WATER CHEMISTRY
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Water Structural Molecule
- Each water molecule has one oxygen atom and two hydrogen atoms.
- The chemical formula for water is H2O.
- Covalent bond occurs between oxygen and hydrogen atoms.
- H—O—H angle is 104.5°.
- Dipole forms across the water molecule as a result of the polar covalent bonding between hydrogen and oxygen.
- Bonding electrons are shared unequally by the hydrogen and oxygen atoms, a partial negative charge (ð-) forms at the oxygen end of the water molecule, and a partial positive charge (ð+) forms at the hydrogen ends.
- Each water molecule has one oxygen atom and two hydrogen atoms.
- The chemical formula for water is H2O.
- Covalent bond occurs between oxygen and hydrogen atoms.
- H—O—H angle is 104.5°.
- Dipole forms across the water molecule as a result of the polar covalent bonding between hydrogen and oxygen.
- Bonding electrons are shared unequally by the hydrogen and oxygen atoms, a partial negative charge (ð-) forms at the oxygen end of the water molecule, and a partial positive charge (ð+) forms at the hydrogen ends.
Hydrogen bonding in water
- The H2O molecule is electrically neutral, but the positive and negative charges are not distributed uniformly.
- Since the hydrogen and oxygen atoms in the molecule carry opposite (though partial) charges, nearby water molecules are attracted to each other like tiny little magnets.
- The electrostatic attraction between the ð+ hydrogen and the ð- oxygen in adjacent molecules is called hydrogen bonding.
- Hydrogen bonding makes water molecules "stick" together.
- Hydrogen bond are relatively weak but they are strong enough to hold the water molecules together.
- The H2O molecule is electrically neutral, but the positive and negative charges are not distributed uniformly.
- Since the hydrogen and oxygen atoms in the molecule carry opposite (though partial) charges, nearby water molecules are attracted to each other like tiny little magnets.
- The electrostatic attraction between the ð+ hydrogen and the ð- oxygen in adjacent molecules is called hydrogen bonding.
- Hydrogen bonding makes water molecules "stick" together.
- Hydrogen bond are relatively weak but they are strong enough to hold the water molecules together.
Properties of Water
1. Thermal
1. Thermal
- Ice can float on its liquid form. Ice is less dense than water because water molecules form crystalline structures at freezing (32 degrees Fahrenheit or 0 degrees Celsius) temperatures.
- To maximize the attractions among water molecules, when liquid water cools down to become solid form (ice) the slower moving molecules move farther apart, increasing the overall volume and decreasing the density.
- The density is lower as the water molecules arranged themselves in hexagon form, leaving more space between the molecules than in the liquid form.
- Water has a very high specific heat capacity, which is the amount of heat per unit mass required to raise its temperature by one degree Celsius.
- Water also has a high heat of vaporization, which means that it can take a lot of heat without its temperature rising much. This plays a huge part in the climate, because it means that oceans take a long time to warm up.
- The energy required to raise the temperature of water by one degree Celsius is 4.2 joules per gram.
- Water's ability to absorb and lose heat without a large temperature change provides an organism with a protection against the ill effects of a large external temperature change.
- Water in our bodies stabilizes the temperature of our bodies.
2. Solvent
- The partial charge that develops across the water molecule helps make it an excellent solvent.
- Water dissolves many substances by surrounding charged particles and "pulling" them into solution.
- Eg: Common table salt, sodium chloride, is an ionic substance that contains alternating sodium and chlorine ions
- Partial charges on the water molecule are attracted to the Na+and Cl- ions.
- The water molecules will actually line up differently depending on which ions are being pulled into solution.
- The negative oxygen ends of water molecules will surround the positive sodium ions; the positive hydrogen ends will surround the negative chlorine ions.
- Any substance that carries a net electrical charge, including both ionic compounds and polar covalent molecules (those that have a dipole), can dissolve in water.
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Ionization of water
- The purest water contains some hydronium ions and hydroxide ions. These two are formed by the self-ionization of two water molecules.
The equation is
H2O + H2O <---> H3O1+ + OH1-
The equilibrium expression is the normal products over reactants.
K = [H3O1+] [OH1-] / [H2O] [H2O]
The molarity for the water is a constant at any specific temperature. This means the equation can be rewritten as
K[H2O] [H2O] = [H3O1+] [OH1-]
The quantity on the right hand side of the equation " K[H2O] [H2O] = Kw " is formally defined as Kw. The numerical value for Kw is different at different temperatures.
At 25oC Kw = 1.0 x 10-14
Kw = K[H2O] [H2O]
Kw = [H3O1+] [OH1-] = 1.0 x 10-14
- The purest water contains some hydronium ions and hydroxide ions. These two are formed by the self-ionization of two water molecules.
The equation is
H2O + H2O <---> H3O1+ + OH1-
The equilibrium expression is the normal products over reactants.
K = [H3O1+] [OH1-] / [H2O] [H2O]
The molarity for the water is a constant at any specific temperature. This means the equation can be rewritten as
K[H2O] [H2O] = [H3O1+] [OH1-]
The quantity on the right hand side of the equation " K[H2O] [H2O] = Kw " is formally defined as Kw. The numerical value for Kw is different at different temperatures.
At 25oC Kw = 1.0 x 10-14
Kw = K[H2O] [H2O]
Kw = [H3O1+] [OH1-] = 1.0 x 10-14