Chemistry - Acids and Bases - Quick Guides

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Study GuideChemistry-Acids and Bases1. Quiz: The pH Scale1. QuestionThe neutral value on the pH scale isAnswer Choices•less than 7•greater than 7•equal to 7Correct Answerequal to 7Why This Is CorrectA neutral solution has equal concentrations of hydrogen ions (H⁺) and hydroxide ions (OH⁻). On thepH scale, this balance corresponds to a pH of 7. Pure water at 25°C is neutral and has a pH of 7.2. QuestionWater dissociates into H⁺and OH⁻ionsAnswer Choices•to a large degree•to a small degree•not at all

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Study GuideCorrect Answerto a small degreeWhy This Is CorrectOnly a very small fraction of water molecules dissociate into ions at any given time. This limiteddissociation is why pure water is a poor conductor of electricity.3. QuestionThe value of the ionization constant for water, Kw isAnswer Choices•1.0 × 10⁻⁷•1.0 × 10¹⁴•1.0 × 10⁻¹⁴Correct Answer1.0 × 10⁻¹⁴Why This Is CorrectKw is defined as the product of the concentrations of H⁺and OH⁻ions in water. At 25°C, this constantvalue is always 1.0 × 10⁻¹⁴.4. QuestionThe pH scale isAnswer Choices•a logarithmic scale•a linear scale•an exponential scale

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Study GuideCorrect Answera logarithmic scaleWhy This Is CorrectEach change of one pH unit represents a tenfold change in hydrogen ion concentration. Because it isbased on powers of 10, the pH scale is logarithmic.5. QuestionWhat is the [H⁺] of a solution if the pH is 3.5?Answer Choices•3.16 × 10⁴•3.16 × 10⁻⁴•3.5 × 10⁻⁴Correct Answer3.16 × 10⁻⁴Why This Is CorrectThe hydrogen ion concentration is calculated using the formula:pH = −log[H⁺]So, [H⁺] = 10⁻³⋅⁵= 3.16 × 10⁻⁴ M.6. QuestionA solution has a [H⁺] = 7.0 × 10⁻¹¹. What is its pH?Answer Choices•10.15•−10.15•11.00

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Study GuideCorrect Answer10.15Why This Is CorrectUsing pH = −log[H⁺]:pH = −log(7.0 × 10⁻¹¹) ≈ 10.15.A low hydrogen ion concentration corresponds to a high pH.7. QuestionA solution has a [OH⁻] = 1.3 × 10⁻³. This solution would be classified asAnswer Choices•acidic•basic•neutralCorrect AnswerbasicWhy This Is CorrectA high hydroxide ion concentration indicates a basic solution. Since [OH⁻] is much greater than 1.0 ×10⁻⁷, the pH will be above 7.8. QuestionBases have a pHAnswer Choices•less than 7•equal to 7•greater than 7

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Study GuideCorrect Answergreater than 7Why This Is CorrectBasic solutions contain more hydroxide ions than hydrogen ions, resulting in a pH above 7 on the pHscale.9. QuestionAn acidic solution hasAnswer Choices•a greater concentration of hydroxide ions than hydrogen ions•a greater concentration of hydrogen ions than hydroxide ions•equal concentrations of hydrogen ions and hydroxide ionsCorrect Answera greater concentration of hydrogen ions than hydroxide ionsWhy This Is CorrectAcids release hydrogen ions in solution. A higher [H⁺] compared to [OH⁻] is the defining characteristicof an acidic solution.10. QuestionA solution has a hydrogen ion concentration of 3.7 × 10⁻². What is its hydroxide ion concentration?Answer Choices•2.7 × 10⁻¹³•3.7 × 10⁻¹⁶•3.7 × 10¹²

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Study GuideCorrect Answer2.7 × 10⁻¹³Why This Is CorrectUsing the equation Kw = [H⁺][OH⁻]:[OH⁻] = (1.0 × 10⁻¹⁴) ÷ (3.7 × 10⁻²) ≈ 2.7 × 10⁻¹³ M.2. Strong and Weak Acids2.1What Happens When Substances Dissolve in Water?When a substance is placed in water, it can behave in different ways:•Some substancesbreak completely into ions.•Somedo not break at all.•Othersbreak only partially.Understanding this behavior helps us classify substances asstrong electrolytesorweakelectrolytes.2.2Strong Electrolytes: Complete DissociationSubstances thatcompletely separate into ions in waterare calledstrong electrolytes.Because they produce many ions, their solutions canconduct electricity very well.•Mostionic compoundsbehave this way.•A common example issodium chloride (NaCl).2.3Compounds with Both Ionic and Covalent BondsSome compounds containboth ionic and covalent bonds.A good example issodium carbonate (Na₂CO₃).

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Study GuideWhy is Na₂CO₃a strong electrolyte?•In water, each formula unit of Na₂CO₃completely dissociatesinto three ions:oTwo sodium ions (Na⁺)oOne carbonate ion (CO₃²⁻)Dissociation equation:•Thesodium ionsare free in solution.•Thecarbonate ion stays intact internallybecause its atoms are held together bycovalentbonds.Figure1.Ionic and covalent bonding in Na2CO3.2.4Weak Electrolytes: Partial DissociationSome substances containpolar covalent bondsandonly partially dissociatein water.These are calledweak electrolytes.An example issulfurous acid (H₂SO₃).Equilibrium reaction:•Most of the solution remains asundissociated H₂SO₃molecules.•Only a small number ofionsare formed.

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Study Guide2.5Strong vs Weak Electrolytes: Key Difference•Strong electrolytes→ completely dissociate into ions.•Weak electrolytes→ only partially dissociate.This difference becomes very important when studyingacids and bases.2.6Strong Acids and Weak AcidsAcids are classified asstrong or weakbased on how much they ionize in water.The general dissociation of an acid is written as:•HA = acid molecule•H⁺= hydrogen ion•A⁻= conjugate base (anion)2.7Acid Ionization Constant (Ka)The extent to which an acid ionizes is measured by theacid ionization constant, calledKa.Ka expression:•Large Ka→ strong acid (more ionization)•Small Ka→ weak acid (less ionization)2.8Common Acids and Their Ka ValuesDifferent acids have very different Ka values.

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Study GuideWhat the table shows:•Strong acids likeHCl, HNO₃, and H₂SO₄havevery large Ka values.•Weak acids likeacetic acid and carbonic acidhavesmall Ka values.•Every acid producesH₃O⁺(hydronium ions)in solution.2.9Calculating Ion Concentrations: Example with Carbonic AcidLet’s look at a1 M solution of carbonic acid (H₂CO₃).Equilibrium reaction:Letx= amount of H₂CO₃that dissociates.Ka expression:x² / (1 − x) = 4.3 × 10⁻⁷Because carbonic acid is weak,x is very small, so:

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Study Guidex² ≈ 4.3 × 10⁻⁷x = 6.56 × 10⁻⁴ M•This is thehydronium ion concentration•pH =3.182.10Organic Acids: Carboxylic AcidsCarboxylic acids contain a–COOH functional group.•Onlyone hydrogenin this group can dissociate.•As a result, these acids areweak acids.Examples:•Acetic acid•Propionic acidSummary•Strong acidsdissociate almost completely.oExample: 0.5 M HCl → [H₃O⁺] ≈ 0.5 M•Weak acidsdissociate only slightly.oTheir ion concentrations must be calculated usingKa.•Thesmaller the Ka, theweaker the acid.

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