Today s Objectives: Section 13.3 (pp )

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. Section 13.3 (pp )

2 2 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.

3 3 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.

4 4 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.

5 5 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)

6 6 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.

7 7 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

8 8 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

9 9 Homework Practice Qs p. 593 #11-15 Section 13.3 Review p. 595 #1-12 Section 13.3 Extra Exercises handout