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:– 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)

1. NH3 + HCl NH4Cl2. CaCO3 ∆→ CaO + CO2

3. BaO + H2O Ba(OH)2

4. CH3CHO CH4 + CO5. Zn + Cu(NO3)2 Zn(NO3)2 + Cu6. CaSO3 ∆→ CaO + SO2

7. Fe + H2SO4 FeSO4 + H2

8. C2H6N2 ∆→ C2H6 + N2

9. CO + Cl2 COCl210. MnI2 Mn + I2

HW Answers (side 2)

11. N2O5 + H2O HNO3

12. Mg + TiCl4 MgCl2 + Ti13. C + ZnO Zn + CO2

14. Br2 + NaI NaBr + I2

15. PCl3 + Cl2 PCl516. P + Br2 PBr3

17. CaH2 + H2O Ca(OH)2 + H2

18. H2SO4 + KOH K2SO4 + H2O19. C3H8 + O2 ∆→ CO2 + H2O20. C6H6 + O2 ∆→ CO2 + H2O

Chemical Reactions:

• Conserve Mass– 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

1. Identify reactants & products– Write equation, predict products as necessary

2. Count atoms– Look for polyatomic ions

3. 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

4. Verify your results

Example:

Let’s try these

• Ca2Si + Cl2 CaCl2 + SiCl4– Ca2Si + 4Cl2 2CaCl2 + SiCl4

• P4 + O2 P2O5

– P4 + 5 O2 2 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• Yes• Yes• No• Yes• No

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• 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) + SO4

2-(aq)

• Net Ionic Equation:

– Zn(s) + Cu2+(aq) Cu(s) + Zn2+

(aq)