1. 1 CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2 Adapted from www.chemistrygeek.com

2. 2 Evidence for a Chemical Reaction Color change Gas production –Bubbles, smoke, odor Heat transfer –Spontaneous change in temperature, hot or cold Formation of Precipitate –Solid that settles from mixing two AQUEOUS solutions Change in pH

3. 3 Evidence for a Chemical Reaction

4. 4 –occur when bonds between the outermost parts of atoms are formed or broken –involve changes in matter, the making of new materials with new properties, and energy changes. –Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction Chemical Reactions

5. 5 Chemical Equations Represent the kind of reactants and products in a reaction and their relative amounts. 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) The numbers in the front are called stoichiometric coefficients

6. 6 reactantsproductsshows the conversion of reactants into products Reactants are the starting materials (left) Products are the ending materials (right) + sign separates molecules on the same side The arrow is read as “yields” Example C(s) + O 2 (g)  CO 2 (g) Parts of a Chemical Equation

7. 7 The charcoal used in a grill is carbon The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O 2  CO 2

8. 8 Solid (s) Liquid (l) Gas (g) Aqueous solution (aq) Catalyst H 2 SO 4 Escaping gas (  ) Change of temperature (  ) Symbols Used in Equations

9. 9 Learning Check Write the unbalanced equation (including symbols for states) for each of the following statements –Solid magnesium reacts with liquid water to form solid magnesium hydroxide and hydrogen gas –Solid ammonium dichromate decomposes to solid chromium (III) oxide, nitrogen gas, and water vapor –Gaseous ammonia reacts with gaseous oxygen to form gaseous nitrogen monoxide and water vapor

10. 10 Because of the principle of the conservation of matter, an equation must be balanced.Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides.It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788 Chemical Equations

11. 11 not –When balancing a reaction you may add coefficients in front of the compounds, but you may not change the subscripts. –Changing the subscripts changes the compound. –Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent) Balancing Equations

12. 12 Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

13. 13 Balancing Equations 4 Al(s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) This equation means 4 Al atoms + 3 O 2 molecules  2 molecules of Al 2 O 3

14. 14 There are three basic steps to balancing a chemical equation. 1.Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. 2.Adjust coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element. 3.Check your answer to see if: –The numbers of atoms on both sides of the equation are now balanced. –The coefficients are in the lowest possible whole number ratios. (reduced) Steps to Balancing Equations

15. 15 Some Suggestions to Help You Some Helpful Hints for balancing equations: Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last. IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element) (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

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17. 17 Balancing Equations ___ H 2 (g) + ___ O 2 (g) ---> ___ H 2 O(l) 22 What Happened to the Other Oxygen Atom????? This equation is not balanced!

18. 18 Balancing Equations ___ Al(s) + ___ Br 2 (l) ---> ___ Al 2 Br 6 (s) 23

19. 19 Balancing Equations K + H 2 O  H 2 +KOH NH 3 +O 2  NO+H 2 O SiO 2 +HF  SiF 4 +H 2 O NH 4 NO 2  N 2 +H 2 O NO  N 2 O+NO 2

20. 20 Balancing Equations C 3 H 8 + O 2  CO 2 + H 2 O B 4 H 10 + O 2  B 2 O 3 + H 2 O Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate