Ch. 8 – Chemical Reactions Intro to Reactions

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

B.Law of Conservation of Mass mass is neither created nor destroyed in a chemical reaction total mass stays the same atoms can only rearrange 4 H 2 O 4 H 2 O 36 g 4 g 32 g

C. Chemical Equations A+B  C+D REACTANTS PRODUCTS

C. Chemical Equations Pt a catalyst is present (in this case, platinum)

2H2(g) + O2(g)  2H2O(g) D. Writing Equations Identify the substances involved. Use symbols to show: How many? - coefficient Of what? - chemical formula In what state? - physical state Remember the diatomic elements.

D. Writing Equations 2 Al (s) + 3 CuCl2 (aq)  3 Cu (s) + 2 AlCl3 (aq) 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? 2 Al (s) + 3 CuCl2 (aq)  3 Cu (s) + 2 AlCl3 (aq)

E. Describing Equations Describing Coefficients: individual atom = “atom” covalent substance = “molecule” ionic substance = “formula unit” 3CO2  2Mg  4MgO  3 molecules of carbon dioxide 2 atoms of magnesium 4 formula units of magnesium oxide

Subscripts vs. Coefficients

E. Describing Equations Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g) How many? Of what? In what state? One atom of solid zinc reacts with two molecules of aqueous hydrochloric acid to produce one f.u. of aqueous zinc chloride and one molecule of hydrogen gas.

II. Balancing Equations

Coefficient  subscript = # of atoms 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 Balance one element at a time. Update ALL atom counts after adding a coefficient. If an element appears more than once per side, balance it last. Balance polyatomic ions as single units. “1 SO4” instead of “1 S” and “4 O”

C. Balancing Example 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 3 3 2 2  1 1 Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 3 3 2 2  1 1 2 3  2  6 3  6   3

Balancing Equations ____C3H8(g) + _____ O2(g) ----> _____CO2(g) + _____ H2O(g) ____B4H10(g) + _____ O2(g) ----> ___ B2O3(g) + _____ H2O(g)

Balancing Equations Na3PO4 + Fe2O3 ----> Na2O + FePO4 Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate Na3PO4 + Fe2O3 ----> Na2O + FePO4