Difficulty: Intermediate | Prerequisites: Basic thermodynamics (ΔG), energy concepts
A reaction can be thermodynamically favourable (negative ΔG) and still not happen at a noticeable rate. Kinetics explains why. This topic bridges the gap between "will it react?" (thermodynamics) and "how fast?" (kinetics). The collision model and Maxwell-Boltzmann distribution show up repeatedly on the AP exam, usually as a explain-and-sketch question worth easy marks if you know the shape of the curve and what shifts it.
Reactions require collisions with enough energy to overcome the activation energy barrier. Raising the temperature shifts the Maxwell-Boltzmann distribution so that more molecules clear that barrier, which is why temperature increases reaction rate even when thermodynamics already favours the reaction at lower temperatures.
Activation energy (Eₐ)
The minimum kinetic energy that colliding molecules must have for the collision to result in a reaction. Think of it as the height of a hill: even if the destination is downhill overall, you still have to get over the crest first.
Thermodynamically favourable
A reaction with a negative Gibbs free energy change (ΔG < 0). In simple terms, the products are more stable than the reactants, so the reaction "wants" to happen, but that says nothing about how quickly it will occur.
Collision theory
The model stating that for a reaction to occur, reactant molecules must collide with sufficient energy and proper orientation. Think of it as two conditions that must both be met: enough speed and the right angle of approach.
Maxwell-Boltzmann distribution
A graph showing the spread of kinetic energies (or speeds) across a population of gas molecules at a given temperature. In simple terms, it shows that most molecules have middling energy, with a few very slow ones and a few very fast ones, and the shape changes with temperature.
Reaction rate
The speed at which reactants are consumed or products are formed, typically measured as a change in concentration per unit time.
At 30°C, the reaction CS₂ + 3 Cl₂ → CCl₄ + S₂Cl₂ is thermodynamically favourable (ΔG < 0), yet no observable reaction occurs.
The molecules simply do not have enough kinetic energy to overcome the activation energy barrier at that temperature.
This distinction between "can it happen?" and "does it happen fast enough to notice?" is central to AP Chemistry.
Raising the temperature from 30°C to 120°C increases the average kinetic energy of the molecules.
A higher fraction of collisions now exceed the activation energy threshold.
The result: the reaction proceeds at an observable rate at 120°C but not at 30°C.
Crucially, the activation energy itself does not change with temperature. What changes is the proportion of molecules that can clear it.
The x-axis is collision energy (or molecular kinetic energy). The y-axis is the fraction of molecules (or collisions) at that energy.
A vertical dashed line marks the activation energy.
At higher temperature (e.g. 120°C), the curve is lower, broader, and shifted to the right. The peak is lower, but the tail extends further past the activation energy line.
At lower temperature (e.g. 30°C), the curve is taller, narrower, and shifted to the left. The peak is higher, but very little area lies beyond the activation energy line.
The area under the curve to the right of the Eₐ line represents the fraction of molecules with enough energy to react.
Peak should be higher and further left than the 120°C curve.
The curve should be narrower.
It must still start at the origin (zero energy, zero fraction).
Very little of the curve's tail should extend past the activation energy marker.
The total area under both curves is the same (both represent 100% of the molecules).
No numerical formulas are tested here, but you should be able to sketch and label:
Two Maxwell-Boltzmann curves (one for each temperature) on the same axes.
The activation energy as a vertical line.
The shaded region beyond Eₐ for each curve, showing which temperature has the larger reactive fraction.
This is why striking a match works: the friction provides the activation energy to start combustion, even though the wood and oxygen are thermodynamically ready to react at room temperature. It is also why food spoils more slowly in a fridge: lower temperature means fewer molecules exceed the activation energy for decomposition reactions.
Students often say "higher temperature gives molecules more energy to react" without specifying that it increases the fraction of molecules above Eₐ. The AP rubric requires you to connect temperature to the distribution, not just say "more energy."
Some students draw the 30°C curve as simply a smaller version of the 120°C curve. The 30°C curve should be taller at the peak and narrower, not just scaled down.
A common error is stating that increasing temperature lowers the activation energy. Temperature does not change Eₐ; only a catalyst does that.
Students sometimes forget that both curves must have equal total area, since each represents the same total number of molecules.
⚠️ "Explain" questions on this topic require you to mention activation energy, the fraction of molecules exceeding it, and the role of temperature in shifting the distribution. Vague answers like "molecules move faster" earn partial credit at best.
⚠️ Drawing the Maxwell-Boltzmann curve for a different temperature is a classic AP free-response part. Get the relative height, width, and position right.
⚠️ The distinction between thermodynamic favourability and kinetic feasibility is a favourite exam concept. Be ready to explain why a reaction with ΔG < 0 might still not proceed.
True or false: A thermodynamically favourable reaction always occurs at an observable rate.
Fill in the blank: The activation energy is the ________ kinetic energy required for a collision to result in a reaction.
True or false: At a higher temperature, the Maxwell-Boltzmann peak is taller and narrower.
Fill in the blank: A catalyst increases reaction rate by ________ the activation energy.
True or false: The total area under the Maxwell-Boltzmann curve changes with temperature.
Answers: 1. False. 2. Minimum. 3. False (it is lower and broader). 4. Lowering. 5. False (it stays the same).
Q: At 30°C, the reaction CS₂ + 3 Cl₂ → CCl₄ + S₂Cl₂ is thermodynamically favourable but does not proceed. Explain why.
A: Although ΔG < 0, the molecules at 30°C do not have sufficient kinetic energy to overcome the activation energy barrier. Too few collisions exceed Eₐ, so no observable reaction occurs.
Q: Explain how raising the temperature to 120°C causes the reaction to proceed at an observable rate.
A: At 120°C, the Maxwell-Boltzmann distribution shifts so that a larger fraction of molecules have kinetic energy exceeding the activation energy. More collisions are energetic enough to be effective, increasing the reaction rate to an observable level.
Q: If you added a catalyst instead of raising the temperature, how would the Maxwell-Boltzmann distribution change?
A: The distribution itself would not change, because the temperature is unchanged. Instead, the catalyst lowers the activation energy, so the same distribution now has a larger fraction of molecules above the new, lower Eₐ threshold.
Q: When sketching the 30°C distribution on a graph that already shows the 120°C curve, list three features your curve must have.
A: (1) A higher, taller peak than the 120°C curve. (2) A narrower spread. (3) The peak shifted to the left (lower energy), with very little area extending past the activation energy line.
This connects directly to catalysis: a catalyst provides an alternative pathway with a lower Eₐ, which you can visualise as moving the vertical line to the left on the Maxwell-Boltzmann graph. It also links to the Arrhenius equation (k = Ae^(−Eₐ/RT)), which quantifies how rate constants change with temperature. Thermodynamics tells you whether a reaction is spontaneous; kinetics tells you whether you will live to see it happen.
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