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Redox – Oxidation Numbers

Redox – Oxidation Numbers . Definition General Rules Calculating Oxidation Number Changes. Oxidation Numbers. When an atom loses or gains electrons we say that it has changed its oxidation number e.g. Na → Na + In this example Sodium changes from an oxidation state of zero to one plus

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Redox – Oxidation Numbers

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  1. Redox – Oxidation Numbers Definition General Rules Calculating Oxidation Number Changes

  2. Oxidation Numbers • When an atom loses or gains electrons we say that it has changed its oxidation number e.g. Na → Na+ In this example Sodium changes from an oxidation state of zero to one plus • What this means is that it has changed its charge; • Elements always have an oxidation number of zero • Anions always have a negative oxidation number e.g. Cl-(oxidation number of -1), O2-(oxidation number of -2), N3-(oxidation number of -3) • Cations always have a positive oxidation number e.g. K+(oxidation number of +1), Ca2+(oxidation number of +2), Al3+(oxidation number of +3) • Sometimes the oxidation numbers are not visible as charges. They can be hidden inside a compound e.g. MnO2. We will look at calculating these numbers later

  3. General Rules • Before calculating oxidation numbers we must memorise some general rules: • In compounds, each oxygen has an oxidation number of -2 (except in H2O2 = -1) • In compounds, each hydrogen has an oxidation number of +1 (except in metal hydrides = -1) • Halogens (group 17 elements) have an oxidation number of -1 (except when reacted with other halogens and in oxyanions) • The sum of all the oxidation numbers in a compound will equal the charge e.g. O (= -2) and H (= +1) means that OH has a charge of -1 and H2O has a charge of zero.

  4. Calculating Oxidation Numbers The following steps can be used to calculate a ‘hidden’ oxidation number. We will use SO42- as an example: • 1. Find the charge of the compound- SO42- has a charge of -2 • Let the compound equal the charge- SO4 = -2 • Substitute in the oxidation numbers that you know e.g. Oxygen = -2 S + (-2x4) = -2 • Make the ‘element’ the subject of the equation S = -2 + 8 S = 6 • This number is the oxidation number of that ‘element’

  5. Using Oxidation Numbers • Oxidation numbers can be used to work out if oxidation or reduction occurred. For example: MnO4- → MnO2 Manganese changes from an oxidation number of 7+ to 4+ Question: What happen to the manganese to make it go from 7+ to 4+ ? Answer: It gained two electrons – this means that it has been reduced

  6. Exam Practice - 2008 Can’t see the exam paper below? Go to the NCEA website and search for 90311 • Have a go at Questions: • One • Two (a)

  7. Exam Practice - 2007 • Have a go at Questions: • Two • Three (a) • Four (a) • Five Can’t see the exam paper below? Go to the NCEA website and search for 90311

  8. Exam Practice - 2006 Can’t see the exam paper below? Go to the NCEA website and search for 90311 • Have a go at Questions: • One (b)(i) • Two (a) and (b) • Three (a) and (b)

  9. Exam Practice - 2005 Can’t see the exam paper below? Go to the NCEA website and search for 90311 • Have a go at Questions: • One (a) – (d) • Four (b)

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