Unit 3: Topic 2 - Properties of Solids Flashcards
Which molecule has a greater boiling point, KBr or MgO?
Looking at the formula for coulombic attraction, as you increase the charge of the particles, the force of attraction between particles is greater. In this case, K has a charge of +1, and Br has a charge of -1. On the other hand, Mg has a charge of +2, and O has a charge of -2. Since the magnitude of the charges of Mg and O are greater than K and Br, Mg and O will face greater attraction between each other. This means MgO is held together by stronger intramolecular forces and, therefore, has a higher boiling point than KBr.
It is important to mention that although the distance between Mg and O atoms would be greater than K and Br, it is still okay to assume that Mg and O face greater attraction since you would need the distance between K and Br to be half that of Mg and O for the attraction forces to be comparable and that is not reasonable considering the elements are only one period apart.
Note: KBr has a boiling point of 1,435°C while MgO has a boiling point of 3,600°C.
Describe the intermolecular interactions occurring to water during phases 1-5 of its heating curve.
Remember, the temperature is only a measurement of how fast molecules are moving. During phase 1,
the water is in its solid form, and those molecules move faster and faster until they can’t move any
faster unless they break the intermolecular forces holding them together.
During phase 2, the water is converted to its liquid form, and the heat added to the system is being
used to break intermolecular forces.
During phase 3, water is in its liquid form, and as you add more heat to the system, those liquid water
molecules move faster and faster (hence the increased temperature). Eventually, they can’t move faster unless
they break more intermolecular forces.
During phase 4, heat is used to break more intermolecular forces and covert the liquid water into its
gaseous form.
During phase 5, there are negligible intermolecular forces to be broken, so all heat is used to make those
gaseous water molecules move faster.
During phases 2 and 4 of the heating curve of water, heat is still being added to the system, but the
temperature does not go up as it does in phases 1, 3, and 5. This is because, during phases 2 and 4,
the intermolecular forces between the water molecules are being broken.
The heating curve of water
Describe, at a particulate level, the movement of molecules in their solid phase.
At a particulate level, the movement of molecules is highly restricted in their solid phase. Molecules are not allowed to move, but they do vibrate in their place frequently. This is due to the strong intermolecular forces that hold them together but also prevent them from moving around.
Molecules in their solid state are very close together (hence their strong intermolecular forces), and they are only allowed to vibrate
Define an ionic solid and the intermolecular forces that hold them together.
An ionic solid is a solid made up of oppositely charged ions that are held together by electrostatic force. The intermolecular force holding them together is this vast network of ionic bonds between a positively charged ion and a negatively charged ion.
Ionic bonds hold together ionic solids, and the great strength of this intermolecular force is responsible for the properties of ionic solids
Describe the properties of an ionic solid and how they relate to its intermolecular forces.
- Ionic solids have high melting points because the intermolecular forces holding them together is pure ionic bonds. Ionic bonds are much stronger than covalent bonds because ionic bonds involve exchanging electrons rather than sharing them. This allows for the Coulombic attraction to be much higher.
- They are hard, brittle, and cleave easily.
Figure 1: Cleavage of an ionic solid at a particulate level - In solid form, ionic solids are poor conductors. This is because the ionic bonds that hold together their electrons don’t allow those electrons to flow freely, preventing them from being good conductors of electricity.
- When dissolved in water or melted, ionic solids can conduct electricity. When they are dissolved or melted, the ions are separated, and those electrons are free to flow again.
Define a covalent network solid and the intermolecular forces that hold them together.
A covalent network solid is a solid formed from strong directional covalent bonds.
Diamond is a type of covalent network solid in which a carbon is covalently bonded to 4 other carbons in a large network
Describe the properties of a covalent network solid and how they relate to its intermolecular forces.
- Covalent network solids are very hard and have high melting points. This is because the covalent bonds that hold them together are very strong, and since the entire solid is made up of these covalent bonds, it requires a lot of energy to break or change the phase of these solids.
- Covalent network solids are poor conductors of electricity. The covalent bonds that hold together these solids don’t allow for the free movement of electrons, and therefore, they are not able to conduct electricity.
- Covalent network solids are insoluble in water. This is because the attraction between the covalently bonded atoms and the solvent molecules is not strong enough to overcome the covalent bonds holding together the atoms. However, some network covalent solids are soluble in certain solvents (not water).
Diamond and graphite are both covalent network solids made entirely of carbon, yet diamond is one of the hardest materials on Earth, while graphite is a soft, brittle mineral. Explain the reason for this difference using intermolecular forces.
Diamond contains a vast lattice of carbon atoms, each of which is covalently bonded to 4 other carbons. Graphite, on the other hand, forms in layers, with each carbon being covalently bonded to 3 other carbons. The layers of graphite called graphene are held together by weak London dispersion forces. Therefore, it is easy to break apart layers of graphene but nearly impossible to break apart diamond.
Diamond forms a vast lattice while graphite forms layers held together by weak London dispersion forces
Define a molecular solid and list intermolecular forces that can hold them together.
Molecular solids are solids with discrete molecular units at each lattice position. They are held together by London dispersion forces, and if the molecules that make them up are polar, molecular solids can also experience dipole-dipole intermolecular forces. Additionally, if the molecules have an H covalently bonded to a very electronegative atom such as F, O, or N, the molecular solid can be held together by hydrogen bonds.
Molecular solids have molecules at each lattice position
Describe the properties of molecular solids and how they relate to their intermolecular forces.
- Molecular solids have low melting points. Most molecular solids are held together by relatively weak London dispersion forces and dipole-dipole forces. Molecules that experience H bonding may have higher melting points, but the strength of even these bonds pales in comparison to a covalent or ionic bond.
- Molecular solids are soft and flexible because the intermolecular forces that hold them together are relatively weak, allowing for some movement along these intermolecular bonds.
- Molecular solids are poor conductors of electricity. The molecules that make up a molecular solid are held together by covalent bonds, which force valence electrons to be tightly held within each molecule. Since these electrons can’t move freely, they cannot conduct electricity.
Caffeine, C8H10N4O2, is the main active ingredient in coffee. It is a large, slightly polar organic molecule with a relatively low melting point of 236°C. In its solid form, it is easily broken apart and does not conduct electricity. What type of solid is caffeine?
Caffeine is a molecular solid. Most large organic solids are molecular solids. These types of solids have low melting points, are soft, and do not conduct electricity. The main intermolecular force holding together caffeine molecules is London dispersion forces due to the sheer size of the molecule but the fact that caffeine is slightly polar means that the molecule would also face dipole-dipole forces.
Define a metallic solid and the intermolecular forces that hold them together.
Metallic solids are solids made from metal atoms that are held together by metallic bonds. Metallic bonding is the sharing of delocalized valence electrons that move freely through a solid. These valence electrons are sometimes referred to as a sea of electrons, and unlike ionic or covalent bonds, they are not localized to the atom they originate from.
Metallic solids have a sea of delocalized electrons that move freely throughout the entire compound
Describe the properties of metallic solids and how they relate to their intermolecular forces.
- Metallic solids tend to have high melting points. This is because the delocalized electrons are electrostatically attracted to all the positively charged metal atoms around them. This lack of bond directionality, combined with the fact that the metal atoms are densely packed, means that metallic bonds are extremely strong and require lots of energy to overcome them. To put the strength into perspective, they are slightly weaker than covalent and ionic bonds but still require a lot of heat to overcome, thus the high melting point of metallic solids.
It is interesting to note that there are some exceptions to this property. For example, gallium, a blue-gray metallic solid, has a melting point of just 29.8°C, allowing it to melt in the palm of your hand. However, cases like gallium are relatively uncommon, and most metals have very high melting points. - Metallic solids are hard and malleable. The hardness is due to the great strength of metallic solids, and the fact that their electrons are delocalized allows the solid to reconfigure itself into different shapes without breaking the solid.
- Metallic solids are very good conductors of electricity. The delocalized sea of electrons allows for the free movement of electrons, making metallic solids the prime choice for conducting materials. For example, copper, the most common conductor material, is a metallic solid. We take advantage of its malleability and conductivity to create thin copper wires that are used in almost all electronics over the world.
Define an alloy and describe the two types of alloys.
An alloy is a substance made by melting two or more elements together, with at least one of them being metal. For example, bronze is an alloy made of copper and tin, and steel is an alloy made of iron and carbon (trace amounts of manganese, silicon, phosphorus, sulfur, and oxygen are frequently added to steel too). Alloys typically retain their sea of electrons, so they tend to conduct electricity. The two types of alloys are interstitial alloys and substitutional alloys.
An interstitial alloy is an alloy in which the space between large host atoms is filled in with a different, smaller atom. Alloys made up of vastly different-sized atoms are almost always interstitial alloys.
A substitutional alloy is an alloy where some of the host metal atoms are replaced with a different, similarly sized atom. Alloys made up of similar-shaped atoms are almost always substitutional alloys.
Interstitial alloys (like steel) are made up of differently shaped atoms, while substitutional alloys (like brass) are made up of similarly shaped atoms
