Reaction Types A process that changes one or more substances into one or more new substances. O -2 H +1 O -2 H +1 2H 2 + O 2 2H 2 O Five observations that indicate a chemical reaction has occurred are: a)permanent color change b)formation of a precipitate c) gas is produced d)an endergonic or exergonic reaction e)give off light Molecules must collide in order to react. O -2 H +1 O -2 H +1 A chemical equation is a shorthand method used by scientists to show the changes that take place in a chemical reaction. 2H 2 + O 2 2H 2 O A chemical reaction can be shown by writing a word equation. For example, Carbon plus oxygen yields carbon dioxide More often symbols and chemical formulas are used in place of words. This is called a formula equation. C + O 2 CO 2 . In a chemical equation reactants are written to the left of the arrow and products are written to the right of the arrow ( reactants products). 2H 2 + O 2 2H 2 O A synthesis reaction is a reaction in which two or more substances combine to form only one product. General equation: A + B > AB Ex. Mg + S > MgS A decomposition reaction is a reaction in which a compound breaks down into two or more products. General equation: AX > A + X Ex. BaS > Ba + S A single replacement reaction takes place when one element replaces another element in a compound. Zn + CuCl 2 ZnCl 2 + Cu This forms a new compound and leaves the less active element by itself. If the free element is less active, then no reaction occurs. Cu + ZnCl 2 When the free element is a metal: General equation: AX + B > BX + A [only if B is stronger than A] Ex. ZnS + Mg > MgS + Zn Most Reactive Least Reactive A double replacement reaction takes place when the compounds in a reaction exchange ions. General equation: XY + AB > XB + AY Ex. BaS + CaO > BaO + CaS