Precipitation Reactions in Analytical Chemistry

ANALYTICAL CHEMISTRY LABORATORY • PRECIPITATION REACTIONS PRECIPITATION REACTIONS • • Precipitation refers to a chemical reaction that occurs in aqueous solution when two ions bond together to form an insoluble salt, which is known as the precipitate. A precipitation reaction can occur when two solutions containing different salts are mixed, and a cation/anion pair in the resulting combined solution forms an insoluble salt; this salt then precipitates out of solution. SOLUBILITY RULE • • • Certain ions easily dissolve, others tend to be insoluble. They all follow certain rules. We can use these rules in order to predict what ions are present in an ‘unknown’ solution. Rules o All Group IA and NH4 + salts are soluble. o All nitrates are soluble. o All chlorides are soluble. → (except Ag+, Pb2+, Hg2 2+) o All sulfides are insoluble. → (except Grp IA and IIA) IONIC EQUATIONS & PRECIPITATION REACTIONS • SOLUBILITIES OF SOME IONIC COMPOUNDS IN WATER • • Mainly water-soluble o NO3-1 All nitrates are soluble o C2H3O2-1 All acetates are soluble o ClO3-1 All chlorates are soluble o Cl-1 All chlorides are soluble except AgCl, Hg2Cl2, PbCl2 o Br1 All bromides are soluble except AgBr, Hg2Br2, PbBr2, HgBr2 o I-1 All iodides are soluble except AgI, Hg2I2, PbI2, HgI2 o SO4-2 All sulfates are soluble except CaSO4, SrSO4, BaSO4, ObSO4, HgSO4, Ag2SO4 o SCN-1 All thiocyanates are soluble except AgSCN, HgSCN, Pb(SCN)2, Hg(SCN)2 Mainly water-insoluble o S-2 All sulfides are insoluble except those of the IA and IIA elements and (NH4)2S o CO3-2 All carbonates are insoluble except those of the IA elements and (NH4)2CO3 o SO3-2 All sulfites are insoluble except those of the IA elements and (NH4)2SO3 o PO4-3 All phosphates are insoluble except those of the IA elements and (NH4)2PO4 o OH-1 All hydroxides are insoluble except those of the IA elements, Ba(OH)2, Sr(OH)2, Ca(OH)2 The following is a common laboratory example of a precipitation reaction. o Aqueous silver nitrate (AgNO3 ) is added to a solution containing potassium chloride (KCl), and the precipitation of a white solid, silver chloride (AgCl), is observed as molecular equations: AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3 (aq) o Note that the product silver chloride is the precipitate, and it is designated as a solid. This reaction can be also be written in terms of the individual dissociated ions in the combined solution. This is known as the complete ionic equation: Ag+ (aq) + NO3− (aq) + K+ (aq) + Cl− (aq) → AgCl (s) + K+ (aq) + NO3− (aq) o A final way to represent a precipitation reaction is known as the net ionic equation. In this case, any spectator ions (those that do not contribute to the precipitation reaction) are left out of the formula completely. Without the spectator ions, the reaction equation simplifies to the following: Ag+ (aq) + Cl− (aq) → AgCl (s) • • • Overall Equation – Shows reactants and products as undissociated, electrically neutral compounds. AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3 (aq) Complete Ionic Equation – Shows the state of reactants and products as hydrated or other phases. Ag+1 (aq) + Ag+ (aq) + NO3− (aq) + K+ (aq) + Cl− (aq) → AgCl (s) + K+ (aq) + NO3− (aq) Net Ionic Equation – Shows only the reactants and products that are directly involved in the reaction. Ag+ (aq) + Cl− (aq) → AgCl (s) Spectator Ions – Ions which are not directly involved in the net ionic equation are called spectator ions. In the previous equation, this would be the sodium and nitrate ions. INTRODUCTION TO CHEMICAL REACTIONS GENERAL FEATURES OF PHYSICAL AND CHEMICAL CHANGES • • A physical change alters the physical state of a substance without changing its composition. A chemical change (a chemical reaction) converts one substance into another. o Chemical reactions involve: ▪ Breaking bonds in the reactants (starting materials) ▪ Forming new bonds in the products WRITING CHEMICAL EQUATIONS • • A chemical equation uses chemical formulas and other symbols showing what reactants are the starting materials in a reaction and what products are formed. o The reactants are written on the left. o The products are written on the right. o Coefficients show the number of molecules of a given element or compound that react or are formed. • o Balance the H’s next: o Finally, balance the O’ s: Step [3] Check to make sure that the smallest set of whole numbers is used The law of conservation of mass states that atoms cannot be created or destroyed in a chemical reaction. o Coefficients are used to balance an equation. o A balanced equation has the same number of atoms of each element on both sides of the equation. TYPES OF PRECIPITATES • • Crystalline o Well-defined / regularly shaped crystals whose shapes are a function of the composition of the salt; o Relatively pure; o Ready to filter; o Are filtered rapidly, easily and completely; o Since these crystals are not pure and are too small to filter effectively, they are heated to achieve larger and purer crystals (digestion); o Examples are calcium oxalate and also the alkaline earth sulfates such as barium sulfate. • Gelatinous Precipitate o Flocculated colloids; jelly-like, hydrous masses o Trap a great deal of water and many impurities; o Very difficult to work with; o The cation & anion react to form a soluble colloid and then coagulate to filterable-sized particles; o Examples are metal hydroxides such as ferric hydroxide; • Curdy Precipitate o Aggregates of small porous particles; o The cation & anion react to form a soluble colloid and then coagulate to filterable-sized particles; o When reasonable in size, they are filtered rapidly, easily and completely; Symbols used in Chemical Equations o → Reaction arrow o ∆ Heat o (s) Solid o (l) Liquid o (g) Gas o (aq) Aqueous solution BALANCING CHEMICAL EQUATIONS HOW TO BALANCE A CHEMICAL EQUATION • • • Example: Write a balanced chemical equation for the reaction of propane (C3H8 ) with oxygen (O2 ) to form carbon dioxide (CO2 ) and water (H2O). Step [1] Write the equation with the correct formulas. C3H8 + O2 CO2 + H2O o The subscripts in a formula can never be changed to balance an equation, because changing a subscript change the identity of a compound. Step [2] Balance the equation with coefficients one element at a time. o Balance the C’s first: o Examples are the silver halides compounds;