CHEMICAL REACTIONS CHAPTER 8

DESCRIBING CHEMICAL CHANGE 8.1

Chemical Reaction REACTANTS  PRODUCTS One or more substances change into one or more new substances Reactants = substances that are reacting or changing Products =substances being formed from the reactants REACTANTS  PRODUCTS

Chemical Equations REACTANTS  PRODUCTS Describe chemical reactions using words instead of symbols The  means “yields” or “reacts to produce” REACTANTS  PRODUCTS

Symbols Used in Chemical Equations Symbols used to show state of matter l=liquid (only water), s=solid, g=gas Other symbols used aq = aqueous solution = made with water  or heat = written over the  to show heat was added

Catalysts and Chemical Reactions Catalyst = A substance that speeds up the rate of a reaction NOT a reactant or a product NOT used up in a reaction If present, the name of the catalyst will be written over the  in a chemical equations Ex: MnO2, Pt, heat, 

Skeleton Equations Do not indicate relative amounts Show just the formulas of the reactants and the products unbalanced

Chemical Equation Example #1 Iron reacts with oxygen to produce iron (III) oxide Translates into… Iron + oxygen  Iron (III) oxide

Back to Chemical Equation Example #1 Iron reacts with oxygen to produce iron (III) oxide Translates into… Iron + oxygen  Iron (III) oxide Fe (s) + O2 (g) Fe2O3 (s)

Chemical Equation Example #2 Hydrogen peroxide reacts to produce water and oxygen gas Translates into… Hydrogen peroxide  water + oxygen

Skeleton Equation Example #2 Hydrogen peroxide reacts to produce water and oxygen gas Translates into… Hydrogen peroxide  water + oxygen H2O2 (l)  H2O (l) + O2 (g)

Chemical Equation Example #3 Carbon tetrahydride (methane) burns in the presence of oxygen gas to produce carbon dioxide and water Translates into… Methane + oxygen  carbon dioxide + water

Back to Chemical Equation Example #3 Gaseous Carbon tetrahydride (methane) burns in the presence of oxygen gas to produce carbon dioxide and water Translates into… Methane + oxygen  carbon dioxide + water CH4 (g) + O2 (g)  CO2 (g) + H2O (g)

Your turn…write a skeleton equation Sulfur burns in the presence of oxygen to form sulfur dioxide Heating potassium chlorate in the presence of the catalyst manganese (IV) oxide produces oxygen gas and solid potassium chloride

1. S (s) + O2 (g)  SO2 (g) MnO2, Δ 2. KClO3 (s)  O2 (g) + KCl (s) Check your Answers… 1. S (s) + O2 (g)  SO2 (g) MnO2, Δ 2. KClO3 (s)  O2 (g) + KCl (s)

Your turn…write a sentence 3. KOH (aq) + H2SO4 (aq)  H2O (l) + K2SO4 (aq) 4. Na (s) + H2O (l)  NaOH (aq) + H2 (g)

Check your Answers… 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 Chemical Equations (1) Law of Conservation of Matter Matter cannot be created nor destroyed but only rearranged The number of atoms on each side of the equation must be equal How many Hydrogen atoms? How many oxygen atoms? How many Hydrogen atoms? How many oxygen atoms? 2 H 2 O 2 H 1 O

Balancing Chemical Equations (2) A balanced equation has the same # of atoms of each element on each side of the equation Coefficients are numbers placed in front of the symbols for reactants and products to balance

Balancing Chemical Equations (3) C (s) + O2 (g)  CO2 (g) C (s) + O2 (g)  CO (g) This equation is balanced because each side of the equation has the same # of carbon and oxygen atoms This equation is NOT balanced because each side of the equation has different # of oxygen atoms

Rules for Balancing Chemical Equations Determine correct formulas for all the reactants and products Write the skeleton formula Count the # of each element on each side of the equation (atomic inventory) NEVER CHANGE A SUBSCRIPT!!

Rules for Balancing Chemical Equations Recheck atomic inventory Make sure the coefficients are in the lowest whole number ratio

Helpful Hints for Balancing Equations POLYATOMICS: treat polyatomic ions as units if present on both sides DIATOMICS: remember seven at seven? can have ½ as a coefficient

Example #1 And the answer… H2 (g) + O2 (g)  H2O (l)

2 NaCl (s) + H2SO4 (aq)  Na2SO4 (s) + 2 HCl (aq) Example #2 NaCl (s) + H2SO4 (aq)  Na2SO4 (s) + HCl (aq) And the answer… 2 NaCl (s) + H2SO4 (aq)  Na2SO4 (s) + 2 HCl (aq)

And the answer… Fe (s) + O2 (g)  Fe2O3 (s) Example #3 Fe (s) + O2 (g)  Fe2O3 (s) 4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s) And the answer…

Practice in Packet…page 13 – finish for homework YOUR TURN… Practice in Packet…page 13 – finish for homework Balancing Equations

Types of Reactions 8.2 8.3 Predicting Reactions

Combination (synthesis) Decomposition Single-replacement Types of reactions Combination (synthesis) Decomposition Single-replacement Double-replacement Combustion Oxidation/reduction (LATER) Acid/Base (LATER)

1. Combination reactions (synthesis) general Two or more substances react to form a single substance Reactants are normally 2 elements General equation: (marriage) A + B  AB

1. Combination reactions (synthesis) EXAMPLES 4Al + 3O2  2Al2O3 3Mg + N2  Mg3N2

If you see two elements A + B  1. Combination reactions (synthesis) predicting products If you see two elements A + B  Determine charges of each atom (review ion sheet) Form a neutrally balanced product Balance equation

2. Decomposition reactions (divorce) general single compound broken down into two or more elements or compounds Difficult to predict Requires energy (heat, light, electricity) General equation: (divorce) AB  A + B

2. DEComposition reactions EXAMPLES 2H20(l)  2H2(g) + O2(g) 2 HgO (s)  2 Hg (l) + O2 (g) electricity

If you see one compound AB  2. DEComposition reactions predicting products If you see one compound AB  Break compound into component elements Be mindful of diatomic elements Balance equation Look for presence of heat () or electrolysis Practice…Page 18 in packet

Practice

Identify… synthesis or decomposition? NaNO3  NaNO2 + O2 CO + O2  CO2 Al + S  Al2S3 KClO3  KCl + O2 Fe + O2  Fe2O3 HgO  Hg + O2 decomposition Synthesis (combination) Jump in packet notes to page 7

3. single-replacement (the other woman/man) general Atoms of an element replace the atoms of a second element in a compound Two reactants and two products Two types Metal replacing metal A + BC  AC + B Nonmetal replacing nonmetal A + BC  BA + C

3. single-replacement METAl replacing metal The element that is displaced must be less active than the element that displaces it Activity Series of metals: lists metals in order of decreasing reactivity A non-reactive metal will not replace a more reactive metal = NO REACTION

ACTIVITY SERIES OF METALS

3. single-replacement EXAMPLES – metal replacing metal iron will replace copper Fe + CuCl2  FeCl2 + Cu Fe2+ Cu2+ Cl- but copper will not replace iron… Cu + FeCl2  NO RXN

3. single-replacement nonmetal replacing nonmetal Nonmetals can only replace nonmetals (second substance in the compound) Limited to the halogens Activity decreases as you go down group 17 on periodic table

3. single-replacement EXAMPLES – nonmetal replacing nonmetal Cl2 + 2 NaBr  2 NaCl + Br2 Cl- Na+ Br- (Cl more active than Br) I2 + NaBr  NO RXN I- Na+ Br- (Br more active than I)

Will the single replacement reaction occur? Mg(s) + 2 HCl(aq) → Br2 + 2KI → 2AgNO3(aq) + Zn(s) → Ag + Cu(NO3)2 → Cl2 + 2NaBr → Au + HCl → I2 + 2KBr → Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g) Br2 + 2KI → 2KBr + I2 2AgNO3(aq) + Zn(s) → 2Ag(s) + Zn(NO3)2(aq) Ag + Cu(NO3)2 → No reaction Cl2 + 2NaBr → 2NaCl + Br2 Au + HCl → No reaction I2 + 2KBr → no reaction Predicting…go to page 7 in notes

Predicting single replacement Read Only Predicting single replacement If you see one element and one compound A + BC  Identify ions of each atom Determine what can replace what (cation- to-cation or anion-to-anion) Be mindful of diatomic elements Consult activity series chart Make neutral compound and atom Balance equation!!! Practice in packet…page 19

4. Double replacement (wife swap) General Exchange of positive ions between ionic compounds in solution General equation: (wife swap) AB + CD  AD + CB

4. Double replacement (wife swap) Solubility table: table that shows solubility of two ions Where they intersect is either: S=soluble  meaning an aqueous substance is formed and a (aq) follows the formula I=insoluble  meaning ppt is formed and a ↓follows the formula

Solubility table

4. Double replacement (wife swap) Solubility rules: All nitrates are soluble All potassium compounds are soluble Others are on solubility chart in packet If there is no ppt, gas or water made and only aqueous substances – NO RXN AgNO3 (aq) + KCl (aq)  AgCl (aq) + KNO3 (aq) NO RXN!

4. Double replacement (wife swap) examples 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

Predicting double replacement Read Only Predicting double replacement If you see two compounds AB + CD  Identify ions of each atom Switch the cations by bonding the outside ions together and the inside ions together Make new neutral compounds Consult solubility table and match up new ion pairs S=soluble (place an (aq) after the compound) I=insoluble (place a  after the compound) Balance the equation

4. Combustion reactions General Info An element or compound reacts with oxygen Produces heat and light Combustion of an element produces the element oxide A + O2  AO Combustion of a hydrocarbon ALWAYS produces carbon dioxide and water CxHy + O2  CO2 + H2O

4. Combustion reactions EXAMPLES Benzene: C6H6 (l) + 7.5 O2  6 CO2 + 6 H2O Methyl alcohol: CH3OH (l) + 1.5 O2  CO2 + 2 H2O Magnesium: 2Mg + O2  2MgO

Predicting combustion Read Only Predicting combustion If you see two hydrocarbon and oxygen CxHy + O2  Products are always CO2 and H2O Balance the equation If you see an element and oxygen A+ O2  Combustion and/or combination Follow steps for combination

Types of reactions…summary Combination (synthesis)…marriage Decomposition …divorce Single-replacement…the other man Double-replacement…wife swap Combustion…the argument Oxidation/reduction (LATER) Acid/Base (LATER)

Examples… Predicting reactions Write the following examples in your notes 1. PbO2  2. Ca + S  3. C5H12 + O2  4. NaCl + K2SO4  5. Ca + Fe2O3 

Examples… Predicting reactions Describe the reactants… 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

Examples… Predicting reactions Name the reaction type… 1. PbO2  decomposition divorce 2. Ca + S  synthesis marriage 3. C5H12 + O2  combustion argument 4. NaCl + K2SO4  double replace. wife swap 5. Ca + Fe2O3  single replace. the other man

Examples… Predicting reactions Predict the products and balance…. 1. PbO2  Pb (s) + O2 (g) 2. Ca + S  CaS 3. C5H12 + 8O2  5CO2 + 6H2O (balance!) 4. NaCl + K2SO4  Na2SO4 (aq) + KCl (aq) NO RXN! = BOTH (aq) 5. 3Ca + Fe2O3  3CaO + 2Fe (s)

COMPLETE IONIC EQUATIONS 8.4

Things to remember…double replacement Two ionic compounds react by exchanging cations AB + CD → AD + CB AgNO3 + NaCl → AgCl + NaNO3 One of the products must be: A gas Water A precipitate (solid)

Aqueous solutions Many important reactions take place in water Ionic compounds dissociate in aqueous solutions Therefore, they are written as aqueous ions AgNO3 (aq) = Ag+ (aq) + NO3- (aq)

COMPLETE IONIC EQUATION An equation that shows ionic compounds as their free ions EX: 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)

Spectator ions Spectator Ions = Na+ (aq) + NO3- (aq) Ions that appear on both sides do 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)

Net ionic equations Net ionic equation: Ag+ (aq) + Cl- (aq)  AgCl (s) Spectator ions can be omitted from both sides of the original equation Shows the particles that actually take part in the reactions Spectator ions = Na+ (aq) and NO3-(aq) Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) … AgCl (s) + Na+ (aq) + NO3- (aq) Net ionic equation: Ag+ (aq) + Cl- (aq)  AgCl (s)

Practice Write the complete ionic equation and net ionic equation for the following reactions. Identify Spectator ions as well. BALANCE!!! EX: aqueous calcium hydroxide reacts with aqueous phosphoric acid