IIIIIIIVV Intro to Reactions Chemical Reactions

Signs of a Chemical Reaction n Evolution of heat and light n Formation of a gas n Formation of a precipitate (solid) n Color change

Law of Conservation of Mass n mass is neither created nor destroyed in a chemical reaction 4 H 2 O 4 H 2 O 4 g32 g 36 g n total mass stays the same n atoms can only rearrange

Chemical Equations A+B  C+D REACTANTSPRODUCTS

Chemical Equations

Subscripts n Subscripts are the numbers proceeding an element. n The number represents how many atoms are present of that specific element. n Examples: n H 2 O = 2H, 1O n AgNO 3 = 1Ag, 1N, 3O n Mg(NO 3 ) 2 = 1Mg, 2N, 6O

Coefficients n Coefficient – is the number in front of an element or compound. n The number represents how many of that specific compound is present. n Examples:  2H 2 O = 4H, 2O  3CO = 3C, 3O  2Mg(NO 3 ) 2= 2 Mg, 4N,12O

Writing Equations n Identify the substances involved. n Use symbols to show: 2H 2 (g) + O 2 (g)  2H 2 O(g)  How many? – coefficient & subscript  Of what? - chemical formula  In what state? - physical state

Writing Equations Two atoms of aluminum react with three units of aqueous copper(II) chloride to produce three atoms of copper and two units of aqueous aluminum chloride. How many? Of what? In what state? Al 2(s)(s)+ 3CuCl 2 (aq)  3 Cu(s)(s)+ 2AlCl 3 (aq)

Describing Equations to produce How many? Of what? In what state? Zn(s) + 2HCl(aq)  ZnCl 2 (aq) + H 2 (g) One atom of solid zinc reacts with two molecules of aqueous hydrochloric acid one unit of aqueous zinc chlorideand one molecule of hydrogen gas.

Balancing Steps 1.Write the unbalanced equation. 2.Count atoms on each side. 3.Add coefficients to make #s equal. Coefficient  subscript = # of atoms 4.Reduce coefficients to lowest possible ratio, if necessary. 5.Double check atom balance!!!

Helpful Tips n Balance one element at a time. n Update ALL atom counts after adding a coefficient. n If an element appears more than once per side, balance it last. n Balance polyatomic ions as single units.  “1 SO 4 ” instead of “1 S” and “4 O”

Al + CuCl 2  Cu + AlCl 3 Al Cu Cl  3  6   Balancing Example Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2  2  6