Ionic Chemical Reactions

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.

Ionic Chemical Reactions

Lecture 12 (Ch. 10)10/1/12

HW: 1, 9, 35, 39, 41, 45, 51, 61

Introduction

To date, we have learned about: Chemical elements and their classifications Valence electron configurations and their effect on chemical

properties and reactivity Chemical bonding Molecular geometries

Now, we will begin to learn about chemical reactions involving ionic compounds

Ionic Compounds (recap)

Na+

Cl-

Electrostatic interactions between NaCl molecules holds them together in a lattice, which is why all ionic compounds are solid at room temperature.

waterNaCl (s)

NaCl (s) NaCl (aq)H2O (L)

NaCl (aq) Na+(aq) + Cl-(aq)=

Dissociation of Ionic Compounds in Water

• An ionic compound fully dissolves in water to form an aqueous solution

• The compound will split into cations and anions.

Polyatomic Ions

Polyatomic ions are covalent molecules with an overall charge. These molecules behave as normal ions.

Polyatomic ions do NOT break apart in water

Ex. Al(PO4) (s) Al3+ (aq) + PO43-(aq)

Aluminum Phosphate

Aluminum cation

Phosphate anion

H2O (L)

6

1-

2-

3-

1+

You DEFINITELY want to know these (Polyatomic Ions)

Types of Reactions

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

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

Combination Reaction

(E+E) 3Li(s) + P(g) Li3P(s)

(E+C) 2Na(s) + Cl2(g) 2NaCl(s)

(C+C) SO3(g) + H2O(l) H2SO4(aq)

Examples Predict the products of the following combination reactions. Also,

predict the phase of each reactant and product.

Hints: You will form ionic compounds. Also, pay attention to possible polyatomic ions.

Li (s) + ½ O2 (g) MgO (s) + CO2 (g)

Li2O (s) Lithium oxide2MgCO3 (s) Magnesium Carbonate

Decomposition ReactionIn a decomposition reaction, one substance splits into two or more simpler substances

2HgO(s) 2Hg(l) + O2(g)

2KClO3(s) 2KCl(s) + 3O2(g)

Single Replacement ReactionIn a single replacement reaction, An element reacts with a salt, and two elements switch places. When

one metal replaces another in an ionic compound, this is also called a transmetallation reaction.

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

Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)

Transmetallations Transmetallations occur because one metal is more active (less stable)

than the other.

In the reaction below, Zn displaces Ag because Zn is more active : Zn(s) + 2AgCl (aq) ZnCl2(aq) + 2Ag(s)

A metal of greater activity will displace a less active metal. The opposite will NOT occur. An activity series is provided on pg. 325 of the text.

Group Examples Activity Series

Predict the products. Include phase. Balance if necessary.

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

Na (s) + ZnSO4(aq)

K (s) + LiCl (aq)

Single Replacement Reactions involving Metals and Strong Acids

• The acids above are known as the strong acids. They are referred to as “strong” because they fully dissociation in water. KNOW THESE.

• When a metal reacts with a strong acid, the metal replaces the hydrogen atom to yield an ionic compound and hydrogen gas.

Zn(s) + 2HCl (aq) ZnCl2(aq) + H2(g)

STRONG ACIDS

Zn and HCl Combine in a Single Replacement Reaction

Single Replacement Reactions Don’t Just Involve Metals

A more reactive nonmetal can also replace a less reactive one, as shown below.

F2(g) + 2KCl --> 2KF (s) + Cl2 (g)

The general rule of thumb with nonmentals is: reactivity increases up a group.

F > Cl > Br > I

Double Replacement ReactionIn 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)

Example Balance the following double replacement reactions

A. CaBr2 (aq) + K2CO3(aq)

B. NH4Cl (aq) + MgSO4 (aq)

ReviewClassify each of the following reactions as:Combination, decomposition, single replacement, or double replacement

A. 2Al(s) + 3H2SO4(aq) Al2(SO4)3(aq) + 3H2(g)

B. Na2SO4(aq) + 2AgNO3(aq) Ag2SO4(s) + 2NaNO3(aq)

C. 2NaClO3(s) 2NaCl(s) + 3O2(g)

D. 3Mg(s) + N2(g) Mg3N2 (s)

Most Double Replacement Reactions are Precipitation Reactions An easy way to identify a chemical reaction is if there is a change in

phase. In a Precipitation Reaction, an insoluble 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 (does not dissociate). The result is the formation of a solid product.

Solubility Rules

1. All group 1 and ammonium salts are soluble!

2. All nitrates, acetates, and perchlorates are soluble

3. Ag, Pb, and Hg(I) salts are all insoluble (except for those mentioned in 2)

4. Carbonates, sulfides, oxides, and phosphates are insoluble (except group 1)

5. MOST hydroxides are insoluble, EXCEPT for hydroxides of Ba, Ca, and Sr (and group 1)

6. All sulfates are soluble EXCEPT for Ca and Ba

Examples of Precipitates Use solubility rules to predict the products of the following double

replacement reactions. If there is no change of phase, say ‘no reaction’:

BaCl2 (aq) + Na2SO4(aq) MgBr2 (aq) + K2CO3 (aq) NaCH3COO (aq) + CaBr2 (aq)

Net Ionic Equations• It is proper practice to use NET IONIC EQUATIONS when describing a

double replacement reaction, especially one involving the formation of a precipitate

• 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

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

NET IONIC EQUATION

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

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

-(aq)

Example Identify the spectator ions, then write the net ionic equation

corresponding to the following reactions:

Na2S(aq) + 2HCl(aq) 2NaCl (aq) + H2S(g)

2AgClO4(aq) + (NH4)2SO4 ?

Part II. Introduction to Red-Ox Reactions

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

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

In an oxidation process, the oxidation state of an element/ion becomes more positive during a reaction

Introduction to Red-Ox 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.

Oxidizing and Reducing Agents

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

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

Zn0 Zn2+ + 2e-

Cu2+ + 2e- Cu0

RED-OX REACTIONS

• Because Zn gets oxidized, it is the reducing agent. In other words, the oxidation of Zn causes the reduction of Cu2+

• Because Cu2+ gets reduced, it is the oxidizing agent. Zn is oxidized because Cu2+ takes electrons away from more active Zn.

Zn-Cu Transmetallation

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

Example of Red-Ox Reactions in Everyday Life: Rust

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

Oxidized

Reduced