Signs of Chemical Reactions There are five main

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

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.

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

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. methane + oxygen Reactant CH 4 + O 2 carbon dioxide + water Product CO 2 + H 2 O

Balancing Equations

Visualizing a Chemical Reaction 2 Na + Cl 2 2 Na. Cl

Visualizing a Chemical Reaction 2 Na + Cl 2 2 Na. Cl

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

Balancing Equations

Unbalanced and Balanced Equations H Cl Cl H H Cl H 2 + Cl 2 HCl (unbalanced) reactants H Cl 2 2 H H Cl Cl Cl H 2 + Cl 2 2 HCl (balanced) reactants products 1 1 Cl H Cl 2 2 products 2 2

Chemical Equations 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. Lavoisier, 1788

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

Symbols Used in Equations • Solid ___ • Liquid (l) • Gas ___ • Aqueous solution (aq) H 2 SO 4 • Catalyst • Change of temperature ( )