Chapter 4 Reactions in Aqueous Solution Observing and Predicting Reactions How do we know whether a reaction occurs? What observations indicate a reaction has occurred? In your groups, make a list of changes that indicate a chemical reaction has occurred. 1 2 Properties of Aqueous Solutions Substances behave differently when they are placed in water, specifically ionic versus covalent compounds. One breaks apart in water, the other does not. Which one is more likely to be pulled apart by water molecules? Electrolytes are ionic and strong acid solutions (e.g., Gatorade TM ); Nonelectrolytes are covalent compounds (e.g., sugar); weak electrolytes are in between. Electrolytes in Aqueous Solutions Strong/Weak Electrolytes Electrolytic Properties Strong electrolyte: substance that, when dissolved in water, results in a solution that can conduct electricity (NaCl) Weak electrolyte: substance that is a poor conductor of electricity when dissolved in water (CH 3 COOH vinegar) Nonelectrolyte: substance that doesn t conduct electricity when dissolved in water (CH 3 OH methanol) Strong, weak, or nonelectrolyte Place the following substances into groups. NaCl C 6 H 12 O 6 CH 3 COOH NH 3 HCl NaOH 1
Electrolytes in Aqueous Solutions Electrolytes 3 substances: A 2 X, A 2 Y, and A 2 Z. Which one is the strongest electrolyte? The weakest? (Water has been omitted for clarity.) Strong/Weak Electrolytes Properties of Aqueous Solutions Most reactions in general chemistry take place in an aqueous environment. What does that mean? Terms: Solution: homogeneous mixture of two or more substances Solute: substance present in smaller amount Solvent: substance present in greater amount Aqueous solution: solvent is water Ways Reactions Occur Three general categories: Precipitation: insoluble (solid) product is formed from aqueous solutions Acid-base neutralization: acid and base react to form water and a salt (ionic compound) Oxidation-Reduction: electrons are transferred between atoms in reaction Combination Decomposition Single-replacement (metal or hydrogen) Products of Precipitation Reactions Precipitation reactions always begin with two ionic compounds. Example: NaCl (aq) AgNO 3 (aq)? Draw these compounds in two separate aqueous environments. What are the possible products when they are combined? Predict Products and Types of Reactions NO 3 - Ag Ag NO - 3 NO 3 - Ag Na Na Na Write formulas of products (based on charges), predict phases (Solubility Rules), and balance the equation. 2
Solubility Rules Solubility Rules Determine if the following ionic compounds will be soluble (aq) or insoluble (s) in water: K 2 CO 3 BaSO 4 PbI 2 NaClO 4 Ag 2 S (NH 4 ) 3 PO 4 If not covered by the rules, it is probably insoluble. 13 Cu(OH) 2 Molecular, Ionic, and Net Ionic Equations There are 3 ways to represent ppt reactions: As whole compounds (molecular equation) As ionic species (ionic equation) more accurate As participants in reaction (net ionic equation) Any aqueous ionic substance is written as a compound (e.g., AgNO 3 ), but this isn t accurate. What does this look like in water? Solids, liquids, and gases remain as compounds. Formation of Silver Chloride Molecular equation: NaCl(aq) AgNO 3 (aq) AgCl(s) NaNO 3 (aq) Ionic equation (write separate ions for soluble (aq) compounds): Na (aq) (aq) Ag (aq) NO 3- (aq) AgCl(s) Na (aq) NO 3- (aq) Net ionic equation (cancel any identical ion on both sides of the equation, called spectator ions): Ag (aq) (aq) AgCl(s) Note: s, l, and g stay together!!!!! Ag / ions 15 Precipitation Reactions Acid-Base Reactions Reaction of lead (II) nitrate and potassium iodide. What is the precipitate? Write the molecular, ionic, and net ionic equations. Worked Ex. 4.2 4.5; Problems 4.4 4.8 Acid: substance that breaks apart in water to form (e.g., HCl, HNO 3, CH 3 COOH, lemon, lime, vitamin C). HA(aq) (aq) A - (aq) Base: substance that breaks apart in water to form OH - (e.g., NH 3, Drano TM, Milk of Magnesia TM ) MOH(aq) M (aq) OH - (aq) strong acid acid weak acid base 18 3
Common Acids and Bases You need to KNOW these!!! Acid-Base Neutralization What does neutralization mean? How might these two solutions react when combined? Na OH - Na OH - Cl- Strong acids: HCl, HBr, HI, HClO 4, HClO 3, H 2 SO 4, HNO 3 Strong bases: LiOH, KOH, NaOH, RbOH, CsOH, Ca(OH) 2, Sr(OH) 2, Ba(OH) 2 19 Acid-Base Neutralization Acid-Base Reaction Neutralization reaction: reaction between acid and base; products are salt (ionic compound) and water HCl(aq) NaOH(aq) NaCl(aq) HO(l) 2 Acid base salt water What are the ionic and net ionic equations for these reactions? HF(aq) NaOH(aq) KOH (aq) H 2 SO 4 (aq) Worked Ex. 4.6; Problems 4.9, 4.10 HCl Acid Base H 2 O Salt Water is created from and OH - 2HCl (aq) Mg(OH) 2(s) The salt is created from spectator ions Mg(OH) 2 2H 2 O (l) MgCl 2(aq) Complete/Balance These Equations Solubility Rules Complete and balance these equations. Write ionic and net ionic equations, if applicable. Na 2 S(aq) CuCl 2 (aq) MgSO 4 (aq) BaCl 2 (aq) KNO 3 (aq) CaCl 2 (aq) CuSO 4 (aq) NaOH(aq) HF(aq) NaOH(aq) KOH(aq) H 2 SO 4 (aq) 23 If not covered by the rules, it is probably insoluble. 24 4
Oxidation-Reduction Reactions Oxidation-Reduction (redox) reactions: electron-transfer reactions When iron rusts, it loses electrons to form a cation, oxygen gain electrons to form an anion: 4 Fe(s) 3 O 2 (g) 2 Fe 2 O 3 (s) Use oxidation number rules to determine gain and loss of electrons. Oxidation numbers are assigned as if elements in compounds completely transferred electrons (like in ionic compounds). Assigning Oxidation Numbers 1) An atom (or molecule) in its elemental state has an oxidation number of 0. 2) An atom in a monatomic ion (Na, ) has an oxidation number identical to its charge. 3a) Hydrogen has an oxidation number of 1, unless it is combined with a metal, in which case it has an oxidation number of 1. 3b) Oxygen usually has an oxidation number of -2. Oxygen in peroxides (O 2-2 ) has an oxidation number of -1. 26 Rules continued 3c) Halogens usually have an oxidation number of -1 (except when bonded to oxygen or in polyatomic ions). 4) The sum of oxidation numbers is 0 for a neutral compound and is equal to the net charge for a polyatomic ion. (Example: NaCl = 0, SO 2-4 = -2) 4a) For binary ionic compounds, the position of the element in the periodic table may be useful: Group IA: 1; Group IIA: 2; Group VIIA: 1; Group VIA: 2; Group VA: 3 27 Example H 2 SO 4 H = 1; O = 2 S is unknown, so leave this for last. The overall charge on this compound is 0. Use algebra to solve for S: 2(1) 1(x) 4(-2) = 0 Solve for each element: MgCr 2 O 7 Worked Ex. 4.7; Problem 4.11 28 Assigning Oxidation Numbers Oxidation-Reduction Reactions Determine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 CCl 4 H 2 O 2 Fe 3 (PO 4 ) 2 MnO - 4 NaNO 3 KClO 4 Oxidized: atom, molecule, or ion becomes more positively charged Loss of electrons is oxidation (LEO) Reduced: d atom, molecule, l or ion becomes less positively charged (reduced charge) Gain of electrons is reduction (GER) Or: OIL RIG (oxidation is loss; reduction is gain) Redox 1 Redox 2 29 5
Assigning Oxidation Numbers Oxidation-Reduction Reactions The substance oxidized causes the other substance to be reduced and is called the reducing agent. The substance reduced causes the other substance to be oxidized and is called the oxidizing agent. 4 Fe(s) 3O 2 (g) 2Fe 2 O 3 (s) Worked Ex. 4.8; Problems 4.12, 4.13 Oxidation-Reduction Reactions Identify the element or ion oxidized/reduced. Also identify the oxidizing agent and the reducing agent. Zn(s) 2HCl(aq) ZnCl 2 (aq) H(g) 2 Reduced Redox Reactions Combination (1 product) Na(s) Cl 2 (g) Decomposition (1 reactant) usually give off gases CuCO 3 (s) Single Replacement (or Displacement) (start and end with an element and a compound) Zn(s) HCl(aq) Oxidized 34 Metals versus Nonmetals Metals tend to react with water to form bases: 2Na (s) 2H 2 O 2NaO H 2 CO 2 (s) H 2 O (l) H 2 CO 3 (aq) MgO (s) H 2 O Mg(OH) 2 Alkali metals water Nonmetals tend to react with water to form acids: 2F 2 (g) 2H 2 O 4HF O 2 CO 2 (g) H 2 O H 2 CO 3 6
Combination Reactions Combination Reactions element element compound H 2 (g) O 2 (g) metal nonmetal ionic compound Na(s) Cl 2 (g) nonmetal nonmetal covalent compound C(s) O 2 (g) Why are these redox reactions? Mg O 2 K Cl 2 37 Decomposition Rxns Produce Gases Decomposition Reactions Compound 2 elements; element compound; or 2 compounds Oxides, peroxides Give off O 2 Nitrates Give off NO 2, NO - 2 Carbonates Give off CO 2 Ammonium salts NH 4 Cl (s) NiCO 3 (s) CuO(s) Give off NH 3 39 Single-Displacement Reactions Single Displacement: Cu AgNO 3 element compound compound element (The more metallic/active element in the compound is displaced.) Metal Displacement Zn(s) Cu(NO 3 ) 2 (aq) Cu(s) Zn(NO 3 ) 2 (aq) Hydrogen Displacement Mg(s) HCl(aq) MgCl 2 (aq) H 2 (g) Alkali Metals ZnSnCl 2 41 42 7
Single Displacement: Fe Cu(NO 3 ) 2 Activity Series The higher the metal on the activity series, the more active that t metal. Translation: higher metals on the chart will form ions as products. 44 Predict whether a reaction occurs Determining Order of Activity Na H 2 O Fe H 2 O Fe Cr(NO 3 ) 2 Ni Pb 2 Ag Mg 2 Zn Co 2 Worked Ex. 4.9; Problems 4.14, 4.16 React w/ cold water React w/ steam React w/ acid Four metals (A, B, C, and D) are tested and the following observations are recorded. A B no reaction C B reaction occurs D A reaction occurs D B no reaction 45 Combustion Reactions Reaction Types Types of Reactions Summary Reaction Types Animated Burning hydrocarbons C x H y O z O 2 (g) CO 2 (g) H 2 O (g) Methanol, CH 3 OH _CH 3 OH (l) _O 2 (g) _CO 2 (g) _H 2 O (g) Precipitation: use Solubility Rules (AB CD AD CB) Acid-Base Neutralization: acid base salt water (AB CD AD H 2 O) Combination: start with elements (A B AB) Decomposition: often produces gas (AB A B) Single Displacement: use Activity Series to predict if a reaction occurs (A BC B AC) Combustion: Hydrocarbon O 2 (g) CO 2 (g) H 2 O(g) 8
Predict Products and Types of Reactions Predict Products and Types of Reactions Cl Cl Na Na Na Na Na Na Ca Ca Ca Ca Ca Ca Ca Predict Products and Types of Reactions OH- K OH- K Classify (and balance) these rxns 1. Co(s) AgNO 3 (aq) 2. Fe(s) HCl(aq) 3. Na 2 CO 3 (s) 4. Ca(s) H 2 O(l) 5. CaCO 3 (s) 6. HClO 4 (aq) KOH (aq) 7. BaCl 2 (aq) Na 2 SO 4 (aq) 52 Classify (and balance) these rxns 8. HgO(s) 9. LiOH(aq) H 2 SO 4 (aq) 10. Na 2 CrO 4 (aq) Ni(NO 3 ) 2 (aq) 11. Li(s) O 2 (g) 12. Mg(OH) 2 (aq) 2HCl(aq) 13. NH 3 (g) HCl(g) 14. NiCO 3 (s) 15. Ca(s) F 2 (g) Classify these reactions by type Single disp. 1. Co(s) 2AgNO 3 (aq) Co(NO 3 ) 2 (aq) 2Ag(s) Single disp. 2. Fe(s) 2HCl(aq) FeCl 2 (aq) H 2 (g) Decomp. 3. Na 2 CO 3 (s) Na 2 O(s) CO 2 (g) Single disp. 4. Ca(s) 2H 2 O(l) Ca(OH) 2 (aq) H 2 (g) Decomp. 5. CaCO 3 (s) heat CaO (s) CO 2 (g) Acid-base neut. 6. HClO 4 (aq) KOH(aq) KClO 4 (aq) H 2 O(l) Precip. 7. BaCl 2 (aq) Na 2 SO 4 (aq) BaSO 4 (s) 2NaCl(aq) Decomp. 8. 2HgO(s) 2Hg(l) O 2 (g) Acid-base neut. 9. 2LiOH(aq) H 2 SO 4 (aq) Li 2 SO 4 (aq) 2H 2 O(l) Precip. 10. Na 2 CrO 4 (aq) Ni(NO 3 ) 2 (aq) 2NaNO 3 (aq) NiCrO 4 (s) Combo. 11. 4Li(s) O 2 (g) 2Li 2 O(s) Acid-base neut. 12. Mg(OH) 2 (aq) 2HCl(aq) 2MgCl 2 (aq) H 2 O(l) Combo. 13. NH 3 (g) HCl(g) NH 4 Cl(s) Decomp. 14. NiCO 3 (s) NiO(s) CO 2 (g) Combo. 15. Ca(s) F 2 (g) CaF 2 (g) 9