Chapter 4 Types of Chemical Reactions and Solution Stoichiometry

Water, the Common Solvent Oxygen has a greater attraction for electrons than does hydrogen The oxygen atom gains a slight excess of negative charge (partial negative charge ), and the hydrogen atoms become slightly positive (partial positive charge ). Water is said to be a polar molecule.

An Ionic Solid Dissolving in Water The positive ends of water molecules are attracted to the negatively charge anions. The negative ends are attracted to the positively charge cations. This process is called hydration.

Polar water molecules

Ethanol molecule in Water

Strong Electrolytes Soluble salts: NaCl, KCl, NH 4 Cl Strong acids: HCl, HNO 3, H 2 SO 4 Strong bases: NaOH, KOH

HCI container

NaOH container

Weak Electrolytes Weak acid: acetic acid HC 2 H 3 O 2(aq) → H + (aq) +C 2 H 3 O 2 - (aq) Weak base: ammonia dissolving in water NH 3(aq) +H 2 O (l) →NH 4 + (aq) +OH - (aq)

Acetic acid

NH 3 in water

Standard Solution A standard solution is a solution whose concentration is accurately known. Primary Standard : A primary standard is a highly purified compound that serves as a reference material in all volumetric and mass titrimetric methods. Secondary Standard: A second standard is a solution of known concentration by titration with a solution of primary standard.

Preparing Standard Acid Solutions Hydrochloric acid is widely used for titration of bases. Dilute solutions of HCl are stable indefinitely and do not cause troublesome precipitation reactions with most cations. 0.1 M solutions of HCl can be boiled for as long as one hour without loss acid. 0.5 M solutions of HCl can be boiled for at least 10 min without significant loss.

Solutions of perchloric acid and sulfuric acid are also stable and are useful for titration where chloride ion interferes by forming precipitates. Solutions of nitric acid are seldom encountered because of their oxidizing properies.

Requirements for a Primary Standard High purity Atmospheric stability Absence of hydrate water － composition of the solid does not change with relative humidity Ready availability at modest cost Reasonable solubility in the titration medium Reasonably large molar mass － the relative error associated with weighing

Several Excellent Primary Standards 1. Potassium Hydrogen Phthalate KHC 8 H 4 O 4 (KHP) 2. Benzoic Acid: its solubility in water is limited, this reagent is ordinarily dissolved in ethanol prior to dilution water. 3. Potassium Hydrogen Iodate KH(IO 3 ) 2 : is an excellent primary standard with a high molecular mass per mole of protons.

Preparation of standard solution or stock solution

Types of Chemical Reactions Precipitation reactions Acid-base reactions Oxidation-reduction reactions

Precipitation Reactions K 2 CrO 4 /Ba(NO 3 ) 2

Reaction of KCI/AgNO 3

Ag/Ba/Fe precipitation