18.1: Reaction Rates Rates measure the speed of any change that occurs within an interval of time. The time required for a chemical reaction to come to completion is known as its reaction rate.

As time passes, the amount of reactant (red squares) decreases and the amount of product (blue spheres) increases. Rates of chemical reactions are often measured as a change in the number of moles during a given interval of time.

Reaction rates vary tremendously Some reactions have very slow rates as seen below with the formation of rust (Iron(II) Oxide and Iron (III) Oxide). Other reactions can have very fast rates, as seen to the right, when a solution of Potassium Iodide is poured into a solution of Lead (II) Nitrate.

Rates of chemical reactions are related to the properties of atoms, ions, and molecules. Atoms, ions, and molecules react to form products when they collide, provided that they have enough kinetic energy. Particles lacking the necessary kinetic energy to react bounce apart when they collide.

Colliding particles will react only when they have enough kinetic energy and collide at the right orientation. The minimum amount of kinetic energy that particles must have in order to react is called the activation energy. In a sense, activation energy is a barrier that reactants must cross to be converted to products.

Temporary arrangement of particles that has sufficient energy to become products.

FactorsAffect Temperature Increasing the temperature speeds up reaction rates. Concentration Increasing the concentration speeds up reaction rates. Particle Size Decreasing the particle size speeds up reaction rates. Catalysts The addition of a catalyst will speed up reaction rates.

Increasing the temp. usually increases the rate of the reaction. At high temperatures, the number of particles that have enough kinetic energy to react is increased.

Higher concentrations = more reacting particles in a given volume. The amount of collisions increases with increasing concentration. As the amount of collisions increases so does the reaction rate.

The smaller the particle size, the larger the surface area for a given mass of particles. An increase in surface area increases the amount of the reactant exposed for reaction. Increased amount of reactant exposed = increased reaction rate.

A catalyst is a substance that increases the reaction rate of a reaction without being used up itself during the reaction. Catalysts permit reactions to proceed at a lower energy than is normally required. (see graph below)