Signs of Chemical Reactions

1. Signs of Chemical Reactions There are five main signs that indicate a chemical reaction has taken place: release input change in color change in odor production of new gases or vapor input or release of energy difficult to reverse

2. 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.

3. CHEMICAL REACTIONS Reactants: Zn + I2 Product: Zn I2

4. Word Equations • A WORD EQUATION describes chemical change using the names of the reactants and products. Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water. carbon dioxide + water methane + oxygen Reactant Product O2 H2O CO2 CH4 + +

5. Balancing Equations

6. Visualizing a Chemical Reaction 2 2 Na + Cl2 NaCl

7. Visualizing a Chemical Reaction 2 2 Na + Cl2 NaCl

8. Chemical Equations 4 Al(s) + 3 O2(g) ---> 2 Al2O3(s) This equation means 4 Aluminum atoms + 3 oxygen molecules produces 2 molecules of aluminum oxide

9. Balancing Equations

10. Cl H H H Cl Cl Cl H reactants products reactants products H H Cl Cl Unbalanced and Balanced Equations H Cl Cl Cl H H H2 + Cl2 2 HCl (balanced) (unbalanced) H2 + Cl2 HCl 2 1 2 2 1 2 2 2

11. Chemical Equations Lavoisier, 1788 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.

12. Chemical Equations 4 Al(s) + 3 O2(g) 2 Al2O3(s) product reactants Depict the kind of reactants and products and their relative amounts in a reaction. The letters (s), (g), and (l) are the physical states of compounds.

13. Symbols Used in Equations • Solid ___ • Liquid (l) • Gas ___ • Aqueous solution (aq) • Catalyst H2SO4 • Change of temperature ()