Solutions, Ions & Acids, Bases (Chapters 3-4) Chem 107 T. Hughbanks Example - Limiting Reagents SiCl 4 is used in making computer chips. It is produced by the reaction: SiO 2 + 2 C + 2 Cl 2 SiCl 4 + 2 CO How much SiCl 4 can be made from 75 g each of the reactants?
Reaction Yields Most real reactions produce less product than equations would predict. Competing reactions, impurities, time, etc. Often report percent yield Percent Yield % yield = actual yield theoretical yield 100% Actual yield: what you really get, from experiment Theoretical yield: maximum you could expect, from calculations actual yield < theoretical yield, % yield < 100%
Yield - example Suppose we ran the reaction from the last example and obtained 80 g of SiCl 4. What would our percent yield be? Clicker Question
How many grams of KClO 3 are needed to produce 42.0 g of O 2 if the percent yield is 65.0%? KClO 3 KCl + O 2 (not balanced!) A. 69.7 g B. 82.5 g C. 165 g D. 371 g Slide 8 of 9 Solutions Homogeneous mixtures, 2 or more components Solids, liquids, gases... Solvent = major component Solute = minor component(s) Aqueous Solution water as solvent
Concentration Solutions have variable composition. To fully describe a solution, we need to know its concentration. MOLARITY: # moles of solute / # liters of solution Other concentration units also used Example: Molarity Cholesterol is C 27 H 46 O Cholesterol count has units of mg per dl (1 dl = 10-1 L, or 100 ml) Values above 200 are considered unhealthy. Express this threshold in terms of molarity.
Ions & Ionic Compounds Ions = atoms or groups of atoms with an electrical charge Cations = positive charge Anions = negative charge Monatomic Ions Formed from neutral atoms by adding or removing one or more electrons Cations - metals, often from first 2 columns of the periodic table Na Na + + e Anions - nonmetals, often halogens Cl + e Cl
Polyatomic Ions (Tables 2.5-2.7) 2 or more atoms held together by chemical bonds electrically charged cations or anions NH 4+, H 3 O +, NO 3, SO 4 2, etc. (Tables 2.5-2.7, p. 55, list several more.) Learn all of Tables 2.5-2.7! Table 2.5
Table 2.6 Table 2.7
Ionic Compounds Combine anions and cations, with charges balancing Held together by Coulomb force (attraction of opposite charges) Ionic compounds are crystalline solids, with high melting points. Ionic Compounds in Solution Many ionic compounds dissolve in water. Solutions of ionic compounds in water conduct electricity. ( free ions)
Ionic compounds in Solution Soluble ionic compounds dissociate in aqueous solution NaCl (s) Ba(OH) 2 (s) Na + (aq) + Cl (aq) Ba 2+ (aq) + 2 OH (aq) Free ions in solution, not molecules. Overall solution still electrically neutral. Ionic Equations For reactions in solution, chemical eqs. usually written in terms of ions. Total Ionic Equation - includes all ionic species present Net Ionic Equation - includes only those that actually react. Omits spectator ions.
Ionic Equations Mix 2 solutions: NaCl + AgNO 3 A white solid will form: AgCl Total Ionic Eq: Na + (aq) + Cl (aq) + Ag+ (aq) + NO 3 (aq) Net Ionic Eq: Na + (aq) + NO 3 (aq) + AgCl (s) Cl (aq) + Ag+ (aq) AgCl (s) Molecular Equations We could also write a molecular eq. : NaCl + AgNO 3 NaNO 3 + AgCl This tell us the compounds that were actually combined to carry out the reaction. However, it doesn t focus on what is actually occurring in solution, so it is less chemically informative as to what the action is.
Problem Mix 75 ml of 0.750 M Pb(NO 3 ) 2 with 125 ml of 0.855 M NH 4 Cl. Find mass of solid PbCl 2 formed and concentrations of the ions still in solution. Solution Stoichiometry AS ALWAYS, focus on moles. Usually use molarity and volume to find number of moles of solutes. Molarity = moles per liter, so: molarity volume = moles
Dilution of Solutions Often, we prepare solutions from concentrated stock solutions, by adding solvent. Concentrated sulfuric acid is 18.0 M H 2 SO 4. In your lab you are using 0.08 M H 2 SO 4. What volume of concentrated H 2 SO 4 should be used to make 1 liter of 0.08 M solution? Reaction Stoich. w/solutions Still MOLES REACT WITH MOLES! Write a balanced equation and use concentrations and volumes to find numbers of moles of reactants. Titration - determination of concentration by careful reaction with a known standard.
Titration Example H 2 SO 4 + 2NaOH Na 2 SO 4 + 2 H 2 O If 20.00 ml of NaOH solution reacts completely with 25.00 ml of 0.0800 M H 2 SO 4, what is the concentration of the NaOH? Acids & Bases What s an acid? a base? Early Observations (17 th century) Acids: powerful solvents change colors of many dyes sour taste
Acids & Bases Bases: slippery, soapy feel change colors of many dyes, but differently than acids bitter taste Acids & Bases Mixing acid + base eliminates the properties of both. Neutralization
Acids & Bases Acids in aqueous solutions form H 3 O + ions. ( hydronium ions ) Bases in aqueous solutions form OH ions. ( hydroxide ions ) Neutralization (net ionic equation): H 3 O + + OH 2 H 2 O Strong Acids Strong acids - dissociate completely in solution HCl + H 2 O H 3 O + + Cl One-way arrow shows that all HCl breaks up into ions. Solution contains H 3 O +, Cl, and H 2 O, but not HCl.
Strong Acid Solution: e.g., HCl HCl + H 2 O H 3 O + + Cl = H 3 O + No intact HCl = Cl molecules present! Strong vs. Weak Acids Weak acids - dissociate only partially in solution. Some molecular acid remains. CH 3 COOH + H 2 O H 3 O + + CH 3 COO Two-way arrow shows that only some of the CH 3 COOH breaks up into ions. Solution contains H 3 O +, CH 3 COO, H 2 O, and intact CH 3 COOH.
Weak Acid Solution: CH 3 COOH CH 3 COOH + H 2 O H 3 O + + CH 3 COO = H 3 O + = CH 3 COO = CH 3 COOH Strong Acids (Table 3.2) HCl, HBr, HI, HNO 3, HClO 4, H 2 SO 4 (also include HClO 3 ) LEARN this list. Note that some weak acids, like HF, are very reactive. Also note that strong/weak does not refer to concentration.
Strong Base Solution: e.g., NaOH NaOH (s) + H 2 O (l) Na + (aq) + OH (aq) = OH No intact NaOH = Na + molecules present! Strong & Weak Bases (Table 3.2) Strong: M I OH (M I = Li, Na, K), M I OH (M II = Ca, Sr, Ba) LEARN this list. Note that strong/weak does not always refer to concentration. e.g., NH 3 and CH 3 NH 2 are weak bases: NH 3(aq) + H 2 O (l) NH 4 + (aq) + OH (aq)
Table 3.2 Strong and Weak Electrolytes Electrolytes are acids, bases, or salts that dissolve into solutions to yield ions. Soluble salts, strong acids, and strong bases are all strong electrolytes. Weak acids or weak bases are weak electrolytes.
Problem - Phosphate Cleanup HPO 4 2 is a water pollutant, but it is used in the treatment of waste in sewage plants. It is removed by treatment with lime (CaO). Lime dissolves in water to give Ca 2+ and OH. Phosphate precipitates as Ca 5 (PO 4 ) 3 OH (hydroxyapatite). Problem - Phosphate Cleanup Write a balanced equation for dissolving the lime. Write a balanced equation for the precipitation reaction. How many kg of lime are needed to remove all the phosphate from a 10 4 L tank which is 0.0156 M in HPO 4 2?