1 CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2

2 –Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken –Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.Introduction

3 Symbols represent elements C = carbon H = hydrogen Al = aluminum O = oxygen One capital letter, one lowercase formulas describe compounds H 2 O NaCl Al 2 O 3 chemical equations describe a chemical reaction Al + O 2 ---> Al 2 O 3 Al + O 2 ---> Al 2 O 3

4 Chemical Equations Depicts the kind of reactants and products and how much there are of eachDepicts the kind of reactants and products and how much there are of each 4 Al + 3 O 2 ---> 2 Al 2 O 3 4 Al + 3 O 2 ---> 2 Al 2 O 3 The numbers in the front are calledThe numbers in the front are called coefficients coefficients

5 reactants products –Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). A “+” sign separates molecules on the same side The arrow is read as “yields” Example C + O 2  CO 2 This reads “carbon plus oxygen react to yield carbon dioxide” Parts of a Reaction Equation

6 The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O 2  CO 2, contains the same information as the English sentence but has quantitative meaning as well.

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

8 notWhen balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. Changing the subscripts changes the compound. Subscripts are determined by the bonding. Balancing Equations

9 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.

10 Chemical Equations 4 Al + 3 O 2 ---> 2 Al 2 O 3 This equation means 4 Al atoms + 3 O 2 molecules --- produces---> 2 molecules of Al 2 O 3 2 molecules of Al 2 O 3

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12 There are four basic steps to balancing a chemical equation. 1.Make sure the formula is written correctly with reactants and products. 2.Make a list of elements for each side of the equation. 3.Count the atoms for each element on both sides. 4.Change coefficients one at a time and do a recount of atoms. 1.Save hydrogen for next to last and oxygen for last. 2.If you still can’t balance at this point, double all coefficents. Steps to Balancing Equations

13 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! Two hydrogen atoms from a hydrogen molecule (H 2 ) combines with one of the oxygen atoms from an oxygen molecule (O 2 ) to form H 2 O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H 2 molecule) to make a second H 2 O molecule.

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

15 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)

16 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