CH.3 Balancing Reactions Reaction Types

A Chemical Reaction Reactants Products Types of Chemical reactions: There are many types of reactions but most can be classified into a few simple reactions types. How do you know a chemical reaction has occurred?

Chemical equations Chemist’s shorthand to describe a reaction. It shows: All reactants and products The state of all substances Any conditions used in the reaction CaCO3 (s) CaO (s) + CO2 (g)  Reactant Products A balanced equation shows the relationship between the quantities of all reactants and products.

Balancing chemical equations Each side of a chemical equation must have the same number of each type of atom. CaCO3 (s) CaO (s) + CO2 (g) Reactants Products 1 Ca 1 Ca 1 C 1 C 3 O 3 O

Balancing chemical equations Step 1 Count the number of atoms of each element on each side of the equation. Step 2 Determine which atom numbers are not balanced. Step 3 Balance one atom at a time by using coefficients in front of one or more substances. Step 4 Repeat steps 1-3 until everything is balanced.

Chemical Changes Temperature Changes Light Gas formation Precipitation Color changes

Types of Chemical Reactions Reactions involving electron transfer synthesis or combination decomposition Electrochemical or single replacement Combustion of hydrocarbons. Reactions that involve rearrangement but not necessarily involve electron transfer Metathesis or Double Substitution Precipitation reactions and Acid Base Reactions Polymerization reaction

Properties of aqueous solutions There are two general classes of solutes. Electrolytic ionic compounds in polar solvents dissociate in solution to make ions conduct electricity may be strong (100% dissociation) or weak (less than 100%) Nonelectrolytic do not conduct electricity solute is dispersed but does not dissociate

Dissolving ionic compounds When an ionic solid dissolves in water, the solvent removes ions from the crystal. NaCl + H2O  Na+(aq) + Cl- (aq)

Dissolving covalent compounds Covalent compounds do not dissociate. C6H12O6  C6H12O6 (aq)

Ionic equations When ionic substances dissolve in water, they dissociate into ions. AgNO3 Ag+ + NO3- KCl K+ + Cl- When a reaction occurs, only some of the ions are actually involved in the reaction. Ag+ + NO3- + K+ + Cl- AgCl(s) + K+ + NO3- H2O H2O

Ag+ + NO3- + K+ + Cl- AgCl(s) + K+ + NO3- Ionic equations To help make the reaction easier to see, we commonly list only the species actually involved in the reaction. Molecular equation KCl + AgNO3 AgCl(s) + KNO3 Full ionic equation Ag+ + NO3- + K+ + Cl- AgCl(s) + K+ + NO3- Net ionic equation Ag+ + Cl- AgCl(s) NO3- and K+ are referred to as spectator ions.

Some simple solubility rules All acids are soluble. All Na+, K+ and NH4+ salts are soluble. All nitrate and acetate salts are soluble. All chlorides except AgCl and Hg2Cl2 are soluble. PbCl2 is slightly soluble. All sulfates are soluble except PbSO4, Hg2SO4, SrSO4 and BaSO4. Ag2SO4 and CaSO4 are slightly soluble. All sulfides are insoluble except those of the Group IA (1), IIA (2) and ammonium sulfide. All hydroxides are insoluble except those of the group IA(1) and Ba(OH)2. Sr(OH)2 and Ca(OH)2 are slightly soluble.

Metathesis Reactions Precipitation reactions the formation of a solid upon mixing two solutions. Gas formation The formation of a gas when two mixtures are reacted Acid Base titration The neutralization of acids and bases to form a salt in water.

Precipitation NaCl + AgNO3 = AgCl (ppt) + NaNO3 Pb(NO3)2 + K2CrO4 = PbCrO4 (ppt) + KNO3

Gas Formation Vinegar + Baking soda = Carbon dioxide CH3COOH + NaHCO3 = NaCH3COO + H2O + CO2

Acid Base Neutralization Vinegar + Ammonia = Salt + water CH3COOH + NH4OH = NH4CH3COO + H2O

Common REDOX Reactions Combustion Corrosion Photosynthesis Kreb’s Cycle Synthesis and Decomposition Single Replacement

Oxidation numbers and the periodic table Some observed trends in compounds. Metals have positive oxidation numbers. Transition metals typically have more than one oxidation number. Nonmetals and semimetals have both positive and negative oxidation numbers. No element exists in a compound with an oxidation number greater than +8. The most negative oxidation numbers equals 8 - the group number

Oxidation Numbers 1. An atom in its elemental state has an oxidation number of 0. 2. An atom in a monatomic ion has an oxidation number identical to its charge. 3. An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have it were a monatomic ion. 4. The sum of the oxidation numbers is 0 for a neutral compound and equal to the net charge for a polyatomic ion.

Identifying oxidation-reduction reactions. Oxidation-Reduction - REDOX A chemical reaction where there is a net change in the oxidation number of one or more species. Both an oxidation and a reduction must occur during the reaction. Mg (s) + Cl2 (g) MgCl2 (s) Here the oxidation number of Mg has changed from zero to +2. Cl has changed from zero to -1.

2 Fe(NO3)3 (aq) + Zn(s) 2 Fe(NO3)2 (aq) + Zn(NO3)2 (aq) REDOX reactions Oxidation An increase in oxidation number. Reduction A decrease in oxidation number. If the oxidation number of any element changes in the course of a reaction, the reaction is oxidation-reduction. Example. 2 Fe(NO3)3 (aq) + Zn(s) 2 Fe(NO3)2 (aq) + Zn(NO3)2 (aq)

Half Reactions The reactions that show the loss or gain of electrons. 2 Mg + O2  2 MgO What are the oxidation states of each of the atoms in the above reaction? Mg  Mg+2 O2  2 O2-

Oxidation 2 Mg + O2  2 MgO Mg  Mg+2 12 p, 12 e 12 p, 10 e Mg  Mg+2 + 2 e The loss of electrons

Single replacement reaction Where one element displaces another in a chemical compound. H2 + CuO Cu + H2O In this example, hydrogen replaces copper. This type of reaction always involves oxidation and reduction (REDOX). Since one species is replacing another, there are no spectator ions.

Single Replacement

Synthesis Reaction The formation of a new compound from several substances 2 H2 + O2 = 2 H2O 2 Na + Cl2 = 2 NaCl CaO (lime water) + CO2 = CaCO3 (s)

Synthesis

Decomposition Reactions HgO = Hg + O2 H2O2= H2O + O2 with MnO2 catalyst

1774: Antoine Lavoisier solves the question of the burning process and chemical processes

Combustion The burning of a substance in the presence of oxygen. Gasoline burns when oxygen is taken into your engine, the products of the reaction are carbon dioxide and water C8H18 + O2 = CO2 + H2O Fe + O2 = Fe2O3 (rust)