Understanding Conjugate Acids and Bases: Strong, Weak, and Indicators, Study Guides, Projects, Research of Chemistry

Download Understanding Conjugate Acids and Bases: Strong, Weak, and Indicators and more Study Guides, Projects, Research Chemistry in PDF only on Docsity! 1 Conjugate acids and bases Different definitions of acids and bases • Arrhenius definition • acids generate H3O+ in water • bases generate OH- in water • Brønsted Lowry definition • Acids are proton donors • Bases are proton acceptors • which is an acid/base? • HF + H2O  H3O+ + F- • CH3NH2 + H2O  CH3NH3 + + OH- • By Arrhenius, HF is an acid, is a CH3NH2 base. Follow the proton • HF + H2O  H3O+ + F- • CH3NH2 + H2O  CH3NH3 ++OH- • What about the reverse reaction? H+ H+ H+ H+ 2 Conjugate acids and bases • When you run the reverse reaction you find the products are also acids and bases. The acids and bases that are formed are called conjugate acids or bases • H2O + HF  H3O+ + F- • base acid conjugate acid conjugate base • CH3NH2 + H2O  CH3NH3 + +OH- • base acid CA CB Label Acid, Base, Conjugate Acid, Conjugate Base • HClO3 + H2O  ClO3 - +H3O+ • • ClO- + H2O  HClO + OH- • HSO4 - + H2O  SO4 2- +H3O+ • NH3 + H2O  NH4 + + OH- • Label Acid, Base, Conjugate Acid, Conjugate Base • HClO3 + H2O  ClO3 - +H3O+ • A B CB CA • ClO- + H2O  HClO + OH- • B A CA CB • HSO4 - + H2O  SO4 2- +H3O+ • A B CB CA • NH3 + H2O  NH4 + + OH- • B A CA CB 5 Weak acids and bases • can be forced the other way • So ammonia… • NH3 + H2O NH4 ++OH- • Ammonia is a gas with a distinct odor • Ammonium and hydroxide are both odorless. • If base is added to the solution you will smell ammonia, if hydroxide is removed you won’t smell anything. Pet “Stain” Problem • Urine has ammonia in it. • Most cleansers are basic. • After cleaning, we still leaves small amounts behind. • If it is small amount of ammonia and a basic cleanser the equilibrium will be shifted to the ammonia side so some thing with a great sense of smell (dog) could pick it up. • A slightly acidic cleanser shifts the equilibrium to the ammonium side to solve this problem Other weak acids and bases • Weak Acids – Acetic Acid (vinegar) – Citric Acid – Ascorbic Acid (vitamin C) – Boric Acid – Carbonic Acid • Weak Bases – Sodium Bicarbonate – Ammonia – Sodium Hypochlorite (bleach) 6 Indicators • Indicators are a substance that change color in the presence of (whatever they check for) • They do this because of Le Châtelier’s principle. All you need an equilibrium reaction with different colored products and reactants. • The pen used to check for counterfeit money is a starch indicator How an acid base indicator works • A generic indicator will follow this reaction, HID is the reactant indicator, and ID- is its product • [HID] + H2O ⇌ H3O+ + [ID]- • The color differences are important, HId is one color and Id- is a different color! • in an acidic solution (high H3O+) you see reactant • [HID] + H2O ⇌ H3O+ + [ID]- • in a basic solution (low H3O+) you see product • H[ID] + H2O ⇌ H3O+ + [ID]- Acid Base indicators • Acid base indicators change color at certain pH levels • They don’t have to change at 7 (most don’t) • Universal indicator solution (phenolphthalein, bromthymol blue and methyl red dissolved in ethanol and water) changes color at each integral pH value 7 Other pH indicators • Litmus and phenolphthalein are indicators • Red cabbage juice has a pigment that changes colors at different pH values Buffers • Buffers are solutions that don’t change in pH when acids or bases are added. • They use weak acids/bases and Le Châtelier’s principle. • You will have a large amount of weak acid and conjugate base • WA = weak acid • HWA + H2O  H3O+ + WA- pH • pH depends on the concentration of hydronium • pH = -log [H3O+] • Concentration of hydronium is the ratio of solute to solvent or in this case H3O+ / H2O