Lab: Model Building with Covalent Compounds



Lab: Model Building with Covalent Compounds

Introduction

Most of our learning is in two dimensions. We often draw representations of molecules on flat paper. Two-dimensional representations include lewis dot structures and structural formulas. In lewis dot structures, dots are used to represent electrons in a bond. The hydrogen molecule, H2, is shown by H:H. In structural formulas, a single bond is represented by a straight line, for example, a hydrogen molecule is H-H. Although such models help us understand the structure of molecules, flat models do not give us the three-dimensional view necessary for an accurate mental picture of most molecules.

Single, double and triple bonds can form between atoms. In some cases, the atoms are arranged in straight chains. At other times, the chains are branched. Still other molecules take the form of a ring of atoms. If a molecule’s structural formula includes a double bond, 2 lines are used, such as X=X.

Sometimes a given molecular formula might represent more than one compound. For example, C2H6O represents both ethanol and dimethyl ether, however compounds with different structural formulas will have different properties.

Ethanol Dimethyl ether

Substances having the same chemical formula but different structures are called isomers.

Scientists who determine the structure of molecules often start with molecular model kits similar to the one used in this experiment. Complicated molecules such as DNA, deoxyribonucleic acid, are most often shown in three-dimensional models. With these models, one can better understand how the atoms of the molecule interact.

Procedure

Part I

1. Draw the lewis dot structure for each of the following molecules.

a. water H2O b. methane CH4

c. ammonia NH3 d. carbon tetrachloride CCl4

e. ethane C2H6

2. Make models of the following molecules. There is only one structural formula for each. To confirm this, try and arrange the atoms in more than one way. When satisfied with the structure, sketch it in the data table. Try to represent the three-dimensional nature of these models in each sketch.

Part II

All of the models of molecules so far include only single bonds. The following group of molecules contain single, double and triple bonds. In the structure, the springs function as double and triple bonds. Repeat steps for part one using these molecules instead.

a. Carbon Dioxide CO2 b. Nitrogen N2 c. Oxygen O2

d. Ethene C2H4 e. Ethyne C2H2 f. Hydrogen Cynide HCN

Part III

A very important group of molecules is derived from a benzene ring (cyclobenzene).

a. cyclobenzene C6H6

Part IV

For the following molecular formulas, there can be more than one arrangement of the atoms. For each one, find as many different structural formulas as indicated. Sketch a structural formula for each isomer in the data table.

For these drawings use the shorthand method shown below. This is frequently used when sketching organic molecules. Carbons tend to be linked together in chains, so one will use lines off the carbons to represent hydrogens bonded them. For example, one of the structural isomers of pentane, C5H12 is a five-carbon chain.

This drawing represents only one isomer of pentane, a five-carbon chain. There are a total of three isomers. Try to build all three. Draw each in the data table.

In the data table draw structural formulas for

a. butane (2 isomers) C4H10 b. dichloroethane (2 isomers) C2H4Cl2

If you are really good…

Draw five different isomers of hexane, C6H14

Reflection:

1. Why would it be important to not only know the 2D model but also the 3D diagram?

2. In Part 2, what would you guess is the correlation between the length of each bond and the type of bond?

3. Hypothesize how the different isomers of a molecule would interact with various substances, such as in air or water.

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