Tuesday, July 26, 2011

Covalent and Ionic Properties

Purpose: In this lab, you can view the properties of ionic compounds and covalent compounds. Features must be examined: the volatility, melting point, solubility in water, and electrical conductivity. You can use these properties to classify substances or analog ion.

Background: The compounds are either ionic or covalent bonds, depending on the nature of the forces that hold them together. In ionic compounds, the attraction between oppositely charged ions. This attraction is called ionic bonding. Compounds with ionic bonds in the form of crystals of a regular pattern of positive and negative ions together with electrical attraction. In covalent compounds, atoms are held together by interactions between adjacent nuclei, and the shared electrons are called covalent bonds. Covalent compounds are present in the form of discrete particles called molecules. The molecules of covalent compounds are weak forces together in groups, usually called the molecular forces. Molecules, the forces are much weaker than the strength of covalent bonds that hold together the factors within molecules or ionic bonds that maintain the positive and negative ions with crystals.

Fusion - To melt an ionic connection, it is necessary to break the ionic bonds. Therefore, ionic compounds generally have high melting points. To melt a covalent bond, there is no need to break the bonds. It is only necessary to overcome intermolecular forces much lower than hold the particles together.

Volatility - the particles of a volatile compound is held together by weaker forces, so that some people can go without a break away from our noses.

Solubility - ionic compounds tend to be soluble (or dissolved in) water, because water is a polar substance which can exert a force sufficient to overcome the ionic bond and a few ions to disintegrate. Generally, covalent compounds are less soluble in water. The tendency of compounds to dissociate or ionize in water, says a lot about how the bonds holding the connection together.

Conductivity - A way to assess the trend of the dissociation of a compound in water is to test the ability of solutions to conduct electricity. If an aqueous solution of the compound does not lead, it is called a non-electrolyte. If driving in aqueous solution, the compound is called electrolyte. The charged particles must be present and to travel freely to the effects of an electric current to flow. The amount of conduction through the solution is an indicator of ionic character of the compound. In fact, driving or not driving the solution provides an indication of the type of link on site. These different forces into account the many properties of ionic and covalent compounds such as solubility, melting point, the degree of volatility and ability to conduct an electrical current.

Procedure (Part I): 

Chemicals to be tested                                                       Materials

                                Sodium chloride                                                                   aluminum boats                                 
                                PDB                                                                                        test tubes                                              
                                Potassium chloride                                                              DI water                                                                                                                           shortening                                                                                                            hot plate  
                                                                                                                                test tube rack
                                                                                                                                stirring rod
  1. Volatility – Carefully smell each compound.  If you can detect an odor, assume that the compound has a high volatility.  Record as high or low volatility.
  2. Melting Point – Place a small amount of each substance.  Heat the sample on a hot plate and record the time it takes for it do dissolve.  The PDB will be heated on a hotplate under the fume hood.  The longer it takes the compound to melt, the higher the melting point.  Record the time it takes to melt, and whether or not it is considered to be a high or low melting point.  If the compound hasn’t melted in 3 minutes consider that the compound has a high melting point. 
  3. Solubility in Water – Put a micro-spatula of each material into a separate test tube.  Stir each with a stirring rod (rinsed between each sample) and record how likely the substance is to dissolve in water.  Record high or low solubility.
Procedure (Part II):

Chemicals to be tested                                                       Materials

                                                Sodium chloride solution                                                   spot plate
                                                Potassium chloride solution                                               conductivity testers
                                                3M HCl                                                                                 wash bottle of DI water
                                                alcohol                                                                                   berel pipettes
                                                paint thinner        
                                                salad oil
                                                sugar solution
                                                tap water
                                                distilled water
  1. Add one berel pipettes’ worth of each of the 9 materials listed to separate wells of a spot plate.
  2. Place the two probes into a well.  Record the results as conductive or not conductive
  3. Rinse probes w/ DI water before putting them into the next sample.  Wipe the probes with a paper towel after testing the oil and then rinse with DI water.
  4. Test each substance and record results.

Conclusion Questions: 
  1. Explain why the type of bond could determine the volatility of a substance?
  2. Does the strength of the bond have anything to do with the melting point?  Explain why.
  3. Water molecules are polar which means one side of the molecule is positively charged and the other side of the molecule is negatively charged.  Which substances tend to dissolve easier in water, ionic or covalent?   Why?
  4. What kinds of elements are in the formulas for the ionic compounds?
  5. What kinds of elements are in the formulas for the covalent compounds?
  6. Explain the difference in conductivity of tap water and distilled water.
  7. List the physical properties that indicate ionic bonding exists in a compound.
  8. List the physical properties that indicate covalent bonding exists in a compound.


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