Water and Energy
Water is present in all three of its phases in the lowest layer of our atmosphere. Its concentration is highly
variable so it is typically not included in a listing of gases in the atmosphere. The structure and physical
properties of water differ greatly with its phase. We can use liquid water's physical and chemical
properties to help us study chemical energy changes through calorimetry.
Outline
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Water in the Atmosphere
•
Energy and Physical Changes
•
Energy and Chemical Changes
•
Homework
Water in the Atmosphere
Forms of Water
Nearly all of the water is located in the lowest layer of the atmosphere, the troposphere. Water is present
in variable amounts in the atmosphere, from 0 % to 4 %. Unlike oxygen and nitrogen, the concentration
of oxygen depends on local weather conditions and changes greatly from place to place on Earth. Clouds
form preferentially over dark vegetation and just downwind of mountain ranges. The water in the
atmosphere makes up only a very small percentage of the total water on Earth.
In the atmosphere, water exists as a gas (water vapor from
evaporation), as a liquid (droplets of rain and liquid water
that coats solid particles), and as a solid (snow and ice). Its
structure depends on its state.
Water in the gas phase has a bent structure with an H-O-H
angle of 104.5 degrees. In the liquid and solid forms, there
are bonds between the hydrogen atoms of one molecule of
H2O and oxygen atoms of other molecules. This gives a 3dimensional structure in which each oxygen atom is
surrounded by a tetrahedral array of 4 hydrogen atoms.
Chemistry 102
Prof. Shapley
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Water and Solar Energy
Water vapor is a clear, colorless gas. It
does not absorb visible light so it is
unaffected by most of the solar radiation
in the troposphere. However, water
vapor is a greenhouse gas. It absorbs
heat energy from the Earth. (More about
this later!)
Clouds and fog are not gas-phase water.
These consist of particles of liquid and
solid water that reflect approximately 20
percent of the incoming solar radiation in
the troposphere. This makes the
atmosphere and the Earth's surface
cooler than it would be otherwise.
Humidity
Humidity is the concentration of water in the gas phase that is present in air. In central Illinois
summertime, the humidity is very high!
Gas concentration relates the amount of gas molecules in a volume. The amount of gas can be expressed
in molecules, moles, or grams. The volume can be in liters, cubic meters, or cubic centimeters. Molar
concentration is the number of moles per liter (mol/L or M).
Imagine that you put some water in an empty container and seal it. After a while, some of the water will
evaporate. Water will continually evaporate and condense but, after a while, the net amount of water in
the gas phase will remain constant. The water(g)-water(l) system is at equilibrium.
Chemistry 102
Prof. Shapley
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The concentration of the water in the gas phase depends on temperature and pressure.
1. Heating the water from room temperature (~20 deg C) to 70 deg C will cause the amount of water in
the gas phase to increase to a higher but constant amount.
2. Decreasing the temperature to 5 deg C will cause some water vapor to condense until a new, lower
constant concentration is reached.
3. Adding air to the container to increase the pressure will cause some water vapor to condense until a
new, lower constant concentration of water(g) is reached.
The troposphere is like a very large container. It
can take a long time for the equilibrium
condition to become established, but the
maximum amount of water(g) present in the will
depend on temperature and pressure.
The relative humidity is the ratio of the actual
water vapor pressure to the saturation water
vapor pressure (equilibrium value) at the
prevailing temperature and is expressed as a
percentage. At 100% relative humidity, the water
(l)-water(g) system is at equilibrium.
The dew point is the temperature to which the
air must be cooled before water condenses from
it.
Note that the values in the graph at right are not
true concentration values but relate the mass of
water per mass of all gas molecules.
Chemistry 102
Prof. Shapley
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Energy and Physical Change
Water Phases
Water is a very complex material. The
diagram at right shows the phases of water
as a function of temperature and pressure.
At low temperature, water is a solid. The
nature of the solid depends on the
pressure. Ice, as we know it, is one of
about 13 different forms of solid water.
As the temperature is increased water
changes state from a solid to a gas at low
pressure (sublimation), and from solid to a
liquid at higher temperature (melting).
This part of the diagram focuses on
water phases most familiar to us at 1
atm or 1013 hPa.
Here we see that water goes from
solid to liquid to gas as the
temperature is increased.
At 1 atm, water melts/freezes (NFP)
at 273.15 K or 0 deg C and water
boils (NBP) at 373.15 K or 100 deg
C.
The solid phase of water has the most
regular structure. Water molecules are in a rigid lattice bonded to other water molecules. The individual
molecules can vibrate but they can't move through the substance.
Liquid water is almost as regular in its structure as solid water. The difference is that the bonds between
molecules are constantly breaking and reforming so that individual water molecules move through the
substance.
The gas phase of water has no overall structure. The water molecules move independently throughout the
volume available to them.
Chemistry 102
Prof. Shapley
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Water Heating Curve
Let's consider what happens to water at 1 atm pressure when heat is added.
1. Heating ice, H2O(s):
As heat is added to solid water, the temperature and the volume increase. It requires 2.108 joules
to increase the temperature of ice by 1 deg C.
2. Phase change, H2O(s)
H2O(l)
It requires 334 J to melt 1 g of ice. The volume decreases in this transformation as ice has a lower
density than the density of water.
3. Heating water, H2O(l):
It requires 4.187 joules to raise the temperature of water by 1 deg C. After about 4 deg C, the
volume of water increases with temperature.
4. Phase change, H2O(l)
H2O(g)
It takes 2260 J to convert each g of liquid water into steam. The volume of increases greatly in
going from liquid to gas.
5. Heating steam, H2O(g):
It requires 1.996 J to raise the temperature of steam by 1 deg C. The volume increases with
temperature.
Energy in Transfer in the Water Cycle
Water has a very high heat capacity and a very high heat of vaporization. Because of this, much of the
solar energy that is absorbed by Earth (12.5 x 1020 kJ/yr) is used to heat and vaporize water.
Consider 1 kg of water near the equator that is vaporized by solar heating. This requires at least 2260 kJ
of energy. The wind carries this water vapor to a cooler region, say Urbana-Champaign Illinois. When the
water condenses and falls as rain, it releases 2260 kJ of heat. Solar energy from the equator is released as
heat over Urbana-Champaign. In this way, heat is effectively moved around the planet and temperature
differences are reduced.
Chemistry 102
Prof. Shapley
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Energy and Chemical Changes
We can use the physical properties of water and the chemistry of water to understand energy changes in
chemical reactions reactions. Because water has a long liquid range (0 - 100 deg C) and a high specific
heat (4.187 J g-1 deg-1), it is a convenient fluid to use in calorimeters. Water is also a product in
combustion reactions when a reactant molecule contains hydrogen atoms.
State Functions
In a chemical reaction, the energy change depends only on the initial state (reactants) and final state
(products) and does not depend on the pathway for the reactions. Consider the two reaction pathways
below:
In the first pathway, A+B goes directly to the product mixture C+D. There is an energy
between the reactants and products of
E. In the second reaction pathway, A+B first
intermediate molecule, Z, then Z is converted into the products C+D. The overall energy
between reactants and products remains the same no matter how many intermediates there
reaction pathway.
difference
forms an
difference
are in the
When a reaction is carried out under constant pressure (typical case where the pressure is 1 atm), the
energy change is the sum of the enthalpy change and a work term.
ΔE = ΔH + (-PΔV), or
ΔH = ΔE + PΔV
For any chemical reaction: ΔH = Hproducts - Hreactants
The enthalpy change, like the total energy change, depends only on the initial state and the final state.
Chemistry 102
Prof. Shapley
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Let's go back to the 2 step reaction:
1. A + B
2. Z
Z
C+D
Because the total enthalpy change is the sum of the
enthalpy changes for each step, we can always
calculate one of these if we know the others.
Calorimetry and Combustion
Using calorimetry, we can determine how much heat
energy is given off or absorbed in a chemical change.
A calorimeter is an instrument for measuring the heat
of a reaction during a well defined process. Some
calorimeters operate at constant pressure while others
operate at constant volume.
As the reaction proceeds, it either gives up heat or
absorbs heat from the environment. The temperature of
the water around the reaction vessel changes as a result
and this is monitored with a thermometer. Knowing the
mass of the water and its specific heat, we can
calculate the amount of heat transferred to or from the
water.
Consider the combustion of 0.100 mol of H2 in a calorimeter that contains 200.0 g water.
H2 + 1/2 O2
H2O (l)
The initial temperature of the water is 20.0 deg. After the reaction is complete, the temperature is 54.2
deg.
Heat released by reaction = -Heat absorbed by water = -(mass of water)(temperature change)
(specific heat of water)
Heat of combustion = -(200 g)(34.2 deg)(4.187 J g-1 deg-1) = -2.86 x 104 J/0.1 mol H2
Heat of combustion = -286 kJ/mol H2
Chemistry 102
Prof. Shapley
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The heat of combustion of hydrogen is the same as the enthalpy of formation (ΔHf) of liquid water
because it forms the molecule water from its elements (hydrogen and oxygen) in their standard states at
25 deg C (H2 and O2).
Through similar procedures, we can obtain the heat of combustion of carbon in graphite (standard state of
the element carbon) and the heat of combustion of methane, CH4. The heat of combustion of carbon is the
same as the enthalpy of formation of carbon dioxide, CO2.
C(s) + O2
CH4(g) + 2 O2
CO2
CO2 + 2 H2O
ΔHrxn = -394 kJ/mol
ΔHrxn = -891 kJ/mol
Hess's Law
It is difficult to monitor the the reaction between molecular hydrogen and graphite (C) in order to
determine the enthalpy of formation of methane. However, we can calculate it from the information
above.
We can use the elements C, 2H2, and O2 in their standard forms as reactants. There are two pathways that
take these elements to CO2 and 2H2O.
Chemistry 102
Prof. Shapley
page 8
Remember that ΔH is a state function. It depends only on the initial state and the final state. So the
enthalpy change for path #1 must equal the enthalpy change for path #2.
If ΔHf values for all reactants and products of a reaction are known, the standard enthalpy change for any
reaction can be calculated.
Chemistry 102
Prof. Shapley
page 9