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Published byClement Higgins Modified over 9 years ago
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Acids and Bases
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What are acids and bases? Arrhenius Acids Hydrogen-containing compounds that ionize to yield hydrogen ions (H + ) in aqueous solutions Arrhenius Bases Compounds that ionize to yield hydroxide ions (OH - ) in aqueous solutions
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What are acids and bases? Bronsted-Lowry Acid A hydrogen-ion donor Bronsted-Lowry Base A hydrogen-ion acceptor
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What are acids and bases? All acids and bases in the Arrhenius theory are also acids and bases based on Bronsted-Lowry theory. Bronsted-Lowry includes some bases not included in the Arrhenius theory. Ex: Ammonia (NH 3 )
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What are acids and bases? Lewis Acid Substance that can accept a pair of electrons to form a covalent bond Lewis Base Substance that can donate a pair of electrons to form a covalent bond
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What are acids and bases? Summary of Acid-Base Definitions TheoryAcidBase ArrheniusH + producerOH - producer Bronsted- Lowry H + donorH + acceptor LewisElectron-pair acceptor Electron-pair donor
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Strong vs. Weak Acids and Bases Strong acids – completely ionised in aqueous solution Ex: HCl; HNO 3 ; H 2 SO 4 Weak acids – ionise only slightly in aqueous solution Ex: Acetic acid – 1% of acetic acid molecules ionised at any instant
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Strong vs. Weak Acids and Bases Strong bases – dissociate completely into metal ions and hydroxide ions in aqueous solution Ex: Ca(OH) 2 ; NaOH; KOH Weak bases – react with water to form hydroxide ion and the conjugate acid of the base (No OH in formula) Ex: CH 3 NH 2, NH 3
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Naming Acids Single Element: Hydro_____ic acid Ex: HCl = Hydrochloric acid Polyatomic Ion: ATEic ITEous Ex: H 2 SO 4 = sulfuric acid Ex: H 2 SO 3 = sulfurous acid
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Naming Bases Bases are named the same way as any other ionic compound Ex: KOH = potassium hydroxide
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Hydrogen Ions from Water Water that LOSES a hydrogen ion becomes a negatively charged hydroxide ion (OH - ) Water that GAINS a hydrogen ion becomes a positively charged hydronium ion (H 3 O + )
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Dissociation of Water Self-ionisation of water: reaction in which TWO water molecules produce ions Ex: H 2 O + H 2 O H 3 O + + OH - Can also be written as a DISSOCIATION: Ex: H 2 O (l) H + (aq) + OH - (aq)
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Dissociation of Water In water or aqueous solution, hydrogen ions (H + ) are joined to water molecules to form hydronium ions (H 3 O + ) H + and H 3 O + are both used to represent hydrogen ions in aqueous solution
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Concentrations Acidity or basicity of a solution is discussed in terms of the concentration of hydrogen ions, [H + ], or the concentration of hydroxide ions, [OH - ] Acidic: [H + ] > [OH - ] Basic (Alkaline): [H + ] < [OH - ] Neutral: [H + ] = [OH - ]
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pH Acidity is measured in pH pH = -log[H + ] Acidic: pH 1 x 10 -7 M Basic: pH > 7;[H + ] < 1 x 10 -7 M Neutral: pH = 7;[H + ] = 1 x 10 -7 M
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pOH Basicity could be measured in a similar manner called pOH pOH = -log[OH - ] pH + pOH = 14
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Measuring pH Acid-Base Indicators An indicator’s acid and base form have different colors in solution Limitations: usually work at 25°C pH Meters Ex: PASCO Probes Make rapid, accurate pH measurements Must be calibrated – put into solution of known pH
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Titrations Neutralisation reaction: hydronium ions combine with hydroxide ions to form water An indicator can be used to show when the neutralisation is complete
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Titrations If the mole ratio is known, you can calculate the concentration of a given acid or base n A A + n B B products This means that n A moles of A reacts with n B moles of B
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To work out concentration C A (moldm -3 ) of unknown solution A of volume V A (dm -3 ) at the end of the titration: C A x V A = nA C B x V B = nB E.g. C A = nA X C B x V B nB x V A
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