0.11 From cells and electrodes to golden pennies

Protocol adapted from 'Chemistry in the Laboratory'

Electrogels - metal corrosion, anodic protection and the golden penny

Objective

The goal of this experiment is:

• Observe the deposition of zinc metal on a penny in contact with metallic zinc and to develop a hypothesis to explain how this happens.
• Observe how iron may be corroded or protected from corrosion depending on whether it is connected to copper or to zinc.
• Observe the plating of a penny with zinc and subsequent color changes.

Grading

You will be assessed on:

• completion of the report form.
• TA evaluation of lab procedure.

 Introduction 

Electrochemistry is everywhere

 In general, whenever two condensed phases (solid or liquid) are brought into contact, a potential (or voltage) difference develops across the interface. Because the interface region is very thin, even transfer of a small amount of charge across the interface can create a very large electric field. For example, transferring about one picomole ( ${\text{10}}^{-\text{12}}$ mole) of electron charge per square centimeter of area will typically create a potential difference of approximately 1 volt across an interface layer about one nanometer thick. The electric field in this interface region would be about 109 volts/meter. Electric fields this large can cause the transfer of electrons across an interface layer or the transfer of ions between the inside and outside of ells in living organisms. Because contacts between condensed phases are very common in nature, electrochemical phenomena are very common, even though we are often unaware of them. At the cellular level, electrochemical phenomena are crucial to the propagation of nerve impulses, the timing of muscular contractions of the heart, and activity in your brain cells.

 Most of the electrical technology created by humans involves the simplest kind of chemical change; electron transfer across an interface. Often, the interface is between a good electron conductor, called an electrode, and a solution containing molecules or ions. The electrode might be a solid (like platinum or copper metal or graphite), or it could be liquid (like mercury metal). When electrons are transferred from the electrode to a molecule, we say the molecule has been reduced. Electron transfer in the opposite sense (from molecule to electrode) is called oxidation.

There are two parts to this lab the first, ANODIC PROTECTION, you will perform in pairs. By coating steel (which is mostly iron metal) with a more active metal like zinc, a process called galvanizing, the steel's corrosion can be retarded or entirely prevented. Often, simply making a good electrical connection between a piece of iron and a piece of zinc is sufficient to keep the iron from corroding. We will study the acceleration and the prevention of iron corrosion by connecting it to various metals.

In the second part of this lab, we will be revisiting THE GOLDEN PENNY EXPERIMENT, which will be set up for you by your TA, so that you can make observations. As you remember, the golden penny experiment involves the plating of a penny with zinc metal. First, the penny is immersed in a solution containing 1 M NaOH and granular zinc. Subsequent heating of the penny for a few seconds on a hot plate causes the silver color of the penny to turn a bright golden yellow. Explaining the details of the process presents a challenge.