CHE-122: Principles of Chemistry II

Laboratory 6 Procedure: Electrochemistry: Galvanic Cells and the Calculations of Cell Potential

Background:

Galvanic cells, also called voltaic cells, or battery cells are type of electrochemical cells that in which spontaneous reaction generates an electric current.

Here there is an example of spontaneous red-ox reaction that is used in a galvanic cell:

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

we can separate above equation in two half-reactions:

Zn(s) → Zn²⁺(aq) + 2e

and

Cu²⁺(aq) + 2e → Cu(s)

As shown in Figure 1. potential, a voltage difference between two half cells, drives electric current.

A Galvanic cell consists of:

Two electrodes, made of metal or graphite, placed in two separate containers (half-cells):

–anode: oxidation process occurs there, has higher potential so electrons flow away from it.

–cathode: reduction process occurs there, has lower potential, electrons flow toward it.

Salt bridge: made of porous get that connects two half cells, only counter ions can flow through it from one solution to another.

Figure 1: Zinc-copper galvanic cell.

Instead of drawing the diagram, we use the following electrochemical cell notation to describe zinc-copper galvanic cell:

Figure 2. Electrochemical cell notation for zinc-copper galvanic cell.

Each half-cell has its own potential. The standard potential of the cell E^o_cell is the resulting potential from combining the potential of each half-cell. Calculation of the standard cell potential can be conducted using either of two methods described below.

First method requires combining half-reaction and their standard electrode potential values, reduction and oxidation.

Second, involves the equation that calculate the difference between standard reduction potentials:

E°cell = E°cathode + E°anode

Figure 3: Formula Used for Calculation of Any Cell Potential Applying the Standard Electrode (Reduction) Potentials –see textbook Table 19.1 p 652.

There are few of objectives of this lab:

1.To enhance your understanding of the construction and principles behind galvanic cell.

1.To gain deeper understanding of spontaneous red-ox reactions involved in the electrochemical process.

2.To get familiar with calculations of cell potential using standard electrode (reduction) potentials, second method.

3.Get familiar with writing the cell diagram.

Before you start this laboratory assignment, you are encouraged to review Section 19.2-19.5 on pages 642-656 in the textbook. Throughout this laboratory assignment, you will be required to analyze an electrochemical reaction in terms of electrons exchange. Be sure to record all observations and any relevant notes that you think you will need to include in your laboratory report.

Take a moment to formulate and write down a hypothesis describing what causes formation of the electric current in a galvanic cell?

Pre-Lab questions:

Answer the following questions in at least 5-10 sentences.

1.Using your own words, explain what does red-ox reaction mean?

2.Using your own words, explain what is a spontaneous reaction?

3.Using your own words, explain what is a cell potential?

Procedure:

Preparing the Lab 6

1.Using all available resources research “Magnesium-Iron Galvanic cells”,

2.Write half page long summary of your research.

3.If you haven’t already done so, formulate a hypothesis describing what causes formation of the electric current in a galvanic cell?

Analyzing the Data

1.Using the research summary from “Preparing Lab 6” section Point 2, on the diagram below, name the following parts of magnesium-iron galvanic cell:

anode, cathode, salt bridge, direction of the electric current flow

2.In 3-5 sentences, describe what parts make a half-cell.

3.Write complete chemical equation for the red-ox process occurring in magnesium-iron galvanic cell.

4.Write oxidation and reduction half-cell reactions for each electrode using proper chemical notation.

5.Based on the half-cell reaction what happens to each of the electrodes in-terms of their mass?

6.Use proper electrochemical cell notation for magnesium-iron galvanic cell.

7.Using 5-10 sentences. Discuss the direction of the electron flow in the magnesium-iron voltaic cell.

8.Using Table 19.1 from p. 652 of your textbook calculate cell potential of your cell applying second method.

Notes

This section should include notes about any observations or data collected during the lab.

Report Requirements

This section contains key information that must be included in your typed report.

1.Define the problem in a manner that is clear and insightful.

1.Identify the strategies and procedures used during the lab.

2.Clear hypothesis statement and other potential solutions that identify any relevant contextual factors (i.e. real-world costs).

3.Clear presentation of data including any tables or other figures that are relevant to understanding your stated conclusions at the end of the report. Include any relevant calculations performed during the lab.

4.Clearly stated results and discussion of possible improvements to the procedure.

5.Conclusive statements arguing in favor of your findings.

Note: All reports will be graded using the rubric embedded within the course.

Here are some questions to consider as you write your report:

1.Does my problem statement make sense?

1.Have I summarized my strategies/procedures well enough to be replicated by an outsider?

2.Did I have a valid hypothesis at the start of the lab? Have I expressed this in my report?

3.Do my tables and/or graphs make sense?

4.Are my conclusions valid based on my supplied data?

5.Did I thoroughly summarize my laboratory experience in a concise, factual way such that the reader can understand my processes and findings in the conclusion section alone?