Chemical Equations and Reactions Section 8.1

What is a chemical reaction? Any change where a new chemical is formed. Indications of a chemical reaction: Change in temperature Production of light/color change Change in state

Changes in State Not all changes in state indicate a chemical reaction. When water melts it is just a physical change. When two liquids are mixed and a solid is formed that solid is called a precipitate.

What is a chemical equation? Chemical equations represent chemical reactions They: Identify reactants and products Contain the correct chemical formulas Satisfy the Law of Conservation of Mass. Coefficients are used to tell how many of each chemical is present.

Types of chemical equations Word Equations methane + oxygen --> carbon dioxide + water Formula Equations CH4(g) + 2O2(g) --> CO2(g) + 2H2O(g)

State Symbols State symbols - subscripted after each element or compound. (s) - Solid (l) - Liquid (g) - Gas (aq) - Aqueous. This means that a substance is dissolved in water.

Other Symbols Yields sign - arrow that separates reactants from products. Triangle above yields sign - reactants must be heated. “Up” arrow- a gas is formed “Down” arrow - a precipitate is formed <--------> - reversible reaction

Law of Conservation of Mass Matter cannot be created nor destroyed in a chemical reaction so we have to have the same number of each element on both sides of the equation. This is done by balancing the equation with coefficients. Zn(s) + 2HCl(aq) --> H2(g) + ZnCl2(aq)

Let’s balance some equations The rusting of Iron: Fe + O2 ----> Fe2O3 4 Fe + 3 O2 ----> 2 Fe2O3 Combustion of octane in an engine: C8H18 + O2 ----> H2O + CO2 2 C8H18 + 25 O2 ----> 18 H2O + 16 CO2

The Diatomic Elements There are seven diatomic elements. They are never found as individual atoms but are bonded as a pair of atoms: H2, N2, O2, F2, Cl2, Br2, I2 You must memorize these!

Let’s write some equations Calcium reacts with sulfur to produce solid calcium sulfide. Ca (s) + S (s) ----> CaS (s) Hydrogen reacts with fluorine to produce hydrogen fluoride gas. H2 (g) + F2 (g) ----> 2 HF (g)

2 Al(s) + 3 ZnCl2(aq) ----> 3 Zn(s) + 2 AlCl3(aq) Here is another Aluminum reacts with a solution of zinc chloride to produce zinc and aqueous aluminum chloride 2 Al(s) + 3 ZnCl2(aq) ----> 3 Zn(s) + 2 AlCl3(aq)