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Acid and Base

Acid and Base. ( 酸和鹼 ). Dr. Wing Fat Chan wfchan@cuhk.edu.hk Department of Chemistry The Chinese University of Hong Kong. Reference Material for 5th IJSO Training Internal Use Only – Do not duplicate or distribute. How much do you know about acid and base?.

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Acid and Base

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  1. Acid and Base (酸和鹼) Dr. Wing Fat Chan wfchan@cuhk.edu.hk Department of Chemistry The Chinese University of Hong Kong Reference Material for 5th IJSO Training Internal Use Only – Do not duplicate or distribute

  2. How much do you know about acid and base? Have you watched the ETV science program “Acid & Base” before?

  3. Definitions of Acid and Base (酸和鹼的定義)

  4. I. Arrhenius Theory (1884) (Swedish) (阿列紐斯學說) Acids - are substances that dissociate in water to give hydrogen ions. HCl (aq) H+(aq) + Cl-(aq) Bases - are substances that dissociate in water to give hydroxide ions. NaOH (s) Na+(aq) + OH-(aq) Arrhenius Acid-base Reaction: Acid + Base  Salt + Water

  5. (Bronsted – Danish) II. Bronsted-Lowry Theory (1923) (Lowry – English) (布侖斯特-路里學說) Acids - are substances that donate protons Bases - are substances that accept protons Bronsted-Lowry Acid-base Reaction: HCl (g) + H2O (l) H3O+(aq) + Cl-(aq) Conjugate Acid Conjugate Base Base Acid Conjugate Acid-Base Pair: consists of two substances related to each other by accepting or donating a proton (共軛酸鹼對)

  6. III. Lewis Theory (1923) (American) (路易斯學說) Acids - are electron-pair acceptors Bases - are electron-pair donors Example of Lewis Acid-Base Reaction: Ag+ + 2 :NH3 [ H3N: Ag :NH3 ]+ Base Acid

  7. Acids: Hydrochloric acid(HCl) Sulphuric acid(H2SO4) Nitric acid(HNO3) Phosphoric acid(H3PO4) (氫氯酸) (硫酸) Mineral acids (硝酸) (磷酸) Ethanoic acid (acetic acid) Citric acid Tartaric acid Oxalic acid (醋酸) Organic Acids (檸檬酸) (酒石酸) (草酸)

  8. Concentration Dilute Acid Concentrated Acid Concentration of acid or base can sometimes affect very much its properties. The usual molarity of three common mineral acids in the laboratory

  9. Concentration Unit Molarity: (摩爾濃度) One mole (摩爾) of substance is the amount corresponding to its formula mass expressed in gram unit. e.g. one mole of CO2 = ? g = 12 + 16 x 2 = 44 g

  10. Calculate the amount of NaCl needed to prepare 100 mL of 1M NaCl solution. (Atomic masses of Na: 23 and Cl: 35.5)

  11. Properties of Dilute Acids • a sour taste(never taste any substance in the laboratory) • turnbluelitmus paper tored • conduct electricity • react with metals higher than copper in the reactivity series and give hydrogen gas • react with metal oxides and hydroxides and give salt and water • react with carbonates and hydrogencarbonates and give salt, carbon dioxide and water

  12. CH3COOH (aq) H+(aq) + CH3COO-(aq) H2SO4(aq) 2H+(aq) + SO42-(aq) H3PO4(aq) 3H+(aq) + PO43-(aq) H2CO3(aq) 2H+(aq) + CO32-(aq) H2C2O4(aq) 2H+(aq) + C2O4-(aq) Basicity (鹽基度)of an acid: the number of ionizable hydrogen atom in one molecule of the acid. e.g. HCl (aq) H+(aq) + Cl-(aq) monobasic HNO3(aq) H+(aq) + NO3-(aq) dibasic tribasic

  13. Properties of Concentrated Acids • highly corrosive Conc. HNO3: • strong oxidizing agent Mg (s) + 2NO3-(aq) + 4H+ Mg2+ + 2NO2(g) + 2H2O (l) C (s) + HNO3(aq) CO2(g) + 4NO2(g) + 2H2O (l) Conc. H2SO4: (吸濕性) • hygroscopic properties • dehydrating properties • strong oxidizing agent (脫水性) conc. H2SO4 C12H22O11(s)12C (s) + 11H2O (l) Cu (s) + 2H2SO4(l) CuSO4(s) + SO2(g) + 2H2O (l)

  14. Bases: Metal oxides Metal hydroxides Ammonia Alkalis : bases that are soluble in water Common alkalis: Sodium hydroxideNaOH Potassium hydroxide KOH Calcium hydroxideCa(OH)2 Ammonia NH3

  15. Properties of Aqueous Alkalis • usually bitter taste(never try to check this yourself) • a soapy feel(conc. KOH & NaOH attack and burn the skin) • turn red litmus paper to blue • conduct electricity • react with acid to give salt and water • react with carbon dioxide to give carbonate and water • react with metal ions in aqueous solutions to give insoluble metal hydroxides e.g. Ca(OH)2(aq) + CO2(g) CaCO3(s) + H2O (l) (limewater test) Cu+2(aq) + 2OH-(aq) Cu(OH)2(s) Mg+2(aq) + 2OH-(aq) Mg(OH)2(s)

  16. Strength (強度) of Acids and Bases Strong acid - fully or highly ionizes in water HCl (aq) H+(aq) + Cl-(aq) e.g. HNO3. H2SO4 Weak acid - ionizes only slightly in water CH3COOH (aq) H+(aq) + CH3COO-(aq) e.g. H2CO3, most organic acids

  17. Strength of Acids and Bases Strong base - fully or highly ionizes in water NaOH (aq) + water  Na+(aq) + OH- (aq) e.g. KOH, Ca(OH)2 Weak base - ionizes only slightly in water NH3(aq) + H2O (l) NH4+(aq) + OH-(aq)

  18. Don’t mix up the terms strengths and concentration for acids and bases! e.g. 1M HCl strongdilute 12M HCl strongconcentrated 1M CH3COOH weakdilute 12M CH3COOH weakconcentrated

  19. Safely Handling of Acid and Base: How to prepare dilute acid or base solution from the concentrated solution? Never add water to the concentrated solution andadd slowly stirring the concentrated solution to water in order to avoid splashing of solution due to local heating Always handle concentrated HCl and HNO3 in the fume hood as corrosive fumes will come out. What to do if any of the concentrated acid or base is split on skin? Wash immediately with plenty of water on the affected skin and then looking for any medical treatment!

  20. H2O (l) + H2O (l) H3O+(aq) + OH-(aq) (酸鹼兩性的) Amphoteric Substance: - Substance that behaves either as acid or as base. e.g. At 25oC, [H3O+] = [OH-],= 1.0 x 10-7 M Water ion-product constant Kw = [H3O+] x [OH-] = 1.0 x 10-14 M2 In all aqueous solutions, Kw = 1.0 x 10-14 M2 When [H3O+]= [OH-], it is neutral. When [H3O+]>[OH-], it is acidic. When [H3O+]<[OH-], it is basic or alkaline.

  21. Acidity and Alkalinity (酸鹼度) How can we compare the acidity or alkalinity of a solution? pH value - is equal to –log [H3O+] [H3O+] = [OH-],= 1.0 x 10-7 M For water at 25oC, pH=-log [1.0 x 10-7] = 7 (neutral) When an acid is added to the water, [H3O+]>[OH-], pH < 7 (acidic) When a base is added to the water, [H3O+]<[OH-], pH > 7 (basic)

  22. Calculate the pH values for the following solutions, • 0.1M HCl • 1M HCl • 0.1M NaOH • 1M NaOH (i) = –log [0.1] = – (–1) = 1 (ii) = –log [1] = – 0 = 0 (iii) = –log [(1.0 x 10-14)/0.1] = – (–13) = 13 (iv) = –log [(1.0 x 10-14)/1] = – (–14) = 14 Kw = [H3O+] x [OH-] = 1.0 x 10-14 M2 Is pH value of 0.1M CH3COOH equal to that of 0.1M HCl?

  23. pH scale and the pH values of some common substances Alkalinity increases Acidity increases 1M NaOH 1M HCl 0.1M NaOH 0.1M HCl Limewater lemon juice NaCO3 solution 0.1M CH3COOH Milk of Magnesia vinegar Household Ammonia soft drinks rain water NaHCO3 solution pure water

  24. How to measure the pH value of solution? (石蕊試紙) Red & blue litmus papers - test only acidic or basic Acid-base indicators (酸鹼指示劑) (酚酞)

  25. Acid-base indicators (百里酚藍) (甲基橙) Only get a rough idea of the acidity or alkalinity of a solution!

  26. (通用指示劑) Universal indicators • a mixture of several indicators • different colors at different pH • in solution or paper ( pH paper) form (pH 計) pH meter Exact pH value: Video

  27. Applications of Acids and Base to Dairy Life (I) Control Soil pH: For acidic soil: (due to acid rain or extensive uses of acidic fertilizers) - add powdered limestone CaCO3 or slaked lime Ca(OH)2 For alkaline soil: - add alums AlK(SO4)2.12H2O or acidic fertilizers (NH4)2SO4

  28. Applications of Acids and Base to Daily Life (II) Neutralization of Excess Acid in Stomach: - excess acid in stomach causing stomach pain Milk of Magnesia (鎂奶)can help to neutralize the excess acid. Mg(OH)2(s) + 2HCl (aq)  MgCl2(aq) + 2H2O (l) weak base

  29. Applications of Acids and Base to Dairy Life (III) Treatment of Insect Sting Relief of uneasy feeling due to insect stings: Acidic ant and Bee Stings: - using a weakly alkaline solution to swollen part Alkaline wasp stings - alkaline - neutralize with a weakly acidic a solution.

  30. Neutralization (中和作用) neutralization acid + base salt + water HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) e.g. Na+(aq) + Cl-(aq) Na+(aq) + OH-(aq) H+(aq) + Cl-(aq) H+(aq) + OH-(aq) H2O (l) Insoluble base: e.g. CuO (s) + H2SO4(aq) CuSO4(aq) + H2O (l) CuO (s) + 2H+ (aq) Cu2+(aq) + H2O (l) H+(aq) + O2-(s) H2O (l) Neutralization is an exothermic reaction

  31. Neutralization Titration: (中和作用滴定法) - to analyze quantitatively the amount of acid or base in a sample solution Procedure: • Preparation of standard solution • Add standard solution (titrant) to the sample solution (analyte) or vice versa • Determine the volume of titrant added at the equivalent point (等效點) Equivalent point for neutralization titration: - occurs when equal amounts of acid and base are reacted.

  32. Strong acid titrated with strong base Equivalent point pH Volume of titrant added Titration Curve: (滴定曲線)

  33. How to determine the equivalent point? (i) Acid/base Indicators Fountain experiment: Video 1 Video 2 Video 3

  34. Color Change for Indicators Strong acid titrated with strong base  Phenolphthalein pH Equivalent point  Methyl orange  Thymol blue Volume of titrant added

  35. Color Change for Indicators Weak acid titrated with strong base  Phenolphthalein pH Equivalent point  Methyl orange Volume of titrant added Using indicators, we can only detect the endpoint and not the equivalent point. (Endpoint: an observable physical change at or near the equivalent point)

  36. Plot the graph for: Weak base titrated with strong acid  Phenolphthalein pH Equivalent point  Methyl orange Volume of titrant added

  37. Titration: • Preparation of standard solution Video (ii) Transfering a fixed amount of solution Video (iii) Performing the titration Video

  38. Strong acid titrated with strong base Equivalent point pH Volume of titrant added (ii) pH meter

  39. Titration Calculations: 0.1000 M NaOH was used to titrate with 25.00 mL HCl solution of unknown concentration. 28.25 mL of NaOH was needed to reach the endpoint. What is the molarity of the HCl solution? HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) 1 mole 1 mole Remember: Mole = (Molarity).(Litres of solution) = C.V

  40. HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) 1 mole : 1 mole C2V2 C1V1 Calculation: The molarity of HCl solution is 0.1130 M.

  41. (i) What happens if H2SO4 is used instead of HCl? Equation: H2SO4(aq) + 2NaOH (aq) Na2SO4(aq) + 2H2O (l) 1 mole : 2 mole C2V2 C1V1 (ii) What happens if Na2CO3 is used instead of NaOH? 2HCl (aq) + Na2CO3(aq) 2NaCl (aq) + CO2(g) + H2O (l) 2 mole : 1 mole

  42. Buffer solutions (緩衝溶液) - solutions that resist sharp change in pH with the addition of small amount of acid or base Applications: • (i) The pH of blood plasma is buffered around pH 7.35-7.45 due to H2CO3 and HCO3-. • Buffer solutions can help to keep enzymes working in organisms • Fermentation processes • Colouring of fabric needs a correct pH condition. How does the buffer solution work?

  43. HA (aq) + H2O (l) H3O+(aq) + A-(aq) Recall: Bronsted-Lowry Theory for Acid and Base Reaction HCl (g) + H2O (l) H3O+(aq) + Cl-(aq) Conjugate Acid Conjugate Base Acid Base Buffer solution • Consists ofweak acidand itsconjugate base(more common) • orweak baseand itsconjugate acid(less common) e.g. Conjugate Base Acid

  44. HA (aq) + H2O (l) H3O+(aq) + A-(aq) CH3COOH (aq) + H2O (l) H3O+(aq) + CH3COO-(aq) Conjugate Base Acid Addition of base: H3O+ is consumed. The equilibrium of system will shift to therightto produce more H3O+. Addition of acid: A-is protonated to give HA. The equilibrium of system will shift to theleft and the pH of the system won’t change much. HA can only produce small amount of A-. In order to have enough A- in the solution, soluble salt of A- has to be added. Acetic acid / sodium acetate e.g.

  45. Acid Rain: (酸雨) - defined as any type of precipitation with an unusual low pH (generally withpH < 5) - formed when the oxides of nitrogen (NOx) and sulphur (SO3) dissolve in water droplets in the air. Where did those oxides come from? Mainly from: Power Plants Automobile Emissions

  46. NOx (g) + H2O (l) + O2(g)   HNO3(aq) SO3(g) + H2O (l)  H2SO4(aq) Effects of Acid Rain • cause severe damage to aquatic environmemt • cause death of leaves, needles and sometimes trees • cause deterioration of buildings and monuments • affect the human health from particulates in air

  47. CaCO3(aq) + H2O (aq) HCO3-(aq) + Ca2+(aq) Solutions to acid rain • To remove the oxides of sulphur from coal-burning emissions by installing scrubbers • To dump the powdered limestone to the lake to increase its buffer capacity • (iii) To reach international agreement on the reduction of sulphur emissions

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