Shapes and Polarities of Molecules CHEM Study
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Shapes and Polarities of Molecules CHEM Study


[Music] This is a production of the chemical education material study. [Female narrator] Two-thirds of the earth’s surface is covered with water, and the present and former oceans are an important source of many water-soluble chemicals. Table salt, bromine, and magnesium, all as ions, when in the ocean, are examples. The existence of water on Earth, predominantly in liquid form, yet able to freeze to ice to vaporize to give clouds and rain, and to dissolve then deposit minerals is directly related to the shape of water molecules. Triatomic H2O might be either linear or bent. It is bent. Were it linear, there would be no oceans, and few ions would dissolve in the liquid. Let’s explore why. [Male narrator] Here are four different liquids. We shall study their properties in developing a model which relates chemical properties to molecular shape. Let’s look at some electrical properties first. Here is a sphere which has been given a positive charge. After rubbing this hard rubber rod with cat’s fur, the sphere and the rod attract each other. Since the sphere is positively charged, this is good evidence that the rod has acquired a negative charge. Now notice how the negatively charged rod attracts a stream of water from the first bottle. Here is a stream of acetone, it too is attracted. Let’s try the other liquids: carbon disulfide and benzene. The charged rod does not attract carbon disulfide, nor does it attract benzene. Why does a negatively charged rod have no effect on carbon disulfide and benzene and yet does deflect water and acetone? Is it possible that the streams which are deflected are charged positively and the others are electrically neutral? [Female narrator] If this is true, a positively charged rod should repel the deflected stream. Let’s try. A glass rod rubbed with silk repels the positively charged sphere. The glass rod must bear a positive electric charge. Will this positively charged rod repel the water? That’s strange. The stream is attracted just as it was by a negative charge. [Male narrator] Let’s try the other liquids. The acetone is also attracted, just as it was by the negative rod. Here is carbon disulfide. No effect from the positive charges. This is the same result as with a negative charge. This is benzene. No deflection, again the same result as with a negative charge. So we see that some liquids are not deflected by either kind of charge while other liquids are attracted by both positive and negative charges. Why? Here is a tabulation of our findings. Water and acetone are deflected, carbon disulfide and benzene are not deflected. How can we account for these different electrical properties? How can water, for example act, in one case as if it has a positive charge and in another case as if it has a negative charge? To answer this question, let’s see if we can construct a model or concept on the molecular level that accounts for the deflection or lack of deflection in these experiments. [Female narrator] As a first step, let’s consider the structure of water on the molecular level. The molecule is bent like this. The hydrogen atoms share electrons with the oxygen atoms. These shared electrons are more strongly attracted by oxygen than by hydrogen. The increase of electron density near the oxygen makes this region of the molecule slightly negative. The lack of electrons around the hydrogen gives this region a small positive charge. Such a separation of positive and negative charge is said to form a dipole. We use an arrow to represent the dipole. The arrowhead points toward the negative charge. In water, there are two such bond dipoles, both pointing toward one side of the bent molecule. This combination results in a molecular dipole. Molecules having such a charge distribution even though overall electrically neutral are called polar molecules. The dipole for any polar molecule can be represented in this manner: one side negative and the other positive. If a positively charged rod is brought near polar molecules, like charges repel, unlike charges attract. This orientation results. With a negatively charged rod, the opposite orientation results. Thus, polar molecules will be attracted toward either positive or negative rods. So we can understand why a molecule with a shape like water can be attracted by both a positively charged rod and by a negatively charged rod. Now let’s see if our model applies to the behavior of our other deflected substance acetone. Acetone consists of a central carbon atom with an oxygen atom on one side and two CH3 groups on the other. Because the oxygen atom has greater attraction for electrons than does carbon, one side of the molecule is negative, the other side is positive. Therefore, acetone has a net dipole and is a polar molecule. Thus far we have accounted for the behavior of water and acetone. Now let’s see if we can account for the behavior of the non deflected substances. We’ll begin with carbon disulfide. The linear molecule consists of two identical sulfur atoms and one carbon atom. There may be a dipole between the carbon atom and each sulfur atom, but because, unlike water, the molecule is linear, the dipoles balance and cancel each other. Such a molecule is called nonpolar. We recall that a polar molecule has a separation of charges which causes the molecule to align itself toward our charged rod. In a non-polar molecule there is no unbalanced distribution of charge, so there is no strong interaction with a charged rod. [Male narrator] Let’s consider the charge distribution of the other non deflected molecule, benzene. C6H6. Because of the highly symmetrical shape, any bond dipoles in this molecule also cancel each other. Thus benzene too is nonpolar. So we see that bond polarities and molecular shapes determine whether molecules are polar or nonpolar. To test our model of molecular dipoles, let’s see if we can predict the effect of shape on the polarity of molecules. [Female narrator] Here are two forms of dichloroethylene. One is called cis-dichloroethylene. The other is called trans-dichloroethylene. They have the same formula, but their structures are different. How will their differences in structure affect their behavior? In the cis, the two chlorine atoms are on the same side of the double bond joining the carbon atoms. Chlorine atoms attract electrons more strongly than do hydrogen atoms. The cis molecule should be polar. In the trans-dichloroethylene, the chlorine atoms are on opposite sides of the double bond. The opposing dipoles cancel one another. The trans molecule should be nonpolar. The trans and cis isomers cannot interconvert because the double bond prevents internal rotation. The polar cis molecules should be deflected by a charged rod. The nonpolar trans should not be deflected. Let’s try the non-polar trans first. It is undeflected. Now the polar cis. It is deflected. Our predictions were correct. Molecular polarity consistently interprets the observed stream deflections. Now let’s be more quantitative. Here is an apparatus which gives more precise measurements of the effects of charges on dipoles The liquids to be tested are placed in this cell. The sides of the cell are metal plates which can be alternately charged plus and minus. The plates are attached to an apparatus which will make the charge oscillate back and forth. In other words, just as a pendulum swings, so here electrons cycle back and forth causing the charge on each plate to alternate plus and minus. A meter called an oscilloscope is used to detect the rapid cycling of charge on each plate. On the scope, time is indicated in the horizontal direction and charge in the vertical direction. A spot on the scope moves up until the maximum charge is reached, then, a little later, the spot moves down until maximum charge in the reverse direction is reached. Let’s watch the apparatus in operation as the oscilloscope warms up. The pattern of the charging cycle appears. Nonpolar trans-dichloroethylene is in the cell. We see the tracing spot as a continuous curve on the scope. The flow of electrons reverses direction many times per second. The distance between vertical lines on the scale corresponds to five microseconds. Thus 17 microseconds is the charging cycle time. Now the cell has been filled with cis-dichloroethylene. With this polar substance between the plates, the charging cycle time is 29 microseconds. Why so much longer? Let’s return to our model. [Male narrator] With a polar molecule between the plates, the electric forces cause the dipole to align in this manner, then the charged ends of the dipole will attract additional electric charge onto the plates. Thus, a polar substance should require more charging time. With nonpolar dichloroethylene, the time was 17 microseconds, but with polar dichloroethylene it was longer, 29 microseconds. Now suppose we further test our model by investigating the effects of temperature. Up till now we have portrayed the molecule at rest, but we know there are actually many molecules present all in random jostling motion. Their jostling motions disrupt the dipole orientation, but at a lower temperature the jostling motion of the molecules is reduced, though they still occasionally tumble. The greater average alignment at the lower temperature should attract more charges onto the capacitor plates. Therefore we predict that the charging time for a polar substance should increase at lower temperatures. We’ll chill the polar substance, cis-dichloroethylene, to 0 degrees centigrade. Insulating lacquer on the outside of the cell prevents electrical conductance through the water in the ice bath. As the cis cools, the charging time does increase. Here are the previous charging times. After thorough cooling, the cis time becomes 37 microseconds, longer than the 29 at room temperature. Since trans-dichloroethylene is a non polar substance, charge on the plates does not affect molecular alignment. Therefore, varying the temperature should have little effect on the cycling time. After chilling, the charging cycle is still 17 microseconds. [Male narrator] So we see that at 0 degrees, the charging time for trans is the same as at room temperature. These effects of temperature upon charging time give further evidence that our model is correct. Now let’s see how our dipole model can explain other differences in physical and chemical behavior. For example, differences between water and benzene. HCl gas, a polar substance, has been placed in the stopper tubes. Now watch what happens inside the tube as we remove the stopper. Well, that was rapid. HCl gas certainly dissolves readily in water. [Female narrator] Now let’s try HCl and benzene. Quite a contrast. The benzene level actually is rising but very slowly. Here is the level about five minutes later, and here about 15 minutes later. Some HCl never does dissolve. Let’s drain out the two solutions and compare some of their properties. We’ll measure the conductivity of the two solutions These silver strips are electrodes connected to a battery. Touching the probes causes a meter deflection which indicates conductivity. The HCl in solution in benzene does not conduct appreciably. By comparison, HCl in water is a conductor. Why the difference? [Male narrator] Let’s consider the molecules of the two solutions. With polar hydrogen chloride in nonpolar benzene, collisions occur but the nonpolar nature of benzene does not encourage ion formation. However, when a polar hydrogen chloride molecule and a polar water molecule collide, a reaction can occur. The surrounding water dipoles hydrate both ions, forming aqueous hydrogen ion and aqueous chloride ion. The resulting ionic solution is a good conductor. Note the orientation of the dipole arrows. The orientation around the positive hydrogen ion is opposite from that around the negative chloride ion. Since HCl is present in water in ions and in benzene as neutral molecules, we might expect the chemical behavior of the two solutions to be different. Let’s test the reactions to magnesium metal. First, the solution of unionized HCl in nonpolar benzene. It produces no visible reaction. Now, the ionic solution of HCL in polar water. Well, that’s different alright. A gas forms a gas which is found to be hydrogen. The hydrated ions in the polar water react rapidly. The unionized molecules in the nonpolar benzene do not. Ions hydrate in polar water for the same reason that water is attracted by a charged rod regardless of whether it is negative or positive. water molecules can align either way. Likewise, the concept of polarity correctly predicts and interprets the effect of temperature on dipole alignment. Our dipole model also correlates differences in solvent properties. It interprets the conductivity of the resulting solutions, and it accounts for their different chemical reactivities. [Female narrator] Ions are much more soluble in polar than in nonpolar liquids as we showed with HCL in polar water and nonpolar benzene. So falling streams of polar liquids may contain many more ions from traces of impurities from the atmosphere, even from their containers, than would the nonpolar liquids. Hydrated ions provide an additional mechanism by which the streams are deflected by a charged rod. In the presence of a charged rod, the ions move to produce a large charge separation and a large deflection In fact the presence of ions can cause a larger effect than does the polarity alone. A knowledge of the shapes and polarities of molecules is a powerful tool for interpreting the chemical properties and reactivity of substances. Polar water molecules not only attract and surround ions to make our seas salty, polarity also causes strong attractions among the molecules of pure water. Consequently, liquid water boils at 100 degrees Celsius and freezes at zero degrees Celsius, both higher values than found for less polar substances with molecules of similar size. Hence we have oceans, lakes, rivers, rain, snow, and ice all held together by polar forces. Were water molecules linear and nonpolar, we would probably have only gaseous water at the surface of the earth. Perhaps you’d like to discuss why this would be so. [Music]

7 thoughts on “Shapes and Polarities of Molecules CHEM Study

  1. 😀 May God bless you 🙂 i studied that at high school and 1st year at med school … but still watching this made me interested ,thrilled and more informed 

  2. Very precisely explained. Great!!! We have teachers as good as this. But, teaching method is different. So, these video tutorials are necessary. Great job, CHEM study and Lawrence Hall of Science!!!! I fully understood polarity thanks to you.

  3. These explanations of the CHEM study have helped me to understand my chemistry class, the old school can teach you what today school cannot. We have to look behind to be able to understand where we came from 🙂

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