# Created by C. Ippolito Jan 2007 Acid Base Reactions Objectives: 1.Calculate hydrogen ion and hydroxide ion concentrations 2.Calculate pH values 3.Explain.

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Created by C. Ippolito Jan 2007 Acid Base Reactions Objectives: 1.Calculate hydrogen ion and hydroxide ion concentrations 2.Calculate pH values 3.Explain the action of buffers 4.Explain why indicators change color 5.Describe the relationship between neutralization reactions and titrations 6.Explain the results of hydrolyses of salt solutions 7.Select appropriate indicators for titrations 8.Use normalities in solving titration problems

Created by C. Ippolito Jan 2007 Self-ionization of Water Even “pure” water has some ions present H 2 O + H 2 O H 3 O + + OH - the longer arrow - water is far greater in number than hydronium ions H 2 O H + + OH -  ion product for water or K w  at 25 o is 1.0 x 10 -14 mole 2 /L 2

Created by C. Ippolito Jan 2007 Self-ionization of Water Calculate molar concentration of ions

Created by C. Ippolito Jan 2007 Sample Problem A mass of 4.0 g of NaOH is dissolved in water to form 500 mL of solution with a temperature of 25 o C. What is the hydrogen ion concentration? NaOH(s)  Na + (aq) + OH - (aq) 1.Find number of moles in 4.0 g 2.Find molar concentration of OH - 3.Use ion product for water constant to find molar concentration of H +

Created by C. Ippolito Jan 2007 Calculations

Created by C. Ippolito Jan 2007 The pH of a Solution [H + ] in scientific notation pH simplifies this –pH values are the negative logarithm in base 10 pH = -log [H + ] –if [H + ] = 1x 10 -12 pH = -(log 10 -12 ) = -(-12) = 12 pH = 12 acidsbasesmeasures the relative strength of acids and bases

Created by C. Ippolito Jan 2007 pH Scale acidsacids have pH values below 7 pH[H + ][0H - ] 010 0 or 110 -14 110 -1 10 -13 210 -2 10 -12 310 -3 10 -11 410 -4 10 -10 510 -5 10 -9 610 -6 10 -8

Created by C. Ippolito Jan 2007 pH Scale water is neutral it has a pH of 7 pH[H + ][0H - ] 710 -7

Created by C. Ippolito Jan 2007 pH Scale basesbases have pH values above 7 pH[H + ][0H - ] 810 -8 10 -6 910 -9 10 -5 1010 -10 10 -4 1110 -11 10 -3 1210 -12 10 -2 1310 -13 10 -1 1410 -14 10 0 or 1

Created by C. Ippolito Jan 2007 Calculations Given [H + ] find pH. –[H + ] = 1 x 10 -5 pH = -log [H + ] = -log [1 x 10 -5 ] = -[-5] pH = 5.00 –[H + ] = 3.5 x 10 -8 pH = -log [H + ] = -log [3.5 x 10 -8 ] pH = -(log3.5 + log10 -8 ) pH = -log3.5 – (-8) pH = -.54 + 8.00 = 7.46

Created by C. Ippolito Jan 2007 Calculations Given pH find [H + ]. –pH = 3.00 pH = -log [H + ] 3.00 = -log [H + ] -3.00 = log [H + ] 1.0 x 10 -3 M = [H + ] –pH = 9.55 pH = -log [H + ] -pH = log [H + ] -9.55 = log [H + ] use antilog function 10 x 2.82 x 10 -10 M = [H + ]

Created by C. Ippolito Jan 2007 Buffers mixtures of chemicals that make a solution resist changes in its pH –types of buffers: weak acid and one of its soluble salts –acetic acid and sodium acetate »HC 2 H 3 O 2 ↔H + + C 2 H 3 O 2 - weak base and one of its soluble salts –ammonia and ammonium chloride –when acid added excess negative ions from salt “absorbs” H + to form more of the weak acid –when a base added H + “absorbs” OH - to form water releasing more negative ions of salt

Created by C. Ippolito Jan 2007 Acid Base Indicators Indicators –change color when in different pH solutions IndicatorBelowRangeAbove methyl orangered3.2-4.4orangeyellow bromthymol blueyellow6.0-7.6blueblue bromcresol greenyellow3.8-5.4greenblue litmusred5.5-8.2redblue phenolphthaleincolorless8.2-10colorlesspink thymol blueyellow8.0-9.6blueblue

Created by C. Ippolito Jan 2007 Titration laboratory procedure to determine concentration of acid/base –a known concentration acid/base is added to the unknown concentration acid/base to neutralize the unknown Standard SolutionStandard Solution –the known concentration acid/base End Point (Equivalence Point) –indicator color change “show” neutralization –recording the volume of the “known” needed to “neutralize” a fixed volume of the “unknown” allows calculation of the concentration of the “unknown”

Created by C. Ippolito Jan 2007 Titration Problems Sample Problem: –20.0 mL solution of strontium hydroxide Sr(OH) 2 with indicator is neutralized when 25 mL of 0.0500 M HCl is added. What is the concentration of Sr(OH) 2 ? –TO SOLVE: write and balance equation to see mole relationship find molar concentration using mole relationships –moles of acid = volume of acid (V a ) x molar concentration of acid (M a ) –moles of base = volume of base (V b ) x molar concentration of base (M b ) if necessary convert ml to L

Created by C. Ippolito Jan 2007 Calculations Equation –HCl + Sr(OH) 2  H 2 O + SrCl 2 –2HCl + Sr(OH) 2  2H 2 O + SrCl 2 Mole Relationship –# of moles of acid is twice the # of moles of base –moles of acid = 2(moles of base) Concentration Equation –V a M a = 2V b M b solve for M b Known: –V a = 25 mL =.0250 L –M a = 0.0500 M –V b = 20 mL =.0200 L

Created by C. Ippolito Jan 2007 Calculations (con’t)

Created by C. Ippolito Jan 2007 Hydrolysis of Salts Hydrolysis –reaction of a substance with water Salts –ionic compounds –from neutralization of acids/bases Neutral Salt Solution –no reaction with water Acidic Salt Solution –reacts with water to produce H + ions Basic Salt –reacts with water to produce OH - ions

Created by C. Ippolito Jan 2007 Hydrolysis of Salts (con’t) Salt of Strong Base and Weak Acid –NaC 2 H 3 O 2 (sodium acetate) formed in neutralization –strong base (NaOH) with weak acid (HC 2 H 3 O 2 ) in water NaC 2 H 3 O 2  Na + + C 2 H 3 O 2 - –the Na + does not attract OH - from water –but the C 2 H 3 O 2 - does attract H+ from water this leads to a slight excess of OH - hydrolysis of this salt causes a basic solution to form

Created by C. Ippolito Jan 2007 Hydrolysis of Salts (con’t) Salt of Weak Base and Strong Acid –NH 4 Cl (ammonium chloride) formed in neutralization –weak base (NH 3 ) with strong acid (HCl) in water NH 4 Cl  NH 4 + + Cl - –the Cl - does not attract from H+ water –but the NH 4 + does donate a proton to water »NH 4 + + H 2 O  H 3 O + + NH 3 this leads to a slight excess of H + hydrolysis of this salt causes a acidic solution to form

Created by C. Ippolito Jan 2007 Gram Equivalent Mass gram equivalent mass of an acid gram equivalent mass of an base

Created by C. Ippolito Jan 2007 Normality makes acid base titration calculations easier Normality –Problem: A solution of 500mL is made with 98 g of H 2 SO 4. What is its normality?

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