Ch. 7 – Chemical Reactions Intro to Reactions

A.Signs of a Chemical Reaction Change in heat and light Formation of a gas Formation of a precipitate Color change Odor

A.Signs of a Chemical Reaction Heat Changes: Exothermic – A release of heat, it feels hot Endothermic – An absorption of heat, it feels cold

A. Heat Transfer A B Why does A feel hot and B feel cold? Heat flows from A to your hand = hot. Heat flows from your hand to B = cold. 80ºC A 10ºC B

B.Law of Conservation of Mass In a chemical reaction, matter is not created or destroyed. Atoms can only rearrange. Discovered by Lavoisier. 4H 2O 4H 2O

A+BC+D REACTANTS PRODUCTS C. Chemical Reaction A change in which one or more substances are converted to different substances. A+BC+D REACTANTS PRODUCTS

C. Chemical Equations Reactants = Starting substances Products = Substances Formed

C. Chemical Equations

C. Chemical Equations A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols. Chemical equations written with symbols makes it much easier to tell what is happening in the reaction.

C. Chemical Equations Aqueous lead(II) nitrate plus two units of aqueous potassium iodide produces solid lead(II) iodide and two units of aqueous potassium nitrate. Pb(NO3)2 2 KI  + PbI2 2 KNO3 + Coefficient - # of units of each substance

II. Balancing Equations Ch.7– Chemical Reactions C. Johannesson

Coefficient  subscript = # of atoms A. Balancing Steps 1. Write the unbalanced equation. 2. Count atoms on each side. 3. Add coefficients to make #s equal. Coefficient  subscript = # of atoms 4. Reduce coefficients to lowest possible ratio, if necessary. 5. Double check atom balance!!!

B. Helpful Tips Balance one element at a time. Update ALL atom counts after adding a coefficient. If an element appears more than once per side, balance it last. Balance polyatomic ions as single units. “1 SO4” instead of “1 S” and “4 O”

C. Balancing Example Aluminum and copper(II) chloride react to form copper and aluminum chloride. Al + CuCl2  Cu + AlCl3 2 3 3 2

Homework Assignment Page 235: 33-36 Show inventory and balance the reactions

D. Writing Balanced Equations Determine the reactants and products Write the reactants on the left side of the equation and the products on the right Show an equal number of atoms for each element on both sides of the equation.

Example When propane gas burns in the air, the reactants are propane (C3H8)and oxygen (O2). The products formed are carbon dioxide (CO2)and water (H2O). Step 1: Find the reactants and products.

Example Step 2: Write a chemical equation C3H8 + O2 CO2 + H2O

Example Step 3: Balance the chemical equation *In balancing a chemical equation, change only the coefficients. Never change the subscripts. C3H8 + O2 CO2 + H2O 5 3 4

D. Writing Reactions Identify the reactants and products and write the equation for each of the following chemical reactions. The key to getting this right is to balance each of the pieces correctly (Write the balanced formula. Remember from chapter 4?) After writing the reactions balance them.

D. Writing Reactions There are a few elements that need to be treated in a special way because of how they bond with each other. Example: Elemental oxygen, is never found by itself. It is always found as O2. Oxygen is one of the diatomic elements. This means they are always paired up. There are 7 diatomic elements: H, I, Br, O, N, Cl, F. (Remember HI BrONClF!) These elements are all gases!

HIBrONClF D. Writing Reactions The Seven Diatomic Elements Br2 I2 N2 Cl2 H2 O2 F2 HIBrONClF

Writing Reactions When writing the balanced formula for elemental metals they are always written by themselves. Example: Elemental Copper is just Cu. Elemental Aluminum is just Al. This will be the case for all elemental metals.

Writing Reactions Example Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas. Circle the reactants, underline the products

Writing Reactions Write the following reactions and then balance them: 1. Solid Ammonium dichromate decomposes to form solid chromium (III) oxide, gaseous nitrogen, and gaseous water. 2. Gaseous ammonia (NH3) reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water.

Assignment Page 234 #’s 9-11, 13, 18, 21, 25 I want you to write the question, circle the reactants, underline the products. Then write out the reaction in symbol form and balance it.

III. Types of Chemical Reactions Ch. 8 – Chemical Reactions III. Types of Chemical Reactions

A + O2  B CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) A. Combustion the burning of any substance in O2 to produce heat (EXOTHERMIC RXN) A + O2  B CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)

A. Combustion Products: contain oxygen hydrocarbons form CO2 + H2O 4 2 4 2 Na(s)+ O2(g)  Na2O(s) C3H8(g)+ O2(g)  5 3 4 CO2(g)+ H2O(g)

B. Composition the combination of 2 or more substances to form a compound only one product A + B  AB

B. Composition H2(g) + Cl2(g)  2 HCl(g) C. Johannesson

Al(s)+ Cl2(g)  2 3 2 AlCl3(s) B. Composition Products: ionic - cancel charges covalent - hard to tell Al(s)+ Cl2(g)  2 3 2 AlCl3(s)

AB  A + B C. Decomposition a compound breaks down into 2 or more simpler substances only one reactant AB  A + B

C. Decomposition 2 H2O(l)  2 H2(g) + O2(g) C. Johannesson

C. Decomposition 2 2 KBr(l)  K(s) + Br2(l) Products: binary - break into elements others - hard to tell 2 2 KBr(l)  K(s) + Br2(l)

A + BC  B + AC D. Single Replacement one element replaces another in a compound metal replaces metal (+) nonmetal replaces nonmetal (-) A + BC  B + AC C. Johannesson

Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s) D. Single Replacement Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s) C. Johannesson

D. Single Replacement Products: metal  metal (+) nonmetal  nonmetal (-) Fe(s)+ CuSO4(aq)  Cu(s)+ FeSO4(aq)

AB + CD  AD + CB E. Double Replacement ions in two compounds “change partners” cation of one compound combines with anion of the other AB + CD  AD + CB C. Johannesson

Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq) E. Double Replacement Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq) C. Johannesson

E. Double Replacement Products: switch negative ions Pb(NO3)2(aq)+ KI(aq)  2 2 PbI2(s)+ KNO3(aq) NaNO3(aq)+ KI(aq)  N.R.

Decide the RXN Type WO3 + 3H2  W + 3H2O RbCl + 2O2  RbClO4 RbBr + AgCl  AgBr + RbCl 2KNO3  2KNO2 + O2 3Mg + N2  Mg3N2 2C2H6 + 7O2  4CO2 + 6H2O

Chemical Changes Chemical reactions can be classified into 5 different types. These 5 types are Single Displacement, Double Displacement, Decomposition, Synthesis, and Combustion. You will be given the general form for each of the 5 reactions.

Single Replacement In a single displacement reaction one element displaces or changes places with another element in a compound. General Form: Element + Compound Element + Compound Ex. Cl2 + 2KBr  2KCl + Br2 Chlorine changes places with Bromine

Single Replacement Look at the reactants in the following single displacement reaction and decide which elements will change places: Al + Fe2O3  Be sure to find the charges of all the elements. Which ones will switch?

Double Replacement In a double displacement reaction the positive and negative parts of two compounds switch places. General form: Compound + Compound  Compound + Compound Ex: PbCl2 + Li2SO4  PbSO4 + 2LiCl

Double Replacement Look at the reactants for the following double displacement reaction and predict what will happen. ZnBr2 + AgNO3 

Decomposition In a decomposition reaction a compound break up or decompose down into simpler substances. General form: Compound  two or more elements or compounds Ex: 2Ag2O  4Ag + O2

Synthesis In a synthesis reaction two or more substances combine to form one new substance. General form: Element or compound + Element or compound  Compound Ex: CaO + SiO2  CaSiO3 Ex: 2H2 + O2  2H2O

Combustion Combustion occurs when an organic compound (a compound of Carbon) reacts with oxygen in the air and burns. When this occurs the products are always Carbon dioxide and Water. General form: Hydrocarbon + oxygen  Carbon dioxide + Water Ex: CH4 + 2O2  CO2 + 2H2O

Examples: Decide the RXN Type WO3 + 3H2  W + 3H2O RbCl + 2O2  RbClO4 RbBr + AgCl  AgBr + RbCl 2KNO3  2KNO2 + O2 3Mg + N2  Mg3N2 2C2H6 + 7O2  4CO2 + 6H2O