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Chemical Reactions Goals: 1. Write balanced equations for simple chemical reactions. 2. Understand the nature of ionic compounds dissolved in water. 3. Recognize common acids and bases and understand their behavior in aqueous solution. 4. Recognize and write equations for the common types of reactions in aqueous solution and net-ionic equations. 5. Recognize common oxidizing and reducing agents and identify oxidation-reduction reactions. Chemical Reactions • A stream of chlorine gas is directed onto solid phosphorus. • The mixture bursts into flame, and a chemical reactions produces liquid phosphorus trichloride. What are Chemical Equations? Depict the kind of reactants and products and their relative amounts in a reaction. P4 (s) + 6 Cl2 (g) - - - > 4 PCl3 (l) reactants products The numbers in the front are called stoichiometric coefficients. The letters (s), (g), and (l) are the physical states of compounds. Chemical Equations P4 (s) + 6 Cl2 (g) - - - > 4 PCl3 (l) This equation means 1 molecule of solid phosphorus (P4) reacts with 6 molecules of chlorine gas producing 4 molecules of phosphorus trichloride. or ___ mol of phosphorus reacts with ___ moles of chlorine producing ___ moles of phosphorus trichloride. What is the Law of Conservation of Matter? • Because the same atoms are present in a reaction at the beginning and at the end, the amount of matter in a system does not change. • Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788 Law of Conservation of Matter We must lay it down as an incontestable axiom, that in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment. …. Upon this principle, the whole art of performing chemical experiments depends. – Lavoisier’s Traité Élémentaire de Chimie. Law of Conservation of Matter and Chemical Equations 6 x 2 = 12 Cl atoms 4 x 3 = 12 Cl atoms P4 (s) + 6 Cl2 (g) - - - > 4 PCl3 (l) 4 P atoms 4 P atoms • The numbers in front of each formula – stoichiometric coefficients - in a balanced chemical equation are required by the law of conservation of matter. Students should become familiar with writing and balancing chemical equations. Write a reaction for Iron reacting with Chlorine producing Iron (III) chloride Notice the stoichiometric coefficients and the physical states of the Visualizing Chemical Reactions • The reaction between reactant A (red) and reactant B (blue) is shown in the diagram: Based on this diagram, which equation best describes the reaction? a) A2 + B A2B b) 2 A + B4 2 AB2 c) A2 + 4 B 2 AB2 d) A + B2 AB2 What is an Aqueous Solution? • Aqueous solution: an homogenous mixture where the solvent is water. KMnO4 in water K+(aq) + MnO4-(aq) How can Water be a solvent for Ionic Compounds? d d Water is a polar molecule: it has a ___________ (one side of the molecule is +, the other is -). This is due to the difference in _________________ (the ability of an atom in a molecule to attract electrons to itself) between oxygen and hydrogen. O is more electronegative than H: O appears partially ________ charged (d-), and H partially __________ charged (d+). How do we know ions are present in aqueous solutions? The solutions conduct _______________! The substances that provide ions in an aqueous solution are called __________________. HCl, CuCl2, and NaCl are _____ _________. What is a Strong Electrolyte? They ionize or dissociate completely (or nearly so) into ions. HCl, CuCl2, and NaCl are strong electrolytes. HCl (aq) H+(aq) + Cl-(aq) What is a Weak Electrolyte? Acetic acid ionizes only to a small extent, so it is a electrolyte. CH3CO2H(aq) weak CH3CO2-(aq) + H+(aq) What is a Nonelectrolyte? Some compounds dissolve in water but do not conduct electricity. They are called nonelectrolytes. Examples include: sugar ethanol ethylene glycol Summary of the Electrolytic Behavior of Common Soluble Ionic and Molecular Compounds Strong Electrolyte Ionic All Weak Electrolyte None Molecular Strong acids Weak acids and Weak bases (NH3) Nonelectrolyte None All others Which Ionic Compounds are Soluble in Water? If one ion from the “Soluble Compd.” list is present in a compound, the compound is water soluble. Book Ed 7: Figure 3.10 (p. 126) Book Ed 6: Figure 5.3 (p. 179) Learn this table! Solubility of Ionic Compounds What is an Acid? An acid - - -> + H PROTON (in water) Because a hydrogen atom is one proton and one electron, a hydrogen ion (H+) is simply a proton. Some strong acids (completely ionized) are: HCl hydrochloric H2SO4 sulfuric HClO4 perchloric HNO3 nitric HNO3 What is an Acid? An acid ---> H+ in water HCl(aq) H+(aq) + Cl-(aq) What is the Hydronium Ion? HCl(aq) H+(aq) + Cl-(aq) HCl H 2O Cl- hydronium ion H3O+ Bronsted – Lowry Theory: An acid donates a proton What is a Weak Acid? CH3CO2H (aq) H+(aq) + CH3CO2- (aq) WEAK ACIDS (only partially ionized) = weak electrolytes CH3CO2H H2CO3 H3PO4 HF acetic acid carbonic acid phosphoric acid hydrofluoric acid Weak Acid What is a Base? Base ---> OH- in water HYDROXIDE NaOH(aq) NaOH is a strong base ---> Na+(aq) + OH-(aq) How is it that Ammonia, NH3 is a base? Bronsted – Lowry Theory: A base accepts a proton Acids and Bases Book Ed 7: p. 132 Book Ed 6: p. 187 Students should become familiar with identifying acids and bases. Representative Trends in Strength of Binary Acids (containing only H atoms and a nonmetal) Higher electronegativity difference of atoms more polar bond Smaller atom bonded to H Shorter bond Weaker acid base weak acid neutral strong acid Larger atom bonded to H Longer bond Stronger acid The Bronsted-Lowry Theory • 1923 – J. N. Bronsted and T. M. Lowry: • Acids – a proton (H+) ________. • Bases – a proton (H+) _________. HCl (g) + H2O (l) • H3O+ (aq) + Cl- (aq) A reaction between an acid and a base consists of a proton transfer from an acid to a base. Conjugate Pairs • The products of an acid-base reaction are also an acid and a base. The overall reaction consists of two combinations of acids and bases – conjugates pairs. CH3COOH acid (1) + H2O base (2) CH3COO- + base (1) H3O+ acid (2) • The acid-base conjugate pairs differs in structure only by a proton (H+): – The conjugate acid of a specie is that specie plus _______. – The conjugate base of a specie is that specie minus __________. Conjugate Pairs NH3 + base (1) H2 O acid (2) NH4+ + acid (1) OH- base (2) Ammonia (NH3) acts as a base by accepting a proton from H2O. In the reverse reaction, ammonium ion (NH4+) acts as acid by losing a proton to OH-. Amphiprotic – a substance that can act ________ __________________________ Dual Role of Water • Amphiprotic – a substance that can act either as an acid or a base – like H2O. pair of e- in bond are kept by O atom lone pair of eforms new bond Water Equilibrium • Self-ionization of water: about 1 water molecule in 500 million transfer a proton to another water molecule: H2O + H2O H3O+ + OH- • Water is in equilibrium with a hydronium ion and a hydroxide ion (or using our convention with a hydrogen ion and a hydroxide ion). H2O H+ + OH- Water Equilibrium • Equilibrium lies far to the left (it is mostly water molecules). Experimentally determined concentrations of ions (25oC) in molarity (moles per liter) are: [H+] = [OH-] = 1.0 x 10-7 M (= 0.0000001 M) • Ion product of water (Kw) – product of the concentrations of H+ and OH-, at 25oC is: Kw = [H+] [OH-] = (1.0 x 10-7) (1.0 x 10-7) = 1.0 x 10-14 • The ion product relationship applies to all aqueous solutions (acids, bases, salts). What are Basic Oxides? (Basic Anhydrides) Metal oxides react with water to form bases CaO(s) + H2O(liq) -- > Ca(OH)2(aq) CaO in water. Indicator shows solution is basic. What are Acidic Oxides? (Acidic Anhydrides) Nonmetal oxides react with water to form acids CO2(aq) + H2O(l) ---> H2CO3(aq) SO3(aq) + H2O(l) ---> H2SO4(aq) And can come from burning coal and oil. = ACID RAIN What is a Net Ionic Equation? Mg(s) + 2 HCl(aq) --> H2(g) + MgCl2(aq) We really should write Mg(s) + 2 H+(aq) + 2 Cl-(aq) ---> H2(g) + Mg2+(aq) + 2 Cl-(aq) The two Cl- ions are ____________ IONS — they do not participate (could have used NO3-). We leave the spectator ions out — to write a NET IONIC EQ: Mg(s) + 2 H+(aq) ---> H2(g) + Mg2+(aq) Review in Exp 8: Six bottles experiment Which Types of Reactions occur in Aqueous Solution? • EXCHANGE REACTIONS The anions exchange places between cations: Precipitation Acid-base; neutralization Gas-forming Driving force Formation of ________ compound Formation of _____ and ______; proton transfer Evolution of _________ ____________ • OXIDATION-REDUCTION REACTIONS Electron Transfer. What are Precipitation Reactions? The “driving force” is the formation of an insoluble compound — a precipitate. Pb(NO3)2(aq) + 2 KI(aq) -----> 2 KNO3(aq) + PbI2(s) Net ionic equation: Net Ionic Equation Complete formula equation: Pb(NO3)2 (aq) + 2 KI (aq) -----> 2 KNO3 (aq) + PbI2 (s) Ionic equation: Net ionic equation: What are Acid-Base Reactions? • The “driving force” is the formation of water. NaOH(aq) + HCl(aq) ---> NaCl(aq) + H2O(liq) • Net ionic equation OH-(aq) + H+(aq) ---> H2O(liq) This applies to ALL reactions of STRONG acids and bases. Acid-Base Reactions Complete formula equation: HCl (aq) + NaOH (aq) products do not have characteristics of acid or base. Ionic equation: Net ionic equation: For strong acids and bases write the ionic form. Write the equations for ammonia added to phosphoric acid to make ammonium phosphate. Complete formula equation: 3 NH3 (aq) + H3PO4 (aq) Ionic equation: Net ionic equation: For weak acids and bases do not write the ionic form. Acid-Base Reactions • Acid-Base reactions are sometimes called NEUTRALIZATIONS because the solution is neither acidic nor basic at the end. • The other product of Acid-Base reactions is a SALT, with general formula: MX. HX + MOH ---> MX + H2O Salt Water Mn+ comes from base & Xn- comes from acid This is one way to make compounds! What are Gas-Forming Reactions? This is primarily the chemistry of metal carbonates. CO2 and water ---> H2CO3 H2CO3(aq) + Ca2+ ---> 2 H+(aq) + CaCO3(s) (limestone) Adding acid reverses this reaction. MCO3 + acid ---> CO2 + H2O + salt carbonates carbon dioxide Gas-Forming Reactions CaCO3(s) + H2SO4(aq) ---> 2 CaSO4(s) + H2CO3(aq) Carbonic acid is unstable and forms CO2 & H2O H2CO3(aq) ---> CO2+ H2O (Antacid tablet has citric acid + NaHCO3) Practice • Write down the complete formula, ionic, and net ionic equations for: Sulfuric acid reacts with calcium carbonate. Students should become familiar with writing ionic and net equations. Gas-Forming Reactions Students should become familiar with identifying Book Ed 7. p. 140 the several types of reactions and predicting Book Ed 6: p. 194 products of the reactions. Redox Reactions Batteries Corrosion Manufacturing metals Fuels Photosynthesis 6 CO2 + 6 H2O C6H12O6 + 6 O2 Combustion, respiration/metabolism C6H12O6 + 6 O2 6 CO2 + 6 H2O Oxidation and Reduction • Oxidation is a ________________. • Reduction is a __________________. Mg + Cl2 Mg2+ + 2 Cl- Mg atom _____________; it is __________. Cl atoms _____________; Cl2 is ___________. LEO says GER What are Redox Reactions? • An oxidation-reduction reaction - must always have both an oxidation and a reduction. Oxidation process – the oxidation number of one or more elements ___________. Reduction process – and the oxidation number of one or more elements _________. Mg (s) + Cu2+(aq) Mg2+(aq) + Cu (s) Half- Reactions • A redox reaction is two half-reactions that occur simultaneously. reduction 0 Mg (s) + +2 +2 Cu2+(aq) 0 Mg2+(aq) + Cu (s) oxidation Oxidation half-reaction: Mg atoms lose 2 eReduction half-reaction: Cu2+ ions gain 2- Mg (s) Mg2+(aq) + _____ Cu2+(aq) + _____ Cu (s) Hydrogen-Oxygen Fuel Cell Hydrogen-Oxygen Fuel Cell 2 H2 + O2 2 H2O • Fuel cell - reactants are supplied continuously from an external source. • Cars can use electricity generated by H2/O2 fuel cells. • H2 carried in tanks or generated from hydrocarbons– (if possible solar) 2 H2 O2 + 4 e4 H+ + 2 O-2 2 H2 + O2 4 H+ + 4 e2 O-2 2 H2O 2 H2O Identifying Redox Reactions • Look for a change in the oxidation number of an element in the course of the reaction. What are oxidation numbers? Is the charge an atom has, or appears to have, as determined by the following guidelines for assigning oxidation numbers. Oxidation Numbers in HF: Animation Students should become familiar with assigning OXIDATION NUMBERS – learn the rules. (Page 200) Follow the rules for Oxidation Numbers 1. 2. 3. 4. 5. 6. Each atom in a pure element has an oxidation number of zero. Cu, I2, S8. For monoatomic ions, the oxidation number is equal to the charge on the ion. NaBr, MgBr2. Fluorine always has an oxidation number of -1 in compounds with all other elements. Cl, Br, and I always have oxidation number of -1 in compounds, except when combined with oxygen or fluorine. NaCl, ClO-. The oxidation number of H is +1 and of O is -2 in most compounds. Exceptions: H in a binary compound with a metal, CaH2. O in peroxides can have an oxidation number of -1, H2O2. The algebraic sum of the oxidation numbers for atoms in a neutral compound must be zero; in a polyatomic ion, the sum must be equal to the ion charge. HClO4 Give the Oxidation Number of each Element in the following: Br2 Na2SO4 CaH2 Fe2O3 P4 H2O2 KMnO4 What is Oxidizing Agent? What is Reducing Agent? • OXIDIZING AGENT: The substance that accepts electrons in an oxidation-reduction reaction and gets reduced. • REDUCING AGENT: The substance that donates electrons in an oxidation-reduction reaction and gets oxidized. Oxidizing and Reducing Agents 2 K (s) + 2 H2O (l) 2 KOH (aq) + H2 (g) Common Oxidizing and Reducing Agents (Table 5.4, p. 203) Metals (Cu) are reducing agents HNO3 is an oxidizing agent Cu + HNO3 --> Cu2+ + NO2 O2, F2, Cl2, Br2, I2, Cr2O2-, MnO4- Metals (Na, K, Mg, Fe) are reducing agents 2 K + 2 H2O --> 2 KOH + H2 H2, Na, K, Fe, Al, C Students should be familiar with common oxidizing and reducing agents. Oxidizing Agents • Oxygen - Oxidizes coal in electric power plants, gasoline in automobiles, “burns” the food we eat. • Potassium dichromate - Lab: aldehydes to ketones; ethanol to acetaldehyde. Cr2O72- + 3 C2H5OH + 8 H+ 2 Cr3+ + 3 C2H4O + 7 H2O acetaldehyde • Potassium permanganate - Lab: color test for many oxidizable substances (purple color disappears). MnO4- + 5 Fe2+ + 8 H+ Mn2+ + 5 Fe3+ + 4 H2O • Halogens and Peroxides – Antiseptics – substance applied to living tissue to kill microorganisms or prevent their growth: Iodine, Hydrogen peroxide (3% sol), Benzoyl peroxide. – Disinfectants – substance that is applied to nonliving tissue to kill microorganisms: Chlorine from [Ca(OCl)2]. Reducing Agents • Carbon - To obtain free metals from their ores. SnO2 (s) + 8 C (s) Sn (s) + CO2 (g) • Metals - To obtain free metals from their ores. Cr2O3 (s) + 2 Al (s) Al2O3 (s) + 2 Cr(s) • Hydrogen - To obtain free metals from their ores and to reduce many kinds chemical compounds. WO3 (s) + 3 H2 (g) W (s) + 3 H2O N2 (g) + 3 H2 (g) • Organic Compounds Fe 2 NH3 (g) – Hydroquinone: it reduces silver ions in photographic films C6H4(OH)2 (aq) + 2 Ag+ (aq) C6H4O2 (aq) + 2 Ag (s) + 2 H+ – Antioxidants – in food chemistry Vitamin C (ascorbic acid) prevents the browning of fruit by inhibiting air oxidation. Vitamin E (tocopherol) - Both vitamins retard oxidation reactions that damage vital components of living cells. Remember • Go over all the contents of your textbook. • Practice with examples and with problems at the end of the chapter. • Practice with OWL tutor. • Work on your assignment for OWL Chapters 4, 5. • Practice with the quiz on CD of Chemistry Now.