che106_lecture10

che106_lecture10 - Chemistry 106 Lecture 10 Topics:...

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Unformatted text preview: Chemistry 106 Lecture 10 Topics: Acid- Base Reac;ons Oxida;on- Reduc;on Reac;ons Chapter 4.3 – 4.5 Announcements •  Class average was 75% (B- ). •  The scores are posted on Blackboard. •  If you scored less than ~50%, you need to seek addi;onal help. •  Reminder: Homework/Tutorial due on Friday (as always) Types of Chemical Reac;ons •  Most of the reactions we will study fall into one of the following categories – Precipitation Reactions – Acid-Base Reactions – Oxidation-Reduction Reactions Acid- Base Reac;ons Acids and bases are some of the most important electrolytes. Acids and bases are very common in household items. Acid- Base Reac;ons •  The Arrhenius concept (~1884) defines acids as substances that produce hydrogen ions, H+, when dissolved in water. •  An example is nitric acid, HNO3, a molecular substance that dissolves in water to give H+ and NO3- . H 2O + − HNO3 (aq) ⎯ྎ → H (aq) + NO 3 (aq) ⎯ྎ Acid- Base Reac;ons •  The Arrhenius concept defines bases as substances that produce hydroxide ions, OH- , when dissolved in water. •  An example is sodium hydroxide, NaOH, an ionic substance that dissolves in water to give sodium ions and hydroxide ions. H 2O + − NaOH(s ) ⎯ྎ → Na (aq) + OH (aq) ⎯ྎ Acid- Base Reac;ons •  The molecular substance ammonia, NH3, is a base in the Arrhenius view because it yields hydroxide ions when it reacts with water. ← ⎯ྎ ⎯ྎ + − NH 3 (aq) + H 2O(l ) →NH 4 (aq) + OH (aq) Acid- Base Reac;ons •  The Brønsted- Lowry concept (~1923) of acids and bases involves the transfer of a proton (H+) from the acid to the base. •  In this view, acid- base reac;ons are proton- transfer reac-ons. Acid- Base Reac;ons •  The Brønsted- Lowry concept defines an acid as the species (molecule or ion) that donates a proton (H+) to another species in a proton- transfer reac;on. •  A base is defined as the species (molecule or ion) that accepts the proton (H+) in a proton- transfer reac;on. Acid- Base Reac;ons •  The Brønsted- Lowry Concept In the reac;on of ammonia with water, ← + − NH 3 (aq) + H 2O(l ) → NH 4 (aq) + OH (aq) H+ the H2O molecule is the acid because it donates a proton. The NH3 molecule is a base, because it accepts a proton. Acid- Base Reac;ons •  The Brønsted- Lowry Concept The dissolu;on of nitric acid, HNO3, in water is also a proton- transfer reac;on − HNO3 (aq) + H 2O(l ) → NO 3 (aq) + H 3O + (aq) H+ where HNO3 is an acid (proton donor) and H2O is a base (proton acceptor). Acid- Base Reac;ons •  The Brønsted- Lowry Concept The H+(aq) ion associates itself with water to form H3O+(aq). + + H (aq) + H 2O(l ) → H 3O (aq) This proton transfer mechanism for the H+ ion is called the hydronium ion. Acid- Base Reac;ons •  In summary, the Arrhenius concept and the Brønsted- Lowry concept are essen;ally the same in aqueous solu;on. –  The Arrhenius concept acid: proton (H+) donor base: hydroxide ion (OH- ) donor –  The Brønsted- Lowry concept acid: proton (H+) donor base: proton (H+) acceptor Acid- Base Reac;ons •  Strong and Weak Acids and Bases –  A strong acid is an acid that ionizes completely in water; it is a strong electrolyte. − + HNO3 (aq) + H 2O(l ) → NO 3 (aq) + H 3O (aq) − + HCl(aq) + H 2O(l ) → Cl (aq) + H 3O (aq) –  Table 4.2 lists the common strong acids. Acid- Base Reac;ons Table 4.2 Acid- Base Reac;ons •  Strong and Weak Acids and Bases –  A weak acid is an acid that only par-ally ionizes in water; it is a weak electrolyte. –  The hydrogen cyanide molecule, HCN, reacts with water to produce a small percentage of ions in solu;on. ← CN − (aq) + H O + (aq) HCN(aq) + H 2O(l ) → 3 Acid- Base Reac;ons •  Strong and Weak Acids and Bases –  A strong base is a base that is present en-rely as ions, one of which is OH- ; it is a strong electrolyte. HO 2 + − NaOH(s) → Na (aq) + OH (aq) –  The hydroxides of Group 1A and 2A elements, except for beryllium hydroxide, are strong bases. (See Table 4.2) Acid- Base Reac;ons Table 4.2 Acid- Base Reac;ons •  Strong and Weak Acids and Bases –  A weak base is a base that is only par-ally ionized in water; it is a weak electrolyte. –  Ammonia, NH3, is an example. ← + NH 3 (aq) + H 2O(l ) → NH 4 (aq) + OH − (aq) Acid- Base Reac;ons •  Strong and Weak Acids and Bases –  You will find it important to be able to iden;fy an acid or base as strong or weak. –  When you write an ionic equa;on, strong acids and bases are represented as separate ions. –  Weak acids and bases are represented as undissociated “molecules” in ionic equa-ons. Acid- Base Reac;ons •  Neutraliza;on Reac;ons –  One of the chemical proper;es of acids and bases is that they neutralize one another. –  A neutralizaEon reacEon is a reac;on of an acid and a base that results in an ionic compound and water. –  The ionic compound that is the product of a neutraliza;on reac;on is called a salt. HCN(aq ) + KOH(aq ) → KCN(aq ) + H 2O(l ) acid base salt Acid- Base Reac;ons •  Neutraliza;on Reac;ons –  The net ionic equa;on for each acid- base neutraliza;on reac;on involves a transfer of a proton. –  Consider the reac;on of the strong acid , HCl(aq) and a strong base, KOH(aq). HCl(aq ) + KOH(aq ) → KCl(aq ) + H 2O(l ) Acid- Base Reac;ons •  Neutraliza;on Reac;ons –  Wri;ng the strong electrolytes in the form of ions gives the complete ionic equa;on. H + (aq) + Cl − (aq) + K + (aq) + OH − (aq) → K + (aq) + Cl − (aq) + H 2O(l ) Acid- Base Reac;ons •  Neutraliza;on Reac;ons –  Canceling the spectator ions results in the net ionic equa;on. Note the proton transfer. + − + − H (aq) + Cl (aq) + K (aq) + OH (aq) → K + (aq) + Cl − (aq) + H 2O(l ) H + (aq) + OH − (aq) → H 2O(l ) H+ Acid-Base Reactions •  Neutraliza;on Reac;ons –  In a reac;on involving HCN(aq), a weak acid, and KOH(aq), a strong base, the product is KCN, a strong electrolyte. –  The net ionic equa;on for this reac;on is − − HCN(aq) + OH (aq) → CN (aq) + H 2O(l ) H+ Note the proton transfer. Acid- Base Reac;ons •  Acid- Base Reac;ons with Gas Forma;on –  Carbonates react with acids to form CO2, carbon dioxide gas. Na 2CO 3 + 2HCl → 2NaCl + H 2O + CO 2 ↑ –  Sulfites react with acids to form SO2, sulfur dioxide gas. Na 2SO 3 + 2HCl → 2NaCl + H 2O + SO 2 ↑ Reac;on of a Carbonate With an Acid NaHCO3 Ace;c Acid Acid- Base Reac;ons •  Acid- Base Reac;ons with Gas Forma;on –  Sulfides react with acids to form H2S, hydrogen sulfide gas. Na 2S + 2HCl → 2NaCl + H 2S ↑ Acid- Base Reac;ons with Gas Forma;on Types of Chemical Reac;ons •  Most of the reactions we will study fall into one of the following categories – Precipitation Reactions – Acid-Base Reactions – Oxidation-Reduction Reactions Oxida;on- Reduc;on Reac;ons – Oxida-on- reduc-on reac-ons involve the transfer of electrons from one species to another. – Oxida-on is defined as the loss of electrons. – Reduc-on is defined as the gain of electrons. – Oxida;on and reduc;on always occur simultaneously. Oxida;on- Reduc;on Reac;ons – The reac;on of an iron nail with a solu;on of copper(II) sulfate, CuSO4, is an oxida;on- reduc;on reac;on – The molecular equa;on for this reac;on is: Fe(s ) + CuSO 4 (aq ) → FeSO 4 (aq ) + Cu(s ) Reac;on of Iron with Cu2+ (aq) Oxida;on- Reduc;on Reac;ons –  The net ionic equa;on shows the reac;on of iron metal with Cu2+(aq) to produce iron(II) ions and copper metal. Loss of 2 e- oxidaEon 2+ 2+ Fe(s) + Cu (aq) → Fe (aq) + Cu(s) Gain of 2 e- reducEon Oxida;on- Reduc;on Reac;ons •  Oxida;on Numbers –  The concept of oxida;on numbers is a simple way of keeping track of electrons in a reac;on. –  The oxida-on number (or oxida;on state) of an atom in a substance is the actual charge of the atom if it exists as a monatomic ion. –  Alterna;vely, it is hypothe-cal charge assigned to the atom in the substance by simple rules. Oxida;on- Reduc;on Reac;ons •  Oxida;on Number Rules Rule: Applies to: Statement: #1 Elements The oxidation number of an atom in an element is zero. #2 Monatomic ions The oxidation number of an atom in a monatomic ion equals the charge of the ion. #3 Oxygen The oxidation number of oxygen is –2 in most of its compounds. (An exception is O in H2O2 and other peroxides, where the oxidation number is –1.) Oxida;on- Reduc;on Reac;ons •  Oxida;on Number Rules Rule: Applies to: Statement: #4 Hydrogen The oxidation number of hydrogen is +1 in most of its compounds. #5 Halogens Fluorine is –1 in all its compounds. The other halogens are –1 unless the other element is another halogen or oxygen. #6 Compounds and ions The sum of the oxidation numbers of the atoms in a compound is zero. The sum in a polyatomic ion equals the charge on the ion. Oxida;on- Reduc;on Reac;ons •  Describing Oxida;on- Reduc;on Reac;ons –  Look again at the reac;on of iron with copper(II) sulfate. 2+ 2+ Fe(s) + Cu (aq) → Fe (aq) + Cu(s) –  We can write this reac;on in terms of two half- reac-ons. Oxida;on- Reduc;on Reac;ons •  Describing Oxida;on- Reduc;on Reac;ons –  A half- reac-on is one of the two parts of an oxida;on- reduc;on reac;on. One involves the loss of electrons (oxida;on) and the other involves the gain of electrons (reduc;on). 2+ − Fe(s) → Fe (aq) + 2e 2+ − Cu (aq) + 2e → Cu(s) oxidation half-reaction reduction half-reaction Oxida;on- Reduc;on Reac;ons •  Describing Oxida;on- Reduc;on Reac;ons –  An oxidizing agent is a species that oxidizes another species; it is itself reduced. –  A reducing agent is a species that reduces another species; it is itself oxidized. Loss of 2 e- oxidaEon reducing agent 2+ 2+ Fe(s) + Cu (aq) → Fe (aq) + Cu(s) oxidizing agent Gain of 2 e- reducEon Oxida;on- Reduc;on Reac;ons •  Displacement Reac;ons –  A displacement reac-on (also called a single- replacement reac;on) is a reac;on in which an element reacts with a compound, displacing an element from it. Zn(s ) + 2HCl (aq ) → ZnCl 2 (aq ) + H 2 (g ) Oxida;on numbers: Zn = 0 → Zn = +2 Zinc is oxidized. H = +1 → H = 0 Hydrogen is reduced. Oxida;on- Reduc;on Reac;ons •  Balancing Simple Oxida;on- Reduc;on Reac;ons –  At first glance, the net ionic equa;on represen;ng the reac;on of zinc metal with silver(I) ions might appear to be balanced. + 2+ Zn(s) + Ag (aq) → Zn (aq) + Ag(s) –  However, a balanced oxida;on- reduc;on equa;on must have a charge balance as well as a mass balance. Oxida;on- Reduc;on Reac;ons •  Balancing Simple Oxida;on- Reduc;on Reac;ons –  Since the number of electrons lost in the oxida;on half- reac;on must equal the number gained in the reduc;on half- reac;on, 2+ − Zn(s) → Zn (aq) + 2e + − 2 Ag ( aq ) + 2 e → 2 Ag(s) oxidaEon half- reacEon reducEon half- reacEon we must double the reac;on involving the reduc;on of the silver. Oxida;on- Reduc;on Reac;ons •  Balancing Simple Oxida;on- Reduc;on Reac;ons –  Adding the two half- reac;ons together, the electrons cancel, 2+ − Zn(s) → Zn (aq) + 2e oxidaEon half- reacEon + − 2Ag (aq) + 2e → 2Ag(s) reducEon half- reacEon 2+ Zn( s) + 2 Ag (aq ) → Zn (aq) + 2 Ag ( s) + which yields the balanced oxida;on- reduc;on reac;on. Al + H+ à༎ Al3+ + H2 When the equa;on above is correctly balanced, the coefficients are: Must be an integer! A.  1, 2 à༎ 1, 1 Al + H+ à༎ Al3+ + H2 B.  1, 3 à༎ 1, 2 1Al + 3H+ à༎ 1Al3+ + 1.5H2 2 x (1Al + 3H+ à༎ 1Al3+ + 1.5H2) C.  2, 3 à༎ 2, 3 2Al + 6H+ à༎ 2Al3+ + 3H2 D.  2, 6 à༎ 2, 3 D is the correct answer Summary: Chemical Reac;ons •  Acid- base reac;ons are proton- transfer reac;ons. •  Oxida;on- reduc;on reac;ons involve a transfer of electrons from one species to another. •  Oxida;on- reduc;on reac;ons usually fall into the following categories: combina;on reac;ons, decomposi;on reac;ons, displacement reac;ons, and combus;on reac;ons. •  A properly balanced oxida;on- reduc;on equa;on must have charge balance as well as mass balance. Next Lecture •  Topics: Molarity and Start of Chap 5:Thermochemistry •  Text Reading: 4.5, 5.1- 5.4 ...
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This note was uploaded on 10/11/2011 for the course CHE 106 taught by Professor Freedman during the Fall '08 term at Syracuse.

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