Shiny Penny Experiment

By Whitney Pye and Jessica Simms

Chemistry 3202 Unit 4: Electrochemistry

Introduction

• Why are the pennies Dirty?

• Pennies get dull and dirty because the copper in the pennies reacts with oxygen in the air to form copper(II) oxide.

Experiment

Materials:

• Four Beakers or cups containing:

Beaker 1: 5 ml soap and 60 ml water

Beaker 2: 60 ml Vinegar

Beaker 3: 5 ml salt and 60 ml water

Beaker 4: 5 ml water and 60 ml vinegar

• About 40 pennies (10 for each beaker

• 2-3 steel screws

Hypotheses

• Predict what will happen to the pennies in each of the four beakers

• Beaker 1: Soap and Water

• Beaker 2: Vinegar  

• Beaker 3: Salt and Water

• Beaker 4: Vinegar and Salt

Experiment

• Place 10 pennies in each beaker/cup, stir and let sit for about 5 minutes

Results

• What did you observe? Which of the solutions “cleaned” the pennies the best?

• Beaker 1: Soap and Water

• Beaker 2: Vinegar

• Beaker 3: Salt and Water

• Beaker 4: Vinegar and Salt

• Explain your observations

Write the balanced equation for the penny reaction

+ NaCl (vinegar) + (salt)

HCl + NaCH3O2

(hydrochloric acid) + (sodium acetate)

CuO + HCl ---> copper(II) oxide + hydrochloric acid --->

CuCl2 + H2O copper(II) chloride + water

HC2H3O2

Part 2: Adding the Screws

• Remove the pennies from the beaker containing the vinegar and salt.

• Take the screws and place them into the beaker containing the vinegar and salt solution.

• Try to place them so that at least one of the screws is only partly submerged in the solution.

• Take note of any activity or changes in the beaker.

What is happening to the screws

When you put your dirty pennies in the vinegar and salt, the copper oxide is dissolved releasing copper ions into solution.

Steel is a metal made by combining iron, other metals, and carbon. When you put the steel screws in the mixture, some of the iron is oxidized into iron ions. As a result the copper (II) ions are reduced and form a copper plating on the screw.

What other observations did you notice when the screws were added to the solution?

Each water molecule is made up of two hydrogen atoms and an oxygen atom. In an acid, lots of hydrogen ions are floating around. In the chemical reactions at the surface of the screw, some of these hydrogen ions join and form hydrogen gas. The bubbles that you see coming off the screw are made of hydrogen gas.

Why did bubbles come off the steel screw?

REDOX (Reduction and Oxidation)

OIL RIG – Oxidation involves loss, Reduction involves gain

• REDOX occurs when there is a transfer of electrons from an ion

• A REDOX equation is the sum of a an oxidation half-reaction and a reduction half-reaction

• Can only occur in single replacement reactions

REDOX

• The species being oxidized, the species being reduced, the oxidizing agent and the reducing agent only apply to reactants in an equation

• They have dual roles: eg. The species being oxidized is losing electrons, and is also the reducing agent

Redox Equation for the formation of copper oxide

2Cu + O2 2CuO 0 0 +2 -2 Oxidizing numbers can be found

on the periodic table or online

Cu Cu2+ + 2e- [O] Oxidizing half-reaction

O2 + 4e- 2O2- [R] Reduction half-reaction

2Cu + O2 2CuO Complete REDOX equation

Copper is being oxidized and the oxygen is being reduced Copper is the species being oxidized and the reducing agent

Write the Redox equation for the copper plated screws

Fe + CuCl2 Cu + FeCl2 0 +2 -2 0 +2 -2

Fe Fe2+ + 2e- [O] Oxidizing half-reaction

Cu2+ + 2e- Cu [R] Reduction half-reaction

Cu2+ + Fe Cu + Fe2+ Complete REDOX equation

Cl2 is a spectator ion, so does not show up in the REDOX equation.

Iron is being oxidized, Copper is being reduced

Write the REDOX equation for the iron and hydrogen gas

Fe Fe2+ + 2e-[O] Oxidizing half-reaction

2H+ + 2e- H2 [R] Reduction half-reaction

2H+ + Fe H2 + Fe2+ Complete REDOX equation

Fe + 2HCl H2 + FeCl2 0 2+ 2- 0 2+ 2-

Iron is being oxidized, Copper is being reduced

References

• Copper Caper https://www.exploratorium.edu/science_explorer/copper_caper.html

• Clean Pennies with Vinegar http://www.sciencebob.com/experiments/pennychem.php

• Shiny Pennies http://www.elmhurst.edu/~chm/demos/ShinyPennies.html

Thank-You