1. Oxidation and Reduction
Oxidation and reduction can be considered in terms of:
- oxygen gain or hydrogen loss,
- electron transfer OR
- change in oxidation number
Consider the following reaction:
Zn(s) + Cu2+(aq) –> Zn2+(aq) + Cu(s)
Take a look below for a model of what is happening on an atomic level.
✍️ Looking at the change in charges, which species is gaining electrons and which species is losing electrons?
✍️ Define the terms oxidising agent and reducing agent. Identify which is which in the above reaction.
To work out which species is being oxidised and which is being reduced, it is possible to assign oxidation numbers. Below is a short summary of the rules for assigning oxidation numbers. Alternatively, click here.
✍️ For each of the following, work out the oxidation number of the underlined element
You can use oxidation number to decide which species is oxidised and which species is reduced in a reaction.
Oxidation number increases = species is oxidised
Oxidation number decreases = species is reduced
✍️ Try question numbers 1-14 on the worksheet found here. Answers are at the bottom of the link.
One of the properties of transition metals, is that they can have variable oxidation states. For example find the oxidation numbers for the metal in the following pairs of compounds.
- CuCl and CuCl2
- Fe2O3 and FeO
To name these compounds using the IUPAC system we must use the oxidation number of the metal as a Roman numeral. Eg:
CuCl = copper(I) chloride
CuCl2 = copper(II) chloride
✍️ Write the IUPAC names of the two iron oxide compounds above.
Reactivity of Metal
Table 25 of the Data Booklet ranks the metals in order of increasing activity. This ranking has the most reactive metals at the top and the least at the bottom.
A metal higher on this list will displace a metal lower on the list from solution.
For example, Zn is above Cu. As we saw in class, the zinc metal displaced the copper metal from solution. This is because zinc is more reactive than copper and therefore more easily oxidised.
✍️ Write an equation for the reaction (if one occurs) between the following pairs of metals and ions:
- magnesium metal and copper sulfate solution
- silver metal and zinc sulfate solution
- lead metal and copper sulfate solution
- tin metal and chromium sulfate solution
- aluminium metal and nickel sulfate solution
Oxidation and Reduction Half Equations
All redox reactions can be split into their component half-reactions. Taking the initial example above with the Zn and Cu2+:
Oxidation 1/2 equation: Zn –> Zn2+ + 2e–
Reduction 1/2 equations: Cu2+ + 2e– –> Cu
Take a look at the rules for balancing half equations in neutral and acidic conditions. (You can also balance half equations in basic conditions but this isn’t covered in our course).
✍️ After studying these rules, try questions 15 and 16 from the worksheet found here.
In our next lesson, we will carry out a redox titration. In order to practice the calculation and to understand the process, try a few of the problems found at the link below. Working is given for you if you get stuck but try to do the problems before looking through the answers!
As an application of redox, the Winkler Method is used to measure biological oxygen demand (BOD) in order to test water quality. We will do this experiment in class later next week.
✍️ Read chapter 9.1 of your text book. Use this and any other sources to add anything important to your notes to make a complete summary of this section.