Chapter 6 Flashcards
What is a lone pair?
A pair of electrons not involved in a covalent bond
What is the electron pair repulsion theory?
All electrons have negative charge. Therefore electron pairs will repel all other electron pairs around an atom such that these pairs will occupy a position that will minimise the amount of repulsion from other pairs.
How is the electron pair repulsion theory used?
It is used to predict and explain shapes of molecules and ions
What are the different shapes a molecule can be?
Linear
Trigonal planor
Tetrahedral
Pyramidal
Non-linear
Octahedral
What is the bond angle of a linear molecule?
180 degrees
What is the bond angle of a trigonal planor molecule?
120 degrees
What is the bond angle of a tetrahedral molecule?
109.5 degrees
What is the bond angle of a pyramidal molecule?
107 degrees
What is the bond angle of a non-linear molecule?
104.5 degrees
What is the bond angle of an octahedral molecule?
90 degrees
How many electron pairs does a linear molecule have?
2
How many electron pairs does a trigonal planor molecule have?
3
How many electron pairs does a tetrahedral molecule have?
4 (all bonding pairs)
How many electron pairs does a pyramidal molecule have?
4 (3 bonding pairs and 1 lone pair)
How many electron pairs does a non-linear molecule have?
4 (2 bonding pairs, 2 lone pairs)
How many electron pairs does a octahedral molecule have?
6
Linear shape:
Trigonal planor shape:
Tetrahedral shape
Pyramidal shape
Non-linear shape
Octahedral shape
Why do lone pairs repel more strongly than a bonding pair?
Lone pairs are slightly closer to the central atom and occupy more space thus repel more strongly than a bonding pair
How does a lone pair affect the bond angle?
For each lone pair, the bond angle is reduced by 2.5 degrees
What are three lines used when drawing a 3D arrangement of molecules and how are they used?
Straight line - used when the bond lies in the plane of the paper
Dashed line - used when the bond extends backwards, away from the reader
Wedged line - when the bond extends forwards, towards the reader
What is the explanation for why a molecule is a Trigonal planor?
Because it has 3 bonding regions and 0 lone pairs and are all repelling each other equally.
What is the explanation for why a molecule is a Tetrahedral?
Because it has 4 bonding regions and 0 lone pairs and are repelling each other equally
What is the explanation for why a molecule is a Linear?
Because it has 2 bonding regions and 0 lone pairs and are repelling each other equally
What is the explanation for why a molecule is Non-linear?
Because it has 2 bonding regions and 2 lone pairs. Lone pairs repel more strongly than bonding pairs
What is the explanation for why a molecule is Pyramidal?
Because it has 3 bonding regions and 1 lone pair. Lone pairs repel more strongly than bonding pairs
What is electronegativity?
The ability of an atom to attract a shared pair of electrons in a covalent bond
What is the pattern of electronegativity in the periodic table according to the Pauling scale?
Electronegativity increases up the periodic table, and towards the right of the periodic table (towards F)
What are the reasons for different electronegativity between atoms?
Nuclear charge
Atomic radius
Electron shielding
Why does electronegativity increase across a period?
Because the nuclear charge increases and this attracts the bonding pair of electrons more strongly
Why does electronegativity decrease down a group?
Because the atomic size increases. The bonding pair of electrons are attracted less strongly to the nuclei
What is a polar bond?
When the electron pair is shared unequally between atoms
What is a permanent dipole?
A separation of opposite partial charge
How can you tell when a bond is polar?
A bond is polar when the electronegativity has a significant difference
When is a molecule described as polar?
When the molecule is asymmetrical so the bonds do not directly oppose each other so they do not cancel out.
When is a molecule described as non-polar?
When the molecule is symmetrical so the bonds act in opposite directions so the two dipoles cancel each other out
What are intermolecular forces?
They are the weak interactions between dipoles of different molecules. They are responsible for the physical properties of atoms.
What are the three main types of intermolecular forces?
Induced dipole-dipole interactions (London forces)
Hydrogen bonding
Permanent dipole-dipole interactions
What are the strengths of the intermolecular forces?
London forces - bond enthalpy of 1-10kJ/mol
Permanent dipole-dipole - bond enthalpy of 3-25kJ/mol
Hydrogen bond - bond enthalpy of 10-40kJ/mol
What are London forces?
Weak intermolecular forces that exist between all simple molecules.
How are London forces produced?
The constant movement of electrons produces an instantaneous dipole, which is constantly changing position
The instantaneous dipole induces a dipole on a neighbouring molecule
The induced dipole induces further dipoles on neighbouring molecules, which then attract one another
What increases the strength of a London force?
The number of electrons in a molecule
What is a permanent dipole-dipole force?
They are interactions between the permanent dipoles in different polar molecules
What is hydrogen bonding?
A hydrogen bond is a special type of permanent dipole-dipole interaction found between molecules containing:
An electronegative atom with a lone pair
A hydrogen atom attached to an electronegative atom
How are hydrogen bonds represented?
With a dashed line
How do hydrogen bonds work?
The hydrogen bond acts between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule.
What is a simple molecular substance?
A simple molecular substance is made up of simple molecules - small units containing a definite number of atoms with a definite molecular formula such as neon, hydrogen or water.
In a simple molecular lattice:
The molecules are held in place by weak intermolecular forces
The atoms within each molecule are bonded together strongly by covalent bonds
What are simple molecular lattices?
They are covalently bonded molecules attracted by intermolecular forces e.g. ice
What are the properties of simple molecular substances?
They typically have low melting and boiling points since the weak intermolecular forces can be broken by low temperatures
They are non-conductors of electricity
Non-polar simple molecular substances tend to be soluble in non-polar solvents and insoluble in polar solvents
Polar simple molecular substances may dissolve in polar solvents however the solubility typically depends on the strength of the dipole.
Why can non-polar simple molecular substances dissolve in non-polar solvents?
When a simple molecular compound is added to a non-polar solvent, such as hexane, intermolecular forces form between the molecules and the solvent
The interactions weaken the intermolecular forces in the simple molecular lattice. The forces eventually break and the compound dissolves
Why are non-polar simple molecular substances insoluble in polar solvents?
When a simple molecular substance is added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent molecules
The intermolecular bonding within the polar solvent is too strong to be broken therefore it remains insoluble
Why can polar simple molecular substances dissolve in polar solvents?
They may dissolve as the polar solute molecules and the polar solvent molecules can attract each other.
Why are simple molecular structures non-conductors of electricity?
There are no mobile charged particles in simple molecular structures
With no charged particles that can move, there is nothing to complete an electrical circuit
What are the anomalous properties of water?
Ice is less dense than water
Water has a relatively high melting and boiling point
They have a high surface tension and viscosity
Why is ice less dense than water?
The oxygen atoms and hydrogen atoms each have 2 lone pairs, therefore each water molecule forms four hydrogen bonds.
The hydrogen bonds extend outwards, holding water molecules slightly apart forming an open tetrahedral lattice full of holes.
The holes in the open lattice structure decrease the density of water on freezing.
When ice melts, the ice lattice collapses and the molecules move closer together, therefore water is more dense than ice
Why does water have a relatively high melting and boiling point?
Water has London forces as well as hydrogen bonds between molecules
Extra energy is required to break the hydrogen bonds in water, therefore water has a much higher melting and boiling point relative to other molecules
