Chemistry - Intermolecular forces, properties and water test

What is the VSEPR theory?

This theory states that the electron pairs are arranged as far apart as possible

•NOTE: a lone pair has greater repulsion ability than a bonding pair

What is a non-polar bond?

A non-polar bond is formed between atoms of the same electronegativity, usually the same atom.

As a result, the electrons are evenly shared between the atoms

What are polar bonds?

•A polar bond is a bond formed between atoms of differing electronegativity.

•This means that the electrons are not evenly shared between the nuclei but are attracted to the more electronegative element to a greater degree.

•This results in a positive ‘end’ and a negative ‘end’ or positive and negative dipoles.

What are non-polar molecules?

Molecules with non-polar bonds or molecules with polar bonds but the molecule is symmetrical meaning there is no net dipole so it is non-polar.

For example: Methane.

•The carbon atom is slightly more electronegative than the hydrogen atom, resulting in polar bonds. However, the methane molecule is a perfect tetrahedron an is symmetrical. There is no lone pair around the central atom. This means that the individual dipoles of the convent bonds cancel each other. The result is a molecule with no overall dipole.

What is a polar molecule?

If there is a lone pair of electrons on the central atom or there are polar bonds AND it is non-symmetrical, the molecule is polar. In non-symmetrical molecules the individual dipoles of the covalent bonds do not cancel each other.

For Example: chloromethane

•The chlorine atom is more electronegative than the carbon atom, therefore the chlorine attracts electrons from the carbon atom while the carbon attracts electrons from the hydrogen atoms. It is non-symmetrical and there is a net dipole. Therefore the bonds are polar.

How is a molecule more polar?

The greater the difference in the electronegativity of the atoms involved in the bond, the more polar the bond is and therefore the more polar the molecule (assuming it is a polar molecule).

How can melting point (m.p) and boiling points(b.p) be explained through intermolecular forces(IMFs)?

The strength of the combined intermolecular forces is what determines the melting and boiling points of the covalent molecular substances and thus the differences in melting and boiling points can be explained in terms of these forces.

What substances/molecules usually have a higher m.p and b.p than others?

A substance that can form hydrogen bonding, dipole – dipole and dispersion forces between its molecules has the strongest combination, giving higher melting and boiling points.

How does having a higher molecular mass lead to a higher b.p?

It is the dispersion forces that will make the difference – the larger the molecular mass of the atom/molecule, the more electrons the atom/molecule has, the stronger the dispersion forces, the higher the boiling point

What is the solubility of a solid as temperature increases?

Solubility increases

What is the solubility of a gas as temperature increases?

Solubility decreases

What are dispersion forces?

Weak forces which exist between the molecules of all covalent molecular substances. They are the only attractive forces between the neutral atoms or compounds that are made up of non-polar molecules. They are the weakest kind of intermolecular force.

How are dispersion forces formed?

1. in a molecule, the electrons are constantly moving. In the case of a non-polar molecule such as fluorine the electrons spend and equal amount of time around each atom.

2. Occasionally the electrons gather more closely together at one end of the molecule, causing one end of the molecule to become negative and the other end to become positive. This is known as a temporary dipole.  

3. These temporary dipoles can then induce dipoles in neighbouring molecules

4. The neighbouring molecules then induce dipole in their neighbouring molecules and so on. The temporary dipoles attract each other to create the IMF known as dispersion forces.

What are dipole-dipole forces?

Weak attractive forces that occur between two polar molecules. These attractive forces occur because the part of the molecule which is slightly negatively charged, can be attracted to the slightly positive part of another molecule. Because the charges involved are only very small, the dipole-dipole forces are only weak. The more polar the molecule the stronger the dipole-dipole forces will be.

what is hydrogen bonding?

A special case of dipole-dipole force between molecules that contain Fluorine, Oxygen or Nitrogen bonded to a Hydrogen atom. This bond means the bond is very polar (due to high electronegativity difference), thus it is a highly polar molecule. The hydrogen bond will form between the FON of one molecule bonded to a H and the H of the other molecule that is bonded to a FON. Hydrogen bonds are generally 10 times stronger than dipole-dipole and about 1/10^th the strength of an ionic or covalent bond

What are the properties of water?

Density

Surface tension

Ability to act as a solvent

High boiling point

What is the density of water?

The density of water is given as 1.0g/ ml meaning 1 g of water has a volume of 1ml

Why does water have surface tension?

Surface tension refers to the forces acting on the surface of liquids and is a measure of the resistance of a liquid to any increase in its surface area. The surface tension of water occurs as the water molecules are attracted to each other. The surface tension of water is high as hydrogen bonding occurs in all directions and is relatively strong. It holds the molecules together.

What are some processes used in Desalination?

-Reverse osmosis: Water is forced through a semi-permeable membrane that blocks salts and impurities.

-Distillation: Seawater is heated to create steam, which is then condensed into fresh water.

What is Chlorination?

Kills bacteria, viruses, and other pathogens inside water. This is done by adding chlorine gas or sodium hypochlorite to water. However this can form potentially harmful by-products if not managed properly.

What is Fluoridation?

Prevents tooth decay and strengthens enamel. This is done by adding fluoride compounds such as sodium fluoride or fluorosilicic acid. Some people think the dental benefits are not enough to ignore some health risks.

What is the kinetic theory?

Kinetic theory states that

-Gases consist of tiny particles moving in rapid, random, straight-line motion until they collide with one another or the walls of the container.

-The size of the particles is negligible compared to the size of the container in which they are moving. The particles have mass but no volume.

-Any attractive or repulsive forces between particles in the gas phase is negligible. (intermolecular forces)

-The average kinetic energy of the particles increases as the temperature increases.

What is gas pressure?

All gases exert a pressure. As the particles move around a container they collide with the walls of the container. These forces over the given area represents the pressure of the gas.

The velocities of the gas particles and the frequency that the particles hit the walls determines the pressure.

What are the changes to gases that can increase its pressure?

-velocity of the particles is increased (temperature increased) the pressure is increased

-the number of particles is increased (concentration increase) the pressure is increased

-space that the particles are moving in is decreased (volume decreased) the pressure is increased

Why do gasses diffuse (spread across a room)?

Because gas particles have rapid, random motion and a lot of empty space between them so they quickly spread and mix.

Why do gasses easily compress?

Because there is a lot of empty space between particles.

Why do gasses fill a container?

Because there are negligible forces between particles. The particles also move around all the time and spread out.

Why do gasses exert pressure?

Because the particles move rapidly and collide with the surface of any container or object.

Why do gasses have low density?

Because there is a lot of empty space between particles.