Summary
Chemical Reactions and Stoichiometry
A chemical reaction can be described by an equation that gives the correct formula of each reactant and product and the relatie numbers of formula units that react or are formed. Symbols in parentheses after the formula of he substance may be used to show the physical state of the substances involved in the reaction. An equation must be balanced to be a quantitative description of a reaction.Reactions can be classified as to type. One such classification divides reactions into four categories: combination, decomposition, displacement, and double displacement. A second classification separates reactions into those that involve a transfer of electrons and those that do not. If electrons are transferred, the reaction is an oxidation-reduction reaction. In such a reaction, one substance is oxidized and loses electrons, and another substance is reduced and gains electrons. Oxidation numbers are useful in identifying oxidation-reduction reactions.
Stoichiometric calculations are based on balanced chemical equations. They allow us to calculate the amount of product that would be formed from a given amount of reactant in the equation. In some instances, where amounts of two reatants are given, one reactant may be limiting. In some instances, only a percent of the theoretical ield is obtained.
Chemical reactions obey the Law of Conservation of Mass/Energy. The change
in enthalpy (
H) for a reaction
measures the heat released or absorbed by a reaction carried out at constant
presure and determines whether the reaction is exothermic or endothermic. The
amount of energy absorbed or released by a particular reaction can be calculated.