Transcript Chapter 4 - Colby College Wiki

Chapter 4
Chemical Reactions and Stoichiometry
Representing Chemical Reactions
Reactants on Left
Products on Right
Indicates a chemical reaction
Balancing Chemical Reactions
• 1st Balance elements that occur in only one
compound on each side
• Balance free elements last
• (Balance unchanged polyatomics as groups)
• Clear fractional coefficients by multiplication
• Never introduce extraneous atoms
• Never change chemical formulas
Equations and the Mole Concept
Na
+
Cl
NaCl
1 atom Na
1 dozen Na
1 mol Na
+
+
+
1 atom Cl
1 dozen Cl
1 mol Cl
1 NaCl
1 dozen NaCl
1 mol NaCl
Reaction Types
Combination Reaction
A
+
B
C
Decomposition Reaction
X
Y
+
Z
CO2
+
H2O
Combustion Reaction
CxHyOz
+
O2
What mass of water is produced when 33.2 g ethanol is
burned completely?
You are a biologist
studying the mating
habits of the Asian
Civet cat.
You manage to isolate 62 mg of the Civet cat sex pheromone
Combustion analysis of the pheromone produces:
138.0 mg CO2 and 49.4 mg H2O
Combustion Analysis
The Sex Pheromone For The Asian Civet Cat
Limiting Reagents
We are making bagel pizzas (pepperoni, of course).
For each pizza, we need 1 bagel, 1 oz. tomato sauce,
2 slices of cheese and 5 slices of pepperoni:
Limiting Reagents
We are making bagel pizzas (pepperoni, of course).
For each pizza, we need 1 bagel, 1 oz. tomato sauce,
2 slices of cheese and 5 slices of pepperoni:
1 bagel + 1 oz. tomato sauce + 2 cheese + 5 pepperoni
1 pizza
Limiting Reagents
We are making bagel pizzas (pepperoni, of course).
For each pizza, we need 1 bagel, 1 oz. tomato sauce,
2 slices of cheese and 5 slices of pepperoni:
1 bagel + 1 oz. tomato sauce + 2 cheese + 5 pepperoni
1 pizza
If we have 13 bagels, 20 oz. of tomato sauce, 26
slices of cheese and 60 slices of pepperoni, how many
pizzas can we make?
If we react 85.0 g of Zn metal with 35.2 g of HCl,
what is the mass of H2 gas that is produced?
Actual & Theoretical Yield
Theoretical Yield: The maximum amount of product that can
be produced (usually reported in mol or g)
Actual Yield: Real (measured) amount of product that was
produced/obtained (usually reported in mol or g)
Percent Yield: The ratio of actual yield to theoretical yield
(reported as a percent, ALWAYS 100% or less)
Percent Yield =
Actual Yield
Theoretical Yield
X 100%
We reacted 12.8 g of Al metal with
excess HBr and obtained 77.9 g of
AlBr3. What is our percent yield?
Al (s) + HBr (aq)
H2 (g) + AlBr3 (s)
Units of Concentration: Molarity
Amount of solute (mol solute)
Molarity (M) =
Volume of solution (L)
Solute - lesser component of a mixture
Solvent - greater component of a mixture
Aqueous - dissolved in water
How many moles of HCl are in 1.00 L of muriatic
acid (a.k.a. concentrated aqueous HCl: 37% HCl by
mass, density = 1.200 g/mL)?
How many grams of AgNO3 do we need to make 500
ml of a 1.0 M AgNO3 solution?
What volume of a 3.0 M HCl solution is needed to
react with 38.4 g of Zn metal?
Compounds in Aqueous Solution
When NaCl dissolves in water – what happens to the NaCl?
It separates into ions
Compounds in Aqueous Solution
A Strong electrolyte dissociates completely.
Soluble ionic compounds – Good electrical conduction.
A Weak electrolyte partially dissociates.
Weak (molecular) acids & bases – Fair conductor of
electricity.
A Non-electrolyte does not dissociate.
Molecular/Covalent Compounds – Poor (non-)
conductor of electricity.
If we dissolve 8.0 g of potassium carbonate in water
and the total solution volume is 100 mL, what is the
concentration of potassium ions in solution? The
concentration of carbonate? Is potassium carbonate a
strong electrolyte?
Precipitation Reactions
A solid (the precipitate) forms when two solutions are
mixed
Ion Exchange (Metathesis)
Why does a precipitate form when the following solutions are mixed?
AgNO3 (aq) + NaI(aq)  AgI(s) + NaNO3 (aq)
What Ionic Compounds Are Soluble In Water?
Alkali metal ion and ammonium ion salts
Li+, Na+, K+, Rb+, Cs+, NH4+
Nitrates, perchlorates and acetates
NO3- , ClO4- , CH3CO2-
Chlorides, bromides and iodides
Cl-, Br-, I-
Except those of Pb+2, Ag+, and Hg2+2
Sulfates
SO4–2
Except those of Sr+2, Ba+2, Pb+2 and Hg2+2
Ca(SO4) is slightly soluble.
What Ionic Compounds Are Insoluble In Water?
Hydroxides and sulfides
HO–, S–2
Except alkali metal (group 1) and ammonium salts
Sulfides of alkaline earth metals (group 2) are soluble
Hydroxides of Sr2+ and Ca2+ are slightly soluble
Carbonates and phosphates
CO3–2, PO4–3
Except alkali metal and ammonium salts
Write The Net Ionic Equation For:
Ca(NO3)2 (aq) + Na3PO4 (aq) 
The reaction of lithium carbonate with magnesium sulfide?
Net Ionic Equations
Overall Equation:
AgNO3 (aq) +NaI(aq)  AgI(s) + NaNO3 (aq)
A net ionic equation contains only the ions undergoing
a change during the reaction
Net Ionic Equations
Net Ionic Equation:
Ag+ (aq) + I– (aq)  AgI (s)
A net ionic equation contains only the ions undergoing
a change during the reaction
Acids and Bases
• Acids provide H+ (protons) in aqueous solution (Arrhenius 1884)
• Bases provide OH- in aqueous solution (Arrhenius 1884)
• Bases react to accept or consume H+ (Bronstead 1923)
HCl (aq) 
H+ (aq) + Cl– (aq)
H2SO4 (aq) 
2 H+ (aq) + SO4–2 (aq)
NaOH (aq) 
Na+ (aq) + OH– (aq)
NH3 + H2O  NH4+(aq) + OH–(aq)
Acids and Bases
• Acids provide H+ (protons) in aqueous solution (Arrhenius 1884)
• Bases provide OH- in aqueous solution (Arrhenius 1884)
• Bases react to accept or consume H+ (Bronstead 1923)
Is water an acid, a base, or neither?
H2O 
?
Autoionization of Water and pH
In pure water at 25 °C, the concentration
of H+ ions is always 1X10-7 M
H2O 
H+ (aq) + OH– (aq)
pH = -log[H+] = -log(1X10-7) = 7
Acids and Bases
Strong acids/bases – Acids and bases that dissociate/react completely
in water to yield ions (including H+ or HO–) [Strong bases: hydroxides
of group 1 & “heavy” group 2 metals (Ca, Sr, Ba)]
Weak acids/bases - Acids and bases that only partially dissociate/react
in water to yield ions (including H+ or HO–)
Acids and Bases
Strong acids/bases – Acids and bases that dissociate/react completely in
water to yield ions (including H+ or HO–) [Strong bases: hydroxides of
group 1 & “heavy” group 2 metals (Ca, Sr, Ba)]
Weak acids/bases - Acids and bases that only partially dissociate/react in
water to yield ions (including H+ or HO–)
Strong acids/bases:
HCl (aq)  H+ (aq) + Cl– (aq)
NaOH (aq) 
Na+ (aq) + OH– (aq)
Weak acids/bases:
acetic acid
CH3 CO2H (aq) 
ammonia
H+ (aq) + CH3 CO2– (aq)
NH3 + H2O  NH4+(aq) + OH–(aq)
Acid/Base Neutralization
The reaction of H+ with HO– yields water:
H+ (aq) + OH– (aq)  H2O
Therefore:
H+ and HO– cannot “co-exist” beyond the amount of
water autoionization
Acids and bases will react to consume each other
Acids and Bases
Monoprotic acids yield 1 H+ ion per formula unit: HCl, HClO4, HNO3
Diprotic acids yield 2 H+ ion per formula unit: H2SO4
Triprotic acids yield 3 H+ ion per formula unit: H3PO4
Acids and Bases
Monoprotic acids yield 1 H+ ion per formula unit: HCl, HClO4, HNO3
Diprotic acids yield 2 H+ ion per formula unit: H2SO4
Triprotic acids yield 3 H+ ion per formula unit: H3PO4
200 mL of aqueous 0.1 M H2SO4 is allowed to react
completely with 7.4 g of solid NaOH. What ions remain in
solution at the end of the reaction? If we assume that the total
volume remains constant at 200 mL, what are the
concentrations of the remaining ions?
Acids and Bases
A student carefully adds 0.05 M potassium hydroxide to 50 mL
of a hydrobromic acid solution with an unknown
concentration. If it takes 17.8 mL of the potassium hydroxide
solution to turn the indicator (phenolphthalein) slightly pink,
what is the concentration of the hydrobromic acid solution?
• The above process is known as a titration – the careful
addition of one solution to another until one component
has exactly consumed another (at the Equivalence Point)
• An indicator is a substance that undergoes an observable
(color) change near or at an equivalence point
Acids and Bases
A student carefully adds 0.05 M potassium hydroxide to 50 mL
of a hydrobromic acid solution with an unknown
concentration. If it takes 17.8 mL of the potassium hydroxide
solution to turn the indicator (phenolphthalein) slightly pink,
what is the concentration of the hydrobromic acid solution?
Oxidation/Reduction Reactions (Redox)
In an oxidation-reduction (redox) reaction, some
atoms undergo changes in oxidation state:
charges change due to the exchange of electrons
Oxidation State
Oxidation state is a term used by chemists to give a general idea of
how electron rich/poor an atom is
• Oxidation state describes the charge an atom would possess if all bonds in
the compound were ionic
• Oxidation states refer to real charges for ions
• Oxidation states do not refer to real charges in covalent (molecular)
compounds
Determination of Oxidation States
1) All free elements = 0
2) The sum of all atomic oxidation states in a compound = the total charge
on the compound
3) For monoatomic ions, the oxidation state = charge on the ion
(Group 1 elements = +1, Group 2 elements = +2, etc.)
4) Fluorine = –1
5) Hydrogen = +1
6) Oxygen = –2
7) Halogens = –1, Group 16 = –2, Group 15 = –3
Determination of Oxidation States
1. H2S
2. S8
3. ClO4–
4. Na2S
5. O3
6. SO4–2
7. Fe2S3
8. ClO2
Redox Vocabulary
During an Oxidation
The oxidation state of a species increases (becomes more positive)
Electrons are lost by the atom
During a Reduction
The oxidation state of a species decreases (becomes more negative)
Electrons are gained by the atom
Oxidation and reduction always occur together.
Zn + HCl  H2 + ZnCl2
Ag+(aq) + Cu(s)  Ag(s) + Cu2+(aq)
Redox Vocabulary
An oxidizing agent (oxidant) oxidizes something it reacts with
Contains an element whose oxidation state decreases (is reduced)
during the redox reaction
A reducing agent (reductant) reduces something it reacts with
Contains an element whose oxidation state increases (is oxidized)
during the redox reaction
In which of the following reactions do
atoms undergo oxidation state changes?
CaCO3 + 2 HCl
C2H6O + 3 O2
2 Al + 3 Cl2
2 HI + Ca(OH)2
CaCl2 + H2O + CO2
2 CO2 + 3 H2O
2 AlCl3
CaI2 + 2 H2O
Identifying Oxidizing and Reducing Agents
H2O2 + Fe+2(aq)  H2O + Fe+3(aq)
H2O2 + MnO4–(aq) 
Mn+2(aq) + O2
2 H2O2 (aq)  2 H2O (l) + O2 (g)
Hydrogen peroxide, H2O2, is a versatile chemical. Its uses include
bleaching wood pulp and fabrics and substituting for chlorine in
water purification. One reason for its versatility is that it can be either
an oxidizing or a reducing agent.
Writing and Balancing Redox Reactions
Cu(s) + Ag+(aq)  Cu+2(aq) + Ag(s)
Is this reaction balanced?
Balancing Redox Reactions
Na2SO3 (aq) + KMnO4 (aq)  Na2SO4 (aq) + K+(aq) + Mn+2(aq)
CN– (aq) + MnO4–(aq)  MnO2 (s) + OCN– (aq)
Balancing Redox Reactions: Half-Reactions
• Write the equations for the half-reactions.
– Balance all atoms except H and O (balance H and O
also if they undergo redox)
– Add e- based on oxidation state changes
– Balance oxygen atoms using H2O
– Balance hydrogen atoms using H+
• Equalize the number of electrons.
• Add the half reactions.
• If in base, add hydroxide to neutralize H+ and cancel water
molecules that appear on both sides of the equation.
• Check the balance.
The Activity Series
We can measure (electrochemically) how easily an element will
accept electrons (reduce) or release electrons (oxidize).
By putting these measurements in order of reduction potential,
we generate an activity series