Mr McCondichie

Unit 1 - Chemical Changes and Structure

Unit 1 - Chemical Changes and Structure Topics:

Oxidation and Reduction

A redox reaction involves two half reactions: oxidation and reduction.

Elements as Oxidising and Reducing Agents

Oxidising Agents

Oxidising agents are substances that cause another species to oxidise. Oxidising agents are reduced. They specialise in electron removal, this means they accept electrons from other substances.

Elements with high electronegativies (non-metals) tend to form ions by gaining electrons (reducting) so can act as oxidising agents.

Reducing Agents

Reducing agents are substances that they cause another species to reduce. Reducing agents are oxidised. They specialise in electron donation, this means they donate electrons to other substances.

Elements with low electronegativities (metals) tend to form ions by losing electrons (oxidising) so can act as reducing agents.

Compounds, Group Ions and Molecules as Oxidising or Reducing Agents

Oxidising agents are widely used because of the effectiveness with which they can kill fungi and bacteria, and can inactivate viruses. The oxidation process is also an effective means of breaking down coloured compounds, making oxidising agents ideal for use as 'bleach' for clothes and hair.

• hydrogen peroxide is a molecule that is an oxidising agent
• dichromate and permanganate ions are group ions that are strong oxidising agents in acidic solutions
• carbon monoxide is a gas that can be used as a reducing agent

Using the databook to identify oxidising and reducing agents

In the periodic table, the strongest reducing agents are in group 1, and the strongest oxidising agents are in group 7.

Ion-electron equations and Redox Reactions

Writing Redox Equations

Displacement reactions are examples of redox reactions.

E.g. Zinc metal displaces silver from silver (I) nitrate solution. This is because zinc is higher than silver in the electrochemical series.

The zinc metal is the reducing agent. It donates electrons and it is oxidised.

The silver ion is the oxidising agent. It accepts electrons and it is reduced.

Oxidation cannot happened without reduction and vice versa. Combining the two equations produces a REDOX equation.

The number of electrons lost in the oxidation must balance the number of electrons gained in the reduction.

Redox equations never contain electrons.

NB -The nitrate ions do not appear in the redox equation. This is because they are spectator ions and are not directly involved in the electron transfer.

More Complex Redox Equations

Oxidation and reduction equations can be represented by ion-electron equations. When molecules or group ions are involved, the ion-electron equation is a little more complicated.

Some are given in the data book but some you will have to write yourselves using the rules.

• Write down the main chemical in its two forms (oxidised and reduced) and balance the central atom (non-oxygen).
• Balance oxygen by adding water molecules.
• Balance hydrogen by adding hydrogen ions (H⁺).
• Balance the charge by adding electrons (e^-).

Worked Example 1: Writing Redox Equations

Potassium permanganate (KMnO₄) oxidises Fe²⁺ ions to Fe³⁺ ions. During this reaction the MnO₄^- ions are reduced to Mn²⁺ ions.

a) Complete the reduction half equation:

b) Write the redox equation for the reaction.

Worked Example 2: Ion electron equations

Complete the ion-electron equation for the reduction reaction:

NO₃^- → NO

Worked Example 3: Writing Redox Equation

Write the redox equation for the reaction between bromine and sodium sulphite. In this reaction sodium (Na⁺) is a spectator ion.

Redox Titrations

A titration is a technique which can be used to determine the concentration of a substance, based on known concentrations and volumes of other reactants.

For many reactants an indicator is required to identify the end-point of the reaction, however for some redox titrations the end-point can be recognised from a colour change in one of the reactants (self - indicating) eg. potassium permanganate solution, the purple permanganate ions are reduced to colourless manganese ions.

Worked Example 4: Redox Titration Calculations

A standard 250cm³ solution of vitamin C was prepared. 25cm³ of this solution was titrated against 0.034mol l^-1 iodine solution using starch as an indicator. The average titre was 16.45cm³. Calculate the mass of vitamin C in the original tablet. The formula mass of vitamin C is 176g.

C₆H₈O₆ + I₂ → 2I^- + C₆H₆O₆ + 2H⁺

Worked Example 5: Redox Titration Calculations

A standard 250cm³ solution of vitamin C was prepared. 25cm³ of this solution was titrated against 0.031mol l^-1 iodine solution using starch as an indicator. The average titre was 17.6cm³. Calculate the mass of vitamin C in the original tablet.

C₆H₈O₆ + I₂ → 2I^- + C₆H₆O₆ + 2H⁺

Worked Example 6: Redox Titration Calculations

25cm³ of acidified potassium permanganate solution reacts with 23.7cm³ of a 0.01mol l^-1 solution of iron(II) sulfate. Calculate the concentration of potassium permanganate solution.

MnO₄^- + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺

Worked Examples from Textbook Questions

Redox Equations Part 1

Redox Equations Part 2