Chapter 5: Introduction to Reactions in Aqueous Solutions

Electrolyte.A substance, such as sodium chloride, that dissolved in water and produces ions to give an electrically conducting solution is called an electrolyte.A substance, such as sucrose, or table sugar (C12H22O11), that is not

ionized and does not conduct electric current when dissolved in water is called a Nonelectrolyte.

Strong electrolyte: is a substance that is almost completely ionized in aqueous solution, and the solution is a good electrical conductor.

Weak electrolyte: is partially ionized in aqueous solution, and the solution is only a fair conductor of electricity. (is an electrolyte that dissolves in water to give an equilibrium between a molecular substance and a relatively small quantity of ions.)

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

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

Strong electrolyte

Weak electrolyte

~ Essentially all soluble ionic compounds and only a relatively few molecular compounds are strong electrolytes.~ Most molecular compounds are either nonelectrolytes or weak electrolytes.

Table 1 Electrolyte Classification of Some Common SubstancesStrong Electrolytes Weak Electrolytes Nonelectrolytes

HCl, HBr, HI CH3COOH H2O

HClO4 HF CH3OH

HNO3 C2H5OH

H2SO4 C12H22O11(sucrose)

KBr Most organic compounds

NaCl

NaOH, KOHOther soluble ionic compounds

Aqueous Reactions

Aqueous reactions can be grouped into three general categories, each with its own kind of driving force: precipitation reactions, acid base neutralization reactions, and oxidation-reduction reactions.

Precipitation Reactions

Precipitation reactions are process in which soluble reactants yield an insoluble solid product that falls out of solution. Most precipitations take place when certain cations and anions combined to produce an insoluble ionic solid called a precipitate.

E.g. the reaction of silver nitrate and sodium iodide in an aqueous water solution yields sodium nitrate in solution and a yellow precipitate of silver iodide. We can write the equation for the reaction as follow:

AgNO3(aq) + NaI(aq) AgI(s) + NaNO3(aq) 

we called this a molecular equation, an equation in which the substances are written as if they were molecular substances, even though they may exist as ions in the solution. Molecular equation is useful because it is explicit about what solutions have been added and what products are obtained; and to calculate the amount of reactants or products.

Ionic Equations

The molecular equation does not tell us that the reaction actual involves ions in solution. However, we know that soluble ionic substances in solution should be represented by their separate ions.To represent this, the above reaction as an ionic equation, in which all the ions are explicitly shown:

An ionic equation:

Ag+(aq) + NO3-(aq) + Na+(aq) + I-(aq) AgI(s) + Na+(aq)+NO3

-(aq)

This is an example of ionic equation, which is a chemical equation for a reaction involving ions in solution in which soluble substances are represented by the formulas of the predominant species in that solution.

Net Ionic Equations

Note that some ions appear on both side of equation. These ions go through the reaction unchanged- does not take part in the reaction. We called them spectator ions. We can cancel them from the equation. The resulting equation is a net ionic equation.

Ag+(aq) + I-(aq) AgI(s)

Net ionic EquationA net ionic equation is an equation that includes only the actual participants in a reaction, with each participant denoted by the symbol or formula that best represent it.

rules for converting molecular equations to ionic equations

The rules for converting molecular equations to ionic equations follow:

1) Make sure the molecular is balanced

2) Ionic substances indicated in the molecular equation as dissolved in solution, such as NaCl(aq), are normally written as ions.

3) Ionic substances that are insoluble (do not dissolve) either as reactants or products (such as precipitate) are represented by formulas of the compounds

4) Molecular substances that are strong electrolytes, such as strong acids, are written as ions. Thus, HCl(aq) is written as H3O+(aq) + Cl-(aq) or as H+(aq) + Cl-(aq).

5) Molecular substances that are weak electrolytes or nonelectrolytes are represented by their molecular formulas.

Acid-Base Reactions

Consider the production of ions in pure water. It produces a small percentage of ions (about 2x10-7% of the molecules react to gives ions).

H2O(l)+ H2O(l) H3O+(aq) + OH-(aq)

Arrhenius definitionAn acid is a substance that provides hydrogen ions (H+) (increase the concentration of H+) in aqueous solution. The symbol H+(aq) does not really represent the structure of the ion present in aqueous solution. H+ is too reactive to exit by itself, it attaches to water to give the more stable hydronium ion, H3O

+.

A base is a substance that produces hydroxide ions (OH-) (increase the conc. of hydroxide ions) in aqueous solution.

HA (aq) H+(aq) + A-(aq)an acid HA is a general formula for an acid

MOH(aq) M+(aq) + OH-(aq)an base MOH is a general formula for a base

Writing Formula, Total and Net Ionic Equations

We must be able to represent a reaction in three different ways:Formula EquationTotal or Complete Ionic EquationNet Ionic Equation

Formula Equation:A balanced chemical equation in which all the reactants and

products are given by their chemical formula.

For Example:

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

Soluble species are indicated by placing “(aq)” after the formula.

Precipitates are indicated by placing “(s)” after the formula.

This way of expressing the reaction is used whenever we want to indicate the chemical formula of the reactants and products.

Total or Complete Ionic Equation Shows all the soluble ionic species broken up into their respective

ions.

For Example:

Ag+(aq)+ NO3-(aq)+ Na+(aq)+ Cl-(aq) → AgCl(s)+ Na+(aq)+ NO3-(aq)

This method is not frequently used because it is cumbersome to write out.

It is used to emphasize the situation which exists before and after the reaction.

Spectator ions:

Ions that remain unchanged during a reaction and do not participate in the reaction.

The spectator ions in the above equation are NO3-(aq) and Na+(aq)

NEVER write a total ionic equation until you have first written a balanced formula equation.

If you proceed directly to a total ionic equation the result may be balanced but still may not represent the actual situation.

Net Ionic Equation: Shows only the reacting species (species which are actively involved in

the reaction…the one making the PPT) in the equation.

The net ionic equation is formed by omitting the spectator ions from the total ionic equation.

For Example:

We had:

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

then:

Ag+(aq)+ NO3-(aq)+ Na+(aq)+ Cl-(aq) → AgCl(s)+ Na+(aq)+ NO3-(aq)

Canceling out spectator ions, we get:

Ag+(aq)+ Cl-(aq) → AgCl(s)

Let’s try an example putting all these ideas together

Example:

Write a formula equation, a total ionic equation, and a net ionic equation for the reaction which occurs when 0.2 M solutions of Al(NO3)3(aq) and MgS(aq) are mixed.

1. Write the complete formula equation. Determine if there is a precipitate using“(s)” to identify the precipitate. The balanced formula equation is:

2 Al(NO3)3(aq) + 3 MgS(aq) → Al2S3(s) + 3 Mg(NO3)2(aq)

2. Write the complete ionic equation by breaking up the balanced formula equation into ions.

Remember: The precipitate remains in molecular form. The number of ions is found by multiplying the subscript

after the ion, by the coefficient in front of the molecule. The final equation must still be balanced.

We get:

2Al3+ (aq)+6NO3-(aq)+3Mg2+ (aq)+3S2-(aq)

Al2S3(s)+3Mg2+ (aq)+6NO3-(aq)

3. Write the net ionic equation by deleting the spectator ions and simplifying the coefficients if necessary.

2 Al3+ (aq) + 3 S2-(aq) → Al2S3(s)

Let’s Try Some More

Write the formula, complete ionic, and net ionic equation for each of the following reactions:

1) Aqueous nickel (II) chloride reacts with aqueous sodium hydroxide to give nickel (II) hydroxide and sodium chloride.

2) Solid potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

3) Aqueous sodium hydroxide reacts with phosphoric acid to give water and sodium phosphate.

Predicting Precipitation Reaction Empirical Rules for the solubilities of Common Ionic Compounds

Soluble compounds Exceptions

Sodium, potassium and ammonium

compounds

Acetate and nitrates

Halides (chlorides, bromides, and iodides) Lead(II), silver and mercury(I) halides are insoluble

Sulfates Calcium, strontium(Sr), barium and lead(II)

sulfate are insoluble

Insoluble compounds Exceptions

Carbonates and phosphates Group 1 metals and ammonium compounds

are soluble

Hydroxides Group 1 metals compounds are soluble and

calcium, Sr2+ and Ba2+ are slightly soluble

Sulfides group 1 and group 2 metals and

ammonium compounds are soluble_________________________________________________________________________________________________

E.g. Predict whether a reaction will occur in each of the following case. If so, write a net ionic equation for the reaction. If no reaction occurs, write NR after arrow.

(a) Al2(SO4)3 + NaOH

i) write down the reactants and interchange of anions to get

product

Al2(SO4)3 + 6NaOH2Al(OH)3 + 3Na2SO4

All common Na compounds are water soluble Na+ remain in

solution. The combination of Al3+ and OH- produce insoluble

Al(OH)3. Then the ionic equation is

2Al3+ +3SO4

2- + 6Na+ + 6OH-2Al(OH)3(s)+ 6Na+ + 3SO4

2-

The net ionic equation is :

Al3+ + 3OH-Al(OH)3(s)

(b) K2SO4(aq)+FeBr3(aq)

(c) CdCl2(aq) + (NH4)2S(aq)