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Chemistry
Reaction Kinetics
Reaction Kinetics
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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.
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.