Chemical Reactions

Evidence of a Chemical Reaction Energy release (heat or light)

Evidence of a Chemical Reaction Color change

Evidence of a Chemical Reaction Odor gas bubbles smoke formation

Evidence of a Chemical Reaction Precipitation

Representing Chemical Equations reactant 1 + reactant 2 → product 1 and product 2 produces Product(s) must be different than the reactants

Representing Chemical Equations Simply a “recipe” + + → Eggs and butter and flour produce a cake Word equations name the reactants and products. What is missing from this recipe? AMOUNTS!!!

Representing Chemical Equations + → Iron (II) and oxygen produce iron oxide Fe (s) + O2 (g) → FeO (s) What is missing? Amounts! How do you figure out the amounts needed and produced?

Balancing Equations Based on the Law of Conservation of Mass “Matter is not created or destroyed in a reaction, just rearranged.” Fe + O-O → Fe O What is wrong here? an oxygen atom is missing

Balancing Equations Atom inventory Fe + O2 → Fe O Reactant side Product side Fe O Fe O 1 2 1 1 Fe + O2 → Fe O Final equation: Fe + O2 → Fe O 2 2 2 2 2 2

Balancing Equations Steps: 1. Write the skeleton equation (can’t escape formula writing) 2. Count atoms in reactants 3. Count atoms in products 4. Change coefficients to make atom counts match. Never change subscripts. 5. Reduce coefficients if necessary.

Practice Problems ____ AlBr3 + ____ K  ____ KBr + ____ Al ____ P4 + ____ Br2  ____ PBr3 ____ P4 + 6 Br2  4 PBr3

Symbols used in Equations + separates two reactants or two products → “Yields” separated reactants from products Designates a reactant in the solid state Designates a reactant in the liquid state (g) Designates a reactant in the gaseous state (aq) Designates a reactant in the aqueous state → → indicates that heat is supplied to a reaction → Indicates that a catalyst is used (here, platinum) heat Pt

Symbols used in Equations #1 separates two reactants or two products #2 “Yields” separated reactants from products #3 Designates a reactant in the solid state Designates a reactant in the # 4 state (g) Designates a reactant in the # 5 state #6 Designates a reactant in the aqueous state # 7 indicates that heat is supplied to a reaction → Indicates that a # 8 is used (here, platinum) # 9 name the diatomic elements Pt

Classifying Chemical Reactions Five general types Synthesis one new product Solid sodium and chlorine gas form solid sodium chloride 2Na(s) + Cl2(g) → 2NaCl(s) + → http://jchemed.chem.wisc.edu/jcesoft/cca/cca0/Movies/NACL1.html http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/04chemrxnpage/chemicalreactions.htm

Classifying Chemical Reactions Combustion Must have O2 as a reactant 2 Mg + O2 → 2 MgO + → Combustion reactions are exothermic; energy is produced.

Classifying Chemical Reactions Combustion of hydrocarbons (compounds containing hydrogen and carbon) CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) Most fuels are hydrocarbons

Classifying Chemical Reactions Decomposition Only one reactant Hydrogen peroxide decomposes to release oxygen gas and leave water on your skin H2O2(aq) → O2 (g) + H2O(l) 2 H2O2(aq) → O2 (g) + 2 H2O(l)

Classifying Chemical Reactions Single replacement reactions One reactant is an element Copper reacts with silver nitrate to produce silver and copper nitrate. Cu(s) + AgNO3(aq) → Ag(s) + Cu(NO3)2(aq)

Single replacement reactions Activity Series of Metals Lithium Iron Rubidium Nickel Potassium Tin Calcium Lead Sodium Copper Magnesium Silver Aluminum Platinum Manganese Gold Zinc Decreases Decreases

Single replacement reactions Halogens Fluorine Chlorine Bromine Iodine Decreases

Classifying Chemical Reactions Double replacement reactions both reactants are ionic compounds A gas, liquid, or solid must be produced Silver nitrate and sodium chloride react to produce sodium nitrate and silver chloride AgNO3 (aq) + Na Cl (aq) → NaNO3 (aq) + AgCl (s) Ag+ Cl- Na+ NO3- Na+ NO3- AgCl

Summary Reaction Classification Class of Reaction Reactants Probable products Synthesis Two or more substances One compound Combustion Some substance and oxygen An oxide H20 and CO2 for hydrocarbon combustion Decomposition Two or more (elements or compounds) Single-replacement An element and a compound A new compound and the replaced element Double-replacement Two compounds Two different compounds including a s, l, g

Reactions in Aqueous Solutions Review: Solution – homogeneous mixture Solute – substance dissolved Solvent – most plentiful substance in the solution Water is the “universal solvent” Other substances can also be solvents

Reactions in Aqueous Solutions How does water dissolve ionic compounds? Water is a polar molecule. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/molvie1.swf http://programs.northlandcollege.edu/biology/Biology1111/animations/dissolve.html

Reactions in Aqueous Solutions Reactions that form solids Lead nitrate and potassium iodide Two clear solutions – mix – precipitate forms Pb +2 NO3 - K+ I-

Reactions in Aqueous Solutions Reactions that form solids What happened on an atomic level? + → Lead and iodide form a new compound Nitrate and potassium are still dissolved ions – they did nothing but “watch” – spectator ions NO3 – Pb +2 K+ I- K+ NO3 – PbI2

Reactions in Aqueous Solutions Reactions that form solids Complete ionic equation Pb +2 + NO3 - + K+ + I- → PbI2 + NO3 - + K+ Spectator ions Net ionic equation Pb +2 + I- → PbI2

Reactions in Aqueous Solutions Reactions that form liquids HBr (aq) + NaOH(aq) → H2O (l) + NaBr(aq) Complete ionic equation .H+(aq) + Br-(aq) + Na+(aq) + OH -(aq) → H2O(l) + Na+(aq) + Br-(aq) Net ionic equation H+(aq) + OH -(aq) → H2O(l)

Reactions in Aqueous Solutions Reactions that form gases 2HI (aq) + Li2S (aq) → H2S (g) + 2LiI (aq) Complete ionic equation 2H+ (aq) + 2I- (aq) + 2Li+ (aq) + S2-(aq) → H2S(g) + 2Li+(aq) + 2I- (aq) Net ionic equation 2H+ (aq) + S2-(aq) → H2S(g)