Chapter 4 Three Major Classes of Chemical Reactions Solution Stoichiometry Many reactions (biochemical, marine, etc.) take place in solution. We need to be able to express the number of moles of particles in a certain volume of solution. Concentration refers to quantitative ways of expressing the relative amounts of substances in a solution. The most common expression of concentration in chemistry is molarity. molarity = moles solute liters solution commonly (if incorrectly) symbolized M (standard: M) o Molarity of a solution is an intensive property. o Molarity depends on temperature. o We use [square brackets] to denote molar concentrations. e.g., Calculate the molarity of a solution prepared by dissolving 2.00 g of calcium bromide in enough water to make 100.0 ml of solution. Preparing Molar Solutions 1. Weigh solid needed. 2. Transfer solid to a volumetric flask containing some solvent. 3. Dissolve solid. 4. Add solvent to make total volume of solution. MUST NOT just measure a volume of solvent and then add the solute! Dilution A physical process, not a chemical process. (The same number of moles of solute is present before and after the dilution.) M CONC V CONC = M DIL V DIL or M I V I = M F V F or M 1 V 1 = M 2 V 2 e.g. 25 ml of a 0.100 M calcium bromide solution was diluted to make 175 ml of solution. What was the concentration of the dilute solution? 4-1
Mole-Mole & Gram-Mole Calculations with Molar Solutions Same approach as for mass-mass calculations; key step is the use of the mole ratio from the balanced chemical equation! GRAMS of A NO DIRECT CALCULATION! GRAMS of B M of A M of B mole ratio MOLES of A moles B / moles A MOLES of B Molarity Volume Solution A = Molarity Volume Solution B e.g., What is the molarity of a solution of NaOH if 17.36 ml of the solution was needed to neutralize 10.00 ml of 1.00 M H 2 SO 4? 2NaOH(aq) + H 2 SO 4 (aq) 2H 2 O(l) + Na 2 SO 4 (aq) moles NaOH L solution 1 L 1.00 mol H SO 2 mol NaOH = 1.15 M NaOH 1 L 17.36 ml NaOH soln 1000 ml 2 4 10.00 ml acid soln 1000 ml L acid soln 1 mol H 2 SO 4 PROTIP Some people use the dilution formula to solve problems like this; doing so for this problem would give the wrong answer! Only use the dilution formula for dilution. 4-2
Thousands and thousands of chemical reactions are known. To simplify and declutter our lives, we classify these reactions by common patterns of behavior. Three major types of reactions, most often in aqueous solution: Precipitation Acid-Base Reduction-Oxidation Water as a Solvent Water is a polar molecule contains separation of partial r and s charges Water primarily dissolves ionic compounds and covalent compounds with their own polar groups (esp. NH, OH groups) Nonpolar compounds (e.g., benzene, methane, fats, oils, etc.) do not dissolve in water Electrolytes strong electrolyte: a substance that dissociates into many ions (a stoichiometric quantity) when dissolved in water weak electrolyte: a substance that dissociates into few ions (a small quantity) when dissolved in water nonelectrolyte: a substance that does NOT dissociate into ions when dissolved in water Soluble ionic substances dissociate into their ions in solution. e.g., consider 0.10 mol of sodium sulfate dissolved in water to make 1.00 L of solution: [Na + ] = 0.20 M [SO 2 4 ] = 0.10 M [Na 2 SO 4 ] = 0 M (being a strong electrolyte, every bit of sodium sulfate that dissolves also dissociates into ions. No intact formula units.) Covalent Compounds in Water MOST covalent compounds do not dissociate into ions when dissolved in water. Exception: ACIDS and BASES o hydrochloric acid: HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl (aq) o ammonia: NH 3 (aq) + H 2 O(l) º NH 4 + (aq) + OH (aq) 4-3
Precipitation Reactions Driving force: removal from solution of certain ions which combine to form a solid product. The other ions that remain in solution are called spectator ions. Example: mixing silver nitrate (aq) + sodium chromate (aq) Predicting Whether a Precipitation Reaction Will Occur ***MUST LEARN Solubility Rules below! Game Plan 1. Note ions present in reactant solutions. 2. Consider possible cation-anion combinations. 3. Using solubility rules, determine if new combination(s) is/are insoluble. 4-4
Acid-Base Reactions Also known as neutralization reactions Driving force: transfer of H + ions; frequently (not always) via removal of OH and H 3 O + ions from solution as water. Arrhenius definitions: acid releases H + ions in water. base releases OH ions in water. Brønsted definitions: acid donates H + ions. base accepts H + ions. The one product you ALWAYS get from an acid-base reaction: a salt Cation (+) of salt from the base Anion ( ) of the salt from the acid The definition of a salt is an ionic compound that can be produced by an acid-base reaction. Water is also produced when acids react with strong bases. When weak bases are involved, water may not appear. e.g., What are the products of the reaction of solutions of potassium hydroxide and perchloric acid? Some acid-base reactions are driven by production of a gas as well as water and a salt. ***MUST LEARN the selected acids and bases above! (Table 4.2) PROTIP If you know your solubility rules, the easy way to remember the strong bases is that they are the soluble metal hydroxides. Ionic Equations for Acid-Base Reactions Weak acids & bases are written in their molecular forms because they are mostly undissociated in solution. e.g., What are the molecular, total ionic, and net ionic equations for the aqueous reaction of ammonia with nitric acid? 4-5
Reduction-Oxidation (REDOX) Reactions Driving force: transfer/shift of electrons LEO the lion says GER Loss of Electrons is Oxidation; Gain of Electrons is Reduction OIL RIG Oxidation Is Loss; Reduction Is Gain The substance that is oxidized is ALWAYS the reducing agent. The substance that is reduced is ALWAYS the oxidizing agent. Tracking Movement of Electrons: Oxidation Numbers ON are assigned as if electrons were transferred completely in a chemical reaction. ***MUST LEARN the Rules for Assigning Oxidation Numbers (Table 4.4). PROTIP The ON rules give the ON for each atom in a formula. For example, each F in AlF 3 has an ON of 1. A reaction is a redox reaction if the oxidation numbers of one or more atoms changes. e.g., P 4 (s) + 5O 2 (g) P 4 O 10 (s) ON of P changes from 0 to +5; ON of O changes from 0 to 2 the reaction is redox 4-6
Fig. 4.17, Summary of redox reaction electron movement and vocabulary The highest ON for an A-group element equals the element s group number. The lowest ON for an A-group nonmetal equals the element s group number minus 8. Elemental Substances in Redox Reactions In general, when an elemental substance appears as a reactant or a product in a reaction, the reaction is REDOX. Combustion Reactions involve reaction with elemental oxygen (O 2 ) usually involve release of energy (heat/light) are always redox reactions Combustion of a compound containing only C and H (or only C, H, and O) will produce CO 2 & H 2 O as products. (Recall the use of combustion analysis to determine composition of an unknown compound of C, H, and O.) e.g., propane: C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(l) Activity Series of Metals Li(s) Li + (aq) + e K(s) K + (aq) + e Ba(s) Ba 2+ (aq) + 2e Ca(s) Ca 2+ (aq) + 2e Na(s) Na + (aq) + e Mg(s) Mg 2+ (aq) + 2e Al(s) Al 3+ (aq) + 3e Mn(s) Mn 2+ (aq) + 2e Zn(s) Zn 2+ (aq) + 2e Cr(s) Cr 3+ (aq) + 3e Fe(s) Fe 2+ (aq) + 2e Co(s) Co 2+ (aq) + 2e Ni(s) Ni 2+ (aq) + 2e Sn(s) Sn 2+ (aq) + 2e Pb(s) Pb 2+ (aq) + 2e H 2 (g) 2H + (aq) + 2e Cu(s) Cu 2+ (aq) + 2e Ag(s) Ag + (aq) + e Hg(l) Hg 2+ (aq) + 2e Pt(s) Pt 2+ (aq) + 2e Au(s) Au 3+ (aq) + 3e INCREASING ACTIVITY The list of metals at left is arranged with the most active metal, i.e., the strongest reducing agent (most easily oxidized) at the top and the least active metal, i.e., the weakest reducing agent (least easily oxidized) at the bottom. e.g., Will a displacement reaction occur when solid copper is placed in a silver nitrate solution? 4-7