2.2.6 [Intermolecular Interactions and physical properties Flashcards
Why does the boiling temperature of the first 10 alkanes increase?
As Mr increases the number of electrons per molecule increases so the instantaneous and induced dipole increases
As the length of the carbon chain increases the number of points of contact between adjacent molecules increases. Instantaneous dipole induced dipole forces exist at each point of contact between the molecules, so more points of contact greater overall intermolecular forces of attraction
Why do branched alkanes have a lower boiling point temperatures than unbranched isomers?
The more branching in a molecule the fewer points of contact between adjacent molecules. Do not pack together as well. leads to a decrease in overall intermolecular force of attraction between molecules and a decrease in boiling temperatures.
Why do Alcohols have high boiling points than alkanes?
Alcohols contain group O-H therefore can form intermolecular hydrogen bonds in addition to London forces. In case of Methanol and Ethane, both molecules have a similar chain length and both have the same number of electrons. Boiling points should be similar. However boiling point of the alcohol is higher than that of the alkane. This increase is due to the hydrogen bonding that exists between the methanol molecules that is not in the ethane molecules. Additional forces of attraction increases energy required to separate molecules.
What is the trend between the enthalpy change of alcohols and alkanes?
Enthalpy change vaporisation is amount of energy required to completely separate the molecules of a liquid and convert it into a gas at the same temperature. Direct measure of strength of the intermolecular forces. Greater the enthalpy change of vaporisation, greater the forces of attraction are between the molecules.
The alcohols are much higher enthalpy change of vaporisation this is because of its higher boiling due to the hydrogen bonds that it posses as well as the intermolecular forces it posses. For the first 2 alcohols the predominant force is the hydrogen bonding and for the last 2 it is the intermolecular or London forces.
What are the anomalous properties of water?
It has a relatively high melting and boiling temperature for a molecule with few electrons.
The density of ice at 0 is less than that of water at 0oc
Why does water have such a high melting point?
The hydrogen bonds between the water molecules are relatively strong. As a result the intermolecular forces of attraction in water are greater than would be expected for the number of electrons in the molecule.
Why does water have a higher boiling temperature than that of other molecules with hydrogen bonds?
Number of electrons per molecule is identical, London forces are similar. Hydrogen bonding in water is stronger, this is because…
HF forms an average of one hydrogen bond per molecule whereas water molecules form an average of 2 hydrogen bonds per molecule. So the hydrogen bonding is much more extensive in the water molecule.
Not all the hydrogen bonds in the HF molecule is broken on vaporisation since the HF is substantially polymerised.
Why does water have higher density than ice?
Molecules in Ice are arranged in rings of six, held together by hydrogen bonds. Structure creates large areas of space inside the rings when the ice melts the ring structure is destroyed and the average distance between the molecules decreases causing an increase in density of the water.
What are the features for choosing a suitable solvent?
The solute particles must be separated from each other and then become surrounded by solvent particles.
The forces of attraction between the solute and the solvent particles must be strong enough to overcome the solvent solvent-solvent forces and the solute-solute forces
How are ionic substances dissolved?
Many ionic solids dissolve in water. Energy required to separate ions in the solid is either completely, or partially supplied by the hydration of the ions.
The - end of the water molecule attracts the sodium ions sufficiently to remove them from the lattice. The sodium ions then become surrounded by the water molecules. Interaction between the sodium ions and the water molecules is called ion dipole interaction.
The + end of the water molecule attracts the chloride ions. One in solution the chlorine ions become surrounded by water molecules. Chlorine ions are hydrogen bonded to the water molecules
This process is known as the hydration and the energy released is known as hydration energy.
Which compounds can form hydrogen bonds with water?
Alcohols contain -O-H group therefore form hydrogen bonds with water.
Ethanol and water mix the hydrogen bonding between ethanol and wate molecules is similar in strength to the hydrogen bonding in pure ethanol and water. Solubility of alcohols decreases with increasing hydrocarbon chain length because of London forces.
Which compounds do not form hydrogen bonds with water?
Non polar molecules such as the alkanes do not dissolve in water. attraction between the alkane molecules and the water molecules is not sufficiently strong to disrupt the hydrogen bonds.
Many polar molecules have limited solubility in water because they either do not form hydrogen bonds and with water, or the hydrogen bonds they form are weak compared to the hydrogen bonds in water.
What are the bonding temperature trends of the Hydrogen Hailides?
Steady increase in boiling temperature from HCL to HI is the result of the increasing number of electrons per molecule, which in turn results in an increase in London forces.
HF only hydrogen halide that forms hydrogen bonds between molecules giving it the highest boiling point because hydrogen bonds are much stronger than the IM forces. Increase due to more London forces between the molecules so more energy is required to separate them
