Why does heat speed up reactions?
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One way to think of heat is the speed and frequency at which atoms and molecules are moving within that substance. Hotter objects have more movement than cooler one. So if you heat something up you are creating more collisions between atoms and molecules every second. More collisions means more potential reactions.
Heat represents the speed at which the particles in a fluid are bouncing around. A chemical reaction requires that the molecules come near to each other to attract atoms and break and re-form molecular bonds. If you increase the heat, the molecules move and bump faster and encounter each other sooner.
And reworded in reverse …. would any interaction cause heat?
We express chemical reactions using stoichiometry; A + B -> C means 1 molecule of A reacts with 1 molecule of B to make 1 molecule of C.
But realistically, we don’t care about that 1 molecule of A/B/C, we care about huge numbers of molecules (Avogadro’s number, 6.022×10^22 molecules). In order for those vast numbers of reactions to happen, we need tons of A molecules hitting tons of B molecules all the time. Heat helps this because a substance at higher temperature mixes more easily and moves (internally) faster. That means more A’s bumping into B’s making more C’s.
For substances to react, the atoms/molecules need to collide with enough speed at the right orientation.
Imagine a large room filled with people. You tell them to walk in a straight line even if it means colliding with another person.
Now imagine the same room with the same number of people, only this time you tell them to sprint as fast as they can. The number of collisions is going to greatly increase and the “force” of those collisions are going to be much greater.
This is effectively what happens when you increase the heat. The molecules are moving faster so you increase the number of collisions and with each collision having a greater impact.
Atoms are surrounded by a “cloud” of electrons. Electrons all have the same charge and like charges repel, and the closer like charges are, the greater this repelling force. Therefore for the atoms to get close enough together to form a bond they need enough energy to overcome the repulsive force.
Temperature is essentially a measure of the average kinetic energy of the constituent particles. The higher the temperature the greater the average kinetic energy. Since the average is higher, there will be a larger population of atoms with sufficient kinetic energy to overcome the repulsive force, so there are more opportunities for the reaction to occur.
It should be noted that even in an exothermic reaction (a reaction in which energy is liberated) the reactants must still be provided enough energy to overcome the repulsion. This is why gasoline vapor and oxygen doesn’t spontaneously combust when mixed. It requires the flame to provide the initial push of kinetic energy. Once you light the vapors, the exothermic reaction provides all the extra energy needed to keep the reaction going until you run out of fuel or oxygen.
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