CHEMICAL REACTIONS Fall 2013

CHEMICAL REACTIONS 1.A chemical reaction is a process that changes one or more substances into one or more new substances. 2. Five observations that indicate a chemical reaction has occurred are: –a) permanent color change b) formation of a precipitate c) gas is produced d) endergonic or exergonic (energy) e) production of light 3. A chemical equation is a shorthand method used by scientist to show the changes that take place in a chemical reaction.

CHEMICAL REACTIONS 4.A chemical reaction can be shown by writing a word equation. magnesium plus sulfur yields magnesium sulfide –More often symbols and chemical formulas are used in place of words. This is called a formula equation. Mg + S ----> MgS

CHEMICAL REACTIONS 5.The starting substances in a chemical reaction are called reactants. The new substances that are formed in a reaction are called products. –In a chemical equation reactants are written to the left of the arrow and products are written to the right. reactants -----> products

CHEMICAL REACTIONS 6.A balanced chemical equation has the same number and type of atoms on both the reactant and product side of the equation. –A coefficient is a whole number placed in front of a symbol or formula to show the number of atoms or molecules involved. –When balancing an unbalanced equation, only coeffients can be changed. The subscripts are not changed.

CHEMICAL REACTIONS 7.A diatomic molecule is made up of two atoms of a single element. The seven diatomic molecules are Bromine, Iodine, Nitrogen, Chlorine, Hydrogen, Oxygen and Fluorine. –These elements are so reactive, they are never found alone. They will always be with another element, OR they travel in pairs. –Br 2 I 2 N 2 Cl 2 H 2 O 2 F 2

CHEMICAL REACTIONS 8.The Law of Conservation of Mass states: In a chemical reaction the total mass of the reactants equals the total mass of the products. –The number of atoms of each element on each side of the equation stays the same. –A balanced equation satisfies the Law of Conservation of Mass.

CHEMICAL REACTIONS 9. Types of Reactions –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

CHEMICAL REACTIONS 9. Types of Reactions –A decomposition reaction is a reaction in which a compound breaks down into two or more products. General equation: –AX > A + X –BaS > Ba + S

CHEMICAL REACTIONS 9. Types of Reactions –A single displacement reaction takes place when one element replaces another element in a compound. This forms a new compound and leaves the weaker element by itself. When the free element is a metal: General equation: – A + BC > B + AC [only if A is stronger than B] –ZnS + Mg > MgS + Zn

CHEMICAL REACTIONS 9. Types of Reactions –A double displacement reaction takes place when the compounds in a reaction exchange ions. General equation: –AB + CD ---> AD + CB –BaS + CaO > BaO + CaS