Chapter 15.2 - Properties And Uses Of Water Flashcards
Explain the structure of water
- H2O = non metals so they bond with each other by sharing electrons - COVALENT
- Oxygen has 2 lone pairs of electrons (non bonding pairs) making it a BENT SHAPE (‘V’)
* oxygen lone pairs have greater repulsion than electrons forming the single bonds between O and H, this pushes atoms closer together giving the BENT shape. - Are POLAR; oxygen has higher electronegativity than H atoms, meaning O-H bonds are more strongly attracted to oxygen atom.
* oxygen partially - and Hydrogen partially + - Bonding of electrons are distributed unevenly across water molecule = DIPOLE
Explain the Hydrogen Bond Between Water Molecules
Main type of INTERMOLECULAR FORCE between molecules in water = HYDROGEN BONDING
HYDROGEN BONDING = formed by an ELECTROSTATIC ATTRACTION between partially (-) H on one molecule and lone pair of electrons on O from neighbouring molecule.
Due to large electronegativity difference between O and H atoms, the partial charges on water molecule are relatively large.
The electrostatic attraction between theses opposite partial charges make hydrogen bonds between water molecules STRONG.
What are Hydrides?
A compound in which hydrogen is bonded to another element.
Water is a Group 16 hydride.
Eg H2O, H2S, H2Se, H2Te, H2Po
Why does water have a RELATIVELY HIGH MELTING AND BOILING POINTS?
H2O = group 16 molecular compound
The melting and boiling points reflect the size of the forces between their molecules.
The higher the melting and boiling point = the STRONGER INTERMOLECULAR forces must be.
* because more energy is required to overcome the forces and allow molecules to move apart from each other.
Despite being the smallest hydride, water has significantly higher melting and boiling points when compared to other group 16 hydrides.
- Due to STRONG HYDROGEN BOND between water molecules
- a significant amount of energy is needed to disrupt all hydrogen bonds between water molecules
Explain the trend in group 16 hydrides.
- apart from water there is a trend in the melting and boiling points of group 16 hydrides.
Melting and boiling points increase down the group.
This increase indicates that the molecular forces responsible for this trend are getting stronger Down the group
The intermolecular forces responsible for this = dispersion forces.
- they increase in strength with increasing mass
How many hydrogen bonds can water make?
Each water molecule has the potential to form 4 hydrogen bonds with surrounding water molecules.
- 2 are partially charged hydrogen atoms
- 2 from the 2 lone pair electrons on oxygen
So all hydrogen atoms and lone pairs in the molecule can be involved in hydrogen bonding
Why does water expand on freezing?
When liquid water freezes to a ice crystal lattice it has greater volume.
* property; lower density in solid phase than liquid phase
Eg frozen lake; top frozen but insulates the water below from cold temps thus reducing possibility that entire lake will freeze - allow aquatic life to live
As liquid water is cooled the water molecules move more slowly. upon approaching freezing temp of water the molecules arrange in a way where each water molecule forms 4 hydrogen bonds to four neighbouring water molecules.
- creating a very open arrangement of molecules, meaning water molecules on ice are more widely spaced than in liquid water.
- therefore ice is less dense than liquid water
- when ice melts the water molecules move more freely and closer together
What is surface tension? Why does water have high surface tension?
Surface tension= measure of the resistance of a liquid to increasing its surface area.
- reason why falling water tends to form water droplets
- also why insects are able to walk across water
Water molecules form hydrogen bonds with neighbouring molecules on the sides and below but not above.
- the sideways forces of attraction to neighbouring molecules are equal in all directions = No NET FORCE
- attraction of surface water molecules to the molecules below them have no opposite force, so water molecules at the surface are PULLED DOWNWARDS
Eg in a beaker of water in room temperature, a water molecule will spend on average on 10^-9 seconds or 1 nanosecond at surface before being pulled back into bulk of the liquid.
The SURFACE TENSION OF WATER IS IN A CONSTANT STATE OF TENSION
What is heat capacity?
Heat capacity of a substance is a measure of the substance’s ability to absorb and store heat energy.
- measurement of how much energy the substance absorbs as its temperature’s increase.
When the same quantity of heat energy is applied to 2 substances with different heat capacities they will undergo different temperature changes.
What is specific heat capacity?
The specific heat capacity of a substance measures the amount of energy in joules needed to increase the temperature of 1 gram of a substance by 1°C
Symbol = C
Expressed = Jg-1 °C-1
* Joules per gram per degree
The specific heat capacity of a substance reflects the type of bonding in that substance.
Eg. for covalent molecules - depends on the strength of the INTERMOLECULAR forces.
What is the specific heat capacity of water? And why is it so high?
4.18 Jg-1 C°
This means 4.18 joules of heat energy are needed to increase the temperature of 1 gram of Water by 1°C
It is High = presence of hydrogen bonds between water molecules.
They are stronger than intermolecular forces and are able to absorb and store large amounts of heat energy before they break.
Conversions to use when calculating S.H.C
- 1ml of water = 1g
- 100ml = 100g
- 1000ml (1L) = 1kg
What is latent heat? What is latent heat of fusion? What is latent heat of evaporation?
Latent heat is the energy required to change a fixed amount of substance (1 mole), from either a solid to a liquid or liquid to gas state.
*Over a period of time that latent heat is being absorbed, the temperature of a substance will not change.
Latent heat of Fusion; heat needed to change 1 mole of a substance from a solid to a liquid at its melting point.
Latent heat of evaporation; heat needed to change 1 mole of a substance from liquid to gas at its boiling point
What is the latent heat of water (fusion + evaporation) and why so high?
Latent heat of fusion; 6.0kj mole -1
* 6.0kj of energy is needed to change 1 mole of water from a solid state to liquid at 0°C.
DUE TO - this energy is need to disrupt the ice lattice by breaking SOME hydrogen bonds between water molecules
Latent heat of evaporation; 44.0Kj mol-1
* 44.0Kj energy is needed change the state of 1 mole of water from a liquid to a gas at 100°C
DUE TO - relatively large amounts of energy is required to COMPLETELY break the hydrogen bonds between the water molecules so they can SEPARATE and form a gas.
Why is latent heat so high?
Latent heat of water is relatively high due to the strength and number of water’s hydrogen bonds relative to its molecular size.
- the specific heat temp will remain constant until the substance has completely changed its state.
If heating continues when state is fully changed, then the temp of the substance will start to increase again.
Calculations for specific heat capacity
Heat energy = specific heat capacity x mass x temp change
Q = C x m x 🔼T
Q = amount of heat energy (J), C = specific heat capacity (Jg-1 C-1) m = mass in grams 🔼T = temp change (C°)
List the properties of water
- Able to dissolve many substances easily
- High specific heat capacity
- High melting and boiling points
- Density in the liquid and solid state
* water expands on freezing
* high surface tension of water - High latent heat
