Rucete ✏ SAT Chemistry In a Nutshell
9. Rates of Chemical Reactions
This chapter explains what affects the speed of chemical reactions and introduces key ideas like activation energy, rate laws, and the role of catalysts. Understanding reaction rates helps explain why some reactions are fast and others slow—critical for real-world chemistry and SAT questions.
Factors Affecting Reaction Rates
- Nature of the reactants:
Ionic reactions in aqueous solution occur very quickly (e.g., AgNO₃ + NaCl).
Reactions involving covalent bonds tend to be slower (e.g., decomposition of H₂O₂). - Surface area:
More surface exposed = faster reaction, due to increased particle contact. - Concentration:
Higher concentration = more collisions = faster rate. - Temperature:
Higher temperature = higher kinetic energy = more collisions with sufficient energy to react.
Rule of thumb: A 10°C increase roughly doubles or triples the rate. - Catalyst:
A catalyst lowers the activation energy, providing an alternate reaction pathway.- Positive catalyst: speeds up reaction
- Negative catalyst (inhibitor): slows down reaction
- Catalyst is not consumed in the reaction.
Activation Energy and Reaction Graphs
- Activation energy (Ea) = minimum energy required to start a reaction.
-
On a reaction diagram:
- Without catalyst: higher energy barrier
- With catalyst: lower energy barrier
- Catalysts do not change ΔH (overall energy change)—they only affect the path.
Rate Law and Law of Mass Action
- Rate of reaction depends on concentrations of reactants:
r = k[A]^a[B]^b -
k = specific rate constant
a, b = reaction orders (determined experimentally, not always equal to coefficients) -
Example:
H₂ + I₂ → 2HI
r = k[H₂]¹[I₂]¹ → second-order reaction (1 + 1 = 2)
Reaction Mechanisms
- A reaction may proceed in multiple steps (elementary steps).
- The slowest step is the rate-determining step.
- Knowing the mechanism helps predict how changes in concentration affect the overall rate.
-
Example:
If step 2 is slow and involves A, increasing [A] increases overall rate.
If [C] only affects a fast step, changing [C] won't affect overall rate much.
In a Nutshell
Reaction rate is influenced by factors like temperature, concentration, surface area, and catalysts. Understanding activation energy and the Law of Mass Action lets us predict how fast a reaction will proceed. The reaction mechanism reveals the step that limits rate, and catalysts speed things up without being consumed. These principles help explain and control chemical change.