Redox Reactions and electrochemical cells Chapter 13 and 14 – Unit b (30%) Redding 2014

13.1 Oxidation and reduction Lets define the following terms: Reduction: Oxidation: Oxidizing agents: Reducing agents:

Question:

Electron transfer theory If we want to examine electron transfer we use half-reactions We split a reaction into its half reactions by dissociating it. Ex: lets write the half reactions for the reaction between zinc metal and hydrochloric acid.

Write and label two balanced half reactions to describe the reaction of zinc metal with aqueous lead (ii) nitrate.

Write and label two balanced half reactions to describe the reaction of zinc metal with aqueous lead (ii) nitrate.

Question:

Writing Complex Half Reaction Equations If we don’t have an overall reaction to refer to, and we want to construct a half reaction; we are able to follow this set of rules to properly construct one: Write chemical formulas for reactants and products Balance all atoms other than oxygen and hydrogen Balance oxygen by adding water Balance hydrogen by adding h+ Balance the charge by adding electrons * add oh- to both sides to equal the number of h+ present *combine h+ and oh- to make water, add and cancel if necessary

Example: write the half reaction nitrous acid can be reduced in an acidic solution to form nitrogen monoxide gas

Example: write the half reaction nitrous acid can be reduced in an acidic solution to form nitrogen monoxide gas

Example: Copper metal can be oxidized in a basic solution to form copper (i) oxide. What is the half reaction for this process?

Example: chlorine is converted to perchlorate ions in an acidic solution. Write the correct half reaction.

Example: aqueous permanganate ions are reduced to solid manganese (iv) oxide in a basic solution. Write the half reaction and indicate if it is oxidation or reduction.

Questions: (when you are done, you will get your worksheet for 13.1)

13.2 predicting redox reactions Developing a redox table: Usually we write these tables as a series of reduction reactions, starting with the strongest oxidizing agent  weakest oxidizing agent. Consider this:

The spontaneity rule We can predict spontaneity to also examine/construct a redox table: As long at the oxidizing agent is above the reducing agent in the table of relative strengths. Example:

Predicting redox reactions in solution. In order to figure out what will actually be reacting IN SOLUTION, it is important that you dissociate and label all of the entities in a solution. Example – when copper is placed into an acidic potassium permanganate solution what will happen?

Predicting redox reactions in solution. Back to the first example: when copper is placed into an acidic potassium permanganate solution what will happen? Now identify your SOA and SRA (using table) and write your half reactions.

Example: in a chemical industry, could copper pipe be used to transport a hydrochloric acid solution?

Predicting redox reactions by constructing half reactions What if the table of relative strengths does not have the half reaction you are looking for? You can build your own! You will also be able to use these reactions to predict what visible changes will occur

A common example of the application of redox reactions is the technology of a breathalyzer. A person suspected of being intoxicated blows into this device and the alcohol in the person’s breath reacts with an acidic dichromate ion solution to produce acetic acid (ethanoic acid) and aqueous chromium(III) ions. Predict the balanced redox reaction equation. The information provided tells you the skeleton equation for the major reactants and products

Example:

13.3 oxidation states

Examples & practice!

To do: Page 585 Questions 1-5 Page 589 Question 7 Hand them in for review when you are done Worksheet – oxidation numbers

Balancing using oxidation numbers You can balance equations through inspection, as we have always done, but we can also use oxidation numbers to balance! assign oxidation numbers Specify the change in the number of electrons per molecule Determine the simplest whole number coefficients that will balance the number of electrons – this number will become the coefficient for the reactant. Use those coefficients to get the coefficients for the products ***If the reaction is occurring in aqueous solution sometimes we have to add water and hydrogen to balance out ions. ***for basic solutions you will still need to add oh- and combine to make water

Balancing using oxidation numbers

Balancing using oxidation numbers

Balancing using oxidation numbers

13.4 redox stoichiometry There are many industrial and laboratory applications of redox stoichiometry. For example: a mining engineer must know the concentration of iron in a sample of iron ore in order to decide whether or not a mine would be profitable. Chemical technicians in industry, monitoring the quality of their companies’ products, must determine the concentration of substances such as sodium hypochlorite in bleach, or hydrogen peroxide in disinfectants

Recall from last year: The general stoichiometry procedure is as follows: 1. Write a balanced chemical equation with measurements and conversion factors. 2. Convert given measurements into a chemical amount. 3. Calculate the amount of the required substance using the mole ratio. 4. Convert this calculated amount to the final requested quantity

Problem: A freshly prepared solution of potassium permanganate is titrated against samples of acidic tin(II) chloride solution, which has a known concentration. The tin(II) chloride solution is the primary standard. Given the following information, What is the concentration of the potassium permanganate solution?

Problem: Extensive long-term research has found that treating children’s teeth with fluoride significantly reduces tooth decay. When this was first discovered, toothpastes were produced containing tin(II) fluoride. Complete the Analysis of the investigation report. Given the following information determine the amount concentration of tin(II) ions in a solution prepared for research on toothpaste?

Problem: Pure iron metal may be used as a primary standard for permanganate solutions. A 1.08 g sample of pure iron wire was dissolved in acid, converted to iron(II) ions, and diluted to 250.0 mL. In the titration, an average volume of 13.6 mL of permanganate solution was required to react with 10.0 mL of the acidic iron(II) solution. Calculate the amount concentration of the permanganate solution.