Chemical Reactions

1
Chapter 8

• Chemical Reactions

2
Chemical Reactions

• Chemical reactions are chemical changes.
• Chemical reactions are represented by chemical
  equations.
• Much information about the qualitative and
  quantitative aspects of a reaction can be
  determined from the equations.

3
Reactants and Products

• The Reactants are the substances that are the
  starting materials. Their formulas are written
  on the left side of the arrow.
• The Products are the substances produced in a
  chemical reaction. Their formulas are written on
  the right side of the arrow.
• The arrow is interpreted as yields in an equation.

4
Three Indicators of a Chemical Reaction

• Formation of a precipitate( an insoluble compound
  in water).
• Release of heat and/or light.
• Release of a gasCO2 and H2 being the most
  common gases.

5
Characteristics of Chemical Equations

• The chemical equation must represent known facts.
• The equation must contain the correct formulas
  for reactants and products.
• The law of conservation of mass must be
  satisfied.
• Coefficients are used to balance chemical
  equations.
• Coefficients are small whole numbers that appear
  in front of a formula in a chemical equation.

6
Word and Formula Equations
7
Other Symbols in Equations

• N2(g) 3H2(g) 2NH3(g)

8
Significance of a Chemical Equation

• The coefficients of a reaction indicate relative,
  not absolute amounts of reactants and products.
• The relative masses of the reactants and products
  can be determined from the reactions
  coefficients.
• The reverse reaction for a chemical reaction has
  the same relative amounts of substances as the
  forward reaction.

9
Balancing Equations

• Atoms the number of atoms of each element are
  equal in the products and reactants.
• Mass the mass of products and reactants are
  equal. (Law of Conservation of Mass)
• Moles the total number of moles of reactants
  and products are not necessarily equal.
• Coefficients the numbers placed in front of
  atoms and compounds to balance chemical equations.

10
Balancing Atoms
N2 H2 NH3
11
Moles and Reactions

• Instead of counting molecules in a reaction, we
  count moles of molecules.
• N2(g) 3 H2(g) 2 NH3(g)
• 1 mole of N2 reacts with 3 moles of H2 to
  produce 2 moles of NH3.

12
What does this equation mean?

• N2 3H2
  2NH3

3 molecules of hydrogen (each containing 2 atoms)
to form
2 molecules of ammonia. ( Each molecule contains
1 atom of nitrogen and 3 atoms of hydrogen.)
1 molecule of nitrogen (each containing 2 atoms)
reacts with
3 moles of hydrogen (H2) to form
1 mole of nitrogen (N2) reacts with
2 moles of ammonia (NH3)
13
(No Transcript)
14
Example CH4 O2 ? CO2 H2O

• On reactant side you have
• C 1
• H 4
• O 2
• On product side you have
• C 1
• H 2
• O 3
• Balance each element on each side
• CH4 2O2 ? CO2 2H2O

15
Balancing Chemical Equations

• Identify the names of the reactants and products,
  and write a word equation.
• Write a formula equation by substituting correct
  formulas for the names of the reactants and
  products.
• Balance the formula equation, using coefficients,
  according to the law of conservation of mass.
  (See additional steps)
• Count atoms on the reactant and product sides to
  be sure that the equation is balanced.

16
Balancing by Inspection

• Check for Diatomic Molecules - H2 - N2 - O2 - F2
  - Cl2 - Br2 - I2
• If these elements appear by
  themselves in an equation, they must
  be written with the subscript 2.
• Balance Metals
• Balance Ions (polyatomic)
• Balance Nonmetals
• Balance Oxygen
• Balance Hydrogen
• Recount All Atoms Balance equations by changing
  coefficientsnever by changing subscripts.
• If the atoms are not balanced at this point,
  there is a problem somewhere.
• Work your way back up the steps, from bottom to
  top, until youfind the problem, and correct it.
  
• If every coefficient will reduce, rewrite in the
  simplest whole-number ratio.
• An equation is not properly balanced if the
  coefficients are not written in their lowest
  whole-number ratio.

MINOH
17
Example Equation 1

• Zinc hydrochloric acid zinc
  chloride hydrogen
• Zn(s) HCl(aq) ZnCl2
  H2 (g)

18
Example Equation 2

• Al4C3(s) HOH(aq) CH4 (g)
  Al(OH)3 (s)

19
Example Equation 3

• Al2(SO4)3 (aq) Ca(OH)2 (aq)
  Al(OH)3 (s) CaSO4 (s)

20
Example Equation 4

• KI (aq) Pb(NO3)2 (aq)
  PbI2 (s) KNO3 (aq)

21
Example Equation 5

• C3H8 (g) O2 (g) ?
  CO2 (g) H2O (l)

22
There are 5 types of reactions

• Synthesis or Direct combination
• Decomposition or Analysis
• Single replacement or displacement
• Double replacement or ionic reactions
• Combustion

23
Combination or Synthesis

• Number of reactants 2
• Number of products 1
• General Expression A Y ? AY
• Examples
• Metal Nonmetal ? Ionic compound
• 2 Al 3 Cl2 ? 2 AlCl3
• Nonmetal Nonmetal ? Covalent Compound
• 2 NO O2 ? 2 NO2

24
Synthesis Reactions - Producing Compounds From
the Elements

• Producing Ammonia (Haber Process)
• N2(g) 3H2(g) 2NH3(g)
• Producing water from the elements
• 2H2(g) O2(g) 2H2O(l)
• Rusting of iron
• 4Fe(s) 3O2(g) 2Fe2O3(s)

25
Decomposition

• Number of reactants 1
• Number of products 2 or more
• General expression AY ? A Y
• Decomposition reactions have only one reactant,
  but varying products.
• 2 HgO(s) ? 2Hg(l) O2(g)

26
Decomposition Reactions(Breaking Compounds into
the Elements)

• Electrolysis of water
• 2H2O(l) 2H2(g) O2(g)
• Decomposition of metal oxides
• 2HgO(s) 2Hg(l) O2(g)
• Decomposition of chlorates
• 2 KClO3(s) 2KCl(s) O2(g)

DC current
Heat
MnO2
heat
27
Single Replacement

• Number of reactants 2
• Number of products 2
• General Expression AX B ? A BX
• Based on the Metal Activity Series (see slide 29)
• Two Possibilities
• Metal ionic compound ? metal Ionic compound
• Zn Pb(NO3)2 ? Zn(NO3)2 Pb
• Metal Acid ? H2 Ionic compound
• Mg 2 HCl ? MgCl2 H2

28
Single Replacement Reactions

• Cu(s) 2AgNO3(aq) Cu(NO3)2(aq)
  2Ag(s)
• Cu is oxidized to Cu2 ions (LEO- lose
  electrons oxidization)
• Ag1 is reduced to Ag atoms (GER gain electrons
  reduction)
• Cu2 ions have replaced the Ag1 ions in AgNO3
• To determine which metals will replace which ions
  in aqueous solution, use the activity series.
• Elements toward the top of the series will
  replace elements below them. Cu is above Ag in
  the series.

29
Activity Series
30
Double Replacement

• Number of reactants 2
• Number of products 2
• General Expression AX BY ? AY BX
• 2 ionic compounds ? 2 ionic compounds
• K2CrO4 2 AgNO3 ? Ag2CrO4 2 KNO3
• Acid Base ? Ionic compound (salt) H2O
• KOH HCl ? KCl H2O

31
Double Replacement Reactions

• Two types of Double Displacement Reactions
• 1. Formation of an insoluble precipitate.
• 2. Formation of a gas.
• Gas Formation
• Acid salt gas another salt
• H2SO4(aq) Na2S(aq) H2S(g) Na2SO4(aq)
• 2HCl (aq) CaCO3(s) CO2(g) H2O(l)
  CaCl2(aq)

32
Reactions of Non-metals

• Non-metals will react with oxygen to form
  non-metal oxides
• S8(s) 8O2(g) 8SO2(g)
• Non-metals will also react with Cl2 or Br2
• P4(s) 6Br2(l) 4PBr3(l)
• Non-metal oxides react with water to form acids
• SO2(g) H2O(l) H2SO3(aq) (sulfurous
  acid)

33
Combustion

• Reactants a hydrocarbon and Oxygen
• Products Carbon dioxide, water and the release
  of energy
• C3H8 5 O2 ? 3 CO2 4 H2O energy

34
Single Displacement Reactions of Non-metals

• Halogens will displace other halogens or halide
  ions.
• The activity series for halogens follows the
  periodic table F2 gt Cl2 gt Br2 gt I2.
• Cl2 comes before I2 and will displace I in
  compounds
• Cl2 2NaI I2 NaCl
• I2 will not displace Cl in compounds
• I2 NaCl N. R. (no reaction)

35
Links to Balancing Equations

• Animated site
• Balancing by Inspection