Polar & Non-Polar Molecules: Crash Course Chemistry #23

Summary

This script delves into the fascinating world of molecular polarity, explaining the distinction between polar and non-polar molecules through the lens of electronegativity and geometric asymmetry. It illustrates the concept with everyday examples like butter and water, highlighting the importance of polarity in solubility and the unique properties of water that support life. The video also touches on the significance of hydrogen bonding and the role of polarity in the structure of cell membranes and the effectiveness of soap.

Takeaways

• 🧬 Molecules are classified as polar or non-polar based on their symmetry and charge distribution.
• 🔍 Polar molecules have an asymmetrical electron distribution and are more interesting due to their off-balance nature.
• 🌟 Non-polar molecules have a symmetrical charge distribution and are useful for their orderly structure.
• 🚫 Water, a polar molecule, does not mix with non-polar substances like butter due to their different charge affinities.
• 🔬 Polarity in a molecule arises from a combination of asymmetrical electron distribution and a difference in electronegativity between atoms of more than 0.5.
• 📊 Electronegativity increases from left to right across a period in the periodic table and decreases as you move down a group.
• 🌐 Geometric asymmetry is required for a molecule to be polar; symmetrical molecules with polar bonds are non-polar due to charge cancellation.
• 📍 The dipole moment indicates the separation of charge in a polar molecule, signified by arrows or partial charge notations.
• 💧 Polar liquids, like water, are effective at dissolving polar and ionic compounds due to their ability to interact with partial charges.
• ❄️ Hydrogen bonding, a result of water's polarity, is crucial for the unique properties of water, including its high specific heat capacity and the fact that ice floats.
• 🧼 Hybrid molecules with both polar and non-polar regions, like surfactants, can interact with both water and oils, facilitating processes like cleaning.

Q & A

What is the primary classification discussed in the script regarding molecules?

The primary classification discussed in the script is whether a molecule is polar or non-polar.

Why is the classification of molecules as polar or non-polar important?

The classification is important because it helps us understand the behavior of molecules, especially in terms of their interactions with other molecules and their solubility properties.

What are the two main factors that contribute to a molecule being polar?

The two main factors are asymmetrical electron distribution around the molecule and the presence of different elements with a significant difference in electronegativity.

What is electronegativity and how does it relate to polarity?

Electronegativity is a measure of an atom's ability to attract electrons towards itself. Polarity arises when there is a difference in electronegativity between the atoms in a molecule, leading to an uneven distribution of electrons and creating a dipole moment.

Why can't a molecule made up entirely of the same element be polar?

A molecule made up entirely of the same element cannot be polar because all atoms will have the same electronegativity, resulting in a symmetrical electron distribution and no dipole moment.

What is the significance of the difference in electronegativity for a molecule to be considered polar?

For a molecule to be considered polar, the difference in electronegativity between its atoms must be 0.5 or greater, which results in a significant charge asymmetry.

Why are polar molecules more interesting according to the script's narrator?

The narrator finds polar molecules more interesting due to their 'wonky, off-balance' nature and the complex interactions they can have with other molecules, including solubility and hydrogen bonding.

What is the 'like dissolves like' principle mentioned in the script, and how does it relate to polarity?

The 'like dissolves like' principle states that polar solvents dissolve polar solutes, and non-polar solvents dissolve non-polar solutes. This is due to the interactions between the molecules based on their polarity.

How does the script explain the phenomenon of water's high surface tension?

Water's high surface tension is explained by the cohesive forces between the polar water molecules, which are at their lowest energy state when lined up together, positive to negative, forming a kind of liquid crystal.

What role do hydrogen bonds play in the properties of water as described in the script?

Hydrogen bonds play a crucial role in water's properties, including its high specific heat capacity, its ability to dissolve many substances, and the fact that ice is less dense than liquid water, causing it to float.

What is a hybrid molecule as mentioned in the script, and how does it relate to polarity?

A hybrid molecule, as mentioned in the script, is a molecule that has both polar and non-polar areas. This allows it to interact with both polar and non-polar substances, such as the surfactants in soap that can dissolve fats and then be washed away by water.

Outlines

00:00

🌟 The Wonders of Molecular Polarity

This paragraph introduces the concept of molecular polarity, emphasizing the importance of classifying molecules as either polar or non-polar based on their symmetry and charge distribution. It highlights the distinction between polar and non-polar molecules using the example of water and butter, explaining how water's polarity causes it to interact differently with non-polar substances like butter. The paragraph delves into the factors that contribute to polarity, such as asymmetrical electron distribution and differences in electronegativity between atoms within a molecule. It also touches on the periodic trends that influence electronegativity and the concept of electronegativity itself, using a humorous analogy to explain its significance. Finally, it outlines the conditions necessary for a molecule to be considered polar, including the presence of different elements with a significant electronegativity difference and geometrical asymmetry that prevents the cancellation of charge asymmetry.

05:01

💧 The Role of Polarity in Dissolution and Hydrogen Bonding

This section explores the implications of molecular polarity, particularly focusing on how it affects the solubility of substances in water. It explains the 'like dissolves like' principle, attributing it to the cohesive forces between polar molecules and the disruption caused by non-polar molecules. The paragraph discusses the concept of surface tension in water, which is a result of polar molecules' tendency to minimize their surface area. It also examines the behavior of water when mixed with non-polar substances like oil, and how this relates to the interactions between water and air. The discussion then moves to the phenomenon of hydrogen bonding, a unique aspect of polar molecules like water, which allows for additional interactions between molecules. This leads to the explanation of why ice is less dense than liquid water, the significance of water's highest density at 4 °C, and the role of hydrogen bonds in various processes, including their impact on water's specific heat capacity and its ability to dissolve a wide range of compounds. The paragraph concludes with an example of a hybrid molecule, illustrating how molecules with both polar and non-polar regions, like surfactants, can have unique properties and uses.

10:03

🎬 Behind the Scenes of Crash Course Chemistry

The final paragraph provides credits and insights into the production of the Crash Course Chemistry episode. It acknowledges the writer of the script, the editor, and the chemistry consultants who ensured the accuracy of the content. The paragraph also recognizes the crew involved in filming, editing, and directing the episode, as well as the script supervisor, sound designer, and graphics team responsible for the visual elements. This section serves as a closing note, giving due credit to the team behind the educational series and highlighting the collaborative effort that goes into creating each episode.

Keywords

💡Polar Molecules

Polar molecules are those in which there is an unequal distribution of electron density, resulting in a molecule with a positive and a negative end, also known as a dipole moment. In the video, the theme of polarity is central to understanding how different substances interact with water. For instance, water is described as a polar molecule that does not mix with non-polar substances like butter due to their differing electron distributions.

💡Non-Polar Molecules

Non-polar molecules exhibit symmetrical electron distribution, meaning that the electrons are evenly spread around the molecule, resulting in no net dipole moment. The script contrasts non-polar molecules with polar ones, using butter as an example of a non-polar substance that does not dissolve in water, highlighting the importance of polarity in solubility.

💡Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. It's a key factor in determining the polarity of a molecule. The video explains that atoms with high electronegativity, like fluorine, create polar bonds when combined with atoms of lower electronegativity, leading to polar molecules.

💡Asymmetry

In the context of molecules, asymmetry refers to the uneven distribution of electron density or molecular geometry. The script explains that both charge asymmetry and geometric asymmetry are required for a molecule to be polar, using CO2 and CH4 as examples of molecules with polar bonds but non-polar overall due to their symmetrical geometry.

💡Dipole Moment

A dipole moment is a measure of the separation of positive and negative electrical charges in a molecule. The video emphasizes the importance of dipole moments in polar molecules, illustrating how they contribute to the molecule's overall polarity and its interactions with other polar or non-polar molecules.

💡Hydrogen Bonding

Hydrogen bonding is a type of dipole-dipole interaction that occurs when the hydrogen atom of one molecule is attracted to the electronegative atom (like oxygen or nitrogen) of another molecule. The video describes how hydrogen bonding in water contributes to its unique properties, such as high specific heat capacity and surface tension, which are crucial for life on Earth.

💡Surface Tension

Surface tension is the elastic tendency of a fluid surface which makes it acquire the least surface area possible. The script uses the example of water's surface tension to explain how the cohesive forces between polar water molecules create a 'skin' on the water's surface, allowing more water to be poured into a container than its volume would suggest.

💡Specific Heat Capacity

Specific heat capacity is the amount of heat energy required to raise the temperature of a unit mass of a substance by one degree Celsius. The video points out that water's high specific heat capacity is due to the energy needed to break or form hydrogen bonds, which helps regulate temperatures on Earth.

💡Hybrid Molecules

Hybrid molecules are those that contain both polar and non-polar regions within the same molecule. The script mentions surfactants in soap as an example of hybrid molecules, which can dissolve fats due to their non-polar regions while their polar regions allow them to be washed away by water.

💡Cell Membranes

Cell membranes are the outer structures of cells, composed of a phospholipid bilayer. The video describes how the polar heads of the phospholipids in cell membranes interact with the aqueous environment, while the non-polar tails maintain the integrity of the cell by preventing it from dissolving in water.

💡Cohesive Forces

Cohesive forces are the intermolecular forces that pull molecules of a liquid together. The script explains that these forces are responsible for the minimized surface area of water in a beaker, demonstrating how water molecules prefer to interact with each other rather than with non-polar substances like oil.

Highlights

Molecules are classified as polar or non-polar based on their symmetry and charge distribution.

Polar molecules have asymmetrical electron distribution and are more interesting due to their off-balance nature.

Non-polar molecules have symmetrical electron distribution and are useful for their beauty in symmetry.

Water is a polar molecule, which is why it doesn't mix well with non-polar substances like butter.

Electronegativity differences cause polar molecules, with a minimum difference of 0.5 required for polarity.

Electronegativity trends in the periodic table show higher values from left to right and lower from top to bottom.

Geometric asymmetry is necessary alongside charge asymmetry for a molecule to be polar.

Molecules like CO2 and CH4 have polar bonds but are non-polar due to their symmetrical geometry.

Polar molecules have a dipole moment, indicating a separation of charge into positive and negative areas.

Polar molecules are indicated with arrows and partial charge symbols, showing charge distribution.

Polar liquids dissolve polar and ionic solids due to their ability to interact with partial charges.

The 'like dissolves like' principle is explained by the interaction of partial charges in polar and non-polar molecules.

Water's polarity leads to high surface tension and the ability to hold more volume than expected.

Oil and water do not mix due to the difference in polarity, with water preferring to minimize its surface area.

Hydrogen bonding in water is responsible for its high specific heat capacity and temperature stability.

Water's ability to dissolve a wide range of compounds is due to its polarity and dipole moment.

Hybrid molecules with both polar and non-polar areas, like surfactants, can dissolve fats and interact with water.

Cell membranes utilize the properties of polar and non-polar molecules to maintain structure and interaction with the environment.

The episode concludes with a summary of the importance of polarity in chemistry and its role in fostering life on Earth.

Transcripts

Molecules! So many of them in their infinite and beautiful variety,

but while that variety is great, it can also be pretty dang overwhelming.

And so, in order to help this complicated chemical world make a little more sense, we classify and we categorize.

It's our nature as humans, and it's extremely useful.

One of the most important of those classifications is whether a molecule is polar or non-polar.

It's a kind of symmetry, not just of the molecule, but of the charge.

It's pretty easy to see when you're just lookin' at 'em.

You got polar and non-polar, polar, non-polar, polar, non-polar.

I'm gonna take sides right now. I'm on team polar.

I think polar molecules are way more interesting, despite their wonky, off-balance selves.

Non-polar molecules are useful, and their symmetry has a kind of beauty,

but polar, in my humble opinion, is where it's at.

[Theme Music]

All right. Now here are two very different types of chemicals.

Right here I have a stick of butter, and then in this bowl, that's just normal water.

So I'm just gonna go ahead and squeeze this butter, which if you're wondering is both a terrible and wonderful feeling.

And then I'm going to [laughs] just drop that.

Now I'm going to attempt to wash that butter off my hand.

But that is just not hap... that's just, it's not going anywhere, ever.

Ever. It's just beading up on me.

Why? Because water is a polar molecule, and the various chemicals that make up butter are non-polar,

and water wants nothing to do with that.

So. What makes a molecule polar? Well, two things.

First, asymmetrical electron distribution around the molecule.

You can't have a polar molecule made up entirely of the same element

because those atoms will all have the same electronegativity,

and thus the electron distribution will be completely symmetrical.

Electronegativity is usually thought of as how much an element wants electrons around it,

but I think it's more about how much electrons want to be near that element.

If electrons were 13-year-old girls, fluorine would be Niall Horan.

They'll do anything just to be near it. Why? Some simple periodic trends.

Electronegativity increases from left to right because there are more protons in the atoms,

and more protons means more boys in the band.

Meanwhile, it decreases as you move from top to bottom

because as the crowd of electrons gets bigger, they start to shield each other from the effects of the protons.

What I'm trying to say is that electrons are hipsters.

If a bunch of other electrons are into that thing, they're less interested.

Now there are a number of other factors here,

but just like the relationship between tweens and their latest boy band fixation,

it's complicated and weird and you probably don't want to think too much about it.

But in this nice little map, you can see that the trend is pretty clear.

The upper-right is where all the superstars of electro-fame are.

Oxygen, nitrogen, fluorine, chlorine, and bromine are basically the One Direction of the periodic table.

So for polarity to occur in a molecule, you have to have two different elements at a minimum,

and the difference between their electronegativities has to be 0.5 or greater.

If that's the case, the outer electrons spend enough extra time around the element that's more electronegative

that chemists label the molecule polar.

The result is a partially negative charge on the more electronegative part of the molecule

and a partially positive charge on the less electronegative side.

Now in extreme cases, like if the electronegativity is greater than 1.6,

then we end up with two ions in the same molecule.

This isn't what we're talking about here when we talk about polar molecules.

We're talking about differences between 0.5 and 1.6.

Another requirement for polarity: you gotta have geometrical asymmetry.

CO2 here has the charge asymmetry locked up, but because the molecule is linear, in a straight

line, it's a kind of symmetrical asymmetry.

The same thing does for CH4 with its tetrahedron of weakly electronegative hydrogens around

a more strongly electronegative carbon.

These molecules have polar bonds, but the molecules themselves are not polar

because the symmetry of the bonds cancels out the asymmetry of the charges.

In order for a molecule to be polar, there has to be a dipole moment,

a separation of the charge around the molecule into a more positive area and a more negative area.

Lots of molecules are asymmetrical in both electronegativity and geometry.

Those are our polar molecules, the asymmetrical beauties of chemistry.

Look at 'em all! They're so quirky and weird!

We've also got a system for indicating where their charges are.

We draw an arrow with a plus sign at the tail pointing toward the negative side of the molecule.

A little lowercase delta plus (δ+) or delta minus (δ–) by the individual atoms signify

a partial positive pr partial negative charge.

Liquids made up of polar molecules are really good at

dissolving solids that are composed of polar or ionic compounds.

Ionic solids are basically just polarity taken to the extreme,

so far that instead of having a partial positive and partial negative dipole moment,

the electrons have completely transferred, creating two charged ions.

Now I assume we've all heard that like dissolves like,

so the easiest way to figure out if a liquid is polar or non-polar is just to dump it in some water.

But the why of this phenomenon is usually just totally glossed over.

What's actually happening to those molecules?

It seems like they're all just bigots, terrified of anything a little bit different than themselves.

But this is chemistry, so there must be some fundamental reason.

And if it's fundamental, it probably has something to do with decreasing the energy of the system.

And indeed it does.

Those partial positive and partial negative charges of water?

They're at their lowest energy state when they're lining up together, positive to negative,

into a kind of liquid crystal.

There's an arrangement there. It flows, of course,

but the oxygen sides are always doing their best to orient themselves toward the hydrogen

sides of other molecules.

You can even see the effects of that attraction

as the surface tension that allows me to pour more than 100 milliliters of water into a 100 mil container.

The strength of that surface tension depends on the intermolecular forces that pull molecules of a liquid together.

These attractive, also called cohesive, forces pull the surface molecules inward.

And what you see when you look at this pile of water is the result of those cohesive forces,

minimized surface area in the water in this beaker.

When you pit a bit of oil into that mix, the water totally freaks out.

Oils have notoriously non-polar molecules, so suddenly

there's this mass of uncharged gunk interfering with the nice, orderly arrangement of polar water molecules.

But if you take a closer look, the processes are very similar to those between water and air.

Water does everything it can to minimize its surface area and kind of expels the oil droplets.

Rather than the water disliking the oil, it actually just likes itself much more, so it won't mix with the oil.

Now if you put polar stuff in, water is all about that,

and those polar water molecules just go after whatever other partial charges they can find.

Or, in the case of many ionic solids,

the partial negative charges on the oxygen side all gang up on the positive ions,

while the partial positives on the hydrogen side surround the negative ions,

breaking the crystals apart and dissolving them into freely moving ions.

In some cases we can actually witness these interactions in unexpected ways.

Mix 50 milliliters of water with 50 mils of alcohol and what the heck? There's less than 100 mils of liquid!

The arrangement of water mixed with alcohol is actually more structured, and thus more dense,

resulting in a smaller volume.

The polarity of water also results in a phenomenon that makes life possible: hydrogen bonding.

The partially negative oxygen and positive hydrogen atoms in a water molecule are not

100% faithful to each other.

They engage in additional kind of loose relationships with other neighboring hydrogen and oxygen atoms.

These loose, somewhat fleeting relationships are called hydrogen bonds.

In ice, 100% of O and H atoms are involved in hydrogen bonding.

The most energetically favorable spatial arrangement of these bonds

actually pushes the water molecules apart a bit,

resulting in the volume of ice being 10% larger than the volume of water,

which is really weird for solids and liquids.

When ice melts, there are still about 80% of Os and Hs engaged in hydrogen bonding,

creating ice-like clusters that keep the volume of the cold water relatively high.

With rising temperatures, these clusters disappear,

while the volume of the truly liquid water rises resulting in a major characteristic of water:

having its highest density at 4 °C.

And yes, that's why ice floats on lakes in the winter and why the bottom of frozen lakes tends to be about 4 °C.

And also why hockey was invented. And why soda bottles explode when you leave them in the freezer.

But hydrogen bonds are also why taking a warm bath is so great, why steam engines changed the world,

and why temperatures on our planet are so constant when compared to other cosmic temperature fluctuations.

It takes a lot of energy to change the temperature of water

because each little temperature change is associated with breaking or forming lots of hydrogen bonds,

and they absorb or give off a lot of heat.

In fact, the specific heat capacity of water is about five times that of common rocks.

And amazingly, we haven't even finished talking about how powerfully useful these partial charges are.

They also allow water to dissolve pretty much anything that's even partially non-polar,

which includes sugars, proteins, ions, and tons of inorganic chemicals.

Water and its useful little dipole moment can dissolve more compounds than any other chemical on Earth.

Frankly, it's amazing that it doesn't dissolve us from the inside out.

Which brings me to one last little polarity tidbit, the hybrid molecule.

There are lots of different molecules, like the surfactants in soap, for example, that

have both polar and non-polar areas.

Dish soap is thus able to dissolve the fatty parts of my butter catastrophe here, and then stick the polar sides out,

allowing the whole mess to get washed away by Avogadro's numbers of polar water molecules

that I'm sticking on my hand right now.

Oh yeah. That's better, but not...

I'm gonna have to go to the bathroom to get this all the way fixed up. So, be right back.

Likewise, the fatty acids that make up your cell membranes have polar heads,

which keeps them interacting with the aqueous environment of out bodies,

but non-polar tails, which prevent the cells from being just dissolved by the water around them.

Pretty dang elegant if you ask me.

Thanks for watching this episode of Crash Course Chemistry.

If you were paying attention, you learned that

a molecule needs to have both charge asymmetry and geometric asymmetry to be non-polar,

that charge asymmetry is caused by a difference in electronegativities, and that I am totally team polar.

You also learned how to notate a dipole moment or charge separation of a molecule,

the actual physical mechanism behind "like dissolves like",

and why water is just so dang good at fostering life on this planet.

This episode was written by me, edited by Blake de Pastino.

Our chemistry consultants are Dr. Heiko Langner and Edi Gonzalez.

It was filmed, edited, and directed by Nicholas Jenkins.

Michael Aranda is our script supervisor and sound designer, and our graphics team is Thought Café.