IIIIIIIVV Intro to Reactions Unit 3 Stoichiometry

A.Signs of a Chemical Reaction n Evolution of heat and light n Formation of a gas n Formation of a precipitate n Color change

B.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

C. Chemical Equations A+B  C+D REACTANTSPRODUCTS

C. Chemical Equations

D. 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  Of what? - chemical formula  In what state? - physical state n Remember the diatomic elements!

D. 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?

E. Describing Equations n Describing Coefficients:  individual atom = “atom”  covalent substance = “molecule”  ionic substance = “unit” 3 molecules of carbon dioxide 2 atoms of magnesium 4 units of magnesium oxide 3CO 2  2Mg  4MgO 

E. Describing Equations How many? Of what? In what state? Zn(s) + 2HCl(aq)  ZnCl 2 (aq) + H 2 (g)

IIIIIIIVV Balancing Equations

Counting Atoms n When coefficients are included in a balanced chemical equation they influence the total number of atoms of each element present n To determine the number of atoms of each element in a compound multiply the coefficient by the subscript of the atom

Counting Atoms 3NH 4 Cl N  H  Cl  1 x 3 = 3 N atoms 1 x 3 = 4 12 H atoms 3 Cl atoms

Counting Atoms 1) 5H 3 PO 4 2) 3NaOH 3) 2K 2 C 2 H 3 O 2 4) Mg 3 (PO 4 ) 2

More Counting Atoms 1) 3Al(NO 3 ) 3 2) 3Al 2 (SO 4 ) 3 3) 2Ca(OH) 2 4) 3NH 4 NO 3

A. 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!!!

B. 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”

How to Balance n First, you need to count all the atoms H 2 + O 2  H 2 O H O

How to Balance n Add coefficients to change the number of atoms H 2 + O 2  H 2 O H O

C. Balancing Example Aluminum and copper(II) chloride react to form copper and aluminum chloride.

Practice… Sn +HF  SnF 2 + H 2 Cu+ AgNO 3  Cu(NO 3 ) 2 +Ag Sn +2HF  SnF 2 + H 2 Cu+ 2AgNO 3  Cu(NO 3 ) 2 + 2Ag

Now You Try! WO 3 +H 2  W +H 2 O Fe+O 2  Fe 2 O 3 SiO 2 +C  SiC+CO WO 3 +3H 2  W +3H 2 O 4Fe+3O 2  2Fe 2 O 3 SiO 2 +3C  SiC+2CO

n C 6 H 14 + O 2  CO 2 + H 2 O n Pb(NO 3 ) 2 + Fe 2 (SO 4 ) 3  PbSO 4 + Fe(NO 3 ) 3 n Al 2 O 3 + NaOH + H 2 O  NaAl(OH) 4 2C 6 H O 2  12CO H 2 O 3Pb(NO 3 ) 2 + Fe 2 (SO 4 ) 3  3PbSO 4 + 2Fe(NO 3 ) 3 Al 2 O 3 + 2NaOH + 3H 2 O  2NaAl(OH) 4