Ionic Chemical Reactions ( Ch 10)

Lecture 8: Ionic Chemical Reactions (Ch 10)

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Suggested HW: 1, 9, 35, 39, 41, 45,

51, 61

• 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-

• Electrostatic interactions (coulombic attractions) between NaCl molecules holds them together in a lattice

• This is the strongest type of intermolecular force, which is why all ionic compounds are solid at room temperature and have extremely high melting points.

Recap

• Polyatomic ions are covalent molecules that possess charge and behave as normal ions in solution.

• 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 (PO43-)

Na3PO4 (s) 3Na+ (aq) + PO43-(aq)

Sodium Phosphate

Sodium cations

Phosphate anion

H2O (L)

About Polyatomic Ions

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Charge

Name Structure

-1

Hydroxide OH-

Cyanide CN-

Bicarbonate HCO3-

Acetate CH3COO-

Nitrate NO3-

Nitrite NO2-

Perchlorate ClO4-

Charge

Name Structure

-2

Carbonate CO32-

Oxalate C2O42-

Sulfate SO42-

Sulfite SO32-

Charge

Name Structure

-3 Phosphate PO43-

Charge

Name Structure

+1 Ammonium

NH4+

KNOW YOUR POLYATOMIC IONS !!!!!

• 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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Group Examples

Chemical reactions involving ionic compounds can be classified as one of the following:

1. combination reactions2. decomposition reactions3. single replacement reactions4. double replacement reactions

Ionic Reactions

• 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

Combination Reactions

3Li(s) + P(g) Li3P(s)Ca(s) + Cl2(g) CaCl2(s)SO3(g) + H2O(l) H2SO4(aq)MgO(s) + CO2(g) MgCO3 (s)

2HgO(s) 2Hg(l) + O2(g) 2KClO3(s) 2KCl(s) + 3O2(g)

Decomposition Reactions

In a single replacement reaction,

When one metal replaces another, this is also called a transmetallation reaction.

Zn(s) + 2AgCl (aq) ZnCl2(aq) + 2Ag(s) Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)

Single Replacement Reactions

• Single replacement reactions occur because one metal is less stable (more reactive) in its solid state than the other.

• In the reaction below, Zn displaces Ag because Zn is more reactive:

Zn(s) + 2AgCl (aq) ZnCl2(aq) + 2Ag(s)

• More reactive metals prefer to exist as aqueous ions instead of neutral atoms. Therefore, a more reactive metal will displace a less active metal. The opposite will NOT occur.

• An activity series is provided on pg. 325 of the text.

Single Replacement

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.

Predict the products.

Li (s) + Ca(ClO4)2 (aq)

Na (s) + ZnSO4(aq)

Al (s) + KNO3 (aq)

Group Work

• 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) ZnCl2(aq) + H2(g)

Single Replacement Reactions Involving Acids

• Single replacement reactions are examples of red-ox (reduction-oxidation) reactions

• A reduction reaction occurs when the oxidation state (charge) of an element/ion becomes more negative during the course of a reaction (i.e. the species gains electrons)

• In an oxidation process, the oxidation state of an element/ion becomes more positive during a reaction (species loses electrons)

Redox Reactions

• Consider the following single replacement reaction:

Zn(s) + Cu SO4 (aq) Zn SO4 (aq) + Cu (s)

On the reactant side, we have elemental Zn. The charge on any pure element is 0

On the product side, we have a Zn2+ 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 reactant side,

we have a Cu2+ ion.

On the product side, we have elemental Cu, so Cu has undergone a reduction from 2+ to 0 by taking electrons from Zn.

Redox Reactions

Zn(s) + Cu SO4 (aq) Zn SO4 (aq) + Cu (s)

• We have identified the reduction and oxidation processes in the reaction above

O: Zn0 Zn2+ + 2e-

R: Cu2+ + 2e- Cu0RED-OX

REACTIONS

• Because Cu2+ gets reduced, it is the oxidizing agent. Cu2+

takes electrons away from more the more reactive Zn (oxidizes Zn).

• Because Zn gets oxidized, it is the reducing agent. In other words, Zn electrons are used for the reduction of Cu2+

Oxidizing and Reducing Agents

Zn(s) + Cu SO4 (aq) Zn SO4 (aq) + Cu (s)

Oxidizing and Reducing Agents

4Fe(s) + 3O2(g) 2Fe2O3(s)

Oxidized

Reduced

(C.I.R.L) Rust Formation

In a double replacement result, two salts react, and the anions exchange places

AgNO3(aq) + NaCl(aq) AgCl(s) +

NaNO3(aq) ZnS(s) + 2HCl(aq) ZnCl2(aq) + H2S(g)

Double Replacement Reactions

• Balance the following double replacement reactions

A. CaBr2 (aq) + K2CO3(aq)

B. NH4Cl (aq) + MgSO4 (aq)

Examples

• 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.

• In the figure to the left, Na2S (aq) and Cd(NO3)2 (aq) undergo double replacement to form CdS and NaNO3 .

• CdS is insoluble

Double Replacement Reactions Yield Physical Changes

• It is proper practice to use NET IONIC EQUATIONS when there is a change in phase

• Ex. Na2S(aq) + Cd(NO3)2(aq) 2NaNO3(aq) + CdS(s)

• Since we know that ionic solutions dissociate in water, we can rewrite the equation above in ionic form:

2Na+(aq) + S2-(aq) + Cd2+(aq) + 2NO3-(aq) CdS(s) + 2Na+(aq) +

2NO3-(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) + S2-(aq) + Cd2+(aq) + NO3-(aq) CdS(s) + Na+(aq)

+ NO3-(aq)

• The spectators ions cancel out. The remaining reactants and products comprise the net ionic equation.

• In order to write a net ionic equation, you must know which ionic compounds are insoluble. The solubility rules enable this.

NET IONIC EQUATION

Cd2+(aq) + S2-(aq) CdS(s)

Net Ionic Equations

1. All group 1 and ammonium salts are water 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. Most sulfates are soluble EXCEPT for Ca, Sr, Ba, and those cations mentioned in #3.

Solubility Rules

• 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’:

MgBr2 (aq) + K2CO3 (aq)NaCH3COO (aq) + CaBr2 (aq)

Examples