1. Chapter 6: Chemical Reactions Chemistry 1020: Interpretive chemistry Andy Aspaas, Instructor

2. Chemical reactions Chemical reaction: a chemical change in which a new substance is formed Clues that a chemical reaction has occurred: –Color change, solid forms, bubbles form, heat/flame produced, heat absorbed –But, clues are not absolute - physical changes can sometimes produce those observations Chemical reactions involve rearrangement of atoms to form new substances

3. Chemical equations Reactants: chemicals present before a reaction Products: chemicals present after a reaction Chemical equation: representaton of a chemical reaction Reactants  Products CH 4 + O 2  CO 2 + H 2 O –Only carbon, hydrogen, and oxygen exist on both sides of equation

4. Writing and interpreting chemical equations The  in a chemical equation means “produces” or “yields” or “reacts to form” Physical states can also be indicated in a chemical reaction –(s): solid, (l): liquid, (g): gas, (aq): aqueous (dissolved in water) K(s) + H 2 O(l)  H 2 (g) + KOH(aq) –But, this equation is unbalanced (different quantities of atoms on each side of equation)

5. Balancing chemical equations CH 4 + O 2  CO 2 + H 2 O is unbalanced –Reactants: 1C, 4H, 2O –Products: 1C, 2H, 3O Law of conservation of mass: atoms must be neither created nor destroyed in a chemical reaction Coefficients are added to balance the equation –Whole numbers before a compound that indicates quantity –Ex. 2H 2 O means there are 2 water molecules present

6. Balancing chemical reactions Start with an unbalanced equation Start with the most complicated molecule and add coefficients so that the number of atoms it contains equals the number of those same atoms on the other side of the equation Proceed balancing other elements Check your work by totaling all the atoms on each side - the totals for the two sides should be identical

7. Practice balancing CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(g) K(s) + H 2 O(l)  H 2 (g) + KOH(aq) C 2 H 5 OH(l) + O 2 (g)  CO 2 (g) + H 2 O(g) –(start with C) NH 3 (g) + O 2 (g)  NO(g) + H 2 O(g)