Chemical Reactions

Chemical Reactions -Chemical change- a change of one or more substances into other substances -A chemical change is also known as a chemical reaction. Ex.: An iron bar rusts

Signs of a Chemical Reaction 1. formation of a precipitate precipitate- solid formed that is different from the reactants 2. a color change occurs 3. formation of a gas (bubbles) 4. energy change: temperature or light

Expressing Chemical Reactions -Reactions can be expressed as word equations or as chemical equations. Ex.: Silver tarnishing (reaction between silver and the sulfur in air) Word equation: silver + sulfur -> silver sulfide Chemical Equation: Ag +S8 -> Ag2S

Expressing Chemical Reactions Ex.: Silver tarnishing (reaction between silver and the sulfur in air) Word equation: silver + sulfur -> silver sulfide Chemical Equation: Ag +S8 -> Ag2S

Interpreting Chemical Equations Ag +S8 -> Ag2S  - used to separate the products & reactants; reads produces or yields + - separates the different substances on the reactants or products sides

Interpreting Chemical Equations Ag +S8 -> Ag2S Reactants- substances undergoing a change; found on the left side of the equation Reactants: Ag & S8 Products- new substance(s) formed; found on the right side of the equation. Products: Ag2S

HC2H2O2 (aq) + NaHCO3(s) --> NaC2H2O2(s) +H2O(l) + CO2(g) Chemical Symbols -Symbols are often used in chemical equations to describe the physical state of reactants and products. HC2H2O2 (aq) + NaHCO3(s) --> NaC2H2O2(s) +H2O(l) + CO2(g) -The symbols in parenthesis indicate the physical state of each substance. (s)- solid (l)- liquid (g)- gas (aq)- aqueous Aqueous- dissolved in water

Determining Physical State Physical State of Elements: - Metals: solids at room temperature *except for Mercury(Hg) it is a liquid - Nonmetals: solids, liquids or gases Gases: Noble Gases, H2, N2, O2, F2, &Cl2 Liquids: Bromine (Br2) Solids: All others

Determining Physical State Physical State of Compounds ( if not explicitly stated) - All ionic compounds are solids at room temperature - Covalent Compounds can be solids, liquids, or gases at room temperature: Common Compounds to know: H2O- liquid CO2, CO, NH3, CH4: gases - All acids are aqueous

Determining Physical States Aqueous- dissolved in water -Many chemical reactions are occur when one or more of the reactants are dissolved in water; however not all products formed from aqueous reactions are dissolved in water -In order for a product to be aqueous it must be soluble in water; this is determined using solubility rules.

Determining Physical States: Solubility Rules All common compounds of Group I and ammonium ions are soluble All nitrate, acetate, and chlorate compounds are soluble. All metal halide (halogen) compounds are soluble, except those of Ag+, Hg+ & Pb+2 All sulfates are soluble except: Ba+2, Sr+2, Ca+2, Pb+2,Ag+, & Hg+. Carbonate, hydroxides, oxides, silicates, and phosphates are insoluble, except for those listed in Rule 1. Sulfides are insoluble except for those listed in Rule 1 and Ba+2, Sr+2, Ca+2, & Mg+2.

DIATOMIC ELEMENTS Diatomic molecules (X2) -occur in nature bonded to themselves Rule of 7: H N,O,F Cl Br I

Examples Write the chemical equation for the reaction described below. Be sure to include the physical states of each substance. When aqueous solutions of Silver nitrate and Potassium chloride are mixed, potassium nitrate and silver chloride are produced. AgNO3(aq) + KCl(aq)  KNO3(aq) + AgCl(s) When sulfuric acid is added dropwise to mossy zinc, hydrogen and zinc sulfate are produced. H2SO4(aq) + Zn(s)  H2(g) + ZnSO4(aq)

BALANCING CHEMICAL EQUATIONS

Balancing Chemical Equations Law of Conservation of Mass- matter can neither be created nor destroyed For reactions: Reactants(#’s & types of atoms) = Products(#’s & types of atoms)

Balancing Chemical Equations Examples: H2CO3 (aq) --> H2O (l) + CO2 (g) H = 2 H = 2 C = 1 C = 1 O = 3 O = 3 Since both sides are equal the equation is said to be balanced!

Balancing Chemical Equations NaOH(aq) + CO2(g) --> Na2CO3(s) + H2O(l) Na = 1 Na = 2 O = 3 O = 4 H = 1 H = 2 C = 1 C = 1 Since the number of atoms of each element are not equal the equation is not balanced!

Balancing Chemical Equations 2NaOH(aq) + CO2(g) --> Na2CO3(s) + H2O(l) Na = 2 Na = 2 O = 4 O = 4 H = 2 H = 2 C = 1 C = 1 The 2 in front of the NaOH is called a coefficient. Coefficient- indicates how many units are involved. Coefficients are used to balance reactions.

Balancing Chemical Equations NaOH(aq) + CO2(g) --> Na2CO3(s) + H2O(l) Na = 1 Na = 2 O = 3 O = 4 H = 1 H = 2 C = 1 C = 1 Since the number of atoms of each element are not equal the equation is not balanced!

Balancing Chemical Equations NaOH(aq) + CO2(g) --> Na2CO3(s) + H2O(l) Na = 1 Na = 2 O = 3 O = 4 H = 1 H = 2 C = 1 C = 1 Since the number of atoms of each element are not equal the equation is not balanced!

Balancing Chemical Equations Practice: Balance the following reactions: 1. __Na + __Cl2 --> __NaCl 2. __ZnS + __AlP --> __Zn3P2 + __Al2S3 Note: You cannot change the subscripts because it will change the identity of the substance.

Balancing and Writing Chemical Equations (PreAP only) Write a balanced chemical equation for the reaction described below. When solid copper metal is placed in a solution of silver (I) nitrate, silver metal precipitates out and a solution of copper (II) nitrate is formed. Chemical Equation: Cu(s) + AgNO3(aq) → Ag(s) + Cu(NO3)2 Balanced Chemical Equation: Cu(s) + 2AgNO3(aq) → 2Ag(s) + Cu(NO3)2

Types of Chemical Reactions

Two substances combine to form 1 product. Synthesis Two substances combine to form 1 product. A + B → AB 2Na + Cl2 → 2 NaCl

A single compound breaks down into two or more substances. Decomposition A single compound breaks down into two or more substances. AB → A + B 2NaN3 → 2 Na + 3 N2

**An element replaces another element in a compound. An element and a compound react to form a new element & a new compound. **An element replaces another element in a compound. A + BC → AC + B Zn + 2 HCl --> ZnCl2 + H2 Single Replacement

Two compounds react to form two new compounds. Double Replacement Two compounds react to form two new compounds. **The positive ions are exchanged during the reaction. AB + CD → AD + CB NaOH + HCl --> NaCl + H2O

A hydrocarbon plus oxygen yields CO2 and water. Combustion A hydrocarbon plus oxygen yields CO2 and water. Hydrocarbon: CxHy CxHy + O2 → CO2 + H2O CH4 + O2 --> CO2 + H2O