Chemical Bonding - 3
Eg. Explain why Cl2 has a higher boiling point than F2
Both Cl2 and F2 have London forces between their molecules. Cl2 has bigger molecules which have more electrons than molecules of F2 Therefore Cl2 has larger temporary dipoles than F2 Cl2 has stronger London forces which require more energy to overcome ∴ Cl2 has the higher boiling point.
We will be considering the following three models to explain the bonding between atoms: (but In certain cases a bond is actually a mixture of more than one type of bond.)
Covalent bonding Ionic bonding Metallic bonding
Examples of questions 1. Same group, different type of intermolecular force Eg. Explain why HF has a higher boiling point than HCl
HF has hydrogen bonds between its molecules. HCl has dipole-dipole forces between its molecules. HF has stronger intermolecular forces. More energy is needed to overcome the stronger intermolecular forces ∴ HF has the higher boiling point.
2. Same group, same type of intermolecular force Eg. Explain why HI has a higher boiling point than HCl
In addition to dipole-dipole forces both HCl and HI have London forces between their molecules. HI has bigger molecules which have more electrons than molecules of HCl Therefore HI has larger temporary dipoles than HCl HI has stronger London forces which require more energy to overcome ∴ HI has the higher boiling point.
There are three types of van der Waals forces:
London forces Dipole - dipole forces Dipole - induced dipole forces (not for examination purposes)
noble gas configuration can be achieved during chemical change in three ways:
Share electrons with another atom Donate one or more electrons to another atom Gain one or more electrons from another atom.
3. Number of hydrogen bonds Eg. Explain why H2O has a higher boiling point than HF
Water forms FOUR hydrogen bonds per molecule. Hydrogen fluoride forms TWO hydrogen bonds per molecule. More energy is needed to overcome the greater number of hydrogen bonds, therefore water has the higher boiling point.
Hydrogen bonding results from the attractive force between polar molecules in which
a hydrogen atom is covalently bonded to a small very electronegative atom such as a N, O, or F atom with at least one lone pair of electrons
Attractive forces between:
the nucleus of an atom (+) and its own electrons (-). the nucleus of an atom (+) and the electrons of the other atom (-).
Repulsive forces between:
the nucleus of an atom (+) and the nucleus of the other atom (+). the electrons of an atom (-) and the electrons of the other atom (-).
The strength of the London forces in different compounds varies. The strength of London forces is influenced by:
- The number of electrons - The interacting surface of the molecule
2) To obtain a lower state of energy. Since: - - The formation of a chemical bond allows the atoms involved to achieve a lower energy state.
- energy is needed to break chemical bonds and - energy is released when chemical bonds are formed.
Valency of Silver is always
1
Atoms bond with each other for two main reasons:
1) To obtain noble gas electron structure for stability. 2) To obtain a lower state of energy.
Valency of Zinc is always
2
The intramolecular bonds that hold the atoms in H2O molecules together are almost __ times as strong as the intermolecular bonds between water molecules. (It takes 464 kJ·mol-1 to break the H--O bonds within a water molecule and only 19kJ·mol-1 to break the bonds between water molecules.)
25
State whether the following compounds are polar or non polar molecules.
3.1 HBr 3.2 He 3.3 NCl3 3.4 PH3 3.5 CCl4 3.6 HF 3.7 H2 3.8 H2O 3.9 CO2 3.10 CO 3.11 NH3
Non polar molecules to know
CO₂ CCl₄ CH₄ symmetrical shapes ∴ bond dipoles cancel
What kind of molecules do halogens form?
Diatomic, non polar molecules
NUMBER of hydrogen bonds in H2O On average each H2O molecule forms ___ hydrogen bonds to other H2O molecules.
FOUR Each molecule has 2 δ+ hydrogen atoms and 2 lone pairs of electrons so that every one of them can be involved in hydrogen bonding.
Explain why HF has a higher boiling point than NH₃
HF - Hydrogen bonding NH₃ - Hydrogen bonding - HF has 2 hydrogen bonds per molecule - NH₃ has 2 hydrogen bonds per molecule - The greater difference in electronegativity in HF means that HF is a stronger dipole - Hydrogen bonding in HF is stronger - more energy is required to overcome the force in HF - ∴ HF has a higher boiling point
F / O / N bonds with hydrogen to form...
HF, H₂O, NH₃
If we consider the hydrogen atom, each atom has one electron in its outermost energy level. If two hydrogen atoms share a pair of electrons between them - with each atom donating one electron - the two hydrogen atoms can share in an electronic structure that is similar to that of ____.
Helium
Explain why H₂O has a higher boiling point that HCl
H₂O - Hydrogen bonding HCl - Dipole dipole - Hydrogen Bonding is stronger than dipole dipole - More energy is required to overcome the intermolecular force in H₂O - ∴ H₂O has a higher boiling point
Explain why H₂O has a higher boiling point than NH₃
H₂O - Hydrogen bonding NH₃ - Hydrogen bonding - H₂O has 4 hydrogen bonds per molecule - NH₃ has 2 hydrogen bonds per molecule - the greater number of hydrogen bonds in water makes the hydrogen bonding stronger - more energy is required to overcome the force in H₂O - ∴ H₂O has a higher boiling point
How is the hydrogen bonding of hydrogen flouride (HF) limited?
In hydrogen fluoride, the problem is a shortage of hydrogens. On average, then, each molecule can only form one hydrogen bond using its δ+ hydrogen and one involving one of its lone pairs. The other lone pairs are essentially wasted.
How is the hydrogen bonding of ammonia (NH₃) limited?
In the case of ammonia, the amount of hydrogen bonding is limited by the fact that each nitrogen only has one lone pair. In a group of ammonia molecules, there aren't enough lone pairs to go around to satisfy all the hydrogens. That means that on average each ammonia molecule can form one hydrogen bond using its lone pairs and one involving one of its δ+ hydrogens. The other hydrogens are wasted.
Diagram showing the four hydrogen bonds per molecule in H2O
In water, there are exactly the right numbers of δ+ hydrogens and lone pairs (oxygen has two lone pairs) so that every one of them can be involved in hydrogen bonding.
∆ > 2,1
Ionic bond
How does Iodine go straight from a solid to a gas?
It sublimes because it has weak intermolecular forces little energy is required for the phase change.
London forces - longer chains
Larger interacting surface area Greater number of London forces
We can use ______ structures to represent covalent bonding.
Lewis and Couper
We can use the ______ to represent the valence electrons of an atom.
Lewis method
All interaction between molecules involves some degree of ___.
London Forces
The intermolecular force between two He atoms is
London force.
The intermolecular force between two O2 molecules
London force.
Noble gases do not bond with other elements. The atoms are held together by ____
London forces
It is important to note that there are also _____ between these molecules. They are much weaker than ionic or covalent bonds and have a significant effect only when the molecules involved are ____. An example is the intermolecular force between HCl molecules. Because of the force of attraction between oppositely charged particles, there is a small dipole-dipole force of attraction between adjacent HCl molecules. The dipole-dipole interaction in HCl is relatively ___. (The covalent bonds between the hydrogen and chlorine atoms in HCl are 130 times as strong.) The force of attraction between HCl molecules is so small that hydrogen chloride boils at -85°C.
London forces close together (touching or almost touching). weak.
Predominant forces
Non polar - London Polar - dipole dipole
Polarity and difference in electronegativity: ∆ = 0
Non polar covalent bond
Linus Pauling finalised a scale of electronegativities for elements on the Periodic Table. This scale is known as the ____. The scale ranges from 0,7 (____) to 4,0 (____). The electronegativities of the other elements are comparative values and therefore have...
Pauling scale Francium → Flourine no units
0 < ∆ <= 2,1
Polar covalent bond
NUMBER of hydrogen bonds in NH₃ (ammonia) On average each NH3 molecule forms ___ hydrogen bonds to other NH3 molecules.
TWO Each ammonia molecule can form one hydrogen bond using its lone pair and one involving one of its δ+ hydrogens. (The other 2 hydrogen atoms are "wasted".)
NUMBER of hydrogen bonds in HF (fluorine) On average each HF molecule forms ___ hydrogen bonds to other HF molecules.
TWO Each molecule can only form one hydrogen bond using its δ+ hydrogen and one involving one of its lone pairs. (The other 2 lone pairs are essentially "wasted".)
Lewis Structures
The 'Lewis Dot' method is simple. Draw atoms with their valence electrons only as dots, grouped in four possible pairs around the atom. Fill the four places around the atom as if they were four orbitals.
Van der Waals forces
The attractive forces or intermolecular forces between molecules
bond length.
The average distance between nuclei of two bonded atoms in a molecule.
HF, H₂O and NH₃ Which has the strongest hydrogen bonding?
Water, then HF, then ammonia
intramolecular bond
a bond which occurs between atoms within molecules
non-polar covalent bond (pure covalent bond)
a covalent bond where there is an equal sharing of electrons.
polar covalent bond
a covalent bond where there is an unequal sharing of electrons leading to a dipole forming (as a result of electronegativity difference)
molecule
a group of two or more atoms which are bonded together strongly enough to behave as a single unit in a chemical reaction.
Valency of an element
a measure of its ability to combine with other elements.
Valency
a measure of the number of chemical bonds formed by the atoms of a given element.
Electronegativity
a measure of the tendency of an atom to attract a bonding pair of electrons.
Atoms gain stability if they obtain...
a noble gas configuration.
covalent bond
a sharing of at least one pair of electrons by two non-metal atoms
In the Couper method each bond I represented by...
a single line
London Force
a temporary attractive force that results when the electrons in two adjacent atoms or molecules occupy positions that make the atoms or molecules form temporary dipoles.
Hydrogen bonds have about ____ of the strength of an average covalent bond. Hydrogen bonding is responsible for ____ (3)
a tenth the formation of genes in the DNA molecule; the helical structure of proteins; and for the incredible strength of Kevlar, a polymer used for canoe hulls and bullet proof vests.
intermolecular force
a weak force of attraction between molecules, ions, or atoms of noble gases
Water - H2O H2O is _____. The H - O bonds are____ bonds. The bond dipoles do not cancel therefore the molecule is ___.
angular polar covalent polar
If the atoms in a bond are equally electronegative, both have the same tendency to _____, and so the bonding pair of electrons will be found on average _____ between the two atoms. This type of bond tends to form between two atoms of the _____ e.g. H2, Cl2.
attract the bonding pair of electrons half way same element
2. Dipole-dipole forces
attractive forces between the positive end of one polar molecule and the negative end of another polar molecule.
Sulphur dioxide - SO2 SO2 is a ___ shape. The S - O bonds are ___bonds. The bond dipoles do not cancel therefore the molecule is ___.
bent polar covalent polar
This is a stable structure, so the molecule stays together, with the hydrogen atoms a fixed distance apart (known as the ____). The electrons are NOT fixed between the nuclei but do spend most of their time there.
bond length
The polarity of a molecule is the sum of all the _____ in the molecule. A polar molecule is called a ___.
bond polarities dipole
For example, a hydrogen chloride (HCl) molecule is a dipole because _____. (recall electronegativity), producing a partial positive charge on the H end and a partial negative charge on the Cl end
bonding electrons are on average closer to the chlorine atom than the hydrogen
There are________ forces between two atoms that approach each other to form a chemical bond.
both attractive and repulsive
When the bond is polar covalent, the molecule....
can either be polar or non-polar.
Atoms are combined together to form molecules by pairing electrons without...___ They make an '_____' around each atom in this way (except ______ which can only support 2 valence electrons).
changing the total number of electrons. octet Hydrogen
in ions, valency is equal to
charge (Cu²⁺ → valency of Cu = 2)
Forces between atoms in a chemical bond: There must be forces between the atoms in a molecule to make it possible to form such a unit. This mutual binding force is known as a ______
chemical bond.
This chemical change is known as...
chemical bonding.
Most substances occur as ____ in nature so one can deduce that...
compounds the bonded state is generally more stable than the non-bonded (free) state.
London forces are the attractive forces that cause non-polar substances to ______ when the temperature is lowered sufficiently.
condense to liquids and to freeze into solids
For example, the intramolecular force in water keeping the hydrogen atoms and oxygen atoms together is a ____ whereas the force that attracts one water molecule to another water molecule is an intermolecular force.
covalent bond
We have already dealt with intramolecular bonds; these are the forces inside the molecule keeping the atoms in a compound together, namely ______ (we will be dealing with ionic and metallic bonding later).
covalent bonding
Generally speaking, ______ are formed between atoms which have the same electronegativity e.g. H2, Cl2 or with a relatively small difference in electronegativity e.g. HCl (H = 2,1, Cl = 3,0 therefore difference = 0,9).
covalent bonds
If the molecule is not ____, the shape of the molecule is very important in determining if the molecule is polar or non polar.
diatomic
The intermolecular force between two HCl molecules is
dipole - dipole force.
3. Hydrogen bonding Hydrogen bonding is a special case of ______ forces. They are STRONGER than any other van der Waals forces.
dipole-dipole
The bond dipole acts in a ____ and has a ____. The arrow alongside shows the bond dipole in HCl
direction magnitude
The H-F, H-O, and H-N bonds are very polar, because the electronegative atom _______. This leaves the hydrogen nucleus exposed and the resulting ____ is substantial. The positive end of this dipole, consisting essentially of _____, has a high charge density because the proton is so small. Consequently, it strongly attracts the ____ end of a dipole in another molecule.
draws the bonding electron pair strongly to itself. bond dipole an exposed hydrogen nucleus negative
Each atom is assigned a value called the ____
electronegativity
Polar and non-polar covalent bonds The ______ is an indication of the attraction of the atom for a shared pair of electrons in a bond.
electronegativity
In a larger atom or molecule, the valence electrons are, on average, ____ the nuclei than in a smaller atom or molecule. They are less tightly held and can more easily form ___
farther from momentary dipoles.
In a polar covalent bond, the more electronegative atom has a ______ than the less electronegative atom.
greater share of the bonding electrons
Hydrogen bonding is also responsible for other unusual properties of water, such as ____ (3 things). Hydrogen bonding is also responsible for the fact that water in the solid phase is ____ than water in the liquid phase.
high surface tension, specific heat, and heat of vaporisation (vapour pressure). less dense
The fact that H2O can form more hydrogen bonds than either HF or NH3 causes water to have a ____
higher melting point and boiling point.
A stronger type of intermolecular force is the _____.
hydrogen bond.
The intermolecular force in HF, H₂O and NH₃ is...
hydrogen bonding.
Simply, boiling point is a good guide to the strength of ___ forces. If they are __, you don't need to supply much energy to overcome them to produce gas molecules, and the boiling point is low. If the intermolecular forces are __, you have to supply a lot of energy to overcome the forces so that the molecules can move apart, and the boiling point is high.
intermolecular weak strong
Intra and Inter molecular forces: We can distinguish between two main types of forces, ____ and ____
intramolecular bonds intermolecular forces
Bond strength and bond length are ____ related--a short bond is generally ____ than a long one.
inversely stronger
Carbon dioxide - CO2 CO2 is ____ in shape. The C = O bond is ____ but the molecule is _____ because the bond dipoles cancel.
linear polar covalent non polar (bond dipoles cancel due to symmetrical shape)
Another example is the force of attraction between helium atoms. Helium becomes a ____ at temperatures below 4 K which indicates that there must be forces of ___ between the neutral atoms which increase as the atoms get closer together. By itself, a helium atom is perfectly ____. But movement of the electrons around the nuclei of a helium atom can cause the electron distribution to become asymmetrical, producing a _____. A neighbouring helium atom can become an _____. This creates an induced dipole- induced dipole force of attraction between pairs of atoms.
liquid attraction symmetrical momentary dipole induced dipole
A hydrogen bond forms when the strong partially positive (δ+) hydrogen atom of one molecule attracts a ______ on the highly electronegative atom of a neighbouring molecule
lone pair of electrons
Bond lengths follow the expected trend that bonds between larger atoms are ____ and bonds between smaller atoms are ____.
longer smaller
Metallic bonding is in
metals
Ionic bonding is between
metals and non metals
Types of van der Waals forces 1. London forces Also known as - - -
momentary dipole forces induced dipole-dipole forces dispersion forces.
If the strength of London Forces increases...
more energy is needed to overcome these forces of attraction (usually heat energy) ∴ melting/boiling point increases
The more electronegative atom has a partial ____ charge, and the less electronegative atom has a partial ___ charge. This results in the formation of a ____.
negative positive dipole
Covalent Bonding Generally occurs between two _____ ____ A ____ occurs when the half-filled orbitals overlap to form a new filled orbital. The product of a covalent bond is called a ______ The shared pair of electrons is called the _____
non metal atoms sharing of one or more pairs of electrons molecule bonding pair
Covalent bonding is between
non metals
Polar and non-polar molecules When we looked at bonding we defined polar and non-polar covalent bonds. When the covalent bond is non-polar, the molecule is _____
non polar
The van der Waals forces between ___ are the weakest of the van der Waals forces and are called ____.
non polar molecules London forces
This type of bond is a _____ and the molecule is a _____
non-polar covalent bond non-polar molecule.
The bonding force is a coulombic attraction between the _____ (+) and the ____ (-).
nuclei electrons
The electrons that are shared must have...
opposite spins
1) To obtain noble gas electron structure for stability. o In the noble gases all the ____ and ___ are already full which is why they are reluctant to form chemical bonds. o Filled outer energy levels leads to ____
outer energy levels orbitals increased stability
When two oxygen atoms collide, they have six electrons in their_____ - two less than Neon. If they can share two pairs of electrons, with each atom contributing two single electrons to the pairs, they can both share in an electronic structure that is similar to that of Neon. Two covalent bonds are formed between the oxygen atoms and this is called a _______.
outermost energy levels double covalent bond.
The symbol δ is used to represent this ___ So in the HCl molecule on the H end is 𝛿⁺ and the Cl end is 𝛿⁻
partial change
When non metal atoms with different electronegativities bond, the resulting covalent bond is called a _____
polar covalent bond.
If a molecule is diatomic and the electronegativity of the two atoms is different, the molecule will be a _____
polar molecule (because there is definitely a bond dipole)
The strength of London is related to how easily a given electron cloud can be ____. The electrons in small atoms and molecules tend to be held ___. These electrons are not as easily polarised. Electrons in larger atoms are more easily polarised. So, as molecular size and mass ___, the intermolecular forces get stronger.
polarised closer to their nuclei. increases
The ____ (bond dipole) can be indicated by an arrow where the arrow points towards the ______
polarity of a bond more electronegative atom.
Ammonia - NH3 NH3 is ____. The H - N bonds are ____ bonds. The bond dipoles do not cancel therefore the molecule is ____.
pyramidal polar covalent polar
Bond strength and Bond length: A useful aspect to know of a bond is its strength. Weak bonds are easily broken and molecules with such bonds are fairly _____. Conversely, strong bonds are difficult to break and give rise to _____ molecules.
reactive stable
In a polar covalent bond (if the molecule is not diatomic), the ____ of the molecule determines if the molecule is polar or non-polar.
shape
The molecule can either be polar or non-polar depending on the _____ of the molecule
shape
The strength of the intermolecular forces influences certain properties of the compound e.g...
shape, volume, ability to flow, surface tension, rate of evaporation (vapour pressure), melting point and boiling point.
London Forces - Shorter chain length/side chains
smaller interacting surface area smaller number of London forces
For substances of large atomic or molecular mass, London forces are strong enough that the substances are ___ at room temperature. However most London Force things are gases. An example of the effect of an increase in molar mass is provided by the diatomic halogens of group 7. They progress from F2, a gas at room temperature to Cl2, also a gas, to Br2, a liquid, and finally to solid I2. Only ____ are responsible for this.
solid or liquid London forces
HF, H2O and NH3 The strength of the hydrogen bond in HF is _____ than the hydrogen bond in H2O.
stronger
Larger and heavier atoms and molecules exhibit _____ than smaller and lighter ones.
stronger London forces
Averaged over time, non-polar molecules have a ___ distribution of electrons, that is to say there is no ___. However at any instant in time, there is a probability that the electron density is shifted one way or another to give a brief ___ of a temporary dipole. This temporary dipole can induce a similar dipole in an adjacent molecule. These forces are ___ that occur between temporary induced dipoles of adjacent molecules.
symmetrical dipole moment polarisation of charge weak electrostatic interactions
The number of electrons Larger molecules have more electrons can develop larger ____ and stronger London forces. When descending a group in the periodic table the size of the molecules ___ and the strength of the London forces ___.
temporary dipoles increases increases
The interacting surface of the molecule Long unbranched chains can develop larger ___ than branched chains ∴ stronger London forces Long unbranched chains have a ___ than branched chain ∴ stronger London forces.
temporary dipoles larger interacting surface
Carbon tetrachloride - CCl₄ CCl4 is ____ in shape. The C - Cl bond is negligibly ____ but the molecule is ____ because the bond dipoles cancel.
tetrahedral polar covalent non polar
Methane - CH4 CH4 is ___ in shape. The C - H bond is negligibly ____ but the molecule is ___ because the bond dipoles cancel.
tetrahedral polar covalent non polar
Bond strength
the amount of energy needed to break a chemical bond.
A dipole is different from an ion (in an ion ______).
the charge is completely separated. ie. the atom has either lost or gained electrons in an ion, electrons are not SHARED, they are either lost or gained.
Valence electrons and Valency: valence electrons
the electrons in the outermost energy level of the atom.
When the atoms reach a point where______, a bond forms.
the forces of attraction equal the forces of repulsion
The stronger the intermolecular force is,
the greater the amount of energy that needs to be supplied to move the molecules apart again i.e. the greater the amount of energy (heat) required to cause a phase change. Therefore higher melting and boiling point
The fact that HF and NH₃ can form the same number of hydrogen bonds means that...
the higher boiling point (and melting point) in HF is due to fact that the hydrogen bond in HF is stronger than the hydrogen bond in NH₃ (the greater difference in electronegativity in HF results in a covalent bond that is more polar i.e. a stronger dipole forms)
The greater the difference in electronegativity...
the more polar the bond
The strength of the hydrogen bond in these compounds is determined by _____
the number of hydrogen bonds.
Chemical bonding
the process by which atoms bond with each other and in this way achieve a noble gas configuration, with low potential energy.
dipole
the separation of charge between two covalently bonded atoms.
Bonding pair
the shared pair of electrons
The strength of the van der Waals force depends on ___. The larger the molecule, the more electrons the molecule has and the greater the chance of the molecule becoming ____. This results in stronger van der Waals forces More energy will be required to overcome these forces of attraction and the boiling point will increase as the ____ increases
the size of the molecule. polarised. molecular mass
If atoms in a bond have the same electronegativity...
they have the same tendency to attract a bonding pair of electrons
Where do you find the atoms that participate in Hydrogen bonding?
top group, smallest atoms in the group, high electronegativities and each have at least one lone pair. (Fluorine has more lone pairs but won't necessarily make more inter forces)
London forces exist between __ or __
two non polar molecules or between atoms of Noble gases.
lone pairs
unshared pairs of valence electrons
What electrons are involved in bonding?
valence electrons
Why is it easier for larger atoms/molecules to form induced dipoles?
valence electrons are less tightly held
What would happen if water did not have hydrogen bonds?
water would exist as a gas
van der Waals forces are much ___ than chemical bonds and as a result of this, the compounds that have Van der Waals intermolecular forces have low ___ and ___
weaker melting points boiling points
The London force is often named as the _____ but this is not necessarily true. The strength of the London force depends on the ___ and the ___ of the molecule. In small molecules like H2 and between He atoms, the London forces are very weak because of ____
weakest intermolecular force size, shape small number of electrons.
