Transcript Lecture 8
Lecture 8: Ionic Chemical Reactions (Ch 10)
Suggested HW: 1, 9, 35, 39, 41, 45, 51, 61
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Recap
•
To date, we have learned about:
– – –
Chemical elements and their classifications Valence electron configurations and their effect on chemical properties and reactivity Predicting ionic charges
•
Now, we will begin to learn about chemical reactions involving ionic compounds
Recap
Na + Cl -
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Electrostatic interactions (coulombic attractions) between NaCl molecules holds them together in a lattice
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This is the strongest type of intermolecular force, points.
which is why all ionic compounds are solid at room temperature and have extremely high melting
About Polyatomic Ions
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Polyatomic ions are covalent molecules that possess charge and behave as normal ions in solution.
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When a salt containing a polyatomic ion is dissolved in water, the polyatomic ions themselves DO NOT break apart. They are simply separated from the
counter-ion
.
•
Example:
Phosphate
(PO 4 3 ) Na 3 PO 4 H O (L) (s) 3Na + Sodium Phosphate (aq) + PO 4 3 (aq) Sodium cations Phosphate anion
KNOW YOUR POLYATOMIC IONS !!!!!
Charge Name Hydroxide -1 Structure OH Cyanide CN Bicarbonate HCO 3 Acetate CH 3 COO Nitrate Nitrite Perchlorate NO 3 NO 2 ClO 4 Charge Name Carbonate -2 Oxalate Sulfate Sulfite Structure CO 3 2 C 2 O 4 2 SO 4 2 SO 3 2 Charge Name -3 Phosphate Structure PO 4 3 Charge Name +1 Structure Ammonium NH 4 +
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Group Examples
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Write out chemical formulas for the following:
– – – – – –
Calcium nitrate Sodium sulfate Ammonium hydroxide Iron (III) acetate Tungsten (VI) cyanide Aluminum perchlorate
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Ionic Reactions
Chemical reactions involving ionic compounds can be classified as one of the following: 1. combination reactions 2. decomposition reactions 3. single replacement reactions 4. double replacement reactions
Combination Reactions
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In a combination reaction, multiple reactants combine to form a single product
–
The reaction may occur between two elements
– –
Or between an element and a compound Or between two compounds 3Li(s) + P(g)
Ca(s) + Cl 2 (g)
Li 3 P(s) CaCl 2 (s) SO 3 (g) + H 2 O(l)
H 2 SO 4 (aq) MgO(s) + CO 2 (g)
MgCO 3 (s)
Decomposition Reactions
2HgO(s) 2Hg(l) + O 2 (g) 2KClO 3 (s) 2KCl(s) + 3O 2 (g)
Single Replacement Reactions
In a single replacement reaction,
When one metal replaces another, this is also called a transmetallation reaction. Zn (
s
) + 2 Ag Cl (
aq
)
Fe (
s
) + Cu SO 4 (
aq
)
Zn Cl 2 (
aq
) + 2 Ag (
s
) Fe SO 4 (
aq
) + Cu (
s
)
Single Replacement
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Single replacement reactions occur because one metal is less stable ( more active ) in its solid state than the other.
•
In the reaction below, Zn displaces Ag because Zn is more reactive: Zn (
s
) + 2 Ag Cl (
aq
) Zn Cl 2 (
aq
) + 2 Ag (
s
)
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More active metals prefer to exist as aqueous ions instead of neutral atoms
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A more active metal will displace a less active metal. The opposite will NOT occur. An activity series is provided on pg. 325 of the text.
Group Work
As shown in the table, Li, which has the highest activity, is the most likely to react. It will displace any other metal. Potassium will displace any metal except Li. And so on.
Activity Series Predict the products.
Li (s) + Ca(ClO 4 ) 2 (aq) Na (s) + ZnSO 4 (aq) K (s) + LiCl (aq)
Single Replacement Reactions Involving Acids
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When a metal reacts with a binary acid (HX), the metal replaces the hydrogen atom to yield an ionic compound and hydrogen gas.
Zn(s) + 2HCl (aq) ZnCl 2 (aq) + H 2 (g)
Redox Reactions
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Single replacement reactions are examples of red ox (reduction-oxidation) reactions
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A reduction process corresponds to a process in which the oxidation state (charge) of an element/ion becomes
more negative
during the course of a reaction
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In an oxidation becomes process, the oxidation state of an element/ion
more positive
during a reaction
Redox Reactions
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Consider the following single replacement reaction: Zn(s) + Cu SO 4 (aq) Zn SO 4 (aq) + Cu (s)
On the reactant side, we have elemental Zn. The charge on any pure element is 0 On the reactant side, we have a Cu 2+ ion.
On the product side, we have a Zn 2+ ion. Since the charge of Zn has gone from 0 to 2 + , Zn has undergone an
oxidation
. Zn loses 2 electrons. Where did they go??? On the product side, we have elemental Cu, so Cu has undergone a
reduction
from Zn.
from 2 + to 0 by taking electrons
Oxidizing and Reducing Agents
Zn(s) + Cu SO 4 (aq) Zn SO 4 (aq) + Cu (s)
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We have identified the reduction and oxidation processes in the reaction above Zn 0 Cu 2+
Zn + 2e 2+
+ 2e Cu 0 RED-OX REACTIONS
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Because Zn gets oxidized, it is the reducing agent
.
In other words, the oxidation of Zn causes the reduction of Cu 2+
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Because Cu 2+ gets reduced, it is the oxidizing agent . Zn is oxidized because Cu 2+ takes electrons away from more active Zn.
Oxidizing and Reducing Agents
Zn(s) + Cu SO 4 (aq) Zn SO 4 (aq) + Cu (s)
(C.I.R.L) Rust Formation
Reduced
4Fe(
s
) + 3O
2
(
g
) 2Fe
2
O
3
(
s
)
Oxidized
Double Replacement Reactions
In a double replacement result,
two salts react, and the anions exchange places Ag NO 3 (
aq
) + Na Cl (
aq
) Zn S (
s
) + 2H Cl (
aq
) Ag Cl (
s
) + Na NO 3 (
aq
) Zn Cl 2 (
aq
) + H 2 S (
g
)
Examples
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Balance the following double replacement reactions A. CaBr 2 (aq) + K 2 CO 3 (aq) B. NH 4 Cl (aq) + MgSO 4 (aq)
Most Double Replacement Reactions Yield Precipitates
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An easy way to identify a chemical reaction is if there is a change in phase. In a Precipitation Reaction , an insoluble (solid, does not dissolve) ionic product is formed.
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In the figure to the left, Na 2 S (aq) and Cd(NO 3 ) 2 (aq) undergo double replacement to form CdS and NaNO 3 .
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CdS is insoluble
Net Ionic Equations
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It is proper practice to use NET IONIC EQUATIONS a change in phase when there is
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Ex. Na 2 S(aq) + Cd(NO 3 ) 2 (aq) 2NaNO 3 (aq) + CdS(s)
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Since we know that ionic solutions dissociate in water, we can rewrite the equation above in ionic form: 2Na + (aq) + S 2 (aq) + Cd 2+ (aq) + 2NO 3 (aq) CdS(s) + 2Na + (aq) + 2NO 3 (aq) The ions in red undergo a chemical reaction, as indicated by the change in phase.
The remaining ions are called SPECTATOR IONS because they are not involved in the reaction in any way.
Net Ionic Equations
Na + (aq) + S 2 (aq) + Cd 2+ (aq) + NO 3 (aq) CdS(s) + Na + (aq) + NO 3 (aq)
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The spectators ions cancel out. The remaining reactants and products comprise the net ionic equation
.
Cd 2+ (aq) + S 2 (aq) CdS(s) NET IONIC EQUATION
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In order to write a net ionic equation, you must know which ionic compounds are insoluble. The solubility rules enable this.
Solubility Rules
1. All group 1 and ammonium salts are soluble!
2. All nitrates, acetates, and perchlorates are soluble 3. With the exception of all anions mentioned in #2, Ag, Pb, and Hg(I) salts are all insoluble 4. With the exception of those cations mentioned in #1, carbonates, sulfides, oxides, and phosphates are insoluble 5. With the exception of Ca, Sr, and Ba, and those cations mentioned in #1, all hydroxides are insoluble 6. All sulfates are soluble EXCEPT for Ca, Sr, Ba, and those cations mentioned in #3.
Examples
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Use solubility rules to predict the products of the following double replacement reactions. Write the net ionic reaction. If there is no reaction, write ‘no reaction’: MgBr 2 (aq) + K 2 CO 3 (aq) NaCH 3 COO (aq) + CaBr 2 (aq)