IIIIIIIVV I.Intro to Reactions Ch. 8 – Chemical Reactions

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 Pt a catalyst is present (in this case, platinum)

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 formula units of aqueous copper(II) chloride to produce three atoms of copper and two formula 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)

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

Subscripts vs. Coefficients

E. 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 f.u. of aqueous zinc chlorideand one molecule of hydrogen gas.

IIIIIIIVV II. Balancing Equations

A. Balancing Steps 1.Write the unbalanced equation. 2.Count atoms on each side. 3. Add coefficients to make #s equal. Note: you may not change a subscript 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”

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

Balancing Equations ____C 3 H 8 (g) + _____ O 2 (g) ----> _____CO 2 (g) + _____ H 2 O(g) ____ B 4 H 10 (g ) + _____ O 2 (g) ----> ___ B 2 O 3 (g) + _____ H 2 O(g)

Balancing Equations Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate Na 3 PO 4 + Fe 2 O > Na 2 O + FePO 4 Na 3 PO 4 + Fe 2 O > Na 2 O + FePO 4