Chemistry-140 Lecture 9

1
Chemistry-140 Lecture 9

• Chapter 4 Chemical Equations

• Chapter Highlights
• balance simple chemical equations
• mass relationships
• limiting reagents
• percent yield
• chemical analysis

2
Chemistry-140 Lecture 9

• Chemical Equations

reactants
products
3
Chemistry-140 Lecture 9

• Balanced Chemical Equations

• a balanced chemical equation shows
• relative amounts of reactants
• relative amounts of products
• the physical states of all species
• a balanced chemical equation does NOT show
• experimental conditions
• gain or loss of energy (heat, light, etc..)
• the rate of the reaction

4
Chemistry-140 Lecture 9

• Reaction Stoichiometry

the stoichiometric coefficients are 2, 3 1
5
Chemistry-140 Lecture 9

• Balancing Chemical Equations

• a chemical equation must be balanced before
  useful quantitative information can be obtained

• Example
• a combustion reaction occurs when a hydrocarbon
  compound (such as butane C4H10) burns in air to
  give carbon dioxide and water

6
Chemistry-140 Lecture 9
Basic Steps in Balancing a Chemical Equation

• Step 2 Balance the number of C atoms

7
Chemistry-140 Lecture 9
Basic Steps in Balancing a Chemical Equation

• Step 3 Balance the number of H atoms

• Step 4 Balance the number of O atoms

2 C4H10(g) 13 O2(g) 8 CO2(g)
10 H2O(l)

• Step 5 Check that the numbers of atoms of each
  element are balanced

8
Chemistry-140 Lecture 9
Weight Relations in Stoichiometric Reactions
Question The reaction of elemental phosphorus,
P4(s), with chlorine gas, Cl2(g) gives phosphorus
trichloride, PCl3(l). What mass of chlorine is
required to react completely with 1.45 g of
phosphorus and what mass of product will result?
9
Chemistry-140 Lecture 9
Method
direct calculation not possible
grams reactant P4
grams product PCl3
x
x (MW)
moles reactant P4
moles reactant PCl3
stoichiometric factor
10
Chemistry-140 Lecture 9
Answer Step 1 Write the balanced equation
for the reaction.
This determines the stoichiometric factors to be
used in the calculations. That is one mole of P4
requires six moles of Cl2 and four moles of PCl3
will be produced.
11
Chemistry-140 Lecture 9
Answer Step 2 Calculate the number of
moles of P4 in 1.45 g. (1.45 g
P4) Number of moles of P4 in 1.45
g is 1.17 x 10-2 mol.
1.17 x 10-2 mol P4
12
Chemistry-140 Lecture 9
Answer Step 3 Use the stoichiometric
factors to calculate the number of moles
of Cl2 required. (1.17 x 10-2 mol P4)
Number of moles of Cl2 required
for complete reaction is 7.02 x 10-2 mol.
7.02 x 10-2 mol Cl2
13
Chemistry-140 Lecture 9
Answer Step 4 Calculate the number of
grams of Cl2 in 7.02 x 10-2 mol.
(7.02 x 10-2 mol Cl2)
Number of grams of Cl2 required for
complete reaction is 4.98 g.
4.98 g Cl2
14
Chemistry-140 Lecture 9
Answer Step 5 Use the stoichiometric
factors to calculate the number of moles
of PCl3 that will be produced. (1.17 x
10-2 mol P4) Number of moles
of PCl3 produced by complete reaction is 4.68 x
10-2 mol .
4.68 x 10-2 mol PCl3
15
Chemistry-140 Lecture 9
Answer Step 6 Calculate the number of
grams of PCl3 in 4.68 x 10-2 mol.
(4.68 x 10-2 mol PCl3)
Number of grams of PCl3 produced in
the reaction of 1.45 g of P4 with is 4.98 g of
Cl2 was 6.43 g PCl3.
6.43 g PCl3
16
Chemistry-140 Lecture 9

• Stoichiometric Reactions Limiting Reagents

• In practice Reagents are rarely combined in the
  exact stoichiometric ratio described by the
  balanced equation

• For example

In this combustion reaction, excess oxygen is
used. The amount of CO2 and H2O produced is
dependent on the amount of propane ONLY. Propane
is the limiting reagent.
17
Chemistry-140 Lecture 9
A Reaction Involving a Limiting Reagent
Example 4.3 Methanol, CH3OH, is an excellent
fuel and can be made from the direct reaction of
carbon monoxide, CO, and hydrogen, H2.
Suppose 356 g of CO are mixed with 65.0 g
of H2. What mass of methanol will be formed ?
18
Chemistry-140 Lecture 9
Answer Step 1 Write the balanced equation
for the reaction.
19
Chemistry-140 Lecture 9
Answer Step 2 Calculate the number of
moles of each reagent. (356 g
CO) (65.0 g H2)

12.7 mol CO
32.2 mol H2
20
Chemistry-140 Lecture 9
Answer Step 3 What is the ratio of moles
of the reagents. Are the reagents present
in the stoichiometric ratio.
BUT
stoichiometry is 2 moles of H2 to 1 mole of CO,
therefore, H2 is in excess and CO must be a
limiting reagent!
21
Chemistry-140 Lecture 9
Answer Step 4 Calculate the number of
grams of methanol that can be
produced. (12.7 mol CO)
Number of grams of methanol
produced in the reaction was 407 g CH3OH.
407 g CH3OH
22
Chemistry-140 Lecture 9

• Percent Yield

• In practice Reactions rarely (if ever!!)
  produce all the product predicted by the balanced
  equation. The ratio of the actual yield to the
  theoretical yield expressed as a percentage is
  known as the percent yield

23
Chemistry-140 Lecture 9

• Percent Yield

• Actual yield the quantity of material actually
  produced in an experiment
• Theoretical yield the quantity of material that
  would be produced IF the reaction produced the
  maximum product allowable by the stoichiometry
  and limiting reagents

24
Chemistry-140 Lecture 9
Percent Yield
Section 4.5 Suppose you made aspirin (ASA) in
the laboratory according to the following
reaction Suppose you begin with 14.4 g
of salicylic acid, use an excess of acetic
anhydride and obtain 6.26 g of ASA. What was the
percent yield of ASA?
25
Chemistry-140 Lecture 9
Answer Step 1 Calculate the moles of the
limiting reagent, salicylic
acid. (14.4 g C7H6O3)

1.04 x 10-1 mol C7H6O3
26
Chemistry-140 Lecture 9
Answer Step 2 Use the stoichiometric
ratios to determine the expected number of
moles of ASA . (1.04 x 10-1 mol C7H6O3)

1.04 x 10-1 mol ASA
27
Chemistry-140 Lecture 9
Answer Step 3 Calculate the number of
grams of ASA to be produced. (1.04
x 10-1 mol ASA) The theoretical yield is
18.8 g of ASA.
18.8 g ASA
28
Chemistry-140 Lecture 9
Answer Step 3 Calculate the percent yield
of ASA in this particular
experiment. x
100 The percent yield of ASA 33.3 .
33.3
29
Chemistry-140 Lecture 10

• Chapter 5 Reactions in Aqueous Solution

• Chapter Highlights
• definitions of electrolytes nonelectrolytes
• recognize acids bases
• predict solubility of ionic compounds in water
• determine net ionic equations
• learn four basic reactions types predict
  products
• acid base
• precipitation
• gas-forming
• oxidation -reduction

30
Chemistry-140 Lecture 10

• Electrolytes

Definition a substance whose aqueous solution
conducts electricity is called an electrolyte

• a substance can be a strong electrolyte, a weak
  electrolyte or a nonelectrolyte depending on the
  degree of dissociation (ionization) in solution

• Example
• For sodium chloride, the ionic solid
  dissociates 100 in water forming exclusively
  Na and Cl- ions in solution
• 100 dissociation strong electrolyte

31
Chemistry-140 Lecture 10
Electrolytes
pure water
acetic acid solution
potassium dichromate solution
32
Chemistry-140 Lecture 10

• Identifying Electrolytes

• Strong electrolytes Substances that dissociate
  completely in water. Simple salts like NaCl
  that are combination of a metal and a
  nonmetal
• Weak electrolytes Substances that do not
  dissociate fully in water but do form some
  ions. Usually molecular compounds like
  acetic acid (CH3COOH) with ionizable groups
  (H)
• Nonelectrolytes Substances that do not
  dissociate in water to form ions. Molecular
  compounds which are soluble but which
  remain intact as the molecule in solution

33
Chemistry-140 Lecture 10
Question What type of electrolytes are these
compounds? a) Epsoms salt MgSO4 . 7
H2O b) Methanol CH3OH c) Acetic acid
CH3COOH
34
Chemistry-140 Lecture 10
Answer a) b) c)
strong electrolyte
nonelectrolyte
weak electrolyte
35
Chemistry-140 Lecture 10
Understanding Predicting Reactions in Solution

• Driving Force a property of the reaction that
  can be identified as the reason for product
  formation

• Examples

solid formation
36
Chemistry-140 Lecture 10
Types of Reactions

• the reaction type depends on the driving force of
  the reaction. There are four basic types

• Formation of an insoluble compound
• Formation of a nonelectrolyte
• Formation of a gas
• Transfer of electrons

37
Chemistry-140 Lecture 10

• Solubility

• certain combinations of cations and anions are
  soluble that is they dissolve in water..
• if a compound will not dissolve in water it is
  insoluble
• if a combination of anion and cation results in
  the formation of an insoluble solid, this is a
  precipitate

• Example

precipitate
38
Chemistry-140 Lecture 10
Soluble Compounds
Exceptions
Almost all salts of Na, K NH4
Salts of NO3-, ClO3-, ClO4-, CH3CO2-
39
Chemistry-140 Lecture 10
Exceptions
Insoluble Compounds
40
Chemistry-140 Lecture 10

• Net Ionic Equations

• the balanced equation that results from the
  omission of all spectator ions is the net ionic
  equation
• spectator ions are the ions which do not
  participate in the reaction

Example Write a balanced net ionic equation for
the reaction of AgNO3 with CaCl2 to produce AgCl
and Ca(NO3)2.
41
Chemistry-140 Lecture 10
Step 1 Write the complete balanced equation
with appropriate stoichiometry
Step 2 Decide on the physical state (eg
solubility) of each compound.
42
Chemistry-140 Lecture 10
Step 3 Recognize that all soluble ionic
compounds dissociate to form ions in
aqueous solution
43
Chemistry-140 Lecture 10
Step 4 Identify the spectator ions and remove
them from the complete ionic equation
to give the net ionic equation. Simplify
the resulting equation in terms of
stoichiometric coefficients.

• The sum of ion charges is the same on
• both sides of the net ionic equation

44
Chemistry-140 Lecture 10
Precipitation Reactions

• Write the net ionic equation for
• the reaction of Pb(NO3)2 with KI.

45
Chemistry-140 Lecture 10

• Acids Bases

• Acid any substance that , when dissolved in
  water, increases the concentration of hydrogen
  ions, H, in the water
• Base any substance that, when dissolved in
  water, increases the concentration of hydroxide
  ions, OH-, in the water

46
Chemistry-140 Lecture 10

• Strong Vs. Weak

• A strong acid or strong base an acid or base
  which ionizes completely in water a strong
  electrolyte
• A weak acid or base an acid or base which does
  not ionize completely in water a weak
  electrolyte

47
Chemistry-140 Lecture 10
Acid-Base Reactions I

• Write the net ionic equation for
• the reaction of HNO3 with KOH.

Overall Reaction
48
Chemistry-140 Lecture 10
Acid-Base Reactions II

• Write the net ionic equation for
• the reaction of CH3CO2H with Ca(OH)2.

Overall Reaction
2 CH3CO2H(aq) Ca(OH)2(s) Ca(CH3CO2)2(a
q) 2 HOH(l)
49
Chemistry-140 Lecture 10
Some Common Acids Bases
Strong Acids
Strong Bases
HCl, HBr, HI, HNO3, H2SO4, HClO4
NaOH, KOH, Ca(OH)2
Weak Acids
Weak Bases
CH3CO2H, H3PO4, HF, H2CO3
NH3
H2SO4(l) H(aq) HSO4-(aq)
Note
HSO4-(aq) H(aq) SO42-(aq)
50
Chemistry-140 Lecture 10
Gas-Forming Reactions

• The acids of some nonmetal ions are gases and a
  small number of aqueous acids easily decompose to
  form a gaseous product.

Examples
51
Chemistry-140 Lecture 10
Gas-Forming Reactions

• Write the net ionic equation for
• the reaction of HNO3 with NiCO3.

Overall Reaction
2 HNO3(aq) NiCO3(s)
Ni(NO3)2(aq) H2CO3(aq)
52
Chemistry-140 Lecture 10

• Properties of Compounds in Aqueous Solution

• Aqueous solution a solution of any substance or
  substances dissolved in water

• Example
• Solid sodium chloride dissolves in water to
  give an aqueous solution of sodium cations and
  chloride anions

aqueous solution of sodium chloride
53
Chemistry-140 Lecture 10

• Oxides of Metals Nonmetals

• If a nonmetal oxide is dissolved in water an
  acidic solution results. This compounds is known
  as an acidic oxide
• If a metal oxide is dissolved in water a basic
  solution results. This compounds is known as a
  basic oxide

54
Chemistry-140 Lecture 10
Summary Types of Reactions

• the reaction type depends on the driving force of
  the reaction. There are four basic types

Reaction Type Driving
Force Precipitation Reaction Formation of an
insoluble compound Acid-Base Neutralization Forma
tion of a nonelectrolyte (water) Gas-Forming Ev
olution of a water insoluble gas Oxidation
-reduction Transfer of electrons
55
Chemistry-140 Lecture 10

• Textbook Questions From Chapter 5
• Solubility 21, 22, 24
• Precipitation Reactions 32
• Net Ionic Equations 36, 37
• Reaction Types 42, 49, 52, 57
• Concentration/Stoichiometry 70, 75, 76
• Titration 84, 86, 90