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Chemical Equations and Reactions
Unit 8 Chemical Equations and Reactions
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Unit 8 Goals Describe evidence and characteristics of chemical reactions Write balanced chemical equations Identify the types of reactions Determine if an ionic compound is soluble or insoluble in water Describe a precipitation reaction and relate it to solubility Predict if a precipitate will form or not using solubility rules Predict the products and balance molecular equations of precipitation reactions
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Thinker: List observations that indicate a chemical reaction has taken place in the following situations: A cut apple turns brown An egg changes when it cooks A log burns A car rusts
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Reminder: Chemical Reactions are indicated by: Hallmark: Bubbling
Color change Energy released as heat and light Formation of solids Hallmark: At least one chemical has changed its formula
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Writing Reactions Several ways to write equations:
Always have reactants on left Always have products on the right Word Equations Methane + Oxygen Carbon Dioxide + Water Formula Equation CH4 + O2 CO2 + H2O Balanced Formula Equations We’ll learn how to do this tomorrow CH4 + 2O2 CO2 + 2H2O
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Energy in Reactions Some reactions require energy Endothermic
Bonds breaking overall Ex: Dinitrogen tetroxide + energy nitrogen dioxide N2O4 + energy NO2
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Energy in Reactions Other reactions make energy Exothermic
Bonds created overall Ex: Methane + Oxygen Carbon dioxide + Water + Energy CH4 + O2 CO2 + H2O + energy
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Symbols in Reactions Equations are like recipes (s) - solid
(l) - liquid (g) - gas (aq) - aqueous (dissolved in water) - yields - reversible reaction ∆ - reactants are heated Pd, Mn, etc. - catalyst
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Homework Reread p On p. 266 do SR 11-17
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What is Wrong With These Reactions?
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HW Answers (side 1) NH3 + HCl NH4Cl CaCO3 ∆→ CaO + CO2
BaO + H2O Ba(OH)2 CH3CHO CH4 + CO Zn + Cu(NO3)2 Zn(NO3)2 + Cu CaSO3 ∆→ CaO + SO2 Fe + H2SO4 FeSO4 + H2 C2H6N2 ∆→ C2H6 + N2 CO + Cl2 COCl2 MnI2 Mn + I2
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HW Answers (side 2) N2O5 + H2O HNO3 Mg + TiCl4 MgCl2 + Ti
C + ZnO Zn + CO2 Br2 + NaI NaBr + I2 PCl3 + Cl2 PCl5 P + Br2 PBr3 CaH2 + H2O Ca(OH)2 + H2 H2SO4 + KOH K2SO4 + H2O C3H8 + O2 ∆→ CO2 + H2O C6H6 + O2 ∆→ CO2 + H2O
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Chemical Reactions: Conserve Mass Therefore: What happens?
No mass is created No mass is destroyed Therefore: No atoms are created No atoms are destroyed What happens? Bonds (energy) are created, destroyed or rearranged
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What does this mean? Formation of Water: H2 + O2 H2O
The above equation is not correct! It does not conserve matter. H – 2 | H - 2 O – 2 | O - 1 We need to balance the equation # of atoms at start = # of atoms at end
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Balancing Equations Purpose: To show how much of each reactant is needed to make a certain quantity of product. What can we do? Add coefficients to each formula Acts as a multiplier for that formula CO2 2CO2 2Ba(OH)2 C – 1 C – 2 Ba – 2 O – 2 O – O – 4 H – 4
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General Rules for Balancing
Identify reactants & products Write equation, predict products as necessary Count atoms Look for polyatomic ions Insert coefficients Balance one atom/polyatomic at a time Delay balancing elements appearing in more than 1 formula on each side. Do not rewrite formulas! Do not change subscripts! Odd-even technique Verify your results
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Example:
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Let’s try these Ca2Si + Cl2 CaCl2 + SiCl4 P4 + O2 P2O5
C3H8 + O2 CO2 + H2O C3H8 + 5O2 3CO2 + 4H2O Is this correct? 2 P O2 4 P2O5
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Homework: Balancing Chemical Equations Worksheet
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Thinker: What do the following terms mean to you? Synthesis
Decomposition Displacement Talk with others around you. Compare definitions & examples.
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Classifying Reactions
Classified to help predict products Five Basic Types: Synthesis Decomposition Single Displacement Double Displacement Combustion . . . more, but beyond scope of this unit.
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Combustion Often used to generate energy
Gas, coal, starches, alcohol, sugars, cellulose, oil, etc. (has C compound) Ex: CHX + O2 H2O + CO2 + energy C3H8 + 5O2 3CO2 + 4H2O + energy CH3CH2OH + 3O2 2CO2 + 3H2O + energy
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Synthesis Greek - “to put together”
2 or more formulas combine to form 1 formula Ex: A + B AB C + O2 CO2 2C + O2 CO 2Na + Cl2 2NaCl2 CaO(s) + H2O(l) Ca(OH)2(s)
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Decomposition Opposite of synthesis Binaries usually elements
Compounds 3+ usually elements Ex: AB ∆ A + B CaCO3 ∆ CaO + CO2 2KClO3 ∆ 2KCl + 3O2 Mg(OH)2 ∆ MgO + H2O Air Bags: NaN3(s) ∆ Na(s) + N2(g)
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Single Displacement A single atom replaces another in a compound.
Like charges exchange places: Metals replace metals or H Nonmetals replace nonmetals Ex: A + BC AC + B 2Al(s) + 3CuCl2(aq) 2AlCl3(aq) + Cu(s) Cu(s) + AgNO3(aq) CuNO3(aq) + Ag(s)
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Reactivity (Activity Series)
Just because we put chemicals together will there be a reaction? Not necessarilly See the Activity Series (p.281 or Appendix A-9 p.832) More Rx elements will replace less Rx elements in compounds Ex: K will replace H in H2O (forms KOH + H2) Cu will replace Ag in AgNO3
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Will the following React?
Ag in Cu(NO3)2 Cu in AgNO3 K in Mg(ClO)2 Al in ZnCrO4 Zn in Al2(CrO4)3 CuCl2 containing Pb CuCl2 containing Ag No Yes
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Double Displacement Similar to single displacement
2 reactants; 2 products Like charges exchange places Must result in: a solid, a gas, or a molecule Ex: AB + YZ AZ + YB 2KI(aq) + Pb(NO3)2(aq) PbI2(s) + 2KNO3(aq) BaO + H2O Ba2OH + H2
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Questions? Rx Type Flowchart on p 284 HW: Read section 8-3 S.R. 1-10
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Thinker: What is a spectator? What role does a spectator play?
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Writing Net Ionic Equations
What is the purpose to outlining? Summarizing important information How does this relate to spectators and reactions? Hint: Are there any types of reactions where some things do not change (phase)? Yes: Ionic Equations (1 & 2 displacement)
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Ionic Compounds Dissolve in H2O Ions separate from each other Thus:
2KI(aq) + Pb(NO3)2(aq) PbI2(s) + 2KNO3(aq) Looks more like: 2K+(aq) + 2I-(aq) + Pb2+(aq) + 2(NO3)-(aq) PbI2(s) + 2K+(aq) + 2NO3-(aq)
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But Wait! 2K+(aq) + 2I-(aq) + Pb2+(aq) + 2(NO3)-(aq) PbI2(s) + 2K+(aq) + 2NO3-(aq) This is too long to write. Let’s outline the net equation. What does net mean? Hint: What does net profit mean? We can remove the spectator ions. Leaving us with: 2I-(aq) + Pb2+ PbI2(s)
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Another Example: Single Displacement This Time Formula Equation
Word Equation Zinc + Copper (II) Sulfate Copper + Zinc Sulfate Formula Equation Zn(s) + CuSO4(aq) Cu(s) + ZnSO4(aq) All Ions: Zn(s) + Cu2+(aq) + SO42-(aq) Cu(s) + Zn2+(aq) + SO42-(aq) Net Ionic Equation: Zn(s) + Cu2+(aq) Cu(s) + Zn2+(aq)
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