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C3H8O + CrO3 + H2SO4 Cr2(SO4)3 + C3H6O + H2O Balancing equations using oxidation numbers Review: balancing chemical equations • Balance the following chemical reaction: CuCl2 + Al Cu + AlCl3 • In the past (e.g. in grade 11) we balanced equations by “inspection”. • Balancing equations relied on having equal numbers of atoms on each side of the equation • We now also concerned with transfer of electrons or the balance of charge • We can balance equations using oxidation #s. • This relies on the idea that the number of electrons lost by an element must be equal to the number gained by a different element. Using Oxidation Numbers +2 -2 0 0 +3 -3 +2 -1 0 0 +3 -1 CuCl2 + Al Cu + AlCl3 Notice: Cu has gained 2e– (oxidation # by 2) Notice: Al has lost 3e– (oxidation # by 3) But, number of e– gained must equal e– lost Multiply Cu by 3, Al by 2: change is 6 for both +6 -6 change +6 0 0 +6 total +2 -2 0 0 +3 -3 0 0 +3 -1 oxidation # +2 -1 3CuCl2 + 2Al 3Cu + 2AlCl3 total oxidation # • • • • Steps to balancing equations 1. Write the skeleton equation 2. Assign oxidation numbers to all atoms and identify the atom/ion whose oxidation numbers change. 3. Using the change in Oxidation numbers, write the number of electrons transferred per atom. 4. Using the chemical formulas, determine the number of electrons transferred per reactant. 5. Balance the electron transfer by adding coefficients to the reactants. 6. Balance the O atoms by adding water molecules, balance H atoms by adding H+ 7. If the solution is basic, add OH- to both sides to neutralize the H+, combine to for water and reduce. Practice : ClO3-1 + I2 → Cl-1 + IO3-1 in an acidic solution Practice : CH3OH + MnO4-1 CO3-2 + MnO4-2 In a basic solution Homework: Page 613 #1-4