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This is a production of the chemical education material study female narrator two thirds of of the earths surface is covered with water and the present and former oceans. 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. Lets 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. Lets look at some electrical properties.
First here is a sphere. Which has been given a positive charge after rubbing this hard rubber rod with cats 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 lets 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 lets 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 thats strange. The stream is attracted just as it was by a negative charge male narrator. Lets 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.
Lets 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. Lets 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 lets 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 lets see if we can account for the behavior of the non deflected substances. Well 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 lets 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 lets 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 lets 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 lets 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 lets 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 lets 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 well 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 lets 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 lets 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 lets drain out the two solutions and compare some of their properties. Well 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.
Lets 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 lets 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.
Thats 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 youd like to discuss why this would be so music.
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