ch 9- physical and thermal properties of water Flashcards
Water cycle(n.)
-water cycle(n.)=the continuous movement of water on Earth as it transitions between solid, liquid and gaseous state.
Define phase change
Transition of a substance from one state to another
Solid-> liquid
Melting
Liquid-> gas
Evaporation/vaporisation
Gas-> liquid
Condensation
Liquid-> solid
Freezing
Gas-> solid
Deposition
Solid-> gas
Sublimation
Precipitation
Any liquid/frozen water that forms in the atmosphere and falls back to earth
Define potable
Drinkable.
This means that water must have a very low salt content as the human body cannot filter out salt
Define group 16 hydrides
group 16 element covalently bonded to hydrogen
Define heat capacity
Relationship between the heat absorbed by a substance and its temperature change
Define special heat capacity
Energy (J) required to raise the temperature of 1 gram of a given substance by 1 °C
Define latent heat of vaporisation
Amount of energy required to convert 1 mol of a substance from its liquid to gaseous state at the boiling point of the substance
Amount of energy needed for the following state change to occur X(l)-> X(g)
Define boiling point
temperature at which a liquid transitions into a gas
Define solar energy
energy produced by the sun
How does water move throughout our ecosystem?
The water cycle
What are the 3 states of water?
Ice- icecaps, snow
Water vapour- up to 5% of air
Liquid water- seas, rivers, lakes
How much of the earth’s surface is water?
70% of the earths surface 1.386 billion km3 of water on Earth
Distribution of water on earth
All water: 97% salt water, 3% freshwater freshwater: 79% icecaps, 20% groundwater, 1% accessible surface fresh water accessible surface freshwater: 52% lakes, 38% soil moisture, 8% water vapour, 1% rivers, 1% within living organism
How much freshwater is accessible at Earth’s surface?
Only 0.03% of water on earth is both fresh and readily accessible
Is water equally distributed?
No
Is all freshwater safe to drink?
No
Where can the following be found: seawater brackish water fresh water water vapour
Seawater: oceans, seas and salt lakes
Brackish water: estuaries, mangroves, swamps
Fresh water: ponds, lakes, rivers, streams
Water vapour: air
How does hydrogen bonding explain water’s high boiling point?
Water’s boiling point is 100°C which is much higher than the range of temperatures experienced on Earth Water has hydrogen bonding which is a strong intermolecular force. This causes these forces to absorb more energy, requiring more heat to overcome these forces to boil, resulting in a higher BP
How does intermolecular force strength correlate to boiling point?
Stronger intermolecular forces allow for more energy to be absorbed.
More heat energy is needed to overcome intermolecular forces and to boil the substance resulting in a higher BP
Why does water have a higher BP than all the other group 16 hydrides?
Water can form hydrogen bonds with 4 other water molecules making them stronger than the dispersion forces that hold other hydrides together, thus increasing the temperature required to boil it
Why do we use special heat capacity and not heat capacity?
Heat capacity can be useful to describe differences in thermal properties between compounds but is general and has no set units. Thus specific heat capacity defines the heat capacity in a standardised manner
What is the specific heat capacity of water?
4.18Jg-1°C-1
What is specific heat capacity measured in?
Joules per gram per degree Celsius ‘Jg-1°C-1’ OR ‘kJ kg-1°K-1’
Name the specific heat capacity of one other liquid
Milk 3.75Jg-1°C-1 Ethanol 2.46Jg-1°C-1 Coconut oil 2.10Jg-1°C-1 Benzene 1.74Jg-1°C-1
For molecular substances what is a factor in specific heat capacity?
The strength of intermolecular forces within the substance
Why does water have a high heat capacity?
Due to the strong hydrogen bonds between water molecules which are able to absorb a lot of thermal energy before increasing in temperature
What is the formula to determine specific heat capacity?
q = m × c × △T
where
q: amount of energy transferred to the water (joules)
m: mass of the substance (grams)
c: specific heat capacity of the substances (Jg-1°C-1)
△T: change in temperature of the water °C or K
CAN BE FOUND IN THE DATA BOOK
How is the change in temperature calculated?
≈T=Tfinal-Tinitial
How to convert volume (mL) to mass (g)
Volume * the density of water (0.997g mL-1)
Convert 2500mL to grams
2500mL * 0.997g mL-1 = 2492.5g
When combusted, ethanol releases 29.6 kJ per gram. Calculate the final temperature of 350 mL of water heated by the combustion of 1.0 g of ethanol. Assume the temperature of the water is initially at 25°C and all heat energy released from the combustion of ethanol is transferred to the water.
- Determine the mass of water present.
- Convert the amount of heat expressed in kJ to J.
- Using the specific heat capacity of water, substitute the
values into q = m × c × △T to calculate the temperature
change of the water. - Determine the final temperature of the water by using
the equation △T=Tfinal-Tinitial
Answer
Mass of water is 350 mL × 0.997 g mL−1 = 349 g.
1.0 g of ethanol releases 29.6 kJ.
29.6 kJ × 1000 = 29 600 J
The specific heat capacity of water is 4.18 Jg-1°C-1
.
Substitute these values into the equation:
q = m × c × △T
29 600 J = 349 g × 4.18Jg-1°C-1×△ T
△T =29600J/349 g × 4.18Jg-1°C-1
△T = 20 °C
△T=Tfinal-Tinitial
20 °C = T − 25° C
△Tfinal = 45 °C
Therefore, the final temperature of the water is 45 °C.
Why is water denser than ice?
Ice: the water molecules each bond to 4 neighbouring water molecules in a regular lattice. This rigid tetrahedral lattice structure prevents the water molecules from compacting water: random arrangement of molecules as a liquid allows hydrogen bonding to pull water molecules close together, resulting in less empty space liquid water is ~9% denser than ice this is why ice floats on water
How does the strength of intermolecular forces impact a substances latent heat of vaporisation
The stronger the intermolecular forces are, the more energy they can absorb before being overcome and forming a gas. This means that water has a high latent heat of vaporisation relative to others
What is the latent heat of vaporisation of water? (kJ mol-1)
44.0kJ mol-1
Name the latent heat of vaporisation in another substance
H2: 0.9kJ mol-1 O2: 6.82kJ mol-1
How to calculate the latent heat of vaporisation
q=n x Lwhere q: quantity of heat energy (kJ) n: amount of the substance present (mol)
Calculate the heat energy (in kJ) required to convert 450 g of liquid water to water vapour at water’s boiling point (Latent heat of vaporisation of water = 44.0 kJ mol−1).
- Calculate the amount (in mol) of water present.
- Calculate the heat energy with the equation q = × 44.0 kJ mol−1
Answer
(H2O)=m/M
(H2O)=450g/18g mol-1
(H2O) = 25.0 mol
Latent heat of vaporisation of water = 44.0 kJ mol−1
q = n × 44.0 kJ mol−1
q = 25.0 mol × 44.0 kJ mol−1
q = 1.10 × 103 kJ
Therefore, 1.10 × 10^3 kJ of heat energy is required to boil 450 g of water.
Why are waters thermal properties vital for living organisms?
Water’s high specific heat capacity ensures that our internal temperature remains relatively constant even as the external temperature fluctuates- homeostasis
Why is water vital to a multitude of biological processes?
Water’s high BP ensures it remains a liquid even in hot climates allowing life to be found almost everywhere on earth
Why are waters thermal properties vital for aquatic life?
Aquatic organisms are especially reliant on water’s unique thermal properties to survive. This is as many aquatic creatures can only thrive in water of a narrow range of temperatures.
Why are waters thermal properties vital for ocean temperature regulation?
As oceans absorb solar energy, water is constantly evaporating at the surface. Since a large amount of energy is absorbed by liquid water as it becomes a gas, the evaporation process results in a large amount of energy being removed. This keeps ocean temperatures relatively constant, maintaining marine life.
How is global warming impacting thermal regulation?
Due to rising global temperatures, an increase in average ocean surface temperatures worldwide counteract waters natural thermal regulation
Does water exist in a gaseous state at the Earth’s surface
Yes, water vapour makes up some of the air Some say 25% others say 5% so idk
Does water need more or less energy to change from a liquid to a gas at its boiling point than most other molecular substances?
Water has a high latent heat of vaporisation relative to other molecular substances meaning it requires more energy
Identify two factors that must be considered when determining if water from a natural source is
suitable for drinking.
To be potable, water must have a sufficiently low salt content in order for the human body to be able to process it. It is also important to ensure that water is free from contaminants.
Why are burns caused by steam more serious than those caused by boiling water?
Because steam has more energy it has the fusion and vaporisation energy
With reference to the intermolecular forces present, explain why water has high values for its specific heat capacity and latent heat of vaporisation.
Water molecules are held together by dispersion forces, permanent dipole-dipole forces and hydrogen bonds.As a result of the strength of these intermolecular forces, predominantly the hydrogen bonds, water absorbs a large amount of energy in its bonds before its temperature increases or before it changes state, resulting in large values for the specific heat capacity and latent heat respectively
Water is used as a coolant in electronic products to absorb excess heat. What property of water makes it desirable for this purpose? Justify your response.
Water’s high specific heat capacity enables it to store large amounts of energy in the form of heat before its temperature increases.This property of water means that excess heat can be removed more efficiently because more energy is transferred to the water molecules per degree of temperature increase
How much energy does it take to completely convert 2 kg of ice at -5°C into steam at 100°C? Assume no energy loss to the environment.
q = m × c × △T for ice to water q=nL for latent heat (6) q = m × c × △T for water to steam q=nL for latent heat (44) EMERGENCY
Referring to thermal properties, identify and explain one advantage of cooking food in ethanol rather than water.
Ethanol has a lower specific heat capacity than water. As a result, less energy is needed to heat a volume of ethanol to a certain temperature than to heat the same volume of water to this temperature. Accordingly, ethanol will likely take less time to heat up before cooking and less energy will be used in the cooking process
Referring to thermal properties, identify and explain one advantage of cooking food in water, rather than ethanol.
Water has a higher boiling point than ethanol. As a result, water would still be a liquid above 78.4 °C, whereas ethanol would have evaporated. This means that food can be cooked at higher temperatures in water (up to 100 °C) than in ethanol, without the water being boiled away.
Explain why apart from water, boiling point increases down a group.
The strength of intermolecular dispersion forces increases because as one goes down a group, more electrons are added which makes spontaneous dipole-dipole bonds form more often and more strongly
Ethanol can also be combusted to heat water. The combustion of 1.0 g of ethanol releases 29.6 kJ of energy. 300 mL of water at 20 °C is heated by the combustion of 3.0 g of ethanol.
What is the final temperature of the water? Assume all the energy released from the combustion of ethanol is transferred to the water.
m(H2O) = 300 mL × 0.997 g mL−1 m
(H2O) = 299 g
Energy released by ethanol = 3.0 g × 29.6 kJ g−1
Energy released by ethanol = 88.8 kJ
Energy released by ethanol = 88 800 J
q = m × c × ΔT
88 800 J = 299 g × 4.18 J g−1 °C−1 × ΔT
ΔT = 88 800 J / 299 g × 4.18 J g −1 °C−1
ΔT = 71 °C
ΔT = Tfinal − Tinitial
71 °C = Tfinal − 20 °C
Tfinal = 91 °C
Therefore, the final temperature of the water is 91 °C.
