Today s Objectives: Section 13.3 (pg )

Transcription

1 1 Today s Objectives: 1. Define oxidation number 2. Write and balance equations for REDOX reactions in acidic, basic, and neutral solutions with half-reaction equations, developing simple half-reaction equations, and assigning oxidation numbers 3. Identify electron transfer, oxidizing agents, and reducing agents in REDOX reactions that occur everyday in both living and non-living systems. Section 13.3 (pg )

2 2 Oxidation States An oxidation state is defined as the apparent net electric charge an atom would have if electron pairs in a covalent bonds belonged entirely to the most electronegative atom. Useful for keeping track of electrons, but doesn t represent an actual charge on an atom. They re arbitrary charges, not to be confused with actual electric charges. An oxidation number is a positive or negative number corresponding to the oxidation state of the atom in a covalently bonded entity. These are NOT charges! (+2 vs. 2+ for ions) Example: In water, which is the most electronegative atom, H or O? Oxygen, therefore we act as if the oxygen owns both electrons in the electron pair. Each oxygen atom has 8 p + and 8 e -. If the oxygen atom gets to count the two hydrogen electrons (red dots) in the two shared pairs, then 8 p + and 10 e - result in an apparent net charge of 2- Figure 1 p. 583 Each hydrogen atom has 1 p+, but with no additional electron (since oxygen has already counted it), the hydrogen is left with an apparent net charge of 1+

3 3 REDOX in Living Organisms The ability of carbon to take on different oxidation states is essential to life on Earth. Photosynthesis involves a series of reduction reactions in which the oxidation number of carbon changes from +4 in carbon dioxide to an average of zero in sugars such as glucose. The oxidation of glucose then occurs in the process of cellular respiration

4 4 Oxidation Number Provides a systematic way of counting electrons p. 583 Tip: The sum of the oxidation numbers of any entity must equal the net charge on that entity, i.e. net charge of a neutral compound = zero polyatomic ion = ion charge NOTE: This method only works if there is only one unknown after referring to the above table. Lewis formula and electronegativities are required for two or more unknowns.

5 5 Sample Problem 13.7 (p. 584) What is the oxidation number of carbon in methane, CH 4 (g)? Referring to Table 1 Assign hydrogen an oxidation number of +1 Apply some simple math Since a methane molecule is electrically neutral, then the oxidation number of the one carbon atom and the four hydrogen atoms 4(+1) must equal zero. x + 4(+1) = 0 The oxidation number of carbon is = -4 Record oxidation numbers as a integer value, i.e. sign preceding the number.

6 6 Sample Problem 13.8 (p. 584) What is the oxidation number of manganese in a permanganate ion, MnO 4 - (aq)? Referring to Table 1 Assign oxygen an oxidation number of -2 Since a permanganate ion has a charge of 1-, then the oxidation number of the one manganese atom and the four oxygen atoms 4(-2) must equal -1. x + 4(-2) = -1 x + -8 = -1 The oxidation number of manganese is = +7

7 7 Communication Example 1 (p. 584) What is the oxidation number of sulfur in sodium sulfate? We know the oxidation numbers of both Na and O, and solve algebraically 2(+1) + x + 4(-2) = x + -8 = 0 The oxidation number of sulfur is +6

8 8 Summary Determining Oxidation Numbers p Assign common oxidation numbers from Table 1 p The total of the oxidation numbers of atoms in a molecule or ion equals the value of the net electric charge of the molecule or ion. a) The sum of the oxidation numbers for a compound is zero. b) The sum of the oxidation numbers for a polyatomic ion equals the charge of the ion. 3. Any unknown oxidation number is determined algebraically from the sum of the known oxidation numbers and the net charge on the entity.

9 9 Oxidation Numbers and REDOX Reactions Although the concept of oxidation states is somewhat arbitrary (based on assigned apparent charges) it is self-consistent and allows predictions of electron transfer. Chemists believe that if the oxidation number of an atom or ion changes during a chemical reaction, then an electron transfer (REDOX reaction) occurs. Based on the oxidation number: If they do not change no e transfer occurs not a REDOX reaction increase in the oxidation number = oxidation decrease in the oxidation number = reduction Any REDOX reactions has both oxidation and reduction occurring Figure 3 p. 585

10 10 Sample Problem 13.9 (p. 587) Identify the oxidation and reduction in the reaction of zinc metal with hydrochloric acid. Write the chemical equation, since it was not provided. Determine the oxidation numbers of all entities. Look for atoms/ions oxidation numbers that increase in the reaction. Label the change as oxidation. Also identify the atoms/ions whose oxidation numbers decreases. Label this change as reduction.

11 11 Sample Problem (p. 587) Identify oxidation and reduction reaction when natural gas burns in a furnace. Write the chemical equation (as it is not provided) Determine all of the oxidation numbers Look for atoms/ions oxidation numbers that increase in the reaction. Label the change as oxidation. Also identify the atoms/ions whose oxidation numbers decreases. Label this change as reduction. Communication Examples 2 p. 588

12 12 REDOX Reactions the end. Reduction Historically, the formation of a metal from its ore (or oxide) i.e. nickel(ii) oxide is reduced by hydrogen gas to nickel metal NiO (s) + H 2(g) Ni (s) + H 2 O (l) Ni +2 Ni o A gain of electrons occurs (so the entity becomes more negative) Electrons are shown as the reactant in the half-reaction A species undergoing reduction will be responsible for the oxidation of another entity and is therefore classified as an oxidizing agent (OA) Decrease in oxidation number Oxidation Historically, reactions with oxygen i.e. iron reacts with oxygen to produce iron(iii) oxide 4 Fe (s) + O 2(g) Fe 2 O 3(s) Fe 0 Fe +3 A loss of electrons occurs (so the entity becomes more positive) Electrons are shown as the product in the half-reaction A species undergoing oxidation will be responsible for the reduction of another entity and is therefore classified as an reducing agent (RA) Increase in oxidation number

13 13 Homework Practice Questions p. 585 #1-5 Lab Exercise 13.B p. 586 Practice Questions p. 588 #6-10

14 14 Today s Objectives: 1. Define oxidation number 2. Write and balance equations for REDOX reactions in acidic, basic, and neutral solutions with half-reaction equations, developing simple half-reaction equations, and assigning oxidation numbers. Section 13.3 (pg )

15 15 Balancing REDOX Equations using Oxidation Numbers Balancing by inspection becomes difficult with more complex reactants and products With electron transfer in REDOX reactions, the total number of electrons lost by one atom/ion must equal the total number of electrons gained by another atom/ion. As a result, the changes in oxidation numbers must also be balanced i.e. the total increase in oxidation number for a particular atom/ion must equal the total decrease in oxidation number of another atom/ion.

16 16 Sample Problem (p. 590) When hydrogen sulfide is burned in the presence of oxygen, it is converted to sulfur dioxide and water vapor. Use oxidation numbers to balance this equation: H 2 S (g) + O 2(g) SO 2(g) + H 2 O (g) 1. Assign oxidation numbers to all atoms/ions and highlight the numbers that change. Notice that a sulfur atom is oxidized from -2 to +4. This is a change of 6 meaning 6 e - have been transferred. An oxygen atom is reduced from 0 to -2. This is a change resulting from 2e - transferred. These substances are molecules, not atoms, so we need to specify the change in the number of electrons per molecule 2. Determine the simplest whole numbers that will balance the number of electrons transferred for each reactant. These numbers become the coefficients of the reactants. 3. The coefficients of the products can be obtained by balancing the atoms whose oxidation numbers have changed and then any other atoms.

17 17 Sample Problem (p. 591) Chlorate ions and iodine react in an acidic solution to produce chloride ions and iodate ions. Balance the equation for this reactions: ClO 3 (aq) + I 2(aq) Cl (aq) + IO 3 (aq) 1. Assign oxidation numbers to all atoms/ions and highlight the numbers that change. Remember to record the change in the number of electrons per atom and per molecule or polyatomic ion. 2. Determine the simplest whole numbers that will balance the number of electrons transferred for each reactant. These numbers become the coefficients of the reactants. The coefficients of the products can be obtained by balancing the atoms whose oxidation numbers have changed and then any other atoms. 3. Although Cl and I atoms are balanced, oxygen is not. Add H 2 O (l) molecules to balance the O atoms. 4. Add H + (aq) to balance the hydrogen. The redox equation should now be completely balanced. Check your work by checking the total numbers of each atom/ion on each side and checking the total electric charge, which should also be balanced.

18 18 Sample Problem (p. 592) Methanol reacts with permanganate ions in a basic solution. The main reactants and products are shown below. Balance the equation for this reaction. 1. Assign oxidation numbers to all atoms/ions and highlight the numbers that change. Remember to record the change in the number of electrons per atom and per molecule or polyatomic ion. 2. Determine the simplest whole numbers that will balance the number of electrons transferred for each reactant. These numbers become the coefficients of the reactants. The coefficients of the products can be obtained by balancing the atoms whose oxidation numbers have changed and then any other atoms. 3. Add H 2 O (l) to balance the oxygen, add H + (aq) to balance the hydrogen. 4. Include hydroxide ions and cancel water molecules to balance the reaction. H 2 O (l)

19 19 Communication Example 3 (p. 592) Household bleach contains sodium hypochlorite. Some of the hypochlorite ions disproportionate (react with themselves) to produce chloride ions and chlorate ions. Write the balanced redox equation for the disproportionation. For disproportionation reactions, start with two identical entities on the reactant side and follow the usual procedure for balancing equations.

20 20 Communication Example 4 (p. 593) A person suspected of being intoxicated blows into this device and the alcohol in the person s breath reacts with an acidic dichromate ion solution to produce acetic acid (ethanoic acid) and aqueous chromium(iii) ions. Balance the equation for this reaction. Remember to balance the C and Cr first, then add H 2 O (l) to balance O, H + (aq) to balance H and then stop because this is an acidic solution

21 21 Summary Balancing REDOX Equations with Oxidation Numbers p Assign oxidation numbers and identify the atoms / ions whose oxidation numbers change. 2. Using the change in oxidation numbers, write the number of electrons transferred per atom. 3. Using the chemical formulas, determine the number of electrons transferred per reactant. (Use formula subscripts to do this) 4. Calculate the simplest whole number coefficients for the reactants that will balance the total number of electrons transferred. Balance the reactants and products. 5. Balance the O atoms using H 2 O (l), and then balance the H atoms using H + (aq) Add hydroxide ions and balance water molecules if in an alkaline solution

22 22 Homework Practice Questions p. 593 #11-15 Section 13.3 Review p. 595 #1-12 Section 13.3 Extra Exercises worksheet