AQUEOUS SOLUTIONS

AQUEOUS SOLUTIONS Example The solution NaCl(aq) is sodium chloride NaCl(s) dissolved inwater H2O(l) The solute is NaCl(s) and the solvent is H2O(l) A solution is a homogeneous mixture of a solute dissolved in a solvent. In aqueous solutions solvent is WATER. The solvent is generally in excess.

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 • Oxidation-reduction reactions.

Electrolyte:a substance that conducts electricity when dissolved in water. Acids, bases and soluble ionic solutions are electrolytes. Non-electrolyte:a substance that does not conduct electricity when dissolved in water. Molecular compounds and insoluble ionic compounds are non-electrolytes. Electrolyte and Non-electrolyte

Electrolytes • Some solutes can dissociate into ions. • Electric charge can be carried.

Types of solutes high conductivity Strong Electrolyte - 100% dissociation, all ions in solution Na+ Cl-

Types of solutes slight conductivity Weak Electrolyte - partial dissociation, molecules and ions in solution CH3COOH CH3COO- H+

Types of solutes no conductivity Non-electrolyte - No dissociation, all molecules in solution sugar

Table 4.1 Electrolyte Classification of Some Common Substances Strong Electrolytes Weak ElectrolytesNonelectrolytes HCl, HBr, HI CH3COOH H2O HClO4 HF CH3OH HNO3 C2H5OH H2SO4 C12H22O11(sucrose) KBr Most organic compd NaCl NaOH, KOH Other soluble ionic compounds

A weak electrolyte: CH3COOH(aq)← CH3COO-(aq) +H+(aq) → A non-electrolyte: CH3COOH(aq), CH3OH(aq), H2O(l), CO2(g), Zn(s), MgCO3(s) Ionic Equations The molecular equation does not tell us that the reaction actual involves ions in solution. So the soluble ionic substances in solution should be represented by their separate ions. Other ions are SPECTATOR IONS that do not take part in reaction. MgCl2(s) → Mg2+(aq) + 2 Cl-(aq) A strong electrolyte:

Spectator ions Ag+(aq) + NO3-(aq) + Na+(aq) + I-(aq) → AgI(s) + Na+(aq) + NO3-(aq) Writing Net Ionic Equation Overall /Molecular Equations: Complete formulas are written for all the reactants and products, no ions are written. AgNO3(aq) +NaI (aq) → AgI(s) + NaNO3(aq) Complete ionic equation: Strong electrolytes are written in their ionized forms and weak/non-electrolytes as unionized form. Ag+(aq) + NO3-(aq) + Na+(aq) + I-(aq) → AgI(s) + Na+(aq) + NO3-(aq) Net ionic equation:Write only those chemical species which are involved in a chemical reaction. All spectator ions are eliminated. Ag+(aq) + I-(aq) → AgI(s)

Suppose copper (II) sulfate reacts with sodium sulfide. Write out the chemical reaction and name the precipitate. CuSO4 (aq) + Na2S (aq) CuS (s) + Na2SO4 (aq) Write out the net ionic equation. Cu+2(aq)SO4-2 (aq) + 2Na+ (aq) + S-2(aq) CuS (s) + 2Na+ + SO4-2 (aq) Cu+2 (aq)+ S-2(aq) CuS (s) Example of Ionic equations

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. 2Ag NO3 (aq) + Na2CO3 (aq)  AgCO3(s) + 2NaNO3 Soluble Cations Alkali metals: Li+, Na+, K+, Rb+, Cs+, NH4+ Mostly insoluble Metal (other than alkali metals) sulfides, hydroxidescarbonates, phosphates Soluble Anions: Halides: Cl-, Br-, I- except of Ag+, Hg2+2, Pb+2 NO3-, ClO4-, CH3CO2-, SO4-2 except SO4-2 of Ba+2, Hg2+2, Pb+2

Example (a) Al2(SO4)3 + NaOH  i) write down the reactants and interchange of anions to get product Al2(SO4)3 + 6NaOH  2Al(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++3SO42- + 6Na+ + 6OH-2Al(OH)3(s)+ 6Na++ 3SO42- The net ionic equation is : Al3+ + 3OH-Al(OH)3(s)

Acid-Base Reactions An acid is a substance that provides hydrogen ions (H+) (increase the concentration of H+) 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) a base MOH is a general formula for a base

Strong and Weak Acids and Bases A strong acid is an acid that is almost completely ionized in aqueous solution. A weak acid is an acid that only partially ionized (as result of an equilibrium reaction with water) in aqueous solution. HCl(aq)  H+(aq) + Cl-(aq) strong acid CH3CO2H(aq)  H+(aq) + CH3CO2-(aq) weak acid A strong base is a base that dissociate nearly completely in aqueous solution. A weak base is a base that is only partially ionized (as result of an equilibrium reaction with water) in aqueous solution. NaOH(s) Na+(aq) + OH-(aq) strong base NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) weak base

Weak Acids CH3COOH HCOOH HF HCN HNO2H3PO4- Weak Bases NH3 CH3NH2

Acid-Base Neutralization Reaction In a neutralization reaction, an acid and a base react to form water and an aqueous solution of an ionic compound called a salt. A neutralization reaction: HA(aq) + MOH(aq)  H2O(l) + MA(aq) acid base water salt HCl(aq) + NaOH(aq)NaCl(aq) +H2O(l) H+(aq) + Cl-(aq) +Na+(aq) + OH-(aq) Na+(aq) +Cl-(aq) + H2O(l) By eliminating the spectator ions, we discover the actual Reaction of the neutralization of strong acid by a strong base The net ionic equation: H+(aq) + OH-(aq) H2O(l) or H3O+(aq) + OH-(aq) 2H2O(l)

Oxidation-Reduction Reaction • Historically, OXIDATION referred to the combination of an element with oxygen to yield an oxide, and the word REDUCTION referred to the removal of oxygen from an oxide to yield the element. • Today, by using broader definitions, we look at the change of oxidation state of the element involved in the reaction. • An oxidation is defined as the lose of one or more electrons by a substance. (Increase in Oxidation number) • A reduction is the gain of one or more electrons by a substance. (Decrease in Oxidation number) • As a remembrance LEO says GER • Loss Electrons = Oxidation • Gain Electrons = Reduction

What is Redox? • REDOX stands for REDuction/Oxidation • An oxidation and a reduction must occur together, and such a reaction is called an oxidation-reduction reaction or REDOX reaction. A redox reaction is a process in which electrons are transferred between substance or in which atoms change oxidation number. • Not all the redox reactions involve oxygen.

Oxidizing and Reducing Agents An oxidizing agent: contains an element whose oxidation number decreases in a redox reaction by gaining electrons (it make possible for some other substance to be oxidized and itself reduced). A reducing agent: contains an element whose oxidation number increases in a redox reaction by losing electrons (it make possible for some other substance to be reduced and itself oxidized). In general, a substance with an element in one of its highest possible oxidation state is an oxidizing agent. If an element is in its lowest possible oxidation state, the substance is a reducing agent.

OXIDATION NUMBER An oxidation number is the APPARENT charge on an atom in a molecule or a compound. Rules for determining oxidation numbers (ON): 1. The ON of any element in free (uncombined) state is zero. e.g. Na0, H20, O20 etc 2. The ON for any simple, monatomic ion is equal to the charge on the ion and it is same in compound as was in monoatomic ion e.g. +1 for Sodium in Na+,Na2CO3and NaCl 3. The sum of all the oxidation numbers of the atoms for neutral species must be equal to zero and for ions must be equal to the charge on the ion. 4. In its compounds, fluorine always has an ON of –1. 5. In its compounds, hydrogen has an ON of +1 except for metal hydrides, where the ON is -1. 6. In its compounds, oxygen has an ON of -2 except for the peroxides (-1), superoxides (-1/2) and OF2(+2).

AQUEOUS SOLUTIONS