* Kinetic Energy and Collisions: Chemical reactions happen when molecules collide with enough energy to break existing bonds and form new ones. Lowering the temperature reduces the average kinetic energy of molecules. This means they move slower and collide less frequently, with less force.

* Activation Energy: Every reaction has an activation energy, which is the minimum energy required for the reaction to occur. Lower temperatures mean fewer molecules have enough energy to overcome this barrier.

Exceptions:

While most reactions slow down with lower temperatures, there are some exceptions:

* Exothermic Reactions: Some reactions release heat (exothermic). In these cases, lowering the temperature can actually *speed up* the reaction because it provides the heat needed to overcome the activation energy.

* Equilibrium Reactions: For reversible reactions, lowering the temperature can shift the equilibrium towards the side that releases heat (the exothermic side). This can make it *appear* that the reaction is speeding up, even though the rate constant has actually decreased.

Overall:

Lowering the temperature generally slows down chemical reactions. This is due to the decrease in kinetic energy and the reduced number of molecules with enough energy to overcome the activation energy barrier. However, there are some exceptions, especially with exothermic reactions and equilibrium reactions.