According to collision theory, for a chemical reaction to occur, reactant molecules must:

Question

Accepted Answer

Collide with sufficient energy and proper orientation. Collision theory states that for a chemical reaction to occur, reactant molecules must collide with each other with sufficient energy (at least equal to the activation energy) and with proper orientation (the reactive parts of the molecules must be correctly aligned during the collision). Not all collisions lead to reactions; only those that meet both the energy and orientation requirements, called effective collisions, result in the formation of products.

Simply being in the same container is necessary for molecules to encounter each other, but proximity alone does not guarantee a reaction. Molecules collide billions of times per second, but only a tiny fraction of these collisions are effective. The products do not need to be at the same temperature as the reactants; temperature is a property of the reactants that determines whether they have sufficient kinetic energy for effective collisions. Identical molecular masses are not required for a reaction; in fact, most chemical reactions involve different substances with different molecular masses.

Collision theory provides a molecular-level explanation for why reaction rates are affected by concentration (more molecules per volume means more collisions), temperature (higher temperature means faster molecules with more kinetic energy), surface area (more exposed surface means more accessible collision sites for heterogeneous reactions), and catalysts (which lower the activation energy, making a larger fraction of collisions effective). Understanding collision theory and how it connects to observable factors affecting reaction rates is important for the CLEP Chemistry exam.

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