Chemical Equations and Reactions Unit 8 Chemical Equations and Reactions

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

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

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

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

Energy in Reactions Some reactions require energy Endothermic Bonds breaking overall Ex: Dinitrogen tetroxide + energy  nitrogen dioxide N2O4 + energy  NO2 http://commons.wikimedia.org/wiki/Image:Dinitrogen_Tetroxide_as_dimer_of_Nitrogen_dioxide.gif

Energy in Reactions Other reactions make energy Exothermic Bonds created overall Ex: Methane + Oxygen  Carbon dioxide + Water + Energy CH4 + O2  CO2 + H2O + energy http://commons.wikimedia.org/wiki/Image:Methane_combustion.jpg

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

Homework Reread p 260 - 266 On p. 266 do SR 11-17

What is Wrong With These Reactions?

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

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

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

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

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 – 4 O – 4 H – 4

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

Example:

Let’s try these Ca2Si + Cl2  CaCl2 + SiCl4 P4 + O2  P2O5 C3H8 + O2  CO2 + H2O C3H8 + 5O2  3CO2 + 4H2O Is this correct? 2 P4 + 10 O2  4 P2O5

Homework: Balancing Chemical Equations Worksheet

Thinker: What do the following terms mean to you? Synthesis Decomposition Displacement Talk with others around you. Compare definitions & examples.

Classifying Reactions Classified to help predict products Five Basic Types: Synthesis Decomposition Single Displacement Double Displacement Combustion . . . more, but beyond scope of this unit.

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

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)

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)

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)

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

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

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

Questions? Rx Type Flowchart on p 284 HW: Read section 8-3 S.R. 1-10

Thinker: What is a spectator? What role does a spectator play?

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)

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)

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)

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)