Understanding Chemical Reaction.docx

Understanding Chemical Reactions and Equations A chemical reaction is a process through which one or more substances are changed into different substances. The original substances are called reactants, and the resulting substances are referred to as products.

Representing Chemical Reactions Chemical reactions are represented by chemical equations. Consisting of symbols and formulas, chemical equations show what happens during a reaction. The equations present the identities and relative amounts of the reactants and products involved in a chemical reaction. 4Fe (s) + 3O2 (g)  2Fe2O3 (s) 2KClO3 (s)  2KCl (s) + 3O2 (g)

Compounds at the left-hand side of the arrow are the reactants needed for the reaction to take place. Those at the right-hand side are the products formed in the reaction. Notice the types of numbers in the equations-the big numbers written before the chemical formulas and the small ones written after some elements in the formulas. The big numbers are called coefficients, which give an idea of the molecules or formula units of a substance that participate in the reaction. The small numbers are termed subscripts, which tell how many atoms are present in a specific compound. Note that the absence of coefficients or subscripts means that the number is equal to 1.

O2 2O2

SYMBOLS IN CHEMICAL EQUATIONS SYMBOLS +

 (s) (l) (g) (aq)

FUNCTION Written between the symbols and/or formulas of reactants or products. Left side: read as “combines with” or “reacts with” Right side: read as “and” Separates reactants from products Read as “yields” or “produces” Indicates a solid reactant or product Indicates a liquid reactant or product Indicates a gaseous reactant or product Indicates that the reactant or products is in an aqueous sol’n Indicates that heat must be supplied to reactants before a reaction occurs The substance written above the arrow is a catalyst or a solvent.

WORD EQUATIONS Chemical reactions may be written in two-ways-through word equation or chemical equation. As the term suggests, a word equation presents the reactants and products of a chemical reaction using words. It has only qualitative meaning. It only identifies the reactants and products involved in a reaction, and not their quantities. Consider the reaction for hydrogen chloride formation. This particular gas is produced by the combination of hydrogen gas and chlorine gas. The word equation may be written as: Hydrogen gas + chlorine gas  hydrogen chloride gas

CHEMICAL EQUATIONS A word equation can be converted to a chemical equation by replacing the names of the reactants and products with their corresponding symbols or formulas. Usually, the physical state of each reactant and product is indicated using italicized letters in parentheses after each chemical species. Substances dissolved in water are considered aqueous and are labeled (aq). The chemical formulas for hydrogen, chlorine, and hydrogen chloride at H2, Cl2, and HCl, respectively. All these species exist in the gaseous statee. Therefore, the chemical equation representing the previous word equation is H2 (g) + Cl2 (g)  HCl (g)