T3W5 Test

12, 14, 15

What types of bonding are found in and between water molecules?

Polar covalent bonds between H and O atoms. H bonding between 𝛿+ charge on H and 𝛿- charge on O. Dipole-dipole forces and London dispersion forces as well.

Why is H bonding strong in water?

The partial charges on H and O atoms are strong due to a large electronegativity difference.

Why does water have a higher melting and boiling point compared to other group 16 hyrides?

The strong H bonds between molecules require large amounts of energy to disrupt.

Why is ice less dense than liquid water?

When water freezes, each molecule forms 4 H bonds with neighbouring molecules, creating a fixed hexagonal lattice with empty space between molecules, decreasing the density. In liquid water, molecules can fill empty spaces because the bonds break quickly.

Surface tension definition

A measure of the resistance of a liquid to increasing its surface area.

Explain how surface tension occurs in water.

Water molecules on the surface are packed tighter together bc they have no particles above them to form H bonds with. When something is placed on the surface, the molecules are so strongly attracted to each other that the object can float.

Heat capacity definition

A measure of a substance’s capacity to absorb and store heat energy.

Latent heat definition.

The energy absorbed by a fixed amount of substance as it changes state from solid to liquid or liquid to gas at its MP/BP.

Latent heat of fusion/vaporisation definition.

Fusion: heat needed to change 1 mole of a substance from solid to liquid at its MP.

Vaporisation: heat needed to change 1 mole of a substance from liquid to gas at its BP.

What must happen for a substance to dissolve?

The attractive forces between the solvent and solute particles must be sufficient to overcome the solute-solute and solvent-solvent forces.

Describe the process of ionisation.

Dipole-ion attraction between water molecules and the ions in a ionic substance lead to the breaking of the polar covalent bonds between the cation and anion. The ions are surrounded by polar water molecules and held in solution by ion-dipole attractions (hydrated).

Disassociation meaning.

The process of dissolving a solid ionic compound in water and separating the cations and anions to form hydrated ions.

Which ions are always soluble in water?

Sodium, nitrate, ammonium, potassium, ethanoate.

Solubility definition

The maximum amount of a solute that can be dissolved in a given quantity of a solvent at certain temperatures.

Why does increasing temp increase solubility?

It has more energy to overcome the forces of attraction which hold a substance together.

What is the VSEPR theory based on?

The principle that electron pairs in the outer shell of an atom repel each other, so they are arranged as far away from each other as possible.

What are the steps for predicting the shape of a molecule?

Draw the Lewis structure, count the amount of ‘things’ on the central atom, determine shape and bond angles.

What is the bond angle for linear molecules?

180˚

What is the bond angle for trigonal planar molecules?

120˚

What is the bond angle for tetrahedral molecules?

109.5˚

What is the bond angle for bent molecules with 1 lone pair?

<120˚

What is the bond angle for bent molecules with 2 lone pairs?

104.5˚

What is the bond angle for trigonal pyramidal molecules?

107˚

What do the shapes of molecules play a role in determining?

The intermolecular forces between molecules and physical properties such as polarity, solubility, density, vapour pressure, MP and BP.

How does the strength of intermolecular forces compare to intramolecular forces?

They are 10-100 times weaker than bonds like ionic, covalent and metallic bonds.

What is vapour pressure?

The pressure exerted by particles which have escaped from the surface liquid in a closed container colliding with the container walls.

How do intermolecular forces affect vapour pressure?

Liquids with strong IMFs have molecules held together tightly, so it is harder for them to escape the liquid’s surface, meaning vapour pressure is lower.

What is electron density?

The measure of the probability of an electron being present at a particular location within an atom.

How does electrostatic attraction arise in intermolecular forces?

When electrons are unevenly distributed due to the shape of a molecule and the electronegativity of its atoms, partial positive and negative charges occur, which attract opposite charges in other molecules.

What are non-polar molecules?

Molecules in which there is an even distribution of valence electrons among atoms, meaning there is no charge on either end.

Why are bonds between different atoms typically polar?

One atom has a higher electronegativity than the other, meaning it has a stronger pull on the electron, causing it to have a partial negative charge while the other atom has a partial positive charge.

Why are intermolecular bonds weaker than ionic bonds?

The partial charge on a polar molecule are smaller than the charges on ions.

Steps for comparing polarity of molecules

Find electronegativites of each atom, subtract the lower value from the higher value, determine which has a greater difference.

What do properties of polyatomic molecules depend on?

The shape of the molecule and the polarity of the bonds.

What are symmetrical molecules?

Molecules with evenly-distributed polar bonds, causing the bond dipoles to cancel out.

How to determine solubility from polarity?

Polar substances tend to dissolve in polar solvents, non-polar substances tend to dissolve in non-polar solvents - like dissolves like.

What is an electric dipole?

The occurrence of a positive charge and an equal negative charge separated by a fixed distance.

What is a dipole?

A separation of positive and negative charges within a covalent bond or molecule.

At what ∆EN is a bond polar/non-polar?

∆EN ≤ 0.4 = non-polar

1.7 ≥ ∆EN ≥ 0.5 = polar

What are the 3 main types of intermolecular forces?

Dipole-dipole, hydrogen bonding, dispersion.

What do dipole-dipole forces result from?

The attraction between positive and negative ends of polar molecules.

How strong are dipole-dipole forces and why?

Relatively weak because of partial charges

How does polarity of molecules affect MP/BP?

More polar molecules have stronger dipole-dipole forces, meaning more heat energy is required to break them.

What is hydrogen bonding?

A special form of dipole-dipole force which only occurs in molecules in which a H atom is covalently bonded to a N, F or O atom.

How does hydrogen bonding occur?

Because O, F and N atoms are small and highly electronegative, they strongly attract electrons, leaving H with a significant positive charge. This charge is electrostatically attracted to lone pairs of electrons on N, O and F atoms in neighbouring molecules, resulting in a relatively strong intermolecular bond.

What are the 2 key requirements for H bonding to occur?

A H atom covalently bonded to a F, O or N atom, and a lone pair of electrons on the F, O or N atom.

What are dispersion forces?

The forces of attraction between non-polar molecules, resulting from temporary dipoles caused by the random movement of electrons surrounding molecules.

How does molecule size affect dispersion forces?

As molecule size increases, strength of dispersion force increases because larger molecules have more electrons so it is easier to produce temporary dipoles.

How does molecule shape affect dispersion forces?

Molecules that form long chains have more surface area to interact with neighbouring molecules, meaning they have stronger dispersion forces.

In what type/size of molecules can dispersion forces dominate other types of forces?

Large polar molecules.

4 properties of gases

Low density, fill available space, compress easily, mix rapidly.

Why do gases mix rapidly?

The wide spacing and independent movement of particles.

What is the kinetic molecular theory?

A model developed to explain gas behaviour based on the behaviour of the particles of the gas.

What are the 7 assumptions of the kinetic molecular theory?

Gases are composed of small particles (atoms/molecules)

Most of the volume occupied by a gas is empty space bc the volume occupied by particles is negligible compared to the space between them.

Particles move rapidly in straight lines.

Particles collide with each other and the walls of the container

Forces between particles are extremely weak

Collisions between particles are elastic

As the temperature of the gas increases, as does the Ek of the particles.

What is an ideal gas?

A theoretical gas composed of particles which don’t interact except during elastic collisions, which conforms to the criteria of the kinetic molecular theory.

In what conditions do gases behave in a non-ideal way and why?

Cooler temperatures and higher pressures bc they make particles come close and interact more.

Describe the relationship between the average Ek of particles and the temperature of the gas.

Directly proportional - the higher the temperature, the larger the proportion of molecules with a high Ek.

Diffusion definition

The process by which gases in a mixture spread out to uniformly fill the total volume available.

What does rate of diffusion depend on?

The temperature of the gas, and the molar mass and average velocity of particles.

Why do gases made up of smaller particles diffuse faster?

Lighter particles have higher velocities and therefore diffuse faster.

Volume definition

The quantity that is used to describe the space that a substance occupies.

Pressure definition

The force exerted on a unit area of a surface by the particles of a gas as they collide with the surface.

What is the total pressure of a mixture of gases?

A sum of all of the partial pressures of each gas in the mixture.

What does Avogadro’s Law state?

At the same temperature and pressure, equal volumes of gases contain equal numbers of particles.

What are standard conditions?

0^oC (273.15 K) and 100 kPa

What determines how much the volume of a gas differs from an ideal gas?

Intermolecular forces - the stronger intermolecular forces, the greater the difference.