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Acids and Bases

Acids and Bases. Acids and bases are some of the most important industrial compounds on Earth ACIDIC SOLUTIONS: taste sour Carbonic and phosphoric acid give carbonated drinks their sharp taste Citric acid and ascorbic acid give lemons and grapefruit their mouth-puckering tartness

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Acids and Bases

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  1. Acids and Bases • Acids and bases are some of the most important industrial compounds on Earth • ACIDIC SOLUTIONS: • taste sour • Carbonic and phosphoric acid give carbonated drinks their sharp taste • Citric acid and ascorbic acid give lemons and grapefruit their mouth-puckering tartness • Acetic acid makes vinegar taste sour

  2. Acids and Bases • Basic solutions taste sour and feel slippery • Think of soap slipping out of your hands in the shower • The litmus in litmus paper is one of the dyes commonly used to distinguish solutions of acids and bases (turns red = acid; turns blue = base) • Acid and base solutions conduct electricity

  3. Ions in Solution • Why are some aqueous solutions acidic, others basic, and others neutral? • All water (aqueous) solutions contain hydrogen ions (H+) and hydroxide ions (OH-) • The relative amounts of the two ions determines whether an aqueous solution is acidic, basic, or neutral • An acidicsolution contains more hydrogen ions than hydroxide ions • A basicsolution contains more hydroxide ions than hydrogen ions

  4. Ions in Solution • What do you think a neutral solution contains? • The usual solvent for acids and bases is water • Water produces equal numbers of H+ ions and OH- ions in a process known as self-ionization • Self-ionization: two water molecules react to form a hydronium ion (H3O+) and a hydroxide ion according to its equilibrium H2O(l) + H2O(l) H3O+(aq) + OH-(aq) Water molecules Hydronium Hydroxide ion ion

  5. Ions in Solutions • Simplified version of the equation for self-ionization of water is: H2O (l) H+ (aq) + OH- (aq) • ARRHENIUS MODEL: states that an acid is a substance that contains hydrogen and ionizes to produce hydrogen ions in aqueous solution HCl(g) →H+(aq) + Cl-(aq)

  6. Bronsted-Lowry Model • Bronsted-Lowry Model: states that an acid is a hydrogen-ion donor and a base is a hydrogen-ion acceptor HX(aq) + H2O(l) H3O+(aq) + X-(aq) • The water molecule accepts a H+ ion and becomes an acid • On donating its H+ ion, the acid HX becomes a base X-

  7. Conjugates • Conjugate acid: species produced when a base accepts a hydrogen ion from an acid • Conjugate base: species that results when an acid donates a hydrogen ion to a base HX(aq) + H2O(l) H3O+(aq) + X-(aq) Acid Base Conjugate Acid Conjugate Base

  8. Conjugates NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) • Amphoteric: substances that can act as both an acid or base • Monoprotic acid: an acid that donate only one hydrogen ion • Polyprotic acid: an acid that can donate more than one hydrogen ion Conjugate Base Base Acid Conjugate Acid

  9. Equilibrium Constant • Equilibrium constant (Keq): the numerical value of the ratio of product concentration to reactant concentrations, with each concentration raised to the power corresponding to its coefficient in the balanced equation aA + bB cC + dD • A and B are reactants; C and D the products. the coefficients in the balanced equation are a, b, c and d [C]c[D]d [A]a[B]b Keq =

  10. Strengths of Acids and Bases • Acids that ionize completely are called strong acids. • Because strong acids produce the maximum number of ions, they are good conductors of electricity • Weak acid: an acid that ionizes only partially in dilute aqueous solution • Weak acids cannot conduct electricity as efficiently because they produce fewer ions

  11. Strengths of Acids and Bases • Ionization equation for hydrocyanic acid: HCN(aq) + H2O(l) H3O+(aq) + CN-(aq) [H3O+][CN-] [HCN][H2O] • The concentration of liquid water in the denominator is constant, so it can be combined with Keq to give a new equilibrium constant Ka [H3O+][CN-] [HCN] Keq = Keq [H2O] = Ka =

  12. Strengths of Acids and Bases • Ka is called the acid ionization constant • The smaller the Ka, the weaker the acid • Strength of Bases: • Strongbases dissociate entirely into metal ions and hydroxide ions Ca(OH)2(s) Ca2+(aq) + 2OH-(aq) • Weakbase: ionizes only partially in dilute aqueous solution to form the conjugate acid of the base and hydroxide ion

  13. Strength of Bases CH3NH2(aq) + H2O(l) CH3NH3+(aq) + OH-(aq) Base ionization constant: Kb = [CH3NH3+][OH-] [CH3NH2] • Kb is called the base ionization constant • The smaller the value of Kb, the weaker the base Conjugate Acid Conjugate Base Base Acid

  14. pH and pOH • The pH of a solution is the negative logarithm of the hydrogen ion concentration pH = -log[H+] • Acids have a pH below 7 • Base solutions have a pH above 7 • Neutral solutions have a pH = 7 • Using pOH: The pOH of a solution is the negative logarithm of the hydroxide ion concentration pOH = -log[OH-]

  15. pH and pOH • pH + pOH = 14.00 • Knowing the value of one enables you to find the other • Suppose the pH of a solution is 3.50 and you must determine the concentrations of H+ and OH-. pH = -log[H+] • First you need to multiply both sides of the equation by -1 -pH = log[H+]

  16. pH and pOH • To calculate [H+] using this equation you must take the antilog of both sides of the equation. • antilog(-pH) = [H+] • To calculate [H+], substitute 3.50 for pH in the equation • antilog(-3.50) = [H+] • Use a log table or your calculator to determine the antilog of -3.50. The antilog is 3.2 x 10-4. [H+] = 3.2 x 10-4 mol/L • You can calculate [OH-] using the relationship [OH-] = antilog (-pOH)

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