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Balancing Redox Equations

Balancing Redox Equations. Identifying if a redox reaction has occurred In order for a reaction to be classified as a redox reaction there must be a change in the oxidation numbers of the Combination reactions involving at least 1 element in the reactants or products

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Balancing Redox Equations

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  1. Balancing Redox Equations • Identifying if a redox reaction has occurred • In order for a reaction to be classified as a redox reaction there must be a change in the oxidation numbers of the • Combination reactions involving at least 1 element in the reactants or products • if compounds combine to form a new compound it is not classified as a redox reaction • NH3 + HClg NH4Cl (in this reaction the oxidation numbers of the elements do not change) • Decomposition reactionsinvolving at least 1 element in the products • if a compound decomposes to form a new compounds it is not classified as a redox reaction • CaCO3gCaO + CO2 • All single displacement reactions involve an element and are considered redox reactions • Combustion reactions are redox reactions as they all have elemental O2 in the reactants • Double displacement reactions are not considered redox reactions

  2. Balancing - 1st method using oxidation number changes: easy! • best method for balancing reactions where the elements in the reactants and products occur in a single form on each side of the reaction • not for reactions involving acids and bases • write a skeleton reaction (no coefficients) Fe2O3 + CO g Fe + CO2 • assign oxidation numbers to the elements in the equation Fe (+3) gFe (0) reduction (-3 change) C (+2) gC (+4) oxidation (+2 change) O remains -2 on each side of the reaction • balance the oxidation and reduction charges to account for all of the electrons 2 x (-3Fe) = 3 x (+2C) • balance the equation by balancing the charges Fe2O3 + 3CO g2Fe + 3CO2

  3. 2nd method using 1/2 reactions • a half reaction is a reaction that shows just the oxidation or reduction that takes place in the reaction • write a skeleton reaction (no coeficients) S(s) + HNO3(aq)g SO2(g) + NO(g) +H2O(l) (takes place in an acidic solution) S + H+ + NO3-gSO2 + NO + H2O • separate into 1/2 reactions S (0) g SO2 (+4) oxidation NO3-(+5) gNO (+2) reduction • Note: O remains -2 and H+ remains +1 on both sides of the equation and thus are not included in the 1/2 reactions

  4. balance the 1/2 reactions by adding H2O then H+ Oxidation: 2H2O + S gSO2 (add the appropriate amount of water 1st) (balance with the appropriate amount of H+ 2nd) 2H2O + S g SO2 +4H+ Reduction: NO3-gNO + 2H2O 4H+ + NO3-gNO + 2H2O Add electrons to the sides of the 1/2 reactions to balance the charges 2H2O + S gSO2 +4H+ + 4e- 3e- + 4H+ + NO3-gNO + 2H2O (note the neg. charge on the NO3-)

  5. 4. balance the charges on the 2 1/2 reactions 3x (4e-) = 4 x (3e-) 3x (2H2O + S gSO2 +4H+ + 4e- ) 4x (3e- + 4H+ + NO3-g NO + 2H2O) 6H2O + 3S g3SO2 +12H+ + 12e- 12e- + 16H+ + 4NO3-g4NO + 8H2O 5. combine the 1/2 reactions 6H2O+ 3S + 12e- + 16H++ 4NO3-g3SO2 +12H++ 12e- + 4NO + 8H2O 6. subtract any items that are present on both sides of the equation and reduce to balance the overall equation 3S(s) + 4HNO3-(aq)g3SO2(g) + 4NO(g) + 2H2O(l)

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