Chapter 4: Chemical Quantities & Aqueous Reactions CHE 123: General Chemistry I Dr. Jerome Williams, Ph.D. Saint Leo University

Overview Water: The Universal Solvent Electrolytes Solubility Reactions in Aqueous Solution

Water: The Universal Solvent Water is called the universal solvent because –it dissolves a great many substances –widely available and low cost

Water: The Universal Solvent Water has unique properties that make it suitable for sustaining living systems. Polar substance even though it has covalent bonds Capacity to form Hydrogen Bonds Favorable mp, bp, specific heat capacity when compared with other materials of comparable molecular weight (like methane (CH 4 ), ammonia (NH 3 ).

Water: The Universal Solvent Many reactions take place in aqueous solution. Aqueous means dissolved in water.

Hydration of Solids in Water What do we mean by a substance dissolving in a solvent like water? Solid dissolves (falls apart) through interaction of ions with water. Newly formed ions are now “FREE TO MOVE”. Concept gives rise to types of aqueous reactions we encounter.

Water: The Universal Solvent Like dissolves like is a common saying that describes how materials will interact in a solvent. Polar substances dissolve in polar solvents while nonpolar substances dissolve in nonpolar solvents.

Electrolytes Electrolytes are substances that when dissolved in water conduct an electric current.

Electrolytes Electrolytes are classified as follows: –Strong Electrolytes ionize completely in solution. Examples include strong acids & bases, soluble ionic compounds, etc.

Electrolytes Electrolytes are classified as follows: –Weak Electrolytes ionize incompletely in solution. Examples are weak acids & bases. –Nonelectrolytes do not ionize at all. Molecular substances that do not break apart into ions are examples.

Solubility Allow you to predict whether a reactant or a product dissolves in water. Soluble compounds are those which dissolve to more than 0.01 M. There are three basic classes of salts:

Solubility 1. Salts which are always soluble: All alkali metal salts: Cs +, Rb +, K +, Na +, Li + All ammonium ion (NH 4 + ) salts All salts of the NO 3 –, ClO 3 –, ClO 4 –, C 2 H 3 O 2 –, and HCO 3 – ions

Solubility 2. Salts which are soluble with exceptions: Cl –, Br –, I – ion salts except with Ag +, Pb 2+, & Hg 2 2+ SO 4 2– ion salts except with Ag +, Pb 2+, Hg 2 2+, Ca 2+, Sr 2+, & Ba 2+

Solubility 3. Salts which are insoluble with exceptions: O 2– & OH – ion salts except with the alkali metal ions, and Ca 2+, Sr 2+, & Ba 2+ ions CO 3 2–, PO 4 3–, S 2–, CrO 4 2–, & SO 3 2– ion salts except with the alkali metal ions and the ammonium ion

Types of Aqueous Reactions Precipitation Reactions: A process in which an insoluble solid precipitate drops out of the solution. Most precipitation reactions occur when the anions and cations of two ionic compounds change partners. Pb(NO 3 ) 2 (aq) + 2 KI(aq)  2 KNO 3 (aq) + PbI 2 (s)

Types of Aqueous Reactions Acid–Base Neutralization: A process in which an acid reacts with a base to yield water plus an ionic compound called a salt. The driving force of this reaction is the formation of the stable water molecule. HCl(aq) + NaOH(aq)  NaCl(aq) + H 2 O(l)

Types of Aqueous Reactions Oxidation–Reduction (Redox) Reaction: A process in which one or more electrons are transferred between reaction partners. The driving force of this reaction is the decrease in electrical potential. Mg(s) + I 2 (g)  MgI 2 (s)

Solubility Predict the solubility of: –(a) CdCO 3 (b) MgO(c) Na 2 S (d) PbSO 4 (e) (NH 4 ) 3 PO 4 (f) HgCl 2 Predict whether a precipitate will form for: –(a) NiCl 2 (aq) + (NH 4 ) 2 S(aq)  –(b) Na 2 CrO 4 (aq) + Pb(NO 3 ) 2 (aq)  –(c) AgClO 4 (aq) + CaBr 2 (aq) 