Polar & Non-Polar Molecules: Crash Course Chemistry #23
New Section
This section introduces the concept of classifying molecules as polar or non-polar and discusses the importance of this classification in understanding the chemical world.
Classification of Molecules
- Molecules can be classified as polar or non-polar based on their symmetry and charge distribution.
- Polar molecules have an asymmetrical electron distribution, while non-polar molecules have a symmetrical electron distribution.
- The speaker expresses a preference for polar molecules due to their interesting properties.
Polar vs. Non-Polar Examples
- The speaker demonstrates the difference between polar and non-polar substances using butter (non-polar) and water (polar).
- Water does not mix with butter due to the polarity difference between the two substances.
Factors Affecting Polarity
- Two factors determine whether a molecule is polar:
- Asymmetrical electron distribution around the molecule.
- Difference in electronegativity between atoms in the molecule.
- Electronegativity increases from left to right across the periodic table and decreases from top to bottom.
Geometrical Asymmetry and Dipole Moment
- In addition to electron distribution, geometrical asymmetry is necessary for a molecule to be polar.
- Some molecules have polar bonds but are not overall polar due to symmetrical geometry canceling out charge asymmetry.
- A dipole moment occurs when there is a separation of positive and negative charges within a molecule.
Indicating Charges in Polar Molecules
- Polar molecules can be represented by drawing an arrow with a plus sign indicating partial positive charge and lowercase delta symbols indicating partial negative charges on individual atoms.
- Liquids made up of polar molecules are effective at dissolving polar or ionic compounds.
Intermolecular Forces and Surface Tension
- Intermolecular forces, also known as cohesive forces, determine the strength of surface tension in liquids.
- Surface tension allows water to be poured into a container without overflowing due to attractive forces between water molecules.
New Section
This section explores the fundamental reasons behind the dissolving behavior of polar and non-polar substances.
Dissolving Behavior
- Like dissolves like, meaning that substances with similar polarity tend to dissolve in each other.
- The reason for this phenomenon lies in decreasing the energy of the system.
- Partial positive and negative charges in polar molecules align together, reducing their energy state.
Liquid Crystal Arrangement
- Water molecules form a liquid crystal arrangement where positive and negative charges align.
- The oxygen side of water molecules tends to orient towards hydrogen sides of neighboring molecules.
- This alignment contributes to surface tension and cohesive forces within liquids.
Polarity vs. Ionic Compounds
- Polar molecules have partial positive and negative charges, while ionic compounds have completely transferred electrons, resulting in charged ions.
- Liquids composed of polar or ionic compounds can dissolve other substances with similar polarity.
Timestamps are approximate and may vary slightly.
New Section
This section discusses the similarities between the interactions of water and air, highlighting how water minimizes its surface area and does not mix with oil due to its preference for itself. It also explains how polar water molecules interact with other partial charges and dissolve ionic solids.
Water's Interaction with Oil
- Water minimizes its surface area and expels oil droplets because it prefers itself over oil.
- The dislike between water and oil is actually due to water's preference for itself rather than a dislike for oil.
Polar Water Molecules
- Polar water molecules are attracted to other partial charges.
- In ionic solids, the negative charges on the oxygen side attract positive ions, while the positive charges on the hydrogen side surround negative ions, dissolving the crystals into freely moving ions.
Water-Alcohol Mixture
- When water is mixed with alcohol, the arrangement of molecules becomes more structured and dense, resulting in a smaller volume despite having less liquid.
Hydrogen Bonding
- The polarity of water allows for hydrogen bonding.
- Oxygen and hydrogen atoms in a water molecule engage in loose relationships with neighboring hydrogen and oxygen atoms, forming hydrogen bonds.
- In ice, all oxygen and hydrogen atoms are involved in hydrogen bonding, pushing the molecules apart and making ice 10% larger in volume than liquid water.
Temperature Effects on Hydrogen Bonds
- As temperature rises, ice-like clusters disappear in cold water due to breaking of hydrogen bonds.
- The volume of truly liquid water increases as these clusters disappear.
- Water reaches its highest density at 4 °C.
Energy Requirements for Temperature Change
- Changing the temperature of water requires a lot of energy because each temperature change involves breaking or forming many hydrogen bonds, which absorb or release a significant amount of heat.
- Water has a specific heat capacity about five times that of common rocks.
Water's Dissolving Ability
- Water's partial charges allow it to dissolve partially non-polar substances such as sugars, proteins, ions, and inorganic chemicals.
- Water can dissolve more compounds than any other chemical on Earth.
Polar and Non-Polar Areas in Molecules
- Some molecules, like surfactants in soap, have both polar and non-polar areas.
- Dish soap can dissolve fatty substances by interacting with their non-polar parts while allowing polar water molecules to wash away the mixture.
Cell Membranes and Water Interaction
- Cell membranes have polar heads that interact with the aqueous environment but non-polar tails that prevent dissolution by surrounding water.
Conclusion
- A molecule needs charge asymmetry and geometric asymmetry to be non-polar.
- The dipole moment or charge separation of a molecule can be notated.
- Water's polarity and properties make it essential for life on Earth.