Chapter 6 - Shapes of molecules and intermolecular forces Flashcards
What are permanent dipole-dipole interactions?
The δ positive and δ charges on polar molecules cause weak electrostatic forces of attraction between molecules
What is the strongest intermolecular force?
Hydrogen bonding
When will hydrogen bonding occur?
When hydrogen is bonded to something very electronegative, e.g. fluorine, nitrogen or oxygen
or when a lone pair of electrons are attracted to the hydrogen
How to show a hydrogen bond?
A dashed line
Two molecules with hydrogen bonding
Ammonia and water
Why do water, ammonia and hydrogen fluoride have such high m and b points?
A lot of energy is needed to overcome the hydrogen bonds
What happens to covalent bonds during melting and boiling?
They don’t break - only the weak intermolecular forces do
Why is ice less dense than water?
The hydrogen bonds hold the atoms in the lattice far apart from each other. When ice melts, the lattice collapses and the atoms move closer together; water is more dense than ice
Why do boiling points increase as you go down group 7?
The number of electrons in the element increases, so the strength of the induced dipole-dipole interaction increases
Why do simple covalent compounds have such low m and b points?
Little energy is needed to overcome the intermolecular forces
Why are polar molecules soluble in water?
Water is a polar molecule. Compounds with hydrogen bonds form hydrogen bonds with water molecules so will be soluble
Why don’t simple covalent compounds conduct electricity?
Covalent molecules are uncharged
What are the three types of intermolecular forces?
Induced dipole-dipole interactions
Permanent dipole-dipole interactions
Hydrogen bonding
Which is the strongest intermolecular bond?
Hydrogen bonding
What will induced dipole-dipole forces do?
Cause all atoms to be attracted to eachother
How do London forces work?
Electrons in charge clouds are moving rapidly. At any one moment, there will be more electrons on one side than the other. This causes a temporary dipole. This induces a temporary dipole on a neighbouring atom, in the opposite direction. They are then attracted to each other
What will happen to the London forces if there are more electrons?
The stronger the instantaneous dipole will be. This makes the atoms more attracted to eachother
Why do stronger induced dipole-dipole forces cause higher boiling points?
In bigger atoms with more electrons, the dipole-dipole forces are stronger and so take more energy to overcome
What is responsible for holding molecules in a lattice?
Induced dipole-dipole forces - atoms are held together by covalent forces but the molecules are held together by induced dipole-dipole interactions
Why is the boiling point of hydrogen chloride higher than for just fluorine?
Fluorine molecules are non-polar and so only have London forces between
HCl molecules are polar so have London forces AND permanent dipole-dipole interactions between molecules, which requires more energy to overcome
What is a simple molecular substance?
Made up of simple molecules, forming a simple molecular lattice
Bonding in a simple molecular lattice
Atoms within each molecule are held together by strong covalent forces but the molecules themselves are held together only by weak intermolecular forces
Why do simple molecules have such low melting points?
The intermolecular forces are weak
When will non-polar simple molecular structures be soluble and why?
In non-polar solvents - intermolecular forces form between the molecules and the solvent
These weaken the lattice so it dissolves
When will polar molecules be soluble and why?
In polar solvents - the two attract each other
What is electronegativity?
An atom’s ability to attract an electron pair
Strongly electronegative elements
Oxygen, nitrogen, chlorine and fluorine
What will happen in a covalent molecule which is polar?
The electrons will move to the more electronegative element
What happens when a polar bond is created?
A permanent dipole is created
Why is the bond in Cl2 non-polar?
The two atoms have the same electronegativity
When will a bond be non-polar?
When the bonded atoms are the same or the electronegativity of the two atoms is similar
Why is hydrogen chloride a polar bond?
The chlorine is more electronegative, so the electrons move in that direction, creating a permanent dipole
Why is water polar?
The two O-H bonds have a permanent dipole, which act in different directions. The oxygen molecule has the delta negative charge
Bonding in ammonia
One lone pair, all bond angles 107
Bonding in water
Two lone pairs, all bond angles 104.5
Bonding in methane
No lone pairs, all bond angles are 109.5
What is the name of the shape with two bonded and two lone pairs?
Non-linear, 104.5 degrees
What does a solid line represent?
A bond in the plane of the page
What does a solid wedge represent?
Comes out of the paper
What does a dotted line represent?
Goes into the plane of the paper
What happens with each lone pair?
The bond angle decreases by 2.5 degrees
What is a lone pair of electrons?
One which isn’t shared
Why will electrons repel each other?
They are both negatively charged
What is the name of the shape with four bonded pairs?
Tetrahedral, 109.5 degrees
What is the name of the shape with three bonded pairs and one lone pair?
Pyramidal, 107 degrees
Why is carbon dioxide non-polar?
The polar bond are arranged symmetrically so cancel each other out
Why will NaCl be soluble in water?
The Na+ is attracted to the O2-
The Cl- is attracted to the H+, so the structure dissolves
What is the order of repulsion?
Lone pair/lone pair angles are biggest
Lone pair/bonding pair are second biggest
Bonding pair/bonding pair are smallest
What is a hydrogen bond?
The attraction between the lone pair of electrons on one electronegative atom in one molecule and a hydrogen atom in another molecule attached to an electronegative atom
