Differences Between Ionic and Molecular Compounds

of 6 Differences between Ionic and Molecular Compounds Lab Your Name:______Your partner’s name:______

What you will do:

1. Identify three properties of ionic compounds. 2. Identify three properties of covalent compounds. 3. Relate these properties to the ionic and inter-molecular bonds of the compounds. 4. Identify an unknown substance as ionic or covalent.

Materials: ionic compounds 24 well plate (clear plastic) LabPro* molecular compounds hot plate Logger Pro* unknown compound aluminum foil conductivity probe*

metal & plastic scoopulas wash bottle of distilled water (dH2O)

*teacher led demonstration

Safety First: Wear goggles at all times, wear gloves when handling materials, keep your workspace clean!

Obtain Samples 1. 6 identified substances and 1 “unknown” are provided in containers - you will be shown where. Using a plastic scoopula, place a small sample of each substance in its own well in the clear plastic well-plate provided. You don’t need very much! Record the well location on your data sheet. Then perform the following tests for each substance:

Appearance 2. Describe the appearance of the chemical (“small, same-sized crystals” or “different sized chunks”) and any other obvious physical characteristics. Write your description on the data sheet.

Hardness 3. Obtain a length of paper towel about ½ meter in length. 4. Arrange rice-grain-size amounts of the compounds on the paper towel 5-10 cm apart. 5. Carefully press on each sample with the back of your metal scoopula and try to crush it. Always wipe off your scoopula between tests so that you do not contaminate other samples. If the substance compresses, record in the data table that it is ‘soft’. If it crumbles, record it as ‘medium’. If it does not crush easily but fragments, identify it as ‘hard’. 6. Dispose of your paper towel and samples in the garbage can.

Conductivity (teacher led) Page 2 of 6 Melting Comparison 8. Set-up a hot plate and the aluminum foil. Add small samples of the various compounds as shown in the diagram below. Label each sample corresponding to its number on your data sheet.

9. Carefully place the foil on top of the hotplate and turn on the hotplate to 50% of the maximum setting (“5” on most hot plates). Record the start time. Avoid inhaling the vapors from the samples.

10. Record the order and approximate length of time in which the samples melt. Record any other noted changes. Watch sodium chloride carefully: if you’re lucky, your salt may ‘pop’! If any samples have not melted by the end of 10 minutes, record that too and then turn off the heat.

11. Unplug the hot plate and leave the tray and samples to cool.

Clean-up

12. Clean-up your work area ensuring that all drops of water or other materials are removed.

13. WASH YOUR HANDS. Sinks and soap for hand-washing are at the side of the room. Analysis Complete the questions on the pages following the data sheet. Note: You must not add or change anything on your data sheet. Your answers must be based on your observed data and your understanding of ionic and molecular compounds. The textbook will be a big help – read it! Page 3 of 6 Data Sheet Date: ______Your Name: ______Substance # Formula Description of Physical Hardness Conductivity Melting time or Enter: Ionic or Appearance (soft, medium, (μS/cm) “Did not melt” Molecular hard) & observations

NaCl sodium 1 chloride ______

MgCl2 magnesium 2 chloride ______

C7H6O3 salicylic acid 3 ______

KI potassium 4 iodide ______

C20H40 paraffin 5 ______

C12H22O11 sucrose 6 ______

Use your observations! unknown 7 ______

Conductivity Tests: Air ______µS/cm dH20 ______µS/cm Tap water ______µS / cm Page 4 of 6

Data Analysis Your Name: ______0. Use the formula for each compound given in the data table to determine whether each material is ionic or molecular. Write your answer in the underlined space provided in the Formula column (column 3, page 3) It is essential that you get this step correct – if you’re not sure, please refer back to Ch.6! Answer the following questions concisely – a long essay is not required, just use proper terminology.

Physical Properties

Crystals are formed from regular arrangements of particles. Both ionic and molecular compounds can form crystals. The bonds within an ionic crystal are strong attractions between positive and negative ions. In a molecular crystal there are covalent bonds within each molecule, and then weaker attractions between the molecules that allow them to form crystals. The ionic bonds in an ionic crystal are much stronger than the bonds between molecules (“intermolecular bonds”) in a molecular crystal.

1. Which would you expect to form the harder materials, ionic or molecular compounds? Why?

2. What happened when you tried to crush or break the ionic compounds? How did this compare to the molecular compounds?

3. Does this support the model for ionic compounds? (why / why not?)

Melting Time 4a. Which compounds, ionic or molecular, would you expect to melt most easily? Why? - What happens when something melts? Page 5 of 6 4b. Does this match what you observed? Explain & Discuss!

5. Tough Question: When heated, some ionic solids (such as salt) may “spit” or “pop” just before they melt. Use a sketch of an ionic lattice to explain why a salt crystal might “pop” apart when heated. (Hint: What happens to the particles in the lattice when they are heated, and how are ionic crystals held together?)

Solubility & Conductivity When it is dissolved in water, an ionic compound conducts electricity very well. Molecular compounds usually will conduct weakly or not at all. Rank your conductivity data (including the water & air) in this chart: Materials with High Ionic or Molecular? Materials with Low Ionic or Molecular? Conductivity (Highest at top) (from the formula) Conductivity (Highest at top) (from the formula) Page 6 of 6 6. Why would it make sense that solutions of ionic compounds would conduct electricity?

7. Explain the difference in conductivity between distilled and tap water.

8. Was there a clear difference in conductivity between the ionic and the molecular compounds? Discuss.

General Properties of Ionic and of Molecular Compounds This lab activity has compared the general properties of ionic versus molecular compounds. Complete the following table by listing the property and the characteristic shown by each type of compound.

Property (from data table) Ionic Compounds Molecular Compounds 1.

9. Identify your unknown sample Based on your observations in the lab and your answers for the previous questions; Would you classify your unknown substance as an ionic compound or a molecular compound? Why? Discuss!