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Strengths of Acids and Bases

Strengths of Acids and Bases. Main Idea: In solution, strong acids and bases ionize completely, but weak acids and bases ionize only partially. Strength of Acids. Ions carry electric current through the solution

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Strengths of Acids and Bases

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  1. Strengths of Acids and Bases Main Idea: In solution, strong acids and bases ionize completely, but weak acids and bases ionize only partially.

  2. Strength of Acids • Ions carry electric current through the solution • If all of the acid molecules completely dissociate to form ions, then the conductivity will be high • Acids that ionize completely are called strong acids • Strong acid reactions are represented using a single arrow: HCl (aq) + H2O (l)  H3O+ (aq) + Cl- (aq)

  3. List of Strong Acids - MEMORIZE • HCl – hydrochloric acid • HBr – hydrobromic acid • HI – hydroiodic acid • HNO3 – nitric acid • HClO4 – perchloric acid • H2SO4 – sulfuric acid

  4. Weak Acids • Weak acids are weak electrolytes (conduct electricity, but poorly) • Weak acids do not ionize completely in solution • Weak acid reactions are represented by reversible arrows because although some ionization occurs, the ions can also rejoin to make the compound: CH3COOH (aq)  CH3COO- (aq) + H+ (aq) • There are numerous weak acids, so there’s no need to memorize any (although acetic acid, above, is a classic example) – just memorize the strong and if the acid in question is not on that list, then it’s weak!

  5. Strength and the Bronsted-Lowry Model • Conjugate acids and bases are the opposite strength of what it came from: • Strong acid = weak conjugate base • Weak acid = strong conjugate base • Strong base = weak conjugate acid • Weak base = strong conjugate acid

  6. Strengths of Bases • What you have learned about acids can be applied to bases, except that OH- ions, rather than H+ ions are involved. • Strong base = 100% ionization (one-way) • Weak base = <100% ionization (reversible reaction)

  7. List of Strong/Weak Bases - MEMORIZE • STRONG bases: All Group 1 and 2, SOLUBLE hydroxides • EX: LiOH, NaOH, KOH, etc. • NOTE: Ca and Mg have low solubility in water as hydroxides, so technically they aren’t strong bases, BUT to make life easy, we’ll consider them as part of the list • WEAK bases: anything that’s not a hydroxide! The classic example is ammonia, NH3

  8. DID YOU KNOW? (A Deadly Compound) Hydrogen cyanide, HCN, is a poisonous gas found in the exhaust of vehicles, in tobacco and wood smoke, and in smoke from burning nitrogen-containing plastics. Some insects such as millipedes and burnet moths release hydrogen cyanide as a defense mechanism. A solution of hydrogen cyanide is called hydrocyanic acid. Fruits that have a pit, such as cherries or peaches, contain cyanohydrins, which convert to hydrocyanic acid in the digestive system if the pits are eaten. However, no hydrocyanic acid is produced in the flesh of these fruits, so the fruit can be safely eaten.

  9. HOMEWORK • Write chemical equations for the ionization of each acid (think about which arrows are most appropriate!): • HClO2 b) HNO2 c) HIO • Write the first and second ionization equation for H2SeO3. • Write chemical equations for the ionization of each base (think about the B-L definition of bases for what to react each of these with): • Hexylamine (C6H13NH2) • Propylamine (C3H7NH2) • Carbonate ion (CO32-) • Hydrogen sulfite ion (HSO3-)

  10. MORE HOMEWORK 4) Describe the contents of dilute aqueous solutions of the strong acid HI and the weak acid HCOOH. 5) Relate the strength of a weak acid to the strength of its conjugate base. 6) Identify the conjugate acid-base pairs in each equation and identify their strength: a) HCOOH (aq) + H2O (l)  HCOO- (aq) + H3O+ (aq) b) NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

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