Lowering the temperature of a chemical system slows the rate of reaction. The reaction rate is defined as how quickly the starting materials, or reactants, are converted into the final products over time. Understanding why cooling affects this speed requires examining how temperature directly influences the interactions and energy of the particles involved.
The Necessity of Molecular Collisions
For any chemical change to happen, the reactant molecules must first physically encounter each other in the correct way. This concept is known as collision theory.
The speed at which molecules move is directly related to the temperature of the system. A decrease in temperature causes the reactant molecules to move more sluggishly, reducing their overall velocity. This reduction in molecular speed leads to a decrease in the frequency of collisions, meaning fewer opportunities for the molecules to interact chemically.
Furthermore, a successful reaction demands that the molecules collide with the correct spatial alignment, or orientation. Even when two molecules meet, they must be positioned in a specific way for the chemically reactive parts to interact effectively.
The Energy Threshold: Activation Energy
A collision’s frequency and orientation alone do not guarantee a chemical transformation. Every reaction has a minimum energy requirement that must be met for the reactants to convert into products. This energy barrier, which must be overcome, is called the activation energy (\(E_a\)).
This barrier represents the energy needed to break the existing chemical bonds in the reactant molecules before new product bonds can form. Think of it like pushing a heavy object up a ramp; the peak represents the transition state, a temporary, high-energy molecular structure.
Only collisions that supply kinetic energy equal to or greater than the activation energy are considered “effective collisions.” These are the only encounters capable of overcoming the energy barrier and resulting in the formation of new products. Collisions below this specific energy threshold simply bounce off one another, remaining chemically unchanged. The magnitude of the activation energy is specific to each reaction and determines how much energy the molecules must possess to react.
How Temperature Shifts the Successful Collision Rate
Temperature is fundamentally a measure of the average kinetic energy of the molecules within a substance. When the temperature is lowered, the average kinetic energy of all the reactant molecules decreases proportionally.
The true power of temperature in reaction kinetics, however, lies not just in the average energy but in the distribution of energies among all the molecules. Even at a fixed temperature, molecules possess a wide range of kinetic energies.
When the temperature is decreased by a small amount, the entire energy distribution curve shifts toward lower energies. This shift causes a profound, non-linear drop in the number of molecules that possess energy greater than the activation energy (\(E_a\)). The energy barrier itself remains constant, but the population capable of clearing it shrinks dramatically.
The effect is exponential, meaning a small temperature decrease results in a dramatically smaller percentage of molecules being energetic enough to surmount the activation energy barrier. The lowering of temperature first reduces the overall frequency of collisions because the molecules are moving more slowly. Second, and much more significantly, it drastically reduces the fraction of those already less-frequent collisions that possess the minimum activation energy required for a successful transformation.