U2- Water Flashcards
physical properties of water
- polar compound made of discrete molecules
- colourless, odourless liquid
- freezes at 0C and boils at 100C
- on earth, found in all three states of matter: solid, liquid and gas
- less dense in solid phase than liquid phase
- liquid water evaporates at all temperatures above 0C
why does water have polar covalent bonds
due to difference in electronegativity between O and H
what causes water to have poles (dipoles)
its bent shape
what is hydrogen bonding in water’s effect on its properties
- high MP and BP vs other simple covalent substances
- expansion of water upon freezing
- high latent heat
- high specific heat capacity
- ability of water to dissolve other substances
why do we pick molecules of similar size for comparison of MP/BP?
Because the molecules have a similar exposed SA and number of electrons,
then they will have similar dispersion forces
[(therefore) assured that difference in MP/BP is only due to hydrogen bonds]
why does water have a high MP/BP?
Because each water molecule is capable of forming 4 hydrogen bonds, then these bonds require a lot of energy to disrupt/overcome
Compare BP of water vs ethanol (78C) and explain the difference
While both water and ethanol can form hydrogen bonds, dipole dipole forces and dispersion forces,
water forms more h bonds per molecule (4) than ethanol (2) which require more heat energy to disrupt
therefore water (100C) has a higher BP than ethanol (78C)
why does water expand will freezing?
as water cools, molecules slow down and adopt a more regular arrangement due to the rigid intermolecular h bonds
the resulting open, hexagonal crystalline lattice in the ice pushes molecules further apart then in liquid water, expanding the structure
therefore the sane number of molecules occupy a larger space and ice has lower density than water
Why does 1L block ice, when melting, produce roughly 900mL of liquid water?
As water heats up, the open, hexagonal crystalline lattice is disrupted, pushing molecules closer together allowing them to move more freely
therefore liquid water has a higher density than ice and the same amount of H2O will occupy a smaller volume
latent heat of fusion
energy required to convert 1 mol of a substance from a solid to a liquid at its MP
water = 6kJ/mol
latent heat of vaporisation
energy required to convert 1 mol of a substance from a liquid to a gas at its BP
water = 44kJ/mol
why is sweating on a hot day a good mechanism to cool warm bodies?
after sweat is produced it evaporates on skin
the energy required for this to occur (latent heat of vaporisation) is absorbed from the skin which therefore cools the skin.
Specific heat capacity (SHC)
the amount of energy required to raise the temp of 1g of substance by 1 degree Celsius
water = 4.18J/g/C
why does water have a high specific heat capacity?
Water has a relatively high SHC due to the extensive hydrogen bonding present. That is, the hydrogen bonds require lots of energy to break and so water can absorb a lot of heat before the temperature will begin to increase
why does water have high latent heat values?
because strong H bonds need to be disrupted at each change of state which requires a lot of energy
why do we add acid to water rather than water to acid?
- when a large amount of energy is released into water, energy can spread easily through the liquid and can be absorbed due to high SHC without increasing the temperature by much
- when a large amount of energy is released into acid, the energy causes a much higher temp increase potentially boiling the acid and releasing corrosive fumes as the acid cannot absorb/store the nrg as well
density
a measure of the mass of a substance per unit volume (eg g/L)
solution
an evenly distributed mixture of atoms, molecules or ions including liquid solvent and one more solutes
solvent
the primary component of a solution, in which one or more solutes are dissolved
