Study Guide 3
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This 9 page Study Guide was uploaded by Cassidy Zirko on Sunday March 27, 2016. The Study Guide belongs to Chem 143 at University of Montana taught by Dr. Cracolice in Spring 2016. Since its upload, it has received 321 views. For similar materials see General Chemistry 2 in Chemistry at University of Montana.
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Date Created: 03/27/16
Chem 143, Prof Cracolice. Study Guide Exam 2 Chapter 58 Weak acid reaction: Hwk +HOH wk + H O + — 3 Buffer paring is Hwk/wk loss of hydrogen with water forms hydronium pH determination Hydronium concentration is completely depended on the ratio of weak acid to conjugate base Maintain hydronium and pH by adjusting ratio Finding buffer begins with finding the correct ratio of weak acid to conjugate base Acid always in same location as hydronium but on the opposite side of the equal sigh wk− ¿ Hwk o Ka =¿ H 3O o The weak conjugate base of the acid is not the base being added to the solution Procedure o Find hydronium concentration from pH o Then find the ratio of Hwk/wk using Ka and hydronium concentration o Typically have acid concentration between .1M and 1.0M o The ratio must be maintained o M= mol/L or mol/1000ml, can be used as conversion factors to find grams Larger amount of Hwk/wk (bigger ratio) the more acid or base can be added without a change in pH Buffer capacity: amount of acid or base needed to change pH by 1 pH unit overall ratio determines how much acid or based added Smaller ratio less capacity Strong and strong base – all have free floating hydrogen mixed in solution To find the amount of hydronium in solution, you need to figure out how much of the strong acid will be neutralized by the strong base while including the volume change To account for the volume change, make sure you do M V1=M1V 2 2 For strong acids, all of the hydrogen will dissociate Always watch units, only add moles together, not molarity Change in pH is the final pH initial pH Weak acids don’t completely dissociate, calling for use of an ICE Table o A weak acid is any acid that is not one of the seven strong acids, if you cant remember them, in the data pack they are the ones with the exponents that are positive numbers Even with weak acids, you need to account for volume changes When you are adding a common ion, you should use a Before, change, after table because it will allow you to adjust for the amount of the common ion or hydrogen that is added The smallest value in the BCA table will be limiting reactant Chem 143, Prof Cracolice. Study Guide Exam 2 Use the same equation for the BCA table that you would use for an ICE table, but you want to find how much of each species is left after the limiting reactant is used, this requires either adding or subtracting the value depending on what the limiting reactant is If there is a large change in pH it means the buffer capacity is to small o If the ratio is consistent then it will buffer at a particular pH o Amount of acid and base varies with the magnitude of the buffer o The higher the magnitude, the better the buffer 3 criteria on chosing a buffer o 1 the species of the buffer should not interfere with the system (should not form preicipitates) o 2 the buffer should be within + or – 1 pH unit of the pKa of the buffer system (because the better the buffer, the ratio of acid to conjugate base is 1 leaving pH=pKa) o 3 acid and base in the buffer should be 10 times greater than the quantity of strong acid or base that is going to be adde d pH=pKa at halfway to the equivalence point pKa represents the approximate desired buffer pH of the buffer system weak acid and conjugate base is almost equal buffer does best buffering of both acids and bases Chapter 59: Method 1: Adding soluble salt of conjugate base to weak acid solution o All sodium salts are soluble release of ion of conjugate base of weak acid o Concentration of salt = the concentration of conjugate base of weak acid o Using ICEE table with correct concentrations a o Always check assumptions it must be correction with sig fig rules and if its not you have to preform multiple assumptions o Equation: Hwk + H O Wk + H O + 2 3 o Weak acids will always have hydronium as a product because the hydrogen will dissociate allows us to be able to find pH o Strong acids will 100% dissociate Method 2: Adding Limiting Quantity of Strong Acid to solution of Weak base o Strong acids 100% ionize = concentration of strong acid is the concentration of hydronium initially in solution o Strong acid adds hydronium to weak base o Strong acid will react with conjugate weak base: H O + 3k Hwk + H O 2 o Some of the conjugate base will still remain in solution because the acid is the limiting reactant o Keep in mind that a buffer has Hwk/wk, that’s what makes it a buffer o BCA table will help to find how many moles of acid is formed from the hydronium in solution o Always subtract the small value Chem 143, Prof Cracolice. Study Guide Exam 2 o Use the ICEE table to find the concentration of hydronium at equilibrium then can find pH Method 3: Partial neutralization of extra weak acid with strong base o Hydroxide always acids as a strong base o Sometimes ammonia is used as a base o Weak acid/strong base: OH+Hwk H O + Wk 2use for BCA table) o Hydroxide becomes limiting reactant o Use the reaction of the dissociation of the acid for the actual ICE table, not the same equation as the BCA table Key point: ALWAYS MAKE SURE THAT THE BUFFER HAS THE WEAK ACID AND CONJUGATE WEAK BASE IN SOLUTION!!!!!!!!!!!!! Chapter 60 Titration careful addition of one solution to another by means of a buret Buret measure volume of solution required to react with carefully measured amount of another dissolved substance Indicator weak acid/base combination that changes color depending on the pH Standardizing finding the concentration of unknown solution for use in later titrations Primary standard a solution that is used to standardize solutions of unknown concentrations because it can be weighed accurately o Substance needs to be available in a very high purity form, reasonable cost, remains pure over time o Must have a high molar mass minimizes error with weighing Acids standardized with bases, bases used as titrant Titrations usually used to find the quantity of acid or base in sample Strong acids, source of hydronium, strong base is source of hydroxide Monoprotic one hydrogen in the acid (1 mole acid/1mole of base) titrations mainly use stoichiometry (volume and molarity given moles given moles wanted volume or molarity wanted) watch sig figs moles of strong base= moles of strong acid, creates a neutral solution titration curves show ow the pH of a solution changes based on the volume of base added all titration problems need dilutions equivalence point point where in an acid and base titration stoichiomtrically equivalent quantities exist in solution only spectator ions and water present at equivalence point greater concentration of acid and base means greater nearly vertical point on a titration curve will be, sharper change at equivalence point amount of titrant needed at equivalence point is the same as amount of solution originally present in a 1 to 1 ratio Chem 143, Prof Cracolice. Study Guide Exam 2 o ie. Began with 35 mL of a strong acid, you need 35 mL of strong base to reach equivalence point use knowledge of volumes at equivalence point to do a dilution at equivalence point of a strong acid/strong base titration the pH will be 7 at ½ the equivalence point volumes of acid and conjugate base are equal so the pH=pKa dilution based on how much base is being added verses the amount of acid already present in solution remember that pH=14pOH (find pOH if you are above the equivalence point because it is a more basic solution strong base controls the pH in a strong base, weak acid titration strong acidweak base: pH at equivalence point <7 (slightly acidic) o weak base and hydronium form the conjugate weak acid solution so there are more acidic species than basic in the solution strong acid strong base: pH at equivalence point =7 o complete neutralization of all acidic/basic species strong base weak acid: pH at equivalence point is >7 (slightly basic) o weak acid and hydroxide form conjugate weak base of acid more basic species in solution all titrations occur with the titrant of a strong acid or strong base of a known concentration Chapter 61 Equivalence point equal moles of acid and base exist in solution Finding molarity of unknown substance is why a titration occurs Found in 2 ways with a titration curve or an indicator Unknown weak acid reacts with strong base (Hwk + OH Wk + H O) 2 Presence of weak acid and conjugate weak base creates buffer system, resists change in pH After the buffer is all reacted steep change in pH on the titration curve o Cant resist change in pH base cause increase in pH o Small amounts of base will lead to a very large change in pH At the equivalence point Hwk + OH Wk + H O, reaction has gone to completion= 2 basic pH o More strong base added creates a weak basestrong base system o Weak base has no effect on pH so it is only determined by the amount of strong base added Titration curve shows the progress of a titration o Weak acidstrong base – the shape depend on the concentration of each species General form begins at pH appropriate for weak acid, goes through buffer region, rapid change in pH through the equivalence point, rises and levels of with more strong base addition Chem 143, Prof Cracolice. Study Guide Exam 2 Weak acid (smaller Ka value) have higher starting pH values Weak base/strong acid acts as weak acid and strong base but opposite shape + o Weak base initially hydrolyzes: Wk+HOH HWk + OH + + o Forms conjugate weak acid when acid is added: H +Wk HWk o Equivalence point is lower than 7 because solution contains conjugate weak acid adding strong acid lowers pH further Polyprotic acid slightly ionized before titration Addition of strong base produced conjugate base of strong acid H A whic2 is HA, solution remains acidic Has as many equivalence points as there are hydrogens in the acid o 1 moles of OH=amount of H A in so2ution o 2 moles of OH= amount of HA in solution 2 protonsstcurve repetes itself at a higher pH with a higher second equivalence point o 1 equivalence point is usually acidic o 2 equivalence point is usually basic Indicator changes color with a change in acidity, weak acids and bases with different colors in solution Need to know where the steep part of the a titration curve is to choose the correct indicator Chapter 62 Solubility of low solubility salts little amount of salts dissolve, makes them appear insoluble M a b(s) + Xa, equblibrium expression is Ksp= [M ][ X ] a+ b Remember, solids and liquids have a constant concentration and are never included in an equilibrium expression Ksp solubility product constant Effective concentration/activity is about equal to molarity Polyvalent/spectator ions could cause deviation with experimental values Ksp is temperature dependent Procedure: Finding Ksp o 1 write down the equilibrium equation for the reaction o 2 write the solubility constant product expression Feature of equilibrium rate of dissolving is = to rate of crystallization Solubility in mol/L is molarity of solution molarities of each ion is dependent on Ksp In a 1:1 ratio solubility of both ions will be the same Procedure finding Ksp from solubility of each ion o 1 determine molar concentration of ions in solution o 2 substitute into Ksp expression and solve Many things can be found using Ksp and solubility’s, read problem carefully to find which one you need to solve for Procedure finding individual solubility’s Chem 143, Prof Cracolice. Study Guide Exam 2 o 1 assign a variable to one species o 2 determine the concentration of the other species in terms of the variable o 3 substitute concentrations from steps 1 and 2 into Ksp expression and solve o Remember to do proper squaring and algebraic rules Conversion: Mol/L g/100ml : .1L/100ml and g/mol Adding common ion changes the equilibrium according to le chatilers principle o Adding reactant (solid) solution becomes more soluble because there are more ions present, the equilibrium shifts right o Adding product (ions) solution becomes less soluble because there are more ions in solution causing an increase in formation of the solid, equilibrium shifts left Solubility reduced in the presence of a common ion (the Ksp decreases) The smaller the Ksp the less soluble something is Solution pH will have an effect on solubility of any ionic compound that is made up of ions that will react with hydronium or hydroxide ions Greater acidity of the solution, the more soluble the compound will be o Anions will react with the hydronium, allowing greater solubility o More acidic solution, the greater the solubility of a low solubility salt Conjugate bases of strong acids will not react with hydronium ions No effect on solubility of solution Chapter 63 Ksp tells when (and at what concentrations) an equilibrium will exist Ion product, IP the value of Ksp that is obtained from individual ion concentrations that doesn’t equal the known Ksp value Comes into play with le chatilers principle Ion added IP > Ksp for a short while Ksp is the Maximum IP a solution can have without precipitation o IP>Ksp precipitation occurs o IP=Ksp system is at equilibrium o IP<Ksp system is unsaturated and no precipitation will occur because the rate of dissolving is faster than the rate of crystallization Increase concentration of one ion= decrease the concentration of the other inion Dilution factor: mL of one species divided by the total volume of the solution The dilution factor comes into play when there is change in volume State an IP inequality at the end of each problem Procedure: o 1 calculate the moralities of potentially precipitating ions in the final solution (requires dilution factor) o Determine the ion product o Compare to Ksp Chem 143, Prof Cracolice. Study Guide Exam 2 Precipitating at multiple equilibriums: concentration of the common ion between the multiple equilibria must be the same Concentration of other ions will change based ion the corresponding Ksp values Qulitative Analysis testing unknown sample to find what ions are present in solution, not finding the amount of ions Group group of elements that will precipitate out together upon addition of a particular solution Group 1: insoluble chlorides adding 6M HCl to solution o Precipitate out silver, mercury (II) and lead (II), HCl allows for the creation of a low pH environment Group 2: Acid Insoluble Sulfides adding hydrogen sulfide, having a low pH o Bismuth, cadmium, copper(III), Mercury (I), Antimony (III) and (V), tin (II) and IV) o Sulfide concentration very low because of reaction with water o Concentration doesn’t need to be high because the Ksp values are very low o Group 3 sulfides don’t precipitate because they have higher Ksp Group 3: Base insoluble hydroxide and sulfides adding Ammonia and Ammonium sulfide o Aluminum, cobalt (II), Chromium (III), iron (II) and (III), manganese (II) and (III) and zinc o Solution how basic because of adding the ammonia neutralizes acid, ammonium will form o Aluminum, Chromium (III) and Iron (III) will precipitate with hydroxide formed from ammonia and water reaction because of their low Ksp values o After ammonium sulfide is added, sulfide concentration is high enough that it will allow the rest of the ions to precipitate out Group 4: Insoluble Carbonates adding Ammonium carbonate o Group 1A/1 and 2A/2 and ammonium are the only ones that can be left in solution o Barium, calcium and magnesium precipitate out with carbonate Group 5: Alkali Metals and Alkali like ions o Ammonium, sodium and potassium only ions possibly left in solution o Sodium and potassium discovered with flame test, sodium yellow, potassium purple o Ammonium detected by adding NaOH and seeing if there is the smell of Ammonia. Chapter 64: Troposphere lowest layer of atmosphere pressure and temperature decrease with altitude Stratosphere second lowest layer of atmosphere Increasing temperature with increasing altitude Has layered nature of temperature change in second layer mesosphere 3 lowest layer, middle layer Chem 143, Prof Cracolice. Study Guide Exam 2 temperature decreases with increasing altitude thermosphere temperature increases with increasing altitude Two major components nitrogen and oxygen Any ideal gas at any temperature, volume is directly proportional to number of moles Volume percentage is same ration as mole percentage Volume fraction same as mole fraction Ppm and ppb ratios of number of molecule sin sample Convenient to express trace concentrations in ppm Photochemical reactions interactions with electromagnetic radiation caused by reactions Shorter wavelengths greater energy Moles to molecules 1mole=6.02*10^23 molecules 1000 joules= 1 kj Energy of a photon: E=hv, Eenergy, hPlanks constant, vfrequency λv λ c= , c speed of light, v frequency, wavelength Minimum energy needed to break bond = minimum energy of electromagnetic radiation that can break a bond photodissociation breaking of a chemical bond due to absorbance of a photon of electromagnetic radiation photoionizationrequires high energy photons occurs in thermosphere, causes ionization electronically stable molecule with more energy than is normal, unstable, moves back to lower energy level releasing photon or decomposing at high altitudes density is low balance in rate of formation and recombination of electrons is unequal Halogen source gases contain chlorine, bromine and chlorofluorocarbons (CFC) CFC composed of only carbon, chlorine, fluorine ▯ used in refrigeration, air conditioning, propellants, and foam Chlorine used and produced in ozone destruction ▯ leads to more destruction by same chlorine Ozone hole hole like appearance above Antarctic southern hemisphere Polar stratospheric clouds composed of both solid and liquid water, some nitric and sulfuric acid 1 ice crystals, provide surface that allow production of chloride ions at high rate Clouds speed up reaction that would normally slow down the production of chloride ions Chapter 65 acid rain when rain, snow, fog or sleet has pH less than 5.6 sulfur dioxide and nitric acid are primarily responsible Chem 143, Prof Cracolice. Study Guide Exam 2 Smog concentration of smoke and fog, describes polluted air Photochemical smog role of sunlight in initiating chemical change Catalytic converters device that is in exhaust systems of automobiles o has honey comb structure to increase surface area o silicone and aluminum wash coast, gives rougher surface, maximizing surface area o heavy metal, usually platinum, palladium or rhodium, provides surface for molecules to react on Green house effect sun emits energy, half reaches upper limit of atmosphere, half reflected by into space or absorbed by atmosphere Energy radiated from reaching earths surface warms it, forms in infrared radiation Greenhouse gases responsible for green house effect, absorbed infrared energy Kyoto protocol working to reduce green house emissions world wide
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