Rates and Equilibrium Notes, part I Collision Theory Factors Affecting Reaction Rates

Reaction Rates We have talked about many reactions so far, but we have not talked about how fast a reaction occurs. A reaction’s rate is the speed at which it will occur.

Collision Theory Reactions occur when atoms, ions or molecules collide, providing that they have enough energy to do so. Why only if they have enough energy?

Activation Energy Activation energy is the minimum amount of energy particles need to react with each other.

Orientation Atoms also have to collide in the right configuration to react.

Reaction Mechanisms Because molecules have to collide to react, large reactions with lots of reactants do not happen at once. A reaction mechanism is the series of steps that take place. See board for example and graph.

Factors Affecting Reaction Rates Every reaction proceeds at its own rate, but by modifying the conditions a reaction takes place in, we can change the speed reactions occur.

1) Temperature Usually, raising the temperature of a reaction speeds up a reaction (the opposite is true, as well…lowering temperature slows down reaction). Why?

As the temperature of particles increases, their overall energy increases (increasing the chance that there will be enough energy for the reaction). Also, the faster moving particles are more likely to collide. 1) Temperature

2) Concentration Generally, increasing concentration of reactants increases a reaction’s rate. Why? No, not THAT kind of concentration!

Because you have more particles in the same amount of space (the same volume) there are more collisions, thus more chances each second for the reaction to occur! 2) Concentration

3) Particle size Decreasing the particle size for the same amount of reactant will cause the reaction rate to increase. Why?

Decreasing particle size increases the total amount of reactant exposed for reaction. The more exposure, the more likely that collisions will occur and thus, a faster reaction! 3) Particle size

Surface area is the same idea, but looked at in the opposite way. The smaller particle size for the same amount of stuff, the higher the surface area…so the higher the surface area, the faster the reaction. 3) Particle size

4) Catalysts A catalyst is something that speeds up a reaction without being used up. How does it work?

Catalysts allow a reaction to proceed at a lower than normal activation energy. It can do this by forming an “alternate route” for the reaction to occur, sometimes allowing for an intermediary product. 4) Catalysts

Other catalyst terms… An inhibitor is a substance that acts against the effects of a catalyst. An enzyme is specifically a biological catalyst usually composed of a protein. 4) Catalysts