such as concentration, temperature, surface area, and catalysts.
Understanding Reaction Kinetics
A rate law is not usually read straight from a balanced chemical equation. Chemists measure it. They run several trials, changing the starting amount of one reactant while keeping other conditions steady.
If doubling one reactant doubles the initial rate, that reactant has first order behavior. If doubling it makes the rate four times larger, it has second order behavior. A reactant can have zero order behavior when changing its amount does not change the rate during that stage.
The numerical rate constant depends on temperature and on the total order of the reaction. Its units are therefore useful checks on calculations.
Concentration changes over time follow different patterns for different orders. A zero order reaction loses the same concentration in equal time intervals. A first order reaction loses the same fraction in equal time intervals.
This difference matters when reading graphs. For zero order data, concentration plotted against time gives a straight line. For first order data, the natural log of concentration plotted against time gives a straight line.
For second order data, the reciprocal of concentration plotted against time gives a straight line. Students should identify what has been plotted before deciding that a reaction is first or second order.
Half life means the time needed for a reactant concentration to fall to half its current value. In a first order reaction, every half life is the same length. A radioactive sample behaves this way, which is why radioactive dating is based on a predictable pattern.
Zero order and second order reactions behave differently. Their half lives change as the reactant is used up. This is a common source of mistakes.
A fixed half life does not apply to every reaction. Drugs in the body are often approximated as first order over a useful concentration range, so half life can help doctors plan dosing intervals.
Rate laws can give clues about the reaction mechanism, which is the actual sequence of small steps between reactants and products. An elementary step has a rate law related to the particles involved in that single step. A full reaction may contain several elementary steps, so its balanced equation does not reveal the measured rate law.
The slowest step often limits the overall rate, like a narrow doorway limiting movement through a building. Intermediates are made in one step then consumed in another, so they should not appear in the final rate law. Temperature studies add further evidence.
A plot using the natural log of the rate constant against reciprocal temperature can be used to find activation energy. Small temperature changes can noticeably affect cooking, battery performance, enzyme activity, corrosion, and food storage.
Key Facts
- Average reaction rate = -Δ[reactant]/Δt = Δ[product]/Δt
- For aA + bB → products, rate = k[A]^m[B]^n
- Overall reaction order = m + n
- Arrhenius equation: k = Ae^(-Ea/RT)
- A catalyst lowers activation energy Ea and increases rate without being consumed.
- Higher temperature increases the fraction of particles with energy greater than or equal to Ea.
Vocabulary
- Reaction rate
- Reaction rate is the change in concentration of a reactant or product per unit time.
- Activation energy
- Activation energy is the minimum energy particles must have during a collision for a reaction to occur.
- Rate law
- A rate law is an equation that relates reaction rate to reactant concentrations and a rate constant.
- Catalyst
- A catalyst is a substance that speeds up a reaction by providing a lower energy pathway and is not used up overall.
- Reaction mechanism
- A reaction mechanism is the step by step sequence of elementary reactions that explains how reactants become products.
Common Mistakes to Avoid
- Confusing reaction rate with rate constant, because rate depends on concentration while the rate constant k is fixed for a given reaction at a given temperature.
- Using coefficients as reaction orders automatically, because reaction orders must be found experimentally unless the step is an elementary reaction.
- Thinking a catalyst changes the final amount of product, because a catalyst speeds up both forward and reverse reactions and does not change equilibrium position.
- Ignoring units in rate laws, because the units of k change depending on the overall reaction order.
Practice Questions
- 1 The concentration of A drops from 0.800 M to 0.500 M in 60.0 s. What is the average rate of disappearance of A?
- 2 For the rate law rate = k[A]^2[B], the measured rate is 0.036 M/s when [A] = 0.30 M and [B] = 0.20 M. Calculate k with units.
- 3 A reaction is slow at room temperature but much faster when heated. Explain this using collision frequency, activation energy, and the Maxwell-Boltzmann distribution.