metathesis reactions - nassau community college 151/151...•referred to as redox reactions...
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Lecture 6
Professor Hicks
Inorganic Chemistry (CHE151)
Metathesis reactions
• Precipitation reactions
• Acid-base (neutralization) reactions
• Reactions that form a gas
metathesis reactions
ions change partners
• How to write complete balanced reactions
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Solubility
• Max amount of a substance that will dissolve
• Not increased with increased stirring, shaking, etc.
• For molecular compounds educated guesses of
solubility can be made based on the “like dissolves
like” rule
• Solubility of ionic compounds seemingly
unpredictable
• Detailed rules exist to predict solubility for ionic
compounds based on the identity of the ions
I will give you the solubility rules for ionic compounds for the exam
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Precipitation reactions • Only some ionic compounds dissolve in water
• If solutions are mixed that contain ions that can form an
insoluble compound, the compound will form and
precipitate as a solid
AgCl
insoluble AgNO3
soluble
Ag+ (aq)
NO3- (aq)
NaCl soluble
Na+ (aq)
Cl- (aq)
mix
Na+ (aq)
NO3- (aq)
AgCl (s)
NaNO3 is
soluble
AgCl solid
precipitates
initially
cloudy
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Precipitation of an ionic compound
is a metathesis reaction
AgNO3 + NaCl → AgCl + NaNO3
white solid forms
a precipitate
metathesis reaction = ions change partners
Acids • H+ + anion
H+ -
anion
• Ionic compounds must separate into ions to dissolve
• Acids are molecular compound because they can
dissolve without dissociating into ions
• Weak acids have a small percentage of molecules
separated into H+ and an anion, the rest stay together
as one particle
Strong acids
HCl HNO3 H2SO4
H+ Cl- H+ NO3- H+ SO4
2-
hydrochloric acid nitric acid sulfuric acid
Weak acids
HC2H3O2 HF
H+ C2H3O2
- H+ F-
acetic acid hydrofluoric acid
HF ~ 95% H+ and F- 5%
Strong acids separate 100% into H+ and anion in water HCl ~ 0 % H+ and Cl- ~ 100%
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Molecular
compounds
All other
molecular
compounds dissolve
Dissolved
molecules
Ionic
compounds
dissolve
cations
(+ ions)
anions
(- ions)
separated ions H+ + F- ~5%
(separated ions)
HCl → H+ + Cl-
~ 100%
HF ~ 95%
(molecules)
Acids
dissolve
Acids are
Molecular compounds
Electrolytes
- anion
+ cation
• Molecular compounds = Non-Electrolytes
• Soluble ionic compounds and strong
acids are called Strong Electrolytes
- Form ions in water
- Mobile Ions = conduct electricity
• Weak acids = Weak Electrolytes
- Small % molecules → ions
- Conduct electricity poorly
water is a molecular compound and a non-electrolyte
(remember H is an exception it acts like a non-metal)
tap water only conducts b/c
ions are dissolved in it
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Strong Bases
+
cation OH-
any
cation
hydroxide
ion
- Ionic compounds with hydroxide ion (OH-)
Examples: NaOH, LiOH, KOH, Ca(OH)2, Ba(OH)2, Sr(OH)2,
Acid-base neutralization reactions
acid + strong base → H2O (l) + salt
H+
- anion
+
cation
OH-
strong
base
acid
HCl (aq) + Ba(OH)2 (aq) → H2O (l) + BaCl2 (aq)
metathesis
reactions
ions change
partners
acid-base reactions sometimes require balancing
2 2
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Weak Bases (molecular bases)
• Molecules that react with water to produce
OH-
Base + H2O → Base-H+ + OH-
NH3 (aq) + H2O (l) → NH4+ (aq) + OH- (aq)
Molecules that contain nitrogen are weak bases
Methylamine (CH3NH3), amphetamine, cocaine, or heroin
Free-base form is the form that has not accepted an H+ yet
Metathesis reactions that form a gas
acid + CO32- → CO2 (g) + H2O (l) + salt
acid + CN- → HCN (g) + salt
acid + S2- → H2S (g) + salt
strong base + NH4+ → NH3 (g) + H2O (l) + salt
Alka-seltzer
gas chamber
rotten eggs, swamps
cleaning solutions
You must memorize these reactions for the exam
acid + HSO3- or SO3
2- → SO2 (g) + H2O (l) + salt
preservative
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If potassium cyanide is dissolved in water and hydrochloric acid is added a gas is formed. Write the molecular equation for this reaction.
How to write a complete molecular reaction Example: a metathesis reaction to form a gas
KCN (aq) + HCl (aq) → HCN (g) + salt
salt is the combination of the other ions from the metathesis reaction
salt = KCl check solubility rules = KCl is soluble written (aq)
+ KCl (aq) KCN (aq) + HCl (aq) → HCN (g)
HCl HNO3 HClO4 H2SO4
Hydrochloric Nitric Perchloric Sulfuric
Acid Acid Acid Acid
strong acids all soluble written (aq)
check solubility rules = KCN is soluble written as (aq)
memorize these
Strong acids
+ NaClO4 (aq) HClO4 (aq) + NaOH (aq) →
How to write a net ionic equation
Example: a neutralization reaction
Why is water described as liquid (l) not aqueous?
Pure substances - solids that precipitate, also substances
that would form separate liquid layer (oils), or are the
solvent itself are written as their physical state under
those conditions (solid, liquid, gas)
Write the molecular equation, total ionic equation, and net ionic equation
for the neutralization reaction of perchloric acid and sodium hydroxide.
Strong Electrolytes (strong acids and soluble
ionic compounds ) written as separate ions
(aq) in ionic equation
Molecular compounds including weak acids
are written as molecules – not as ions
H+(aq) + ClO4-(aq) + Na+(aq) + OH-(aq) → H2O(l) + Na+(aq) + ClO4
-(aq)
Complete Ionic Equation
H2O (l) + salt Molecular Equation
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Net Ionic Equation from
Complete Ionic Equation
H+(aq) + ClO4-(aq) + Na+(aq) + OH-(aq) → H2O(l) + Na+(aq) + ClO4
-(aq)
H+ (aq) + OH- (aq) → H2O (l)
Complete Ionic Equation
Net Ionic Equation
cross out spectator ions
Spectator ions are ions are dissolved
(aq) on the reactants and products side
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Formic acid (HCO2H) and sodium hydrogen sulfite
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Sulfuric acid and barium cyanide
Chloric acid and strontium hydroxide
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Oxidation-Reduction Reactions
• Oxidation is the loss of electrons
• Reduction is the gain of electrons
• They must always happen together
• Referred to as redox reactions
Oxidation-Reduction Reactions
4Na (s) + O2 (g) 2Na2O (s)
Na is oxidized O2 is reduced
Na is the reducing agent
O2 is the oxidizing agent
How can we tell O has gained / Na lost electrons?
Dissolve Na2O in water and the ions that form are 2Na+ (aq) and O2- (aq)
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Oxidation-Reduction Reactions
2Ag+ (aq) + Cu (s) 2Ag (s) + Cu2+ (aq)
• Cu donates electrons to Ag+
• Cu is oxidized (loses electrons)
• Ag+ is reduced (gains electrons
Oxidation-Reduction Reactions with
molecular compounds
2C (s) + O2 (g) 2CO (g)
CO does not break into ions when dissolved in
water - it is a molecular not ionic compound
This type of reaction is considered an oxidation-
reduction reaction, based on the similarity of the
balanced reaction to a large number of reactions
where the product can be dissolved to yield O2-
ions
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Concept of oxidation numbers
• Used to keeps track of electrons
• Allows fictional “charges” to be assigned
oxidation-reduction reactions identified
Electronegativity is a scale of
metallic/non-metallic character of elements
• Electronegativity scale runs 0-4
• Used to determine which of the elements
in a molecular compound will be thought of
as the metal and which the non-metal
• F has the highest EN
• Fr/Cs are the lowest
• H/B have the lowest EN of any non-metals
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Oxidation numbers in
CO • O in CO is negative because it is more
electronegative than C
• if O is assigned a -2 charge as it forms in
ionic compounds the C must be +2
• these are the Oxidation Numbers aka
Oxidation States for O and C
Rules for the Assignment of Oxidation Numbers with additions from Professor Hicks
1. Oxidation numbers must add up to the charge on the molecule, formula unit, or ion.
2. The atoms of free elements have oxidation numbers of zero.
3. Metals in Groups 1A, 2A, and Al have +1, +2, and +3 oxidation numbers,
respectively. 4. H and F in compounds have +1 and -1 oxidation numbers,
respectively.
5. Oxygen has a -2 oxidation number. Except in peroxides where it is -1
6. Group 7A elements have a -1 oxidation number.
7. Group 6A elements have a -2 oxidation number.
8. Group 5A elements have a -3 oxidation number.
9. When there is a conflict between two of these rules or an ambiguity in assigning
an oxidation number, apply the rule with the lower number and ignore the
conflicting rule.
10. H is +1 with any other non-metal except Boron (H+ in most molecular compounds)
11. H is -1 with any metal or Boron (H- in all ionic compounds and B/H compounds)
NaH, LiH, NaBH4, etc.
The negative ion of hydrogen ,H-, is called the hydride ion.
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a) O22- b)O2
- c) NH4+
a)BH3 b) LiH d) LiMgH3
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O2 + 2 F2 2OF2
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Combustion Reactions are Redox Reactions
Assign all the oxidation states below
C8H18 + 12½ O2 8CO2 + 9H2O
What was oxidized?
What was reduced?
C8H18 + 9 F2 C8F18 + 9H2
ement(s) are being are being
What was oxidized?
What was reduced?
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Combustion Reactions are Redox Reactions
Assign all the oxidation states below
C2H4 + H2 C2H6
What was oxidized?
What was reduced?
Combustion Reactions are Redox Reactions
Assign all the average oxidation states below
C6H12O6 + 6O2 6CO2 + 6H2O
What was oxidized?
What was reduced?
The reaction above is the combustion of glucose.
It is the same reaction referred to as respiration
performed in a living organism. The reverse of this
reaction is photosynthesis.
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Combustion Reactions are Redox Reactions
Assign all the average oxidation states below
C6H12O6 + ½ F2 C6H11O6F + ½ H2
What was oxidized?
What was reduced?
S based respiration/photosynthesis!
Assign all the oxidation states below
S + H2 H2S
What was oxidized?
What was reduced?
Instead of reducing CO2 to C6H12O6
(photosynthesis) purple bacteria reduce S
to H2S to get energy.
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