1 / 9

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

adsila
Download Presentation

Redox – Oxidation Numbers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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)

More Related