Chapt. 9 “CHEMICAL REACTIONS & EQUATIONS” What is a chemical reaction? How do chemists represent what occurs in a chemical reaction? T. Witherup 2006

OBJECTIVES Describe the characteristics of a chemical reaction. Explain why chemical reactions occur. Learn how to write balanced chemical equations. Learn the six basic classifications of chemical reactions.

-What is a chemical reaction? A Chemical Reaction is a process in which one or more substances change into new substance(s) having different physical and chemical properties. -What is the evidence for a chemical reaction? Energy Changes: heat, light, sound or electrical energy changes. New Substances: gas, solid, liquid, water, color changes, odor changes. 9-1 The Nature of Chemical Reactions

Why do chemical reactions occur? The arrangement of electrons in atoms determines the direction and outcome of chemical reactions. –Remember the “Octet Rule”? Energy changes are involved in chemical reactions, and reactions occur because existing bonds are broken, atoms are rearranged, and new bonds are formed. –Energy may be released, exothermic (burning methane). –Energy may be absorbed, endothermic (ice packs). Overall, the energy of the products is lower than that of the reactants when reactions occur, but energy is sometimes needed to start a reaction.

9-2 Chemical Equations Word Equations –“–“Sodium metal burns in chlorine gas to form solid sodium chloride.” Formula Equations –N–Na + Cl 2  NaCl (Why is it Cl 2, not simply Cl?) Balancing Equations –I–If the mass is to remain constant before and after a chemical reaction, the number of atoms of each element must be the same before and after the chemical reaction. (What Law is this?) –T–Therefore we write, 2Na + Cl 2  2NaCl

–Reactants: starting substances (left). –Products: ending substances (right). –Plus sign (+) means “reacts with.” –Arrow sign (  ) means “yields.” –Coefficients: how many units of each substance are present (whole numbers written before reactants & products). –Subscripts: how many of each atom type are in a substance (subscripts in formulas). –REMINDER: Don’t change the subscripts of a correctly written formula! Chemical Equation Definitions

Chemical Equation Labels Definitions: (s) = solid (l) = liquid (g) = gas (aq) = aqueous (water) solution –Therefore our complete equation is: 2Na(s) + Cl 2 (g)  2NaCl(s)

Other Abbreviations Used (s)  means “a solid (precipitate) is formed.” (g)  means “a gas is produced.” (aq) means “dissolved in water solution.” heat means “reacts with heat.” (or Δ ) catalyst means “reacts with a catalyst present.” (A catalyst is a substance that speeds up a reaction, but does not get consumed in the reaction.) Precipitate is a solid that suddenly appears when that phase was not initially present, i.e. two liquids are mixed and you get a solid.

9-3 Classifying Chemical Reactions Chemists recognize six general types of chemical reactions, although there is some overlap among these categories. Learning the types of chemical reactions will help you predict the products that will form.

Types of Reactions - 1 Synthesis - two or more reactants combine to form one product. A + B  AB A + 2 B  AB 2 N H 2  2 NH 3

Types of Reactions - 2 Decomposition - one reactant breaks into two or more products. AB  A + B 2 A 2 B  4 A + B 2 2 KClO 3  2 KCl + 3 O 2

Types of Reactions – 3 Single Replacement/Displacement - an element reacts with a compound, and part of the compound is replaced by the element. (Metals replace metals, and nonmetals replace nonmetals.) A + BC  AC + B Cu + H 2 SO 4  CuSO 4 + H 2 (g) 

Types of Reactions – 4 Double Replacement/Displacement - one compound reacts with another compound, and they change partners. (Each cation goes with the opposite anion.) AB + CD  AD + CB CaCl 2 + (NH 4 ) 2 O  CaO + 2 NH 4 Cl

Types of Reactions – 5 Combustion - a substance reacts with oxygen, and everything ends up attached to oxygen. CH O 2  CO H 2 O

Types of Reactions – 6 Redox (Oxidation/Reduction) - a reaction involving substances that gain or lose electrons; both processes occur together, and the reactions may be simple or complex. (Chapter 20) K 2 Cr 2 O HI  2CrI 3 + 2KI + 3 I 2 + 7H 2 O

HOW TO BALANCE EQUATIONS See my handout sheet for guidelines. PRACTICE!

More About Reaction Type #3 Single Replacement Reactions follow the “Activity Series” In a single replacement reaction, one element substitutes for another in a compound. The “Activity Series” data, collected over many years, helps us predict which elements will replace another and which will not. (Page 295 shows a partial “Activity Series” table.) Each element on the list will replace any element below it from an aqueous solution. –Thus, Li will replace all of those shown. –And Zn will replace Fe, Cd, Co, etc., but it will not replace the Alkali and Alkaline Earth elements.

Mini Quiz - Classify the following: H 2 + O 2  H 2 O BaCl 2 + Br 2  BaBr 2 + Cl 2 BaCl 2  Ba + Cl 2

Did we cover the “OBJECTIVES”? Describe the characteristics of a chemical reaction. Explain why chemical reactions occur. Learn how to write balanced chemical equations. Learn the six basic classifications of chemical reactions.