Water. Water is a polar molecule composed of two polar covalent O-H bonds in a bent or angular molecular geometry with two pairs of nonbonding electrons. water has 4 pairs of electrons arranged tetrahedral around the central oxygen atom 75% of the earth’s surface is covered with water;
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Although the attractive force from the partial charge of a single polar molecule is not as strong as the charge from an ion, it is plausible that a multitude of polar molecules could react on a single ion effectively. The positive end (H+) of several water molecules are attracted to the negative end of the salt crystal (Cl-) while the negative end of several water molecules (O2-) are attracted to the positive end of the crystal (Na+). The ionic bonds of the crystal are weakened by the solvating effect of the water molecules and the ions break away from the bulk crystal. The large number of water molecules in the container prevent the salt ions from re-combining.
The fundamental explanation for water’s unusual properties relates to the polarity of its bonds. Polarity describes the partial charge associated with a bond or molecule. A polar bond or molecule has a charge distribution present (one end positively charged and the other end negatively charged) while a nonpolar bond or molecule has no distinct charge distribution (neutral).
Water is composed of two polar covalent O-H bonds (the difference in electronegativity is 1.4) arranged in a “bent” molecular geometry. Each bond has a dipole moment pointing in an overall similar direction leading to the existence of an overall dipole moment. The oxygen atom pulls the pair of electrons closer towards itself (making it partially negative) and further from the hydrogen atoms (making them partially positive).
This charge distribution allows the partially positive hydrogen atoms from one molecule to be attracted to the partially negative oxygen atom of another molecule. This strong interlocking network between neighboring molecules is called HYDROGEN BONDING.The ability to form strong hydrogen bonds is the main reason for water’s unusual properties.
HYDRATES: Solids that contain water molecules as part of their crystalline structure. The water in the hydrate is known as the water of hydration or the water of crystallization.
HYGROSCOPIC: A substance is hygroscopic if it readily absorbs water from the atmosphere and forms a hydrate.
DELIQUESCENT: A substance is deliquescent if it absorbs water from the air until it forms a solution.
DESICCANTS: Compounds that absorb water and are used as drying agents.
EFFLORESCENCE: The process by which crystalline materials spontaneously lose water when exposed to air.
The energy required to heat (or cool) a solid (or heat/cool a liquid or a gas) can be calculated using q = msDT. It requires additional energy to change states. The energy required to convert a specific amount of the solid to a liquid is known as the heat of fusion (q = DHfus) and the energy required to convert a specific amount of a liquid to a gas is the heat of vaporization (q = DHvap).
The total amount of energy can be calculated from qT = q1 + q2 + q3...
Heating curve for water
Q. How many kilojoules of energy are needed to change 15.0 g of ice at -5.00oC to steam at 125.0 oC?
The first step is to design a pathway:
q1 = msDT for ice from -5.0 to 0.0 oC, the specific heat of ice is 4.213 J/g oC
q2 = DHfus for ice to liquid at 0.0oC
q3 = msDT for liquid 0.0oC to 100.0 oC
q4 = DHvap for liquid to steam at 100.0oC
q5 = msDT for steam 100.0 to 125.0 oC; the specific heat of steam is 1.900 J/g oC
so qT = q1 + q2 + q3 + q4 + q5
The next step is to calculate each q:
q1= (15.0 g) (4.213 J/g oC) (0.0 - (-5.0) oC) = 316 J
q2 = (335 J / g) (15.0 g) = 5025 J
q3= (15.0 g) (4.184 J/g oC) (100.0 - (0.0) oC) = 6276 J
q4 = (2260 J / g) (15.0 g) = 33900 J
q5= (15.0 g) (1.900 J/g oC) (110 - 100 oC) = 285 J
qT = 316 J + 5025 J + 6276 J + 33900 J + 285 J = 45.8 kJ
Ice at 0oC contains less heat than liquid water at the same temperature. Heat must be added to convert ice to water, so the water will contain that much more additional heat energy. Also the liquid state is in motion much more than the solid state. An increase in motion can only be accomplished by an increase in energy.
1. Which contains less heat, ice at 0oC or water at 0oC? Explain your answer.
2. On the basis of KMT, explain why vapor pressure increases with temperature.
3. Write equations to show how the following metals react with water.
a) aluminum b) calcium c) potassium d) iron
According to the kinetic molecular theory, the vapor pressure of a liquid should increase with temperature because of the increase in collisions and kinetic energy that always accompanies an increase in heat energy (temperature). KEm = 3/2 RT. The increase in energy thus motion allows the liquid molecules to escape (overcome the surface tension and other cohesive forces maintaining the liquid state) from the surface of the liquid into the gas phase.
a) Al (s) + 3H2O (g) 3H2 (g) + Al2O3 *requires steam
b) Ca (s) + 2H2O H2 (g) + Ca(OH)2 *slowly at ambient temperature
c) 2K (s) + 2H2O H2 (g) + 2KOH + heat * vigorous at ambient temperature
d) 3Fe (s) + 4H2O (g) 4H2 (g) + Fe3O4 *requires steam
See next slide for essay/answer
1. Explain the physical process of boiling.
2. Why does ice float in water?
3. Why does water have a relatively high boiling point?
4. Explain if ice will float in ethyl alcohol (d = 0.789 g/L)?
5. How much energy is needed to change 62.74 g of water at 15.00oC to steam at 103.0 oC?
6. Magnesium carbonate, MgCO3, forms a hydrate containing 39.1 % water of hydration. Calculate the formula of this hydrate.
1.645 x 105 J or 3.931 x 104 cal
MgCO3 . 3 H2O
At room temperature the water molecules have enough energy to allow the particles to move past each other but not enough to escape the surface tension. As the temperature of water increases, the heat energy (from the burner) is transferred to kinetic energy (for the molecules) leading to an increase in the molecular motion of the molecules. This action results in an increase in the vapor pressure above the surface of the liquid. When the vapor pressure of the water equals the external pressure, boiling begins. Now a sufficient amount of the molecules have enough energy to resist the attractive forces. Bubbles of vapor are formed throughout the liquid and these bubbles rise to the surface to escape.
Ice floats in its own liquid due to the intermolecular force, hydrogen bonding. As water freezes, the molecular motion of the molecules slow down and the partial positive end (hydrogen) of one water molecule is attracted to the partial negative end (oxygen) of another water molecule. Combine this event with the bent shape of water and the molecules become arranged in a 3-D hexagonal array. This array creates pockets of vacuum (empty space) in the lattice structure as well as a decrease in the number of molecules per unit volume. The mass is directly related to the number of molecules therefore, in the solid state, since there are less particles then there must be less mass per unit volume therefore the solid is less dense than the liquid.
Water has a relatively high boiling point because of the amount of intermolecular forces present. Water experiences LDF (London Dispersion Forces) and d-d (dipole-dipole) forces, along with the additional attractive force, Hydrogen bonding. A large amount of heat energy is required to break all of these forces in order for a phase transition to occur, thus the high boiling point.
4. Explain if ice will float in ethyl alcohol (d = 0.789 g/L)?
Ice would not float in pure ethyl alcohol because the density of water is 1.000 g/mL which is greater than 0.789 g/mL for ethyl alcohol. Yet since ethyl alcohol also undergoes a small degree of hydrogen bonding, the sinking effect is not as dramatic as it would be with a nonpolar substance.
1. Can ice be colder than 0.0oC? Justify your answer.
2. Why does a boiling liquid maintain a constant temperature when heat is continually being added?
3. Why does a lake freeze from the top down?
1. Suppose 50.0 g of ice at 0.0oC are added to 285g of water at 22.0oC. Is there sufficient ice to lower the temperature of the system to 0.0oc and still have ice remaining? Show all work.
2. A mixture of 70.0 mL of hydrogen and 50.0 mL of oxygen is ignited to form water. Does any gas remain unreacted?
3. A 25.0 g sample of a hydrate of FePO4 was heated until all the water was driven off. The mass of anhydrous sample is 16.9 g. What is the formula of the hydrate?