Molecules, Compounds, and Chemical Equations

1
Molecules, Compounds, and Chemical Equations

• Chapter 3
• Chapter 4.5-4.9
• And
• Chapter 18.2

2
Molecular View of Elements and Compounds
Practice problems Tro 3.27-3.32
3
P4, S8, and Se8 are the polyatomic elements
4
Combining Elements to make Compounds

• 2 H2 O2 ? 2 H2O
• the properties of the compound are totally
  different from the constituent elements

5
Formation of Water from Its Elements
6
Chemical Bonds

• Forces of attraction holding two or more atoms
  together
• ionic bonds result when electrons are transferred
  from one atom to another, resulting in oppositely
  charged ions that are held together by
  electrostatic attractions
• generally found when metal atoms bonded to
  nonmetal atoms
• covalent bonds result when two atoms share some
  of their electrons
• generally found when nonmetal atoms bonded
  together

7
(No Transcript)
8
Ionic vs. Molecular Compounds
Propane contains individual C3H8 molecules
Table salt contains an array of Na ions and
Cl- ions
9
Chemical Formulas

• compounds are generally represented by a chemical
  formula
• the amount of information about the structure of
  the compound varies with the type of formula

10
Examples of different molecule representations
11
Molar Mass

• The sum of the atomic masses for all the atoms
  represented in the chemical formula of a
  compound.
• AKA formula mass, molecular mass, molecular
  weight

Practice problems Tro 3.57-3.58
12
Molecular Mass Determination

• Nitrogen fixation in the root nodules of peas and
  other legumes occurs with a reaction involving a
  molybdenum containing enzyme named nitrogenase.
  This enzyme contains two Mo atoms per molecule
  and is 0.0872 Mo by mass. What is the molar
  mass of the enzyme?

Practice problems Tro 3.67-3.72,
3.113-3.116, 3.127, 3.134, 3.135
13
Empirical Formulas

• Simplest formula for a compound.
• Can be determined from percent composition data
  and combustion analysis.

14
(No Transcript)
15
Problem 3.81a

• Calculate the empirical formula for nicotine
  given the following mass percent composition
• C 58.80
• H 8.70
• N 17.27

Practice problems Tro 3.79-3.84, 3.126
16
(No Transcript)
17

• A compound of Ca, C, N, and S was subjected to
  quantitative analysis and formula mass
  determination, and the following data were
  obtained. A 0.250 g sample was mixed with NaCO3
  to convert all of the Ca to 0.160 g of CaCO3. A
  0.115 g sample of the compound was carried
  through a series of reactions until all of its S
  was changed to 0.344g of BaSO4. A 0.712 g sample
  was processed to liberate all of its N as NH3,
  and 0.155 g NH3 was obtained. The formula mass
  was found to be 156. Determine the empirical and
  molecular formulas of this compound.

Practice problems Tro 3.85-3.90,
3.117-3.120, 3.124
18

• Some compounds can be decomposed quantitatively
  with water or acid to give known compounds.
  Suppose you have a 0.643 g sample of a compound
  known to be composed of C, H, Al, and Cl.
  Furthermore, you know that it is composed of some
  number of CH3 groups and chlorine atoms per
  aluminum atom. The formula could be written as
  (CH3)xAlCly. To find x and y you decompose the
  sample with acid in water. The CH3 portion is
  evolved as methane gas, CH4, and the chloride
  ions remain in the water. The chloride ions are
  precipitated as AgCl by adding AgNO3 to the
  solution. The data collected in the experiment
  are given here. What are the values of x and y?

19

• (CH3) xAlCly ?? x CH4(g) Al3(aq) y Cl-(aq)
• 0.643g 0.222g ?
• AgNO3
• ?
• AgCl(s)
• 0.996g

Practice problems Tro 3.121-3.122, 3.130
20
Chemical Reactions

• Chemical reactions are processes in which one set
  of chemicals are converted to a new set of
  chemicals
• Chemical reactions are described by chemical
  equations.

21
Chemical Equations

• 2 C8H18(l) 25 O2 (g) ?16 CO2 (g) 18 H2O(g)
• must be balanced to satisfy Law of conservation
  of mass
• state designations
• (g) gas
• (l) liquid
• (s) solid
• (aq) aqueous

22
Pure silicon, which is needed in the
manufacturing of electronic components, may be
prepared by heating silicon dioxide (sand) with
carbon at high temperatures, releasing carbon
monoxide gas. Write the balanced chemical
equation for this process.
23
Carbon tetrachloride was widely used for many
years as a solvent until its harmful properties
became well established. Carbon tetrachloride
may be prepared by the reaction of natural gas
(methane, CH4) and elemental chlorine gas in the
presence of ultraviolet light. Write a balanced
chemical equation for this process.
24

• Crude gunpowders often contain a mixture of
  potassium nitrate and charcoal (carbon). When
  such a mixture is heated until reaction occurs, a
  solid residue of potassium carbonate is produced.
  The explosive force of the gunpowder comes from
  the fact that two gases are also produced (carbon
  monoxide and nitrogen), which increase in volume
  with great force and speed. Write the balanced
  chemical equation for this reaction.

Practice problems Tro 3.91-3.100
25
Classifying Reactions by Type of Chemistry
26
Classifying Reactions by Type of Chemistry

• Precipitation AX BZ ?? AZ BX
• Acid Base HX BOH ?? BX H2O
• Gas Evolution
• H2X BCO3 ? H2O CO2(g) BX
• H2X BSO3 ? H2O SO2(g) BX
• NH4X BOH ? H2O NH3(g) BX
• Oxidation Reduction A2 B ?? A B2
• Combustion CxHxOxO2?CO2 H2O

27
Classifying Reactions by what Atoms Do
28
Classifying Reactions by what Atoms Do

• Combination/Synthesis A Z ?? AZ
• Decomposition AZ ?? A Z
• Single Displacement A BZ ?? AZ B
• Double displacement AX BZ ?? AZ BX
• Neutralization HX BOH ?? BX H2O

29
(No Transcript)
30
REDOX Reactions

• Oxidation number - In order to keep track of
  electrons in chemical reactions, chemists assign
  an oxidation number to each element.

31
Determining Oxidation States

• Oxidation number of an element in its native
  state is zero.
• Alkali metals have an oxidation number of 1
• Alkaline earth elements have an oxidation number
  of 2
• The oxidation number of monatomic ions is the
  same as the charge.

32
Determining Oxidation States

• Fluorine is ?1 except for F2.
• Cl, Br, and I are ?1 in binary compounds.
• O is usually ?2 (except for peroxides O2?2 and
  superoxides O2?1).
• H is usually 1 (except for hydrides H?1)
• The sum of the oxidation numbers equals the
  charge on ion or molecule.

33
Try some

• Ga2O3 Fe2(CrO4)3
• K2MnO4 Hg2(BrO3)2
• H2PO4- KClO4

Practice problems Tro 4.83-4.86
34

• Oxidation process in which an element loses one
  or more electrons with an increase in the
  oxidation number.
• Reduction process in which an element gains one
  or more electrons with a decrease in oxidation
  number.

35

• Oxidizing agent Substance that causes another
  substance to be oxidized. The oxidizing agent is
  always reduced.
• Reducing agent Substance that causes another
  substance to be reduced. The reducing agent is
  always oxidized.

36
WO3(s) 3 H2(g) ? W(s) 3 H2O(l)

• hydrogen
• tungsten
• WO3
• H2

Element oxidized Element reduced Oxidizing
agent Reducing agent
37
SnO2(s) 2 C(s) ? Sn(l) 2 CO(g)

• Carbon
• Tin
• SnO2(s)
• C(s)

Element oxidized Element reduced Oxidizing
agent Reducing agent
Practice problems Tro 4.87-4.88
38
Half Reaction Method of Balancing Redox Reactions

• Write skeleton ionic reaction. (Usually a
  given.)
• Split into 2 half reactions, one for oxidation
  and one for reduction. (Determine what is
  oxidized and what is reduced by calculating
  oxidation numbers. Remember LEO says GER Loses
  Electrons Oxidation, Gains Electrons Reduction)

39
Half Reaction Method of Balancing Redox Reactions

• Balance each half reaction.
• balance all but H and O.
• balance O by adding H2O.
• balance H by adding H.
• balance charge by adding electrons.

40
Half Reaction Method of Balancing Redox Reactions

• Add half reactions together after multiplying by
  a factor to be sure electrons cancel.

41
Half Reaction Method of Balancing Redox Reactions

• This method provides an equation for a reaction
  occurring in acid. To change to a balanced basic
  reaction add
• H OH? ? H2O
• to the reaction to cancel out all Hs.

Practice problems Tro 18.37-18.42
42
Balance in acid

• Cr2O72-(aq) Cl-1(aq) ? Cr3(aq) Cl2(g)
• 14H(aq) Cr2O72-(aq) 6Cl-1(aq)
• ? 2Cr3(aq) 3Cl2(g) 7 H2O(l)

43
Balance in acid

• MnO2(s) Hg(l) Cl-1(aq)
• ? Mn2(aq)
  Hg2Cl2(s)
• 4H(aq) MnO2(s) 2Hg(l) 2Cl-1(aq)
• ? Mn2(aq) Hg2Cl2(s) 2H2O(l)

44
Balance in acid

• Ag(s) NO3-1(aq) ? Ag1(aq) NO(g)
• 4H(aq) 3Ag(s) NO3-1(aq)
• ? 3Ag1(aq) NO(g) 2 H2O(l)

45
Balance in acid

• H3AsO4(aq) Zn(s) ? AsH3(g) Zn2(aq)
• 8H(aq) H3AsO4(aq) 4Zn(s)
• ? AsH3(g) 4Zn2(aq) 4 H2O(l)

46
Balance in acid

• Au3(aq) I2(s) ? Au(s) IO3-1(aq)
• 10Au3(aq) 3I2(s) 18 H2O(l)
• ? 36H(aq) 10Au(s) 6IO3-1(aq)

47
Balance in acid

• IO3-1(aq) I-1(aq) ? I3-1(aq)
• 6H(aq) IO3-1(aq) 8I-1(aq)
• ? 3I3-1(aq) 3 H2O(l)

48
Balance in acid

• HS2O3-1(aq) ? S(s) HSO4-1(aq)
• H(aq) 3HS2O3-1(aq)
• ? 4S(s) 2HSO4-1(aq) H2O(l)

49
Balance in acid

• O2-2(aq) ? O2(g) H2O(l)
• 4H(aq) 2 O2-2(aq) ? O2(g) 2H2O(l)

50
Balance in acid

• Cr2O7-2(aq) I2(aq) ? Cr3(aq) IO3-1
• 34H(aq) 5Cr2O7-2(aq) 3I2(aq)
• ? 10Cr3(aq) 6IO3-1 17H2O(l)

51
Balance in acid

• S2O3-2(aq) I2(aq) ? S4O6-2(aq) I-1(aq)
• 2S2O3-2(aq) I2(aq) ? S4O6-2(aq) 2I-1(aq)

52
Balance in acid

• MnO4-1(aq) H2O2(aq) ? Mn2(aq) O2(g)
• 6H(aq) 2MnO4-1(aq) 5H2O2(aq)
• ? 2Mn2(aq) 5O2(g) 8 H2O(l)

53
Balance in acid

• Hg2Cl2(s) NO2-1(aq) ? Hg2(aq) NO(g)
• 4H(aq) Hg2Cl2(s) 2NO2-1(aq)
• ? 2Hg2(aq) NO(g) 2Cl-1(aq) 2H2O(l)

54
Balance in acid

• MnO4-2(aq) ? MnO2(s) MnO4-1(aq)
• 4H(aq) 3MnO4-2(aq)
• ? MnO2(s) 2MnO4-1(aq) 2H2O(l)

55
Balance in acid

• Pb(s) PbO2(s) SO4-2(aq) ? PbSO4(s)
• 4H(aq) Pb(s) PbO2(s) 2SO4-2(aq)
• ? 2PbSO4(s) 2H2O(l)

56
Balance in base

• Co(OH)3(s) Sn(s)
• ? Co(OH)2(s) HSnO2-1(aq)
• OH-1(aq) 2Co(OH)3(s) Sn(s)
• ? 2Co(OH)2(s) HSnO2-1(aq) H2O(l)

57
Balance in base

• ClO4-1(aq) I-1(aq)
• ? ClO3-1(aq) IO3-1(aq)
• 3ClO4-1(aq) I-1(aq)
• ? 3ClO3-1(aq) IO3-1(aq)

58
Balance in base

• PbO2(s) Cl-1(aq)
• ? ClO-1(aq) Pb(OH)3-1(aq)
• OH-1(aq) H2O(l) PbO2(s) Cl-1(aq)
• ? ClO-1(aq)
  Pb(OH)3-1(aq)

59
Balance in base

• NO2-1(aq) Al(s) ? NH3(g) AlO2-1(aq)
• OH-1(aq) H2O(l) NO2-1(aq) 2Al(s)
• ? NH3(g)
  2AlO2-1(aq)

60
Balance in base

• ClO-1(aq) ? Cl-1(aq) O2(g)
• 2ClO-1(aq) ? 2Cl-1(aq) O2(g)

61
Balance in base

• HXeO4-1(aq) Pb(s)
• ? Xe(g) HPbO2-1(aq)
• 2OH-1(aq) HXeO4-1(aq) 3Pb(s)
• ? Xe(g) 3HPbO2-1(aq)

62
Balance in base

• Ag2S(s) CN-1(aq) O2(g)
• ? S(s) AgCN(aq)
• 2 H2O(l) 2Ag2S(s) 4CN-1(aq) O2(g)
• ? 2S(s) 4AgCN(aq) 4OH-1(aq)

63
Balance in base

• MnO4-1(aq) S-2(aq) ? MnS(s) S(s)
• 8 H2O(l) 2MnO4-1(aq) 7S-2(aq)
• ? 2MnS(s) 5S(s) 16OH-1(aq)

64
Balance in base

• Cl2(g) ? ClO-1(aq) Cl-1(aq)
• 2OH-1(aq) Cl2(g)
• ? ClO-1(aq) Cl-1(aq)
  H2O(l)

65
Balance in base

• MnO4-1(aq) H2O2(aq) ? MnO2(s) O2(g)
• 2MnO4-1(aq) 3H2O2(aq)
• ? 2MnO2(s) 3O2(g) 2OH-1(aq)

66
Balance in base

• ClO2(aq) ? ClO2-1(aq) ClO3-1(aq)
• 2OH-1(aq) 2ClO2(aq)
• ? ClO2-1(aq) ClO3-1(aq) H2O(l)

67
Balance in base

• CrO4-2(aq) N2H4(aq) ? Cr3(aq) N2(g)
• 4CrO4-2(aq) 3N2H4(aq) 4H2O(l)
• ? 4Cr3(aq) 3N2(g) 20 OH-1(aq)

68
Balance in base

• Ag(s) CN-1(aq) O2(g)
• ? Ag(CN)2-1(aq) OH-1(aq)
• 4Ag(s) 8CN-1(aq) O2(g) 2H2O(l)
• ? 4Ag(CN)2-1(aq) 4OH-1(aq)

69
Balance in base

• Co(s) ClO-1(aq)? Co2(aq) Cl-1(aq)
• Co(s) ClO-1(aq) H2O(l)
• ? Co2(aq) Cl-1(aq) 2OH-(aq)

70
Balance in base

• Cd(s) H2O(l) Ni2O3(s)
• ? Cd(OH)2(s)
  NiO(s)
• Cd(s) H2O(l) Ni2O3(s)
• ? Cd(OH)2(s)
  2NiO(s)