Chemical Kinetics and Reaction Mechanisms
How fast do reactions happen?
Rate laws
The rate law relates reaction speed to reactant concentrations, often in the form The exponents and are determined experimentally, not by the balanced equation alone.
Activation energy
Reactions require an energy barrier to be overcome. Catalysts speed reactions by providing an alternative pathway with a lower activation energy, , without changing or the equilibrium position.
Mechanism reasoning
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Step 1: Identify the slow step, which often controls the overall rate.
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Step 2: Add elementary steps to recover the net balanced equation.
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Step 3: Cancel intermediates, which appear in one step and are consumed in another.
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Step 4: Check whether the rate law matches the molecularity of the slow step.
What does a catalyst change?
A catalyst lowers the activation energy and therefore increases the rate, but it does not change equilibrium thermodynamics.
Correct answer: The activation energy
What is the role of the rate-determining step?
The slowest step in a mechanism often governs the observed kinetics.
Correct answer: It limits the overall reaction rate
Rate factors
Higher temperature
- More molecules exceed
- Usually increases rate
Higher concentration
- More frequent collisions
- Often increases rate for concentration-dependent reactions
Kinetics vs thermodynamics
A reaction can be thermodynamically favorable yet slow, or fast yet unfavorable; speed and spontaneity are different ideas.
An intermediate in a mechanism is best described as a species that
Intermediates are formed during one elementary step and used up in another, so they do not appear in the overall equation.
Correct answer: Is produced in one step and consumed in another