Chemical Equations and Reactions

Describing Chemical Reactions A chemical reaction is the process by which one or more substances are changed into one or more different substances. Reactants are the original substances Products are the resulting or new substances Law of Conservation of mass states, the total mass of reactants must equal the total mass of products for any given chemical reaction. Chemical equations represent, with symbols and formulas, the identities and relative molecular or molar amounts of the reactants and products in a chemical reaction

Symbols used in chemical equations

Symbols used in chemical equations (cont.)

Symbols used in chemical equations (cont.)

Indications of a Chemical Reaction Evolution of energy as heat and light Production of a gas (bubbling) Formation of a precipitate A solid that is produced as a result of a chemical reaction in solution and that separates from the solution. Color change

Characteristics of Chemical Equations All properly written chemical equations will share these characteristics The equation must represent known facts. All reactants and products must be identified The equation must contain the correct formulas for the reactants and products. Watch out for HOFBrINCl’s The law of conservation of mass must be satisfied. Atoms are never created or destroyed in a chemical reaction. Atoms on the reactant side must equal atoms on the product side. Coefficients are used to “balance” all the atoms in a chemical equation. They are inserted in front of a chemical formula and act as a multiplier.

Types of Chemical Reactions Categorizing chemical reactions into different types helps us to predict the products which will occur during reaction There are 5 basic types of Reactions we will discuss: Synthesis Decomposition Single displacement (replacement) Double displacment (replacement) Combustion

Synthesis Reaction Also known as a composition reaction Two or more substances react to form a new compound A + B  AB 2Mg(s) + O2(g)  2MgO(s)

Decomposition Reactions A single compound undergoes a reaction that produces two or more simplier substances. AB  A + B 2H2O  2H2 + O2 CaCO3  CaO + CO2

Combustion Reactions A substance combines with oxygen and produces a combination of water and carbon dioxide. Usually involves hydrocarbons CxHy + O2  CO2 + H2O C3H8 + 5O2  3CO2 + 4H2O

Single Displacement Reactions One element in a compound replaces another element in a different compound A + BC  AC + B 2Al + 3Pb(NO3)2  3Pb + 2Al(NO3)3

Single Displacement Reactions (cont.) Not all single displacement reaction will take place. Must refer to the activity series of metals to predict whether the metal is “strong” enough to displace the other metal.

In order for a single displacement reaction to take place the lone metal must be higher on the activity series than the metal in the compound.

Double Displacement Reactions The ions of two compounds exchange places to form two new compounds AB + CD  AD + CB 2KI + Pb(NO3)2  PbI2 + 2KNO3