Lab: Conductivity

Objective

Background

The Nature of Electrolytes

When atoms combine to make compounds they bind to each other by electrical forces of attraction. Positive charges attract negative charges and these strong forces create a bond between atoms. These bonds can take different forms: some are covalent bonds in which atoms share valence electrons. The negatively charged electrons occupy the space between the positively charged nuclei and both nuclei are attracted to the electrons. Other bonds are ionic bonds in which atoms gain electrons to become a negatively charged anion or by lose electrons to become a positively charged cation. These ions then attract each other directly with no electrons between them.

An ionic compound in the act of dissolving in water. See a full explanation with animations at this page: http://www.middleschoolchemistry.com/ multimedia/chapter5/lesson3

Chemical bonds are formed and broken during chemical reactions that change one substance into another. Chemicals bonds may also be temporarily broken by dissolving a compound in water. Some compounds have bonds that can be broken to create two or more freely moving ions when the compound dissolves in water. This is due to the fact that water is a strongly polar molecule. It has two oppositely charged poles that strongly attract other atoms and molecules. In the illustration at right water molecules are shown dissolving ions from a solid crystal. The water molecules’ positively charged poles attract negative ions away from the crystal and then surround them as the ions then become mobile and move around in the solution. The negative poles of the water molecules do the same for the positive ions. The ions remain surrounded by water molecules just as a famous pop star is always surrounded by an entourage. Chemical compounds which break up into ions when dissolved in water are called electrolytes. There are other compounds which dissolve well in water but which do not break into separate ions. Such compounds are called non-electrolytes. Finally, a third class of compounds, called weak electrolytes, are found to produce a small number of ions when dissolved in water. The majority of the molecules of weak electrolytes remain as whole, uncharged molecules but a small fraction break up into cations and anions. This lab is not concerned with weak electrolytes.

Concentration of Solutions

Solutions are homogeneous mixtures of two or more substances. For liquid solutions the solvent is often water. Solvents are materials in which other substances dissolve. Solids, liquids and gases may all dissolve in water and any of them may be called a solute. A solute is any substance that dissolves in a solvent. Solutions may be made to have a variety of different strengths, or concentrations. The concentration

The image on the left shows dissolved ions at low concentration. The image on the right shows dissolved ions at high concentration.

of a solution is a quantitative measure of how much solute is dissolved in the solvent. Often concentration is measure in moles per liter. Moles are a unit related to the number of molecules of a substance and liters are a common unit of volume.

Concentration is higher for solutions containing more of the solute and lower when less material has been dissolved. In the illustration at right there are two boxes showing different concentrations. On the left there are very few particles of the solute and the image represents a solution with a low concentration. On the right there are many more solute particles in a higher-concentration solution.

Conductivity

One interesting consequence of the fact that some compounds produce separate, mobile cations and anions when dissolved in water is that the solution conducts electricity. Imagine a complete circuit containing a light bulb and a battery. The light bulb lights because of the power supplied by the battery. If a wire is cut then the bulb goes out. But if the two cut ends of the wire are placed in an electrolyte solution of high enough concentration the bulb will light again. Very often it is possible to demonstrate this in a classroom. There is also a high-quality simulation available from the PhET project at the University of Colorado. It is called Sugar and Salt Solutions and is available here: http://phet.colorado.edu/en/simulation/sugar-and-salt-solutions.

Electrons in a metal are mobile and can move among the atoms of the metal like water in a pool full of pebbles. When a battery is used to make an electrical current it causes the electrons to move in an organized way, all in one direction, turning the pool into a stream. This motion of the electrons in a piece of metal wire is what we normally think of as an electrical current. The electrical current in an electrolyte solution has a slightly different mechanism. The ions themselves move and carry electrical charge from one wire to the other. Electrons do not leave the metal and travel through the water to the other wire. Instead, cations move toward the negatively charged wire and anions move toward the positively charged wire. Fundamentally, an electrical current is only the movement of charged particles and it doesn’t matter whether the particles are electrons or ions.

The conductivity probes you will use in this lab activity measure how well a solution conducts electricity. It gives a low reading for a low concentration of ions and a higher reading for higher concentrations of ions. In effect, the reading produced by the probe is a measure of the number of ions per liter. The number of ions per liter depends on two things. First, it depends on how much solid is dissolved. The more material that is present in solution, the more conductive the solution will be. Second, it depends on the chemical formula of the dissolved material. Some compounds have more ions in their chemical formulas than others. Consider the following compounds and the number of ions each one makes when they dissolve:

Each equation shows how many ions are produced from each formula when they dissolve in water. Sodium chloride produces two ions, calcium chloride produces three ions, and sodium phosphate produces four ions. Even when these salts are made into solutions with the same concentration, measured as whole formula units per liter, they produce different levels of conductivity. For example, a solution of sodium phosphate will be twice as conductive as a solution of sodium chloride with the same concentration.

Pre-Lab Questions

1. What is an electrolyte and how is it different from a non-electrolyte?
2. Define the terms solute, solvent, and solution in your own words.
3. In your own words, describe the process by which an ionic solute dissolves in water to make an electrolyte solution.
4. In your own words, describe how electrical current is moved through an electrolyte solution and how this is different from how electrical current moves through a metal wire.
5. Do electrolytes carry electricity or produce it? Justify your answer by thinking about the example of the light bulb and the battery from the introduction.
6. Based on your knowledge of ionic compounds, which of the following two compounds will produce a solution with higher conductivity if the same number of formula units are dissolved? AlCl₃ or MgBr₂? Justify your answer.

Materials

• Labquest software on a computer
• Vernier Conductivity Probe
• ring stand
• clamp
• stirring rod
• several 100-mL beakers
• distilled water
• wash bottle with distilled water
• 100-ml graduated cylinder

• dropper bottle of 1.0 M NaCl
• dropper bottle of 1.0 M CaCl₂
• dropper bottle of 1.0 M Na₃PO₄
• dropper bottle of 1.0 M C₁₂H₂₂O₁₁ (sugar)
• dropper bottle of 3% H₂O₂ (hydrogen peroxide)
• Sharpie marker for labelling

Safety

The chemicals used in this activity are all of low toxicity and are used in dilute solutions. Even so, it is best to wear standard laboratory safety equipment such as splash goggles, long pants, and nitrile gloves, if desired. Tie back long hair and do not wear dangling jewelry in the lab.

Part I: Electrolytes vs. Non-electrolytes

Procedure

1. Wear your safety goggles while working in the lab.
2. Set up the Conductivity Probe using the ring stand and clamp. Do not clamp the probe too tightly and do not submerge the probe completely into water at any time.
3. Set the range selector on the Conductivity Probe to “0 - 2000 μS/cm”.
4. Connect the probe to the computer and start the LabQuest software.
5. The software will immediately recognize the probe and produce a data collection screen.
6. Obtain and label five 100-mL beakers with the formulas of the substances you will be testing: NaCl, CaCl₂, Na₃PO₄, C₁₂H₂₂O₁₁, and H₂O₂. You will use these beakers throughout this lab without emptying them until it is time to clean up.
7. Fill each beaker with 50 mL of distilled water by measuring with a graduated cylinder.
8. Test the conductivity of one of the beakers containing distilled water by submerging the electrode at the bottom completely.. Water alone will be the first material you test. Hold the beaker up to the probe and submerge the end of the probe in the water. Record the conductivity value on your screen.
9. Now add just one drop of each solution your teacher has provided for you to each labelled beaker.
10. Stir the NaCl solution with the glass rod. Rinse the rod and dry it before stirring the next solution. Stir each one with the stirring rod, rinsing and drying in between each one.
11. Use the Conductivity Probe to measure the conductivity of the NaCl solution. Record the value in a data table you write down. Don’t forget to include distilled water as one of the substances you tested.
12. Rinse the end of the probe carefully using the wash bottle full of distilled water. This prevents the probe from giving inaccurate readings when you test the next solution.
13. Now use the probe to measure the conductivity of each of the other solutions. Always be sure to rinse the probe between tests!

Part II: Electrolyte Conductivity vs. Concentration

Procedure

In this part of the lab you will collect data to show the proportion between concentration and conductivity for three strong electrolytes.

1. On the Experiment menu, click Data Collection…. Choose Events with Entry.
2. For Name enter Number of Drops and for Units enter drops. Click OK
3. Click the Collect button to start data collection.
4. Raise the beaker containing 50 mL distilled water and one drop of NaCl solution up over the end of the probe. Click the Keep button and enter 1 for the volume in drops.
5. Add one drop of NaCl solution and stir it using your clean stirring rod.
6. Test the conductivity of the solution by holding it so the end of the probe is submerged. Swirl it gently, without spilling and watch the conductivity value for about 5 seconds. Once the reading reaches a steady value, click Keep and enter 2.
7. Repeat this process until you have reached a total of 8 drops.
8. Stop data collection.
9. Next you will use the software to analyze your result by producing a linear regression. This will calculate the equation of the line and plot a best-fit line for your data.
   1. On the Analyze menu choose Linear Fit.
   2. When it plots the line and gives you the equation it will be in the form y = mx + b. The y variable stands for conductivity, the x value stands for volume in drops.
   3. Record the value of the slope in a data table.
   4. Click the File Cabinet icon to store the data from the first run.
10. Rinse the electrode with the wash bottle.
11. Repeat the data collection procedure with the CaCl₂ solution.
12. Remember to store the data from Run 2 using the File Cabinet icon.
13. Repeat the data collection process with the Na₃PO₄ solution.
14. As instructed by your teacher, print this graph and/or export the data for further analysis and formatting in a spreadsheet program. In your report you must include a well-formatted graph with all three runs properly labeled and showing the slope of each line.

Post-lab Questions

Answer the following questions in a typed document on a separate paper. As part of your report you must include the answers to the Pre-lab questions, your formatted and labeled graph, and the answers to the questions below. This lab does not require a formal lab report.

1. Which of the substances you tested in part one were electrolytes? Which of them were non-electrolytes? Use your data to justify your answers and recall that distilled water should have essentially zero conductivity. If your distilled water had a reading slightly above zero then take this value as equivalent to zero conductivity.
2. What kind of proportion exists between the conductivity of a solution and the concentration of ions in the solution? Describe what happens to conductivity as concentration increases.
3. Compare the slopes of the three lines and put them in order from smallest to largest. What explains the difference in the slopes since all three were generated starting with solutions with the same starting concentration?
4. Write equations for when the following salts dissolve in water: Na₂SO₄, Mg₃(PO₄)₂, and Al(OH)₃.
5. Which of the salts in the previous problem would have the steepest slope if it were used in Part II of this lab? Which would have the smallest slope? Justify your answers.