Chapter 5 States of Matter Flashcards
Gas Pressure
Gases in a container exert a pressure as the gas molecule are constantly colliding with the wall of the container
Changing gas volume
- Decreasing the volume (at constant temperature) of the container causes the molecules to be squashed together which results in more frequent collisions with the container wall
- The pressure of the gas increases
- The volume is therefore inversely proportional to the pressure (at constant temperature)
- —-A graph of volume of gas plotted against 1/pressure gives a straight line
Changing gas temperature
- Increasing the temperature (at constant volume) of the gas causes the molecules to gain more kinetic energy
- This means that the particles will move faster and collide with the container walls more frequently
- The pressure of the gas increases
- The temperature is therefore directly proportional to the pressure (at constant volume)
- —A graph of temperature of gas plotted against pressure gives a straight line
The kinetic theory of gases states that
molecules in gases are constantly moving
- The theory makes the following assumptions:
- The gas molecules are moving very fast and randomly
- The molecules hardly have any volume
- The gas molecules do not attract or repel each other (no intermolecular forces)
- No kinetic energy is lost when the gas molecules collide with each other (elastic collisions)
- The temperature of the gas is related to the average kinetic energy of the molecules
ideal gases def
Gases that follow the kinetic theory of gases
The volume that an ideal gas occupies depends on:
Its pressure
Its temperature
Limitations of the ideal gas law
-At low temperatures and high pressures real gases do not obey the kinetic theory
Real gases therefore do not obey the following kinetic theory assumptions at low temperatures and high pressures:
- There is zero attraction between molecules (due to attractive forces, the pressure is lower than expected for an ideal gas)
- The volume of the gas molecules can be ignored (volume of the gas is smaller than expected for an ideal gas)
At low temperatures and high pressures real gases do not obey the kinetic theory as under these conditions:
- Molecules are close to each other
- There are instantaneous dipole- induced dipole or permanent dipole- permanent dipole forces between the molecules
- These attractive forces pull the molecules away from the container wall
- The volume of the molecules is not negligible
Ideal gas equation
pV = nRT
- p = pressure (pascals, Pa)
- V = volume (m3)
- n = number of moles of gas (mol)
- R = gas constant (8.31 J K-1 mol-1)
- T = temperature (kelvin, K)
- The ideal gas equation can also be used to calculate the molar mass (Mr) of a gas
Calculating the volume of a gas
Step 1: Rearrange the ideal gas equation to find volume of gas
Step 2: Calculate the volume the oxygen gas occupies
Lattice Structures
- Most ionic, metallic and covalent compounds are crystalline lattice
- The ions, atoms or molecules are arranged in a regular and repeating arrangement
Giant ionic lattices
- Ionic compounds are arranged in giant ionic lattices (also called giant ionic structures)
- The type of lattice formed depends on the sizes of the positive and negative ions which are arranged in an alternating fashion
Covalent lattices
-Covalent compounds can be arranged in simple molecular or giant molecular lattices
- -Simple molecular lattices: Iodine, buckminsterfullerene (C60) and ice
- -Giant molecular: silicon(IV) oxide, graphite and diamond
Metallic lattices
- Metals form giant metallic lattices in which the metal ions are surrounded by a ‘sea’ of delocalised electrons
- The metal ions are often packed in hexagonal layers or in a cubic arrangement
Effects of Bonding & Structure on Physical Properties
Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility
ionic bonding and giant ionic lattice structures
- Ionic compounds are strong
- They are brittle as ionic crystals can split apart
- have high melting and boiling points
- Ionic compounds are soluble in water as they can form ion – dipole bonds
- Ionic compounds only conduct electricity when molten or in solution
Ionic compounds are strong.why?
The strong electrostatic forces in ionic compounds keep the ions strongly together
Ionic compounds have high melting and boiling points. why?
- The strong electrostatic forces between the ions in the lattice act in all directions and keep them strongly together
- Melting and boiling points increase with charge density of the ions due to the greater electrostatic attraction of charges
- Mg2+O2- has a higher melting point Na+Cl–
Ionic compounds only conduct electricity when molten or in solution. why?
When molten or in solution, the ions can freely move around and conduct electricity
In the solid state they’re in a fixed position and unable to move around
Metallic bonding & giant metallic lattice structures
- Metallic compounds are malleable
- Metallic compounds are strong and hard
- Metals have high melting and boiling points
- Pure metals are insoluble in water
- Metals can conduct electricity when in the solid or liquid state
Metallic compounds are malleable. why
- When a force is applied, the metal layers can slide
- The attractive forces between the metal ions and electrons act in all directions
- So when the layers slide, the metallic bonds are re-formed
- The lattice is not broken and has changed shape
Metallic compounds are strong and hard. why
Due to the strong attractive forces between the metal ions and delocalised electrons
Metals can conduct electricity when in the solid or liquid state. why
As both in the solid and liquid state there are mobile electrons which can freely move around and conduct electricity
Simple covalent lattices
- have low melting and boiling points
- —These compounds have weak intermolecular forces between the molecules
- —Only little energy is required to break the lattice
- Most compounds are insoluble with water
- —Unless they are polar (such as HCl) or can form hydrogen bonds (such as NH3)
- They do not conduct electricity in the solid or liquid state as there are no charged particles
- —Some simple covalent compounds to conduct electricity in solution such as HCl which forms H+ and Cl– ions
Giant covalent lattices
- Giant covalent lattices have melting and boiling points
- The compounds can be hard or soft
- Most compounds are insoluble with water
- Most compounds do not conduct electricity however some do
Giant covalent lattices: Giant covalent lattices have melting and boiling points. why
- These compounds have a large number of covalent bonds linking the whole structure intermolecular forces between the molecules
- A lot of energy is required to break the lattice
Giant covalent lattices: The compounds can be hard or soft. why
- Graphite is soft as the forces between the carbon layers are weak
- Diamond and silicon(IV) oxide are hard as it is difficult to break their 3D network of strong covalent bonds
Giant covalent lattices: Most compounds do not conduct electricity however some do. why
- Graphite has delocalised electrons between the carbon layers which can move along the layers when a voltage is applied
- Diamond and silicon(IV) oxide do not conduct electricity as all four outer electrons on every carbon atom is involved in a covalent bond so there are no free electrons available
Vapour pressure
The pressure exerted by a gas molecules (made from liquid) in equilibrium with its liquid
Lower it’s vaporised pressure = stronger bonds (sometimes hydrogen) intermolecular bonds = high boiling and melting points
