A chemical reaction rate describes the speed at which reactants are consumed and products are formed. Increasing the temperature of a system almost always causes this rate to increase. This relationship is strong, meaning a small change in heat can lead to a large change in reaction speed. Understanding this requires examining the physical requirements molecules must meet for a successful chemical transformation.
The Necessity of Molecular Collision
For any chemical change to occur, the reactant molecules must first physically encounter one another. Collision theory posits that molecules must collide for their atoms to rearrange into new substances. The rate of a reaction is directly related to how often these collisions occur, which depends on the concentration and movement of the particles involved.
Simply colliding is not enough to guarantee a reaction; the molecules must also hit one another with the correct spatial alignment. They must be oriented properly during the impact for the necessary bond-breaking and bond-forming processes to begin. If the molecules collide at an ineffective angle, they simply rebound, and no chemical change takes place. A successful reaction requires both a physical collision and a suitable orientation upon impact.
The Minimum Energy Barrier
Even when molecules collide with the perfect orientation, a reaction will not proceed unless the collision possesses a minimum amount of energy. This threshold energy, known as the activation energy, represents an energy barrier that must be overcome to break the existing chemical bonds. Breaking these bonds requires an initial energy input to push the system into an unstable, high-energy transition state.
If the energy transferred during the collision is less than the activation energy, the molecules bounce apart unchanged. If the collision energy exceeds this barrier, the atoms can proceed to rearrange, forming new, stable product molecules. Reactions with a high activation energy require a larger energy input, making them slower than those with a lower barrier. This energy barrier is a fixed property of the specific chemical reaction and does not change when the temperature is altered.
Heating the Reactants
Temperature is a measure of the average kinetic energy of the particles within a substance. When heat is added to the reactants, the average speed of the molecules increases, causing them to move more rapidly. This higher speed leads to a slight increase in the total frequency of collisions, as molecules cover more distance in a given time.
The primary reason temperature accelerates the reaction rate is its effect on the distribution of molecular energies. At any given temperature, not all molecules move at the same speed. By increasing the temperature, the entire range of molecular energies shifts toward higher values.
This shift means that a significantly larger proportion of molecules now possess kinetic energy equal to or greater than the activation energy. Because the activation energy barrier remains constant, a small temperature increase pushes a much greater number of molecules over this energy threshold. For many reactions, a 10°C increase in temperature can approximately double the reaction rate.
Real-World Control of Reaction Rates
The principle that temperature controls reaction rates is applied constantly in both everyday life and industrial processes. Refrigeration works by lowering the temperature of food, which slows the chemical reactions catalyzed by enzymes in spoilage organisms like bacteria. Conversely, cooking involves raising the temperature to accelerate the chemical reactions that change the texture, flavor, and digestibility of food.
Industrial chemistry relies on precise temperature control to optimize output and efficiency. In large-scale manufacturing, such as the production of ammonia through the Haber process, temperatures are carefully maintained between 400°C and 500°C. This range is chosen to ensure a fast reaction rate without compromising the yield or structural integrity of the equipment. Other applications, like the curing of two-part epoxy or the setting of concrete, are managed by cooling or heating the mixture to control the hardening speed.