Topic 2 Bonding and structure

Question 1

What is ionic bonding?

Answer 1

It is the electrostatic attraction between oppositely charged ions

Question 2

Which molecular ions should you know?

Answer 2

OH- Hydroxide
NO3- Nitrate
NH4+ Ammonium
SO42- Sulfate
CO32- Carbonate

Question 3

How to show something is an ion when drawing with dots and crosses?

Answer 3

You put square brackets around them with the charge on the outside

Question 4

How can you work out the formula of an ionic compound?

Answer 4

Use the swap and drop method.
1. Write the 2 ions
2. Swap the charges
3. Drop the charges
4. Simplify to lowest whole number ratio

Question 5

What are ionic compounds like in water?

Answer 5

Most of them dissolve in water as water molecules are polar they can attract the + and - ions and break up the structure

Question 6

Can ionic compounds conduct electricity?

Answer 6

Yes they can when they are molten or dissolved in solution. This is as the ions are now free to move around

Question 7

What is the MP like in an ionic bonding?

Answer 7

They have high MP as there are lots of strong electrostatic forces between oppositely charged ions. Lots of energy is required to overcome these forces.

Question 8

What factors affect the strength of the ionic bonding?

Answer 8

The size of the charge on the ion - the bigger the charge the stronger the electrostatic attraction between ions. KCL is made of K+ and CL-. It’s MP is only 770 but CaO is made up of Ca2+ and O2- and has a MP of 2572.

The size of the ionic radii. The smaller the ion the greater the electrostatic attraction between the ions. Smaller ions can pack together more closely and more energy is required to overcome the stronger forces. The MP and BP is higher

Generally the smaller the ion and the higher the charge, the stronger the electrostatic attraction and hence the higher the melting point. We can say they have a high charge density

Question 9

What are the ionic bonding trends?

Answer 9

The ionic radius increases as we go down a group

As we go down, the number of electron shells increases and so the ionic radius increases too

Question 10

What is an isoelectric ion? and what are there trends

Answer 10

These are different atoms that have the same number of electrons. For example, N3- O2- F- Na+ Mg2+ and Al3+ all have the same number of electrons, but a different number of protons

The ionic radius in isoelectric ions decreases as the atomic number increases.

The radius decreases as the attractive force from the nucleus increases (more protons). This pulls in the outer electron shell more.

Question 11

What is the evidence for charged particles?

Answer 11

Electrolysis of copper(II)chromate(vi) on wet filter paper shows evidence for charged particles

When a drop of Green copper chromate is placed on wet filter paper and electricity is passed through it, the ions start to separate

The positive cu2+ ions move towards the negative cathode. You can see the blue solution move.

The negative CrO42- ions move towards the positive anode. You can see the yellow solution move.

Question 12

What is covalent bonding?

Answer 12

It is the sharing of outer electrons in order for atoms to obtain a full shell.

There is an electrostatic attraction between the shared electrons and the positive nucleus

There are single, double, and triple covalent bonds. More electrons are being shared.

Covalent bonds can be represented by lines

Question 13

What is dative covalent bonding?

Answer 13

This is where 1 atom donates 2 electrons to an atom or ion to form a bond.

An arrow represents a dative covalent bond. The direction represents where they’re moving from

CO has a double covalent bond and a dative covalent bond

Question 14

What is bond enthalpy?

Answer 14

It is related to the length of a bond. The shorter the bond, the higher the bond enthalpy.

Question 15

Bond enthalpy in covalent bonding

Answer 15

In covalent molecules, there are forces of attraction between positive nuclei and the negative electrons being shared

There are repulsive forces between the 2 positive nuclei and between electrons in the atoms

There is a balance between the 2 forces and the result is something we call bond length

The greater the electron density between the atoms, the stronger the attractive force. This means that the atoms are pulled in further towards each other. This leads to a shorter bond and a higher bond enthalpy

Triple bonds have the highest electron density

Question 16

What determines the shapes of molecules? Why do molecules have specific shapes?

Answer 16

The number of bond pairs and the number of lone pairs of electrons to work out the shape of the molecule

The reason is that bonds repel each other equally. Bonds contain electrons so they will want to be as far apart as possible

Question 17

What is the order of repulsion?

Answer 17

2 lone pairs > 1 lone and 1 bonding > 2 bonding.

Generally for every lone pair, you reduce the remaining bond angles by 2.5 degrees

Question 18

What are the shapes of molecules with no lone pairs?

Answer 18

2 bonding pairs and no lone pairs are linear - 180
3 bonding pairs and no lone pairs is trigonal planar - 120
4 bonding pairs and no lone pairs is tetrahedral - 109.5
5 bonding pairs and no lone pairs is trigonal bipyramidal - 90 and 120
6 bonding pairs and no lone pairs are octahedral - 90

Question 19

What are the shapes of molecules with lone pairs?

Answer 19

3 Bonding pairs and 1 lone pair is a triangular pyramid - 107
2 bonding pairs and 2 lone pairs is bent - 104.5

Question 20

What are some example of giant covalent structures?

Answer 20

Graphite, Graphene and Diamond

Question 21

What are the features of graphite?

Answer 21

Each carbon is bonded 3 times with the 4th electron being delocalised
Layers slide easily as there are weak forces between the layers
Delocalised electrons between the layers allow graphite to conduct electricity as they can carry a charge
Lots of strong covalent bonds means graphite has a very high melting point
Graphite is insoluble. The covalent bonds are too strong to break

Question 22

What are the features of diamond?

Answer 22

Each carbon bonded 4 times in a tetrahedral shape
The tightly packed rigid arrangement allows heat to conduct well in diamonds
Unlike graphite, diamonds can be cut to make gemstones
Very high MP due to many strong covalent bonds. It is also very hard
Diamond doesn’t conduct electricity well as it doesn’t have any delocalised electrons
Diamond is insoluble. The covalent bonds are too strong to break

Question 23

What are the features of graphene?

Answer 23

Graphene is one layer of graphite. It is 1 atom thick and made up of hexagonal carbon rings
Delocalised free moving electrons makes graphene an excellent conductor of electricity as they can carry a charge
The same delocalised electrons strengthen the covalent bonds. This gives graphene a high strength property
As it is only one cell thick, it is lightweight and transparent

Question 24

What is metallic bonding?

Answer 24

It is the electrostatic attraction between positive metal ions and the negative delocalised electrons.
The more electrons an atom can donate to the delocalised system, the higher the MP
Positive metal ions are formed as metals donate electrons to form a sea of delocalised electrons.

Question 25

What are the features of metals?

Answer 25

Metals are good thermal conductors as the delocalised electrons can transfer kinetic energy
Metals have high MP due to the strong electrostatic attractions
Solid metals are insoluble as the metallic bond is too strong to break
Metals are malleable and ductile as the ion layers can slide when hit with a hammer and still retain an attraction between ions and delocalised electrons
Metals are good electrical conductors as the delocalised electrons are mobile and can carry a current

Question 26

What is electronegativity?

Answer 26

Electronegativity is the ability of an atom to attract electrons toward itself in a covalent bond

The further up and right you go, the more EN the element is. Fluorine is the most EN

The bigger the difference in EN, the more ionic the compound will be. A difference of zero will be purely covalent

Question 27

What are polar bonds?

Answer 27

Covalent bonds can become polar if the atoms attached to it have a difference in EN

The bigger the difference in EN, the more polar the bond will be. Cl is more EN than H so it is the more electronegative element

To show a polarity we use a delta - and delta +

Atoms that are bonded with the same or similar EN values aren’t polar.

Look out for whole molecules that may appear to be polar. If the polar bonds are arranged symmetrically then you have no overall polarity

Question 28

What are the Intermolecular forces?

Answer 28

Hydrogen bonding
permanent dipole - dipole
London (instantaneous)

Gets weaker as you go down

Question 29

What is a dipole?

Answer 29

Any molecule or atom with electrons can form a dipole when it moves near another atom or molecule
This occurs as electrons in a molecule or atom can move from one end to another. Hence creating a temporary dipole
This temporary dipole only exists when 2 molecules or atoms are nearby. When they move away the dipole interaction is destroyed
The delta+ atom on one atom or molecule will be attracted to the delta- on another and a force of attraction is created

Question 30

What are London forces?

Answer 30

London forces can hold some molecules in crystal structures
The bigger the molecule or atom, the more London forces as you have larger electron clouds
When we boil a liquid, we are breaking the weak London forces NOT covalent bonds

Longer straighter chain hydrocarbons have more London forces and so more energy is needed to overcome them. This means the BP increases

Hydrocarbons with branches means that they can’t pack together as close. This weakens the London forces between the chains and lowers their BP

Question 31

What are permanent dipole - dipole forces?

Answer 31

There are wea electrostatic forces that exist between molecules with a polarity. The delta - part on one molecule is attracted to the delta + on another.
Unlike London forces, dipole-dipole interactions involve molecules with a permanent dipole and so are stronger.

Molecules that have dipole-dipole interactions also have London too

Question 32

How can you test if a molecule is polar?

Answer 32

Polar molecules (like water) can be tested by placing a charged rod near a steady stream of polar liquid. You should see the liquid bend towards the rod as the molecules align to face the oppositely charged rod

Question 33

What is hydrogen bonding?

Answer 33

This is the strongest IMF and occurs only when you have very EN elements.

It occurs when Hydrogen on one molecule forms a bond with the lone pair of oxygen, nitrogen or fluorine.

You can show hydrogen bonding using dotted lines between lone pairs and hydrogen

Molecules that have hydrogen bonding also have London and Dipole-Dipole too

Question 34

Why is ice less dense than water?

Answer 34

Ice forms a regular structure held by Hydrogen Bonding. The orientation of hydrogen bonds pushes the molecules further apart. This makes ice less dense than water.

Alcohols are not as volatile as alkanes with similar masses due to hydrogen bonding. Alkanes have weaker London forces.

HF has a higher BP than HCL as it has hydrogen bonding. More energy is needed to overcome the electrostatic forces

There is a slight increase in BP from HCL due to the increased mass of molecule, hence a bigger electron cloud and more London Forces.

Question 35

What does a substance require for it to dissolve?

Answer 35

Solvent bonds must break
Substance bonds must break
New bonds should form between the solvent and the substance

Question 36

Polar solvents

Answer 36

Polar substances can dissolve in polar solvents

These are molecules that have a polarity. Some hydrogen bonds. Some have permanent dipole-dipole interactions and London forces

Most ionic compounds dissolve in polar solvents like H2O. The delta H+ is attracted to the negative ions and the delta - oxygen is attracted to the positive ions.

The water molecules surround the ion in a process called hydration

For this to happen, the new bonds formed must be the same strength or greater than those broken. If not then the substance is very unlikely to dissolve

Some non-ionic substances can dissolve too. Alcohols dissolve in polar solvents as they can hydrogen bond with water molecules.

The hydrocarbon part is non - polar so it doesn’t dissolve in water. The bigger the hydrocarbon part, the less soluble the alcohol is.

Some polar molecules don’t dissolve in water. Haloalkanes don’t dissolve as their dipoles aren’t very strong. Water forms stronger hydrogen bonds between each other rather than with haloalkanes so haloalkanes are insoluble. They can dissolve in solvents that interact via permanent-permanent dipole interactions though.

Question 37

Non polar solvents

Answer 37

Non-polar substances can dissolve best in non-polar solvents.

These are molecules that don’t have a polarity. An example would be alkanes as they only have London forces

Non-polar molecules tend not to dissolve well in water as water forms stronger hydrogen bonds between each other rather than interacting with non-polar molecules