Collision Theory

Collision theory states that in order for a chemical reaction to proceed, reactant molecules must collide with each other with a sufficient amount of energy, known as activation energy. Once they collide with enough energy, they break their current bonds and form new bonds to become products.

Collision theory assumes that the rate of a reaction is proportional to the number of collision per unit time, which makes sense. If more collisions occur, then more sufficient energy collisions will happen and more reactants will become products.

The second assumption is that the reactant molecules must collide with a proper orientation in order for new bonds to form, which also makes sense. As you can see below, the carbon monoxide molecules don't collide with the oxygen molecules to form products if the oxygen atom in CO collides with the O₂ molecule. However, when the carbon atom in CO collides with the O₂ molecule, carbon dioxide forms.

The third assumption is that the reactants must be able to penetrate within the valence shells of each other in order to react.

The minimum energy reactant molecules must collide with is called the activation energy(denoted by E_a). If this activation energy is higher than the average kinetic energy of the reactant molecules, the reaction rate will be slower because less molecules will be moving fast enough to collide with enough energy. If the activation energy is less than the average kinetic energy of the molecules, then the reaction will proceed rapidly.

A way to represent many of the kinetic properties of chemical reactions is through a reaction diagram.

The net change in energy between the reactants and products is the change in enthalpy, which in this case is negative(exothermic). The activation energy is the hump at the transition state between reactants and products. The reactants must collide with enough energy to overcome this "hump". Think of the energy curve as a running trail that the molecules "run" on. The molecules have to run fast enough(with enough average kinetic energy) to overcome the hill and come out on the other side of the reaction successfully.

The Arrhenius Equation relates the rate constant k that you see in every rate law with the activation energy of the reaction.

A is a unitless factor known as the frequency factor, which is related to the frequency of collisions and molecular orientations of the reactants.

The exponential term is the rate-determining term. If T is high, then the rate constant is higher because the exponent term is less negative. Similarly, increasing the activation energy will make the rate constant smaller. Both of these are consistent with the reaction rate.

A is a unitless factor known as the frequency factor, which is related to the frequency of collisions and molecular orientations of the reactants.

The exponential term is the rate-determining term. If T is high, then the rate constant is higher because the exponent term is less negative. Similarly, increasing the activation energy will make the rate constant smaller. Both of these are consistent with the reaction rate.

The chemical nature of the reactants affects reaction rates. For example, when calcium reacts with water to form a base, the rate is medium. When sodium undergoes a similar reaction, the rate is rapid.

The physical state of the reactants also is important. Take two solutions of dilute HCl(hydrochloric acid). In the first one, you have powdered iron and in the second, you have an iron nail. The first one will react faster because there is more available surface area of iron to react. The acid can react with every single iron atom in the powder. In the second one, a lot of the iron atoms are nestled inside the nail so they can't react until every atom around them does.

Temperature also affects reaction rates, which makes sense because the reactant molecules need enough average kinetic energy to react. Average kinetic energy itself is proportional to temperature, so increasing temperature increases reaction rates while decreasing temperatures decrease reaction rates. This is why refrigerating milk makes it spoil far less quickly than leaving it outside the fridge.

Concentration affects reaction rates because if molecules are in a more concentrated volume, they're more likely to react.