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Chapter 4 Types of Chemical Reactions and Solution Stoichiometry Water (H2O = HOH) • • • • Solvent - dissolves solute to make solutions Aqueous means dissolved in water Water dissolves many substances Water’s molecular shape: bent d- d+ H2O Water’s polarity • Causes ionic compounds to “fall apart” in water when opposite charges become attracted to one another • Called hydration • Some molecules are more soluble in water than others Electrical Conductivity • A solution’s ability to conduct electricity • The more ions in a solution, the stronger the electric current will be • Pure water does NOT conduct electricity Electrolytes • STRONG electrolytes: solutions that conduct electric current very efficiently • WEAK electrolytes: solutions that conduct a small electric current • NONelectrolytes: solutions that permit no current flow Strong Electrolytes • 100% Dissociation (breaking apart) • Many ions are created in aqueous solution • Examples: soluble salts, strong acids, strong bases NaCl (s) H 2O Na+ (aq) + Cl- (aq) Arrhenius Acid • Substance that produces H+ ions (protons) when dissolved in water • Proton donor HA(aq) + H2O(l) --> H3O+(aq) + A-(aq) Like irrelevant STRONG Acids • • • • HCl: hydrochloric acid - produces H+ and ClHNO3: nitric acid - produces H+ and NO3H2SO4: sulfuric acid - produces H+ and HSO4Not in book - HBr (hydrobromic acid), HI (hydroiodic acid), HClO4 (perchloric acid) • Characteristics: sour taste (vinegar, citrus fruits), cause color changes in plant dyes, pH is low (0-7) Arrhenius Bases • Substance that produces a hydroxide (OH-) ion when dissolved in water Like irrelevant STRONG Bases • NaOH: sodium hydroxide - produces Na+ and OH• KOH: potassium hydroxide - produces K+ and OH• Not in book: LiOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2 (you probably won’t see these) • Characteristics: bitter taste, feel slippery (soaps usually contain bases), cause color changes in plant dyes, pH is high (7-14) • Group I/II hydroxides Weak Electrolyte • Doesn’t completely dissociate in water • Relatively few ions are produced when dissolved in water • Weak acids/bases CH3COOH CH3COO- (aq) + H+ (aq) A reversible reaction can occur in both directions Nonelectrolytes • Create almost no ions when dissolved in water • Ex: sucrose C12H22O11 Molarity • A unit representing concentration • Unit is moles per liter M = moles of solute liters of solution Read as “____ molar” Molarity Example • Grams need to be converted to moles • mL need to be converted to L • How many mL of solution are necessary if we are to have 2.48 M NaOH solution that contains 31.52 g of the dissolved solid? • Answer: 318 mL solution • See book pg. 137-140 More on Molarity… • Multiply molarity by number of ions in the solution to find molarity of specific ions: • Calculate the molarity of all the ions in the following: – 0.25 M Ca(OCl)2 – Anwer: 0.25 M Ca2+, 0.50 M OCl– 2 M CrCl3 – Answer: 2 M Cr3+, 6 M Cl- Dilution • We can add water to solutions and consequently water them down (dilute) them. *only the amount of water changes, not the solvent! M1V1 = M2V2 • M = molarity • V = volume (units must match for 1 and 2) Dilution Example • What volume of 12 M hydrochloric acid must be used to prepare 600 mL of a 0.30 M HCl solution? • Answer: 0.015 L = 15 mL of 12 M HCl Types of Reactions: Review! • • • • • • • Combination/Synthesis Decomposition Single Replacement Double Replacement Combustion *NEW: Acid-Base Reactions *NEW: Oxidation-Reduction Reactions Combination/Synthesis • One product! • Two elements = combine and balance charges 2Al + 3O2 --> 2Al2O3 • Nonmetal oxide + water produces an acid CO2 + H2O --> H2CO3 Decomposition • One reactant • Breaks into its element (diatomics!) 2MgO --> 2Mg + O2 • Metal carbonate --> metal oxide + CO2 Na2CO3 --> Na2O + CO2 • Metal chlorate --> metal chloride + O2 2 Al(ClO3)3 2 AlCl3 + 9 O2 more… Single Replacement • Lone metal (nonmetal) + compound • Metals (or nonmetals) switch • CHECK ACTIVITY SERIES - lone metal must be higher or NR 2Al(s) + 3Pb(NO3)2(aq) 3Pb(s) + 2Al(NO3)3(aq) Double Replacement • Two compounds’ metals switch spots • MUST bubble, form a molecule like water or form a precipitate (precipitate reaction) • USE SOLUBILITY RULES! Solubility Rules • Slightly soluble = Insoluble and means (s) • Soluble means (aq) • MUST BE MEMORIZED!! Acronyms for Solubility • CASHN Gia – C = chlorates, A = acetates, S = sulfates, H = halogens, N = nitrates, and Gia = group 1 A metals (all soluble)…EXCEPT… • For sulfates = Ca, Ba, Sr (remember CBS like TV) • For halogens: Ca, Ba, Sr + Happy-whats happy? Hg Ag Pb…mercury, silver and lead…add a -py to the end and all the first letters spell "happy” • If it’s not part of CASHN Gia, it’s insoluble. • SONG?? Or this one…another… Predict Products/Precipitates • • • • • • • • KCl(aq) + Pb(NO3)2(aq) -> 2KCl(aq) + Pb(NO3)2(aq) -> PbCl2(s) + 2KNO3(aq) AgNO3(aq) + MgBr2(aq) -> 2AgNO3(aq) + MgBr2(aq) -> 2AgBr(s) + Mg(NO3)2(aq) Ca(OH)2(aq) + FeCl3(aq) -> 3Ca(OH)2(aq) + 2FeCl3(aq) -> 2Fe(OH)3(s) + 3CaCl2(aq) NaOH(aq) + HCl(aq) -> NaOH(aq) + HCl(aq) -> H2O(l) + NaCl(aq) Combustion Organic compound (CHO) + O2 --> CO2 + H2O Usually releases energy (light or heat) ***NOTE! • It is assumed from now on that if a reaction isn’t written out you can (and will) write it out… Net Ionic Equations 1. Balance the equation 2. Write the complete ionic equation by breaking up aqueous compounds (ionic) 3. Cancel spectator ions (same on both sides including state) 4. Rewrite what’s left (reduce coefficients if necessary) See example in book pg. 151 Complex Stoichiometry Problems • We can add limiting reagent and molarity to make more complex stoichiometry problems • Example pg. 152153 Acid-Base Reactions • Reminder: Arrhenius (irrelevant) acids/bases • Brønsted-Lowry acids/bases (protons only): Acids are proton donors, bases are proton acceptors • Acid-Base Reactions are called neutralization reactions because the acid/base becomes neutralized by the base/acid added to it. • *OH- will react with both strong and weak acids acid + base salt + water Example • What volume of a 0.100 M HCl solution is needed to neutralize 25.0 mL of 0.350 M NaOH? Write out reaction: HCl + NaOH --> NaCl + H2O 0.250L X 0.350mol NaOH X 1mol HCl X __1L HCl_ 1L 1mol NaOH 0.100 mol HCl Solve for = 0.0875L of hydrochloric acid needed Titrations! • If we know the concentration and amount of either the acid or base (titrant in the buret), we can determine the concentration of the other (base or acid) by adding the titrant until the reaction is neutralized (equivalence point) Titration Setup • An indicator is added to the solution to be analyzed (usually phenolpthalein is added to the acid). It will indicate when the solution is one drop beyond a specific pH (phenolpthalein is 7). This is called the endpoint. • Endpoint is where the color changes vs. equivalence point, where the number of moles of acid = moles of base • Video Titration Steps 1. Record initial volume of the known concentration of base/acid in buret 2. Record amount (usually mL) of unknown concentration acid/base in beaker 3. Add a few drops of indicator to beaker 4. Add titrant until the indicator has just permanently changed color 5. Record the final volume of the known concentration of base/acid in buret 6. You will now have the information needed to calculate the concentration of the unknown Example • Use the following data to calculate the concentration of the HCl solution: • HCl in beaker with indicator: 10.4mL • Initial buret reading: 3.5mL 0.100M NaOH • Final buret reading (pink): 15.4mL 0.100M NaOH **write out the reaction first!! Answer: 0.114M HCl Book Example • More complicated problem on page 159 • TIP pg. 160: The first step in the analysis of a complex solution is to write down the components and focus on the chemistry of each one. When a strong electrolyte is present, write it as separated ions. 4.9 Oxidation-Reduction Reactions Oxidation-Reduction Rxns. (Redox Rxns) • A chemical reaction where one or more electrons are transferred • Photosynthesis, combustion reactions, sugar/fat/protein oxidation in humans Covalent Vs. Ionic Bonds • Covalent - electrons are shared • Between elements in molecular compounds (remember two nonmetals make binary molecular compounds???) • Ionic - opposite charges (cations and anions) are attracted like magnets (strong bond) Oxidation States (oxidation numbers) • Helps show where electrons are transferred in chemical reactions…RULES… • Lone and diatomic elements = zero • Oxygen = -2 except in H2O2 (charge is -1) • In ionic compounds, ions’ #s are same as their charge • Do negative first. Charges add to make zero (or a charge if the compound is charged) Ion charges: n+ or n- (#before charge) Oxidation states: +n or -n (# after charge) Try these… • NO2 • Cl2 – O: -2 each, N: +2 – Cl: 0 • PbS • Fe3O4 – S: -2, Pb: +2 – O: -2 each, Fe: +8/3 • SnCl6-2 • This can happen – Cl: -1 each, Sn: +4 (non whole number • H+ oxidation numbers) – H: +1 Purpose • In reactions, we want to know what elements gain or lose electrons • This is shown through assigning oxidation numbers • The element that becomes more negative gains electrons, decreases oxidation state (reduction) • The element that becomes more positive loses electrons, increases oxidation state (oxidation) • One doesn’t happen without the other. Memorize! • LEO (lose electrons oxidation) the lion goes GER (gain electrons reduction)…or… • OIL (oxidation is lost) RIG (reduction is gained) Example: CH4(g) + 2O2(g) --> CO2(g) + 2H2O(g) 1. Complete/balance equation if necessary 2. Assign oxidation states/numbers 3. Determine which element is oxidized/reduced Also… • C: -4 -> +4 so it’s oxidized – Donates e- so REDUCING AGENT (allows something else to be oxidized) • O: 0 -> -2 so it’s reduced – Accepts e- so OXIDIZING AGENT (allows something else to be reduced) You try… • 1. 2. 3. 4. PbS(s) + O2(g) --> PbO(s) + SO2(g) Assign oxidation #s S: -2 -> +4, O: 0 ->-2 S: oxidized, reducing agent O: reduced, oxidizing agent Copper wire reacts with silver nitrate to form silver metal. What is the oxidizing agent in the reaction? Balancing Redox Rxns. • Fancy way of balancing skeleton equations using electrons • Two ways: – By oxidation states (LEARNING.) – Half-reactions (chapter 18) Steps to follow 1. Assign oxidation states/numbers 2. Use lines to connect oxidation and reduction on both sides of the reaction 3. Add coefficients that will neutralize the number of electrons gained and lost 4. Balance the rest of the equation 5. Make sure your states of matter are appropriate Example Balance the reaction between solid lead(II)oxide and ammonia gas to produce nitrogen gas, liquid water, and solid lead.