Chemical Reaction Rates and Molar Concepts

Reaction Rate

Of a chemical reaction is defined in terms of the time it takes to make a reactant disappear or for a product to appear.

Collinson Theory

Chemical reactions break and form bonds as a result of collision between elemental units of the reactants. Not all cause the breaking or forming of bonds, not all collision are effective. Only collision with sufficient energy and whose molecules have the right orientation may break or form bonds.

Factors That Affect Reaction Rates

  • Temperature: In reactions involving liquid or gas, raising the temperature accelerates the velocity of the elemental units in the reactant. This causes more collision with sufficient energy to contribute to the reaction.
  • Concentration of Reactants: In reactions that are in aqueous solutions, increasing the concentration raises the probability of collision, which accelerates the reaction rates.
  • State of Sub-division of Reactants: In cases where one of the reactants is in a solid state, the size of the portions determines the reaction rate.


Are chemical substances that modify the reaction rate without changing the nature of the reactants or the products. This property, of not changing the reactants or products in a reaction, is the most important since it determines whether or not a catalyst may be used.

  • They are extremely specific. Generally for each chemical reaction there is a specific catalyst. Its presence initiates a specific chemical reaction, and not other reaction, otherwise possible with the same reactants.
  • They take part in the chemical reaction without undergoing permanent changes. Most catalysts reappear after a reaction, so they are not included in the chemical equation as reactants or products.

Amount of Substance

n, is a measure of the number of elementary units contained in a certain amount of substances. Its SI unit is the mole.

A Mole

Is 6.022 x 10^23 elementary unit/mole. It is the amount of substances of a system that contained as many elementary units as there are atoms in 0.0012kg of carbon-12.

n = N divided in N sub A.

Molar Mass

M is the mass in grams of one mole of the substance. n(mole) equals mass (g) divided by molar mass (g per mole^-1).

Molar Volume

Volume occupied by a mole at a certain pressure and temperature. The molar volume is 22.4 liters/mole.


M is the amount of substance in a liter of solution, is mole/L. It is calculated by molarity equals n substance (mole) divided by V solution (L).