Chem - topic 2 Flashcards
What are molecular ions?
Covalently bonded atoms that lose or gain electrons
Which are the 4 elements that don’t tend to form ions and why?
The elements are beryllium, boron, carbon and silicon
Requires a lot of energy to transfer outer shell electrons
What determines the strength of an ionic bond?
- ionic radius and ionic charge
- ionic bonding is stronger and the melting points higher when the ions are smaller and/ or have higher charges
Explain the trend in ionic radius down a group
Ionic radii increases going down the group. This is because down the group the ions have more shells of electrons and thus the outermost electron experience less pull from positive nucleus
Explain the trend in ionic radius for this set of isoelectronic ions, eg N3- to Al3+
There are increasing numbers of protons from N to F and then Na to Al but the same number of electrons. Therefore nuclear attraction between the outermost electrons and nucleus increases and ions get smaller
What are the physical properties of ionic compounds
- High melting points
- non conductor of electricity when solid
- conductor of electricity when in solution or molten
- brittle
Define metallic bonding
Electrostatic attraction between the positive metal ions and the sea of delocalised electrons
Giant ionic lattices have high or low melting and boiling point? Explain your answer
They have high melting and boiling point because a large amount of energy is required to overcome the electrostatic bonds
In what type of solvents do ionic lattices dissolve
Polar solvents eg. water
Why are ionic compounds soluble in water
Water has a polar bond. Hydrogen atoms have a 1+ charge and oxygen atoms have a 2- charge. These charges are able to attract charged ions.
What is it calles when atoms are bonded by a single pair of shared electrons
single bond
What is the effect of multiple covalent bonds on bond length and strength
Double/triple bonds exert greater electron density therefore the attraction between nucleus and electron is greater resulting in a shorter and stronger bond
What is a lone pair
Electrons in the outer shell that are not involved in the bonding
What is a dative covalent bond?
A bond where both of the shared electrons are supplied by one atom
How are oxonium ions formed?
Formed when acid is added to water, H3O+
Draw a dot and cross diagram to show bonding in Al2Cl6
Look at a diagram (2 Cl dative bond towards Al)
What does expansion of the octet mean?
When a bonded atom has more than 8 electrons in the outer shell
What are the types of covalent structure
- Simple molecular lattice
- Giant covalent lattice
Describe the bonding in simple molecular structures
Atoms within the same molecule are held by strong covalent bonds and different molecules are held by weak intermolecular forces
Why do simple molecular structures have low melting and boiling points?
Small amounts of energy is enough to overcome the intermolecular forces
Why do simple molecular structures not conduct electricity
They have no free charged particles to move around
Simple molecular structures dissolve in what type of solvent?
Non polar solvents
Give example of giant covalent structures
- diamond
- graphite
- silicon dioxide, SiO2
List some properties of giant covalent structures (3)
- high melting and boiling point
- non conductors of electricity, except graphite
- insoluble in polar and non polar solvents
How does graphite conduct electricity
Delocalised electrons present between the layers are able to move freely carrying the charge
Draw and describe the structure of a diamond
3D tetrahedral structure of C atoms, with each C atom bonded to four others
What is the shape diagram and bond angle for 3bp 0lp (eg BCl3)
Trigonal planar - 120
What is the shape diagram and bond angle for 2bp 0lp (eg BCl2)
Linear - 180
What is the shape diagram and bond angle for 4bp 0lp (eg CH4)
Tetrahedral - 109.5
What is the shape diagram and bond angle for 5bp 0lp (eg PCl5)
Trigonal bipyramid - 90 and 120
What is the shape diagram and bond angle for 6bp 0lp (eg SF6)
Octahedral - 90
What is the shape diagram and bond angle for 3bp 1lp (eg NH3)
Pyramidal - 107
What is the shape diagram and bond angle for 2bp 2lp (eg H2O)
Non linear - 104.5
What is the shape diagram and bond angle for NH4+
Tetrahedral - 109.5
[diagram]+
Define electronegativity
The ability of an atom to attract the pair of electrons (the electron density) in a covalent bond
What does it mean when the bond is non polar
The electrons in the bond are evenly distributed
How is a polar bond formed
Bonding atoms have different electronegativities
Why is H2O polar, whereas CO2 is non polar?
CO2 is a symmetrical molecule, so there is no overall dipole
What is meant by intermolecular force?
Attractive force between neighbouring molecules
Describe permanent dipole induced dipole interactions
When a molecule with a permanent dipole is close to other non polar molecules it causes the non polar molecule to become slightly polar leading to attraction
Describe permanent dipole- permanent dipole interactions
Some molecules with polar bonds have permanent dipoles → forces of attraction between those dipoles and those of neighbouring molecules
Describe London forces
- London forces are caused by random movements of electrons
- This leads to instantaneous dipoles
- Instantaneous dipole induces a dipole in nearby molecules
- Induced dipoles attract one another
Does boiling point increase or decrease down the noble gases group? Why?
Boiling point increases because the number of electrons increases and hence the strength of London forces also increases
Why is ice less dense than liquid water?
- In ice, the water molecules are arranged in a orderly pattern. It has an open lattice with hydrogen bonds
- In water, the lattice is collapsed and the molecules are closer together
Why does water have a melting / boiling point higher than expected?
Hydrogen bonds are stronger than other intermolecular forces so extra strength is required to overcome the forces
What type of intermolecular forces do alkanes have? Why?
London force → induced dipole-dipole interaction, because the bonds are non polar
What happens to the boiling point as alkane chain length increases? Why?
The boiling point increases because there is more surface area and so more number of induced dipole-dipole interaction. Therefore more energy required to overcome the attraction
Does a branched molecule have lower or higher boiling point compared to equivalent straight chain? Why?
The branched molecule has a lower boiling point because they have fewer surface area and hence less induced dipole-dipole interactions
Are alkanes soluble in water? Why?
Insoluble because the hydrogen bonds in water are stronger than alkanes’ London forces of attraction
Are alcohols soluble in water? Why does solubility depend on chain length?
Soluble when short chain → OH hydrogen bonds to hydrogen bond in water
Insoluble when long chain → non polarity of CH bond takes precedence
Explain the trend of boiling temperatures of hydrogen halides HF to HI
There is a general increase of boiling point from Hal to HI which is caused by increasing London forces because of number of electrons. There is a big drop in boiling point from HF to HCl because fluorine is very electronegative therefore the hydrogen bonding is much stronger
