UNIT 5

Aqueous Reactions and Solution Stoichiometry

Double Replacement Reactions

General Properties of Aqueous Solutions• An aqueous solution is a

homogeneous mixture of water, the solvent, and one or more other compounds, the solutes.

• Solutes are the chemicals that dissolve in the water to form the solution.

• When a solute produces an aqueous solution that conducts electricity, it is called an electrolyte. All soluble ionic compounds are electrolytes.

• When a solute produces an aqueous solution that does not conduct electricity, it is called a nonelectrolyte. Many - but not all ! - organic compounds are nonelectrolytes.

General Properties of Aqueous Solutions• Water is a polar molecule and so is a polar

solvent. As such, it dissolves polar molecules and ionic compounds well.

• The force of attraction between the electropositive H of one water to the electronegative O of another water molecule is a strong dipole-dipole attraction called

hydrogen bonding.

What Happens When Ionic Compounds Dissolve in Water

When ionic compounds

dissolve in water, they separate into their component

ions.

NaCl exists in aqueous solution as Na+ and Cl-.

What Happens When Ionic Compounds Dissolve in Water

The force of attraction between ions and the polar water molecule is called the ion-dipole attraction.

What Happens When Nonelectrolytes Dissolve in Water

• Water is a poor conductor of electricity.

• When molecular compounds that are nonelectrolytes dissolve in water, they remain in molecular form. They do NOT separate into ions.

• This is why solutions of nonelectrolytes do not conduct electricity.

• Sugars and alcohols are nonelectrolytes.Water + methyl alcohol

Predicting What Happens When Ionic Compounds Dissolve in Water

Ionic compounds are strong electrolytes. Strong electrolytes dissociate completely into ions when they dissolve in water.

KClO3 (aq) K+(aq) + ClO3-(aq)

Ca(OH)2(aq) Ca2+(aq) + 2OH-(aq)

(NH4)3PO4(aq) 3NH4+(aq) + PO4

3-(aq)

Al2(SO4)3(aq) 2Al3+(aq) + 3SO42-(aq)

It is important to preserve the stoichiometry of the compound when showing the ions.

Predicting What Happens When Molecular Compounds Dissolve in Water

Some molecular compounds are strong electrolytes. The strong acids are molecular compounds that dissociate completely into ions when they dissolve in water.

Strong acids will lose one acid hydrogen (proton):

HCl(aq) H+(aq) + Cl-(aq)

HNO3(aq) H+(aq) + NO3-(aq)

H2SO4(aq) H+(aq) + HSO4-(aq) Note!!!

HClO4(aq) H+(aq) + ClO4-(aq)

What Happens When Molecular Compounds That Are Weak Electrolytes Dissolve in Water

Weak electrolytes are molecular compounds, some of which dissociate into ions, but most of which remain as molecules when they dissolve in water.

Acetic acid is a weak electrolyte (and a weak acid).

HC2H3O2 (aq) H+(aq) + C2H3O2-(aq)

99% 1% 1%

Solutions of weak electrolytes contain both molecules AND ions! Since there are not many ions, these solutions are weak conductors of electricity.

Weak versus StrongWeak electrolytes, weak acids, weak bases:

These terms refer to electrical conductivity

…NOT REACTIVITY!!!

Ammonia is a weak base but a strong cleaner.

Weak means produces few ions relative to the number of molecules one starts with. The tipoff in a chemical equation is the double arrow, which indicates an equilibrium between products and reactants:

HC2H3O2 (aq) H+(aq) + C2H3O2-(aq)

Weak versus Strong

Molecular compounds that dissolve in water can be • strong electrolytes (the 7 strong acids)

• HCl, HI, HBr, HNO3, H2SO4, HClO3, HClO4

• weak electrolytes • weak acids such as acetic acid or citric

acid• weak bases such as ammonia or the

amines• nonelectrolytes

• sugars, alcohols

Weak versus Strong

Ionic compounds that are soluble are strong electrolytes. Ionic compounds that are not very soluble are considered weak.

• BaSO4 is not very soluble in water. In Chem I, we treat it as a nonelectrolyte (in reality, it is a weak electrolyte).

Metathesis Reactionsaka Exchange Reactions or Double Replacement Reactions

• Metathesis reactions are reactions of the form: AX + BY AY + BX

Why can the exchange occur? Because, in general, AX and BY exist in solution as IONS:

A+ + X- + B+ + Y - • If either of the POSSIBLE new

compounds AY or BX is:• insoluble in water (a precipitate)• a weak electrolyte• a gas

then the metathesis reaction will occur.

Predicting Whether a Metathesis Reaction Occurs – Solubility Guidelines

AX + BY AY + BX

Let’s consider the first case in which this reaction occurs: the case where AY or BX is a precipitate. To predict if a product is a precipitate, you must know its solubility in water. You will be given these guidelines for the test:

• All compounds containing Group 1A metal cations or the ammonium ion are soluble in water.

• All common acetates, nitrates, chlorates and perchlorates are soluble.

• All chlorides, bromides, and iodides are soluble except those containing Ag+, Hg2

2+, or Pb2+.• All fluorides except Group IIA

and Pb2+.• All sulfates are soluble except

those containing Sr2+, Ba2+, Hg22+,

or Pb2+.

Predicting Whether a Metathesis Reaction Occurs – Solubility Guidelines

AX + BY AY + BX

More guidelines that you will be given for the test:

All carbonates (CO32-) and

phosphates (PO43-) are

INSOLUBLE except those containing Group 1A metal cations or the ammonium ion (NH4

+).

All sulfides (S2-) and hydroxides (OH-) are INSOLUBLE except those containing Group 1A metal cations, the ammonium ion, Ca2+, Sr2+, or Ba2+.

Predicting Whether a Metathesis Reaction Occurs – Solubility Guidelines

Will this reaction occur?

HCl + AgNO3 HNO3 + AgCl

Yes, AgCl is insoluble so it will form a precipitate.

HCl + AgNO3 HNO3 + AgCl (s)