Chapter 8: Chemical Reactions College Prep Chemistry Mr. Pompilii Chapter 8: Chemical Reactions

Section 8.1: Describing Chemical Change Word Equations: describes chemical reactions using words instead of symbols

Chemical Equations Definition: chemical formulas written to express chemical and physical changes Skeletal Equation: chemical equation that does not indicate the relative amounts of the reactants and products involved in the reaction

Section 8.1: Describing Chemical Change Reactants -= those substances that are reacting or changing written on the left Products - those substances that are being formed from the reactants written on the right

Section 8.1: Describing Chemical Change Coefficient: number used to represent the amount of a substance in a chemical reaction Located before the chemical

Symbols Used in Chemical Rxns + = used to separate two reactants or two products  = “yields” separates reactants from products ↔ or  = used in place of the  for reversible rxns (s) = solid state (g) = gaseous state

Symbols Used in Chemical Rxns (l) = liquid state; only used for water, mercury, bromine usually (aq) = aqueous soln (made w/ H2O)  or heat = written over the  to show heat was added catalyst = the actual substance is written over the  to show the presence of a catalyst

Catalyst Definition: substance that speeds up the rate of a reaction but that is not used up in the reaction Pt Not a reactant  Not a product Formula written over the “yields” sign Examples: MnO2, Pt, heat, …

Practice Problems Write a skeletal equation: 1. Sulfur burns in oxygen to form sulfur dioxide 2. heating potassium chlorate in the presence of the catalyst manganese (IV) oxide produces oxygen gas and solid potassium chloride

Practice Problem Answers 1. S (s) + O2 (g)  SO2 (g) MnO2, Δ 2. KClO3 (s) --- O2 (g) + KCl (s)

Practice Problem 2 Write a sentence: 3. KOH (aq) + H2SO4 (aq)  H2O (l) + K2SO4 (aq) 4. Na (s) + H2O (l)  NaOH (aq) + H2 (g)

Practice Problem Answers 2 3. aqueous potassium hydroxide and aqueous sulfuric acid produces water and aqueous potassium sulfate 4. solid sodium is added to water hydrogen gas and aqueous sodium hydroxide are produced

Balancing Equations Definition: each side of the equation has the same number of atoms of each element Must obey the Law of Conservation of Mass http://www.physicsgames.net/game/Ba lance.html

Balancing Equations Sometimes we write equations that are already balanced C (s) + O2 (g)  CO2 (g) Other times the skeleton equation is not balanced C (s) + O2 (g)  CO (g)

Rules for Balancing Eqns Write an atomic inventory Balance all non-H’s and O’s first Add a coefficient to the front of the substance with the lesser side. NEVER change a subscript! Re-tally your atomic inventory after putting a coefficient in

Rules for Balancing Eqns Balance all other non-H’s and O’s next. Then balance H’s next Then balance O’s last Recheck inventory!

Example #1 H2 (g) + O2 (g)  H2O (l)

Example #1 Answer 2 H2 (g) + O2 (g)  2 H2O (l)

Example #2 NaCl (s) + H2SO4 (aq)  Na2SO4 (s) + HCl (aq)

Example #2 Answer 2 NaCl (s) + H2SO4 (aq)  Na2SO4 (s) + 2 HCl (aq)

Example #3 Fe (s) + O2 (g)  Fe2O3 (s)

Example #3 Answer 4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s)

Balancing Helpful Hints If a polyatomic ion exists as the same on each side of the , you do not have to split the atoms Only diatomic elements can have ½ or decimal coefficents

Diatomic Elements 7 @ 7… N2, O2, F2, Cl2, Br2, I2, H2

Sect. 8.2 - Types of Rxns 1. Combination (Synthesis) rxns 2. Decomposition rxns 3. Single-Replacement rxns 4. Double-Replacement rxns 5. Combustion rxns (Later) 6. Oxidation / Reduction rxns (Later) 7. Acid / Base rxns

Sect. 8.2 - Types of Rxns In terms of the relationships… 1. Combination = marriage 2. Decomposition = divorce 3. Single-Replacement = “the other man or the other woman” 4. Double-Replacement = “wife swap” 5. Combustion = “the argument”

1.Combination Rxns (synthesis) Definition: two or more substances react to form a single substance The reactants are normally two elements General equation: (marriage) A + B  AB

1. Combination Rxns Examples 1. Synthesis of an element and oxygen to form an oxide 4 Al (s) + 3 O2 (g) 2 Al2O3 Al3+ O2- 2. Synthesis of a metal and a nonmetal forms an ionic compound 3 Mg (s) + N2 (g)  Mg3N2 Mg2+ N3-

How to Predict Equations A + B  (2 elements added together) Combination Rxn 1. Determine charges of each atom (review ion sheet) 2. Form a neutrally charged product 3. Balance equation *Don’t let diatomic elements throw you off!*

2. Decomposition Rxns Definition: a single compound is broken down into two or more elements Don’t forget the 7 diatomics! Requires energy in the form of heat, light, or electricity (written above the  sign) General formula (divorce) AB  A + B

Decomposition Examples electricity 2 H2O (l) --- 2 H2 (g) + O2 (g)  2 HgO (s)  2 Hg (l) + O2 (g)

How to Predict Equations AB  (1 compound) Decomposition Rxn 1. Break compound into elements. 2. Be mindful of diatomic elements. 3. Balance equation. *Usually under the presence of heat () or electrolysis!*

3. Single-Replacement Rxns Definition: atoms of an element replace the atoms of a second element in a compound Two Types (the other man/woman) 1. Metal replacing a metal General Rxn: A + BC  AC + B 2. Nonmetal replacing a nonmetal General Rxn: A + BC  BC + A

3. Single-Replacement Rxns 1. Metal replacing a metal General Rxn: A + BC  AC + B Metals can only replace metals and will be shown as the first substance (cation replaces cation) Must consult the Activity Series of Metals to see if the reaction will take place Activity Series of Metals: lists metals in order of decreasing reactivity

Activity Series of Metals The reactive metals will replace any metals below it A non-reactive metal will not replace a metal above it = NO RXN

3. Single-Replacement Rxns Ex) iron will replace copper Fe + CuCl2  FeCl2 + Cu Fe2+ Cu2+ Cl- Ex) but copper will not replace iron… Cu + FeCl2  NO RXN

3. Single-Replacement Rxns 2. Nonmetal replacing a nonmetal General Rxn: A + BC  BC + A Nonmetals can only replace nonmetals and will be shown as the second substance in the compound (anion replaces anion) Nonmetal replacement limited to the halogens Activity decreases as you go down group 17 on the periodic table

3. Single-Replacement Rxns Cl2 + 2 NaBr  2 NaCl + Br2 Cl- Na+ Br- (Cl can replace Br) I2 + NaBr  NO RXN I- Na+ Br- (I cannot replace Br)

How to Predict Equations A + BC  (1 atom & 1 compound) Single Replacement Rxn 1. Identify ions of each atom. 2. Determine what can replace what (cation-to-cation OR anion-to-anion). *Don’t let diatomic elements throw you off!*

How to Predict Equations Single Replacement Rxn (cont’d) 3. Consult Activity Series Chart! (only elements higher on the chart can replace an atom in a compound!) 4. Make new neutral compound and atom. 5. Balance equation!

4. Double Replacement Rxns Definition: involve an exchange of positive ions between compounds Between two ionic compounds in aqueous solutions Generic formula (wife swap) AB + CD  AD + CB A+ B- C+ D-

4. Double Replacement Rxns For the rxn to occur, at least one of the following must be true for at least one of the products of the rxn 1.) precipitate (ppt or ↓), a solid is formed 2.) gas is formed (↑), ie) H2S ↑ 3.) molecular compound, ie) H2O (l), is formed

4. Double Replacement Rxns Solubility Table: table that shows the solubility of two ions Cations on left & anions on top Where they intersect is either: S = Soluble  meaning an aqueous substance is formed and a (aq) I = Insoluble – meaning ppt is formed and a ↓

4. Double Replacement Rxns 2 NaOH (aq) + H2SO4 (aq)  Na2SO4 (aq) + 2 H2O (l) Driving force is the formation of water BaCl2 (aq) + K2CO3 (aq)  BaCO3  + 2 KCl (aq) Driving force is the formation of a precipitate FeS (s) + 2 HCl (aq)  H2S  + FeCl2 (aq) Driving force is the formation of a gas

4. Double Replacement Rxns Common Solubility Rules: All nitrates are soluble All potassium compounds are soluble Others on Salmon Sheet If there is no ppt, gas, or water made and only aqueous substances made – NO RXN! AgNO3 (aq) + KCl (aq)  AgCl (aq) + KNO3 (aq) NO RXN!

How to Predict Equations AB + CD  (2 aqueous compounds) Double Replacement Rxn 1. Identify ions of each atom. 2. Switch the cations by bonding the outside ions together and the inside ions together.

How to Predict Equations Double Replacement Rxn (cont’d) 3. Make new neutral compounds. 4. Consult Solubility Table and match up new ion pairs. S = soluble, place an (aq) after the cmpd I = insoluble, place a  after the cmpd 5. Balance equation!

5. Combustion Rxns CxHy + O2  CO2 + H2O Definition: hydrocarbons react with oxygen, often producing energy in the form of heat and light Products are always carbon dioxide and water General equation: (the argument) CxHy + O2  CO2 + H2O

benzene: methyl alcohol: 5. Combustion Rxns C6H6 (l) + 7.5 O2  6 CO2 + 6 H2O methyl alcohol: CH3OH (l) + 1.5 O2  CO2 + 2 H2O

How to Predict Equations CxHy + O2  (hydrocarbon + oxygen) Combustion Rxn 1. Products are always CO2 + H2O 2. Balance equation. X + O2  (metal + oxygen) Combustion & Combination Rxn 1. Follow steps for combination rxns.

Your Turn! Predict the Following! 1. PbO2  2. Ca + S  3. C5H12 + O2  4. NaCl + K2SO4  5. Ca + Fe2O3 

Your Turn! Predict the Following! 1. PbO2  1 compound 2. Ca + S  2 elements 3. C5H12 + O2  hyrdrocarbon & O2 4. NaCl + K2SO4  2 compounds 5. Ca + Fe2O3  1 element & 1 compound

Your Turn! Predict the Following! 1. PbO2  divorce 2. Ca + S  marriage 3. C5H12 + O2  argument 4. NaCl + K2SO4  wife swap 5. Ca + Fe2O3  the other man

Your Turn! Predict the Following! 1. PbO2  Pb (s) + O2 (g) 2. Ca + S  CaS 3. C5H12 + O2  CO2 + H2O (balance!) 4. NaCl + K2SO4  Na2SO4 (aq) + KCl (aq)  NO RXN! = BOTH (aq) 5. Ca + Fe2O3  CaO + Fe (s)

Section 8.3: Complete Ionic Eqns Definition: an eqn that shows dissolved ionic compounds as their free ions All aqueous compounds actually exist as ions free floating in the solution Therefore, they are written as aqueous ions AgNO3 (aq) = Ag+ (aq) + Cl- (aq)

Complete Ionic Eqns Reactants really exist as and are written as: AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq) Reactants really exist as and are written as: Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) … Only aqueous products are written as ions: … AgCl (s) + Na+ (aq) + NO3- (aq)

Complete Ionic Eqns Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) … AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq) Ions that appear on both sides did NOTHING in the reaction and are referred to as spectator ions: Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) … AgCl (s) + Na+ (aq) + NO3- (aq) Spectator Ions: Na+ (aq) + NO3- (aq)

Complete Ionic Eqns Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) … AgCl (s) + Na+ (aq) + NO3- (aq) Spectator Ions: Na+ (aq) + NO3- (aq) Can be omitted from both sides of the original eqn Net Ionic Eqn: shows the particles that actually take part in the rxn (making ↓, ↑, or H2O)

Complete Ionic Eqns Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) … AgCl (s) + Na+ (aq) + NO3- (aq) Spectator Ions: Na+ (aq) + NO3- (aq) Net Ionic Eqn: Ag+ (aq) + Cl- (aq)  AgCl (s)

Complete Ionic Eqns Your turn: write the complete ionic equation and net ionic equations for the following reaction. Identify the spectator ions as well. Ex) Aqueous calcium hydroxide reacts with aqueous phosphoric acid

Complete Ionic Eqn Ex Calcium phosphoric Hydroxide (aq) + acid (aq)  Ca2+ OH- H+ PO43- Ca(OH)2 (aq) + H3PO4 (aq)  ? + ?

Complete Ionic Eqn Ex Complete Ionic: Ca(OH)2 (aq)+H3PO4 (aq)  Ca3(PO 4)2 ↓ + H2O (l) Complete Ionic: Ca2+ (aq) + OH- (aq) + H+ (aq) + PO43- (aq)  Ca3(PO 4)2 (s) + H+ (aq) + OH- (aq) Spectator Ions: H+ (aq) + OH- (aq) Net Ionic Eqn: Ca2+ (aq) + PO43- (aq)  Ca3(PO 4)2 (s) Balance 3 Ca2+ (aq) + 2 PO43- (aq)  Ca3(PO 4)2 (s)

Chapter 8 Review Questions Page 232 #32, 34-39, 40, 43-48