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Lecture12221
 

Lecture12221

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a supplemental resource for students

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    Lecture12221 Lecture12221 Presentation Transcript

    • Chemical Reactions: Acid-Base Lecture 12
    • Acid-base reactions everywhere.
    • Acid-base reactions:
    • An acid is a substance that produces H + ions when dissolved in water: HX  H + (aq) + X - (aq)
    • A base is a substance that produces OH - ions when dissolved in water: MOH  M + (aq) + OH - (aq)
    • Strong acids and strong bases dissociate completely into ions when dissolved in water. They conduct a current well.
    •  
    •  
    • Weak acids and weak bases dissociate into ions very little when dissolved in water. They conduct only a small current.
    • Strong and weak acids
    • The key event
      • Molecular: 2HCl (aq) + Ba(OH) 2 (aq)  BaCl 2(aq) + 2H 2 O (l)
      • Total ionic: 2H + (aq) + 2Cl - (aq) + Ba 2+ (aq) + 2OH - (aq)  Ba 2 + (aq) + 2Cl - (aq) + 2H 2 O (l)
      • Net ionic: 2H + (aq) + 2OH - (aq)  2H 2 O (l)
      • or H + (aq) + OH - (aq)  H 2 O (l)
    • The key event:
    • The key event in all aqueous reactions between a strong acid and a strong base is that an H + ion from the acid and an OH - ion from the base form a water molecule.
    • The key event
    • Acid-base reactions occur through the electrostatic attraction of ions and their removal from solution in the formation of the product.
    • An acid and a base are the reactants in the reaction of neutralization.
    • A salt solution and water are the products in the reaction of neutralization.
    • A salt is a substance, an ionic compound that results from the reaction of an acid and a base.
    • The cation comes from the base, the anion comes from the acid:
      • H Cl (aq) + Na OH (aq)  NaCl (aq) + H 2 O (l)
      • H NO 3 (aq) + K OH (aq)  KNO 3 (aq) + H 2 O (l)
      • H 3 PO 4 (aq) + 3 Na OH (aq)  Na 3 PO 4 (aq) + 3H 2 O (l)
      • H 2 SO 4 (aq) + Ba (OH) 2(aq)  BaSO 4 (s) + 2H 2 O (l)
    • Acid-base reaction: Acid + Base  Salt + Water
    • Acid-base reactions are double-displacement reactions, also called double-replacement and metathesis reactions. The ions are exchanging partners.
    • A sample problem on writing ionic equations for acid-base reactions.
    • Titration is a process of finding the quantity of a given chemical by the addition of a liquid reagent of known strength, and measuring the volume of that reagent necessary to cause a reaction that changes the form of the chemical in question.
    • In any titration, one solution of known concentration is used to determine the concentration of another solution through a monitored reaction.
    • Titration technique
    • An acid-base indicator is a substance whose color is different in acid than in base.
    • There are plenty of indicators
    • Litmus paper
    • The equivalence point in the titration occurs when all the moles of H + ions present in the original volume of acid solution have reacted with an equivalent number of moles of OH - ions added from the buret.
    • The end point in the titration occurs when a tiny excess of OH - ions changes the indicator permanently to its color in base.
    • In calculations, we assume that the amount of base needed to reach the end point is the same as the amount needed to reach the equivalence point.
    • There’s some strong acid in our stomachs
    • A sample problem on finding the concentration of acid from an acid-base titration.
    • HCl transfers its proton to H 2 O
    • An acid-base reaction is a proton transfer process.
      • Thomas Martin Lowry (1874-1936),
      • British scientist
      • Johannes Nicolaus Brønsted (1879-1947),
      • Danish scientist
    • Acid is a molecule or an ion that donates a proton.
    • Base is a molecule or an ion that accepts a proton.
    • In acid-base reactions H 3 O + acts as as the acid and OH - acts as the base.
    • Gas-forming reactions are often acid-base, referring to Brønsted and Lowry.
    • A gas-forming reaction:
      • 2HCl (aq) + K 2 CO 3(aq)  2KCl (aq) + [H 2 CO 3(aq) ]
      • [H 2 CO 3(aq) ]  H 2 O (l) + CO 2(g)
      • Overall:
      • 2HCl (aq) + K 2 CO 3(aq)  2KCl (aq) + H 2 O (l) + CO 2(g)
      • Net ionic:
      • H 3 O + (aq) + CO 2- 3(aq)  3H 2 O (l) + 3CO 2(g) 
      • Carbonate ion acts as a base here.
    • Gas-forming reactions occur through the formation of gas and water because both products remove reactant ions from solution.
    • Ionic equations are written differently for the reactions of weak acids: they are shown undissociated.
    • THE END