Water is a Chemical
Water is a Chemical
Water plays an important role as a chemical substance. Its many important functions include being a good solvent for dissolving many solids, serving as an excellent coolant both mechanically and biologically, and acting as a reactant in many chemical reactions. Blood, sweat and tears.. all solutions of water. As chemists we consider water from many perspectives. It is our role to use physical and mathematical laws in application for useful purposes, including diverse perspectives such as living systems, materials and energy. The world of the chemist is a small world - atomic, molecular - which plays a large part in making our lives healthy, comfortable, and hopeful. Because of the diversity of the chemical world, it would be difficult to touch upon all of the applications of water. And for the same reason, it would be impossible to discuss the chemical aspects of water without touching upon the physical, mathematical, and biological aspects of the subject. Let's start our discussion of water as a chemical with a look at its structure. From a molecular perspective, structure is one of the important features of a substance. Just as you might say that the shape of a key determines its function - which doors it can and cannot open - the structure of a molecule and its composition absolutely determines its functions and properties. As chemists we have a vested interest not only in understanding how a substance may be used and broken down, but also in knowing how that substance is created. From this perspective let's look at the chemistry which creates water from its elements, hydrogen and oxygen, and the chemistry of water's breakdown, also known as Electrolysis. Water may be a substance so common that we scarcely make note of it - We waste it, pollute it, let it run down the drain, flush it away.. Certainly we take it for granted! However, chemically speaking, water is really not common at all. When compared to other compounds of similar size, composition, and structure - it is absolutely unique! In fact its properties are so unusual that it would be irreplaceable. Let's take a chemical look at these unusual properties, how they arise and what their implications are. Water is a Chemical!?
Indeed! Water is one of our most plentiful chemicals. Its chemical formula, H20, is probably the most well known of all chemical formulas.
What does the chemical formula tell us?
The formula H20 tells us that one molecule of water is comprised of 2 atoms of hydrogen and one atom of oxygen bonded together. The bonds which hold the hydrogen and oxygen together are called covalent bonds - they are very strong. Let's look at a picture of a molecule of water: In this picture the two hydrogens are represented by white spheres and the oxygen by a red sphere. In this second picture, the hydrogens are shown as white spheres, the oygen as a red sphere. The 'sticks' holding the hydrogens to the oxygen represent covalent bonds. Why does the water molecule look bent?
The water molecule maintains a bent shape (bent at 107.5 degrees actually) because of two considerations. First the tetrahedral arrangment around the oxygen and Second the presence of lone pair electrons on the oxygen. What are Lone Pair Electrons?
These are the electrons that are not involved in the covalent bonds. The pairs of electrons are left alone. In our picture they are represented by the double dots. These lone pairs are very negative - containing two negative electrons each - and want to stay away from each other as much as possible. These repulsive forces act to push the hydrogens closer together
Did you say "Tetrahedral" - What does that mean?
Tetrahedral means "four-sided". In chemistry we interpret this in our imaginations. Draw the central atom in an imaginary space. Next put the atoms attached to the central atom around it such that the distance between them is maximized. The arrangement you'll adopt will be the form of a regular tetrahedron. This molecular shape is shown below. It has regular bond angles of 109.5
If we do a similar arrangement of water, putting oxygen in the center, and using the two hydrogens and two lone pairs at the corners, we also come up with a tetrahedral arrangement. However, there is one important difference - the bond angles for water are not 109.5. Because of the presence of the very negative lone pair electrons, the two hydrogens are squeezed together as the two lone pairs try to get away from each other as far as possible. The resulting angle gives water a 104.5 bond angle. Because we don't "see" the electrons, the resulting tetrahedron "looks" BENT!
What's your Point?
Like many things in the chemical world, the shape and structure of a molecule is an important determinant of its function. The importance of the bent structure of water is that it provides water with two distinct "sides": One side of the water molecule has two negative lone pairs, while the other side presents the two hydrogens.
Let's take another look:
[ fig of electron density map of water ]
Does this make water unusual?
YES! But it's not just that the molecule is bent that makes it special. Water is also highly polar - the two sides of water have very different charge. The lone pairs are negative - Are the Hydrogens positive?
The hydrogens are slightly positive. They get this way because of the "electronegativity" of oxygen. Electronegativity is a measure of how much one atom wants to have electrons, and oxygen wants to have electrons more than hydrogen does. Oxygen has a higher electronegativity. Because of this difference in electronegativity, the electrons in the covalent bonds between oxygen and hydrogen get pulled slightly toward the oxygen. This leaves the hydrogens a little bit electron-deficient and thus slightly positive. We can draw this polarization like this:
Or looking at it from a "net polarization" perspective, like this:
What does the polarization have to do with the properties of water?
Everything! Because water has a slightly negative end and a slightly positive end, it can interact with itself and form a highly organized 'inter-molecular' network. The positive hydrogen end of one molecule can interact favorably with the negative lone pair of another water molecule. This interaction is call "Hydrogen Bonding". It is a type of weak electrostatic attraction (positive to negative). Because each and every one of the water molecules can form four Hydrogen Bonds, an elaborate network of molecules is formed. But if the Hydrogen Bonds are weak, how can they be important?
Think of how many there are! There is strength in numbers!
The polarity also allows water interact with an electric field:
And to interact with other polar molecules - which is how substances become dissolved in water. Hydrogen + Oxygen = Water
The simple statement that water is made from hydrogen and oxygen doesn't give us a very clear picture of what really goes into the creation of a molecule of water. A quick look at the chemical equation for the formation of water tells us more. 2H2 + O2 = 2H2O
It takes two molecules of the diatomic hydrogen gas, combined with one molecule of the diatomic oxygen gas to produce two molecules of water. In other words the ratio of hydrogen to oxygen is 2:1, the ratio of hydrogen to water is 1:1, and the ratio of oxygen to water is 1:2. There's something more though that doesn't show up in the equation. Energy. The formation of water from it's elements produces, in addition to water, a tremendous amount of energy, 572 kJ to be exact.
2H2 + O2 = 2H2O + ENERGY
This is an example of an exothermic reaction, a reaction that produces energy. It is also an example of what is called a combustion reaction, where a substance (in this case hydrogen gas) is combined with oxygen. You are probably familiar with this reaction through two tragic examples of the unleashed energy of the combustion reaction of hydrogen, the Hindenburg, and the spaceshuttle Challenger. Hydrogen Fuel?
Yes - hydrogen is a good, clean fuel, producing only water as a by-product. Unfortunately it produces so much energy that it can get out of control, resulting in an explosion. But let's forget about that explosive part for a minute and think about the possibilities - Hydrogen as a New Clean Fuel - it could be the end of the energy crisis - but where would we get the hydrogen?
Can we create Hydrogen from Water?
Oh Yes! It's the same chemical reaction, but run in reverse:
2H2O + ENERGY = 2H2 + O2
Notice now that the requirement is for energy to be ADDED TO the reactants. This is an example of an Endothermic reaction. This means that we could use Water as a Fuel! IF (and this is a big if) we could find an easy way to convert the water to hydrogen and oxygen, then the hydrogen could be used as a clean fuel. One way to convert Water to Hydrogen and Oxygen is through the process of Electrolysis - using electricity as the source of energy to drive the reaction. Let's take a look at what that might look like:
Isn't this rather circular?
Using Energy to break water to form hydrogen to combine oxygen to form Energy - in this way is rather circular. In fact, because of the laws of thermodynamics, you can't break even in this exchange of energy. However, there exist better ways to disassemble water - namely using CATALYSIS. What does a catalyst do?
A catalyst is a chemical compound that acts to speed up a reaction, but in the process is not itself changed. Therefore the catalyst, at the end of the reaction, is free to act again to assist another reactant through the reaction. Catalysts work by lowering the energy barrier between the reactants and the products. In this case:
2H2O + ENERGY = 2H2 + O2
where it normally takes a tremendous amount of energy to convert reactants to products - the addition of a catalyst can decrease the amount of energy required and therefore speed the reaction up!
2H2O + CATALYST+ energy = 2H2 + O2 + CATALYST
Does this catalyst really exist?
Have you ever wondered how a plant uses water and carbon dioxide to create glucose and oxygen? This too is an endothermic reaction, an energy producing reaction run in reverse. Normally we would think of using glucose as a fuel, through oxidation we could produce carbon dioxide, water and energy - In fact this is what OUR bodies do to provide us with the energy we need for maintaining all of our bodily functions including THINKING!
Glucose (C6H12O6) + Oxygen (O2) = Water (H20) + Carbon Dioxide (CO2) + ENERGY
To run the reaction in reverse, the plant utilizes a catalyst - CHLOROPHYLL - and the energy from the SUN to aid in the decomposition of water. While the chlorophyllic reaction does produce diatomic oxygen gas, it does not produce the hydrogen in a gaseous form. The hydrogen released from the water is used for the formation of glucose. Could we use such a catalyst for converting Water and Sunlight into Fuel?
Scientists often use Nature as a model for the development of new compounds. One such development, which has been studied extensively in this regard, is a molecule known as Rubippy. The structure of Rubippy is shown below. It is similar in structure to the chlorophyll molecule having a metal center (in chlorophyll it's a magnesium ion, in rubippy it's a ruthenium ion) and an attached system of organic rings (in chlorophyll its a porphyrin derivative, in rubippy its a pyridine derivative). Acting as "relay" channel for the transfer of electrons, Rubippy has shown some potential to do just that - convert water and sunlight into a clean, seemingly inexhaustible, source of energy. However, while rubippy has shown promise in this regard, it is not a commercially viable enterprise because of it's high cost, instability, and low efficiency. If Scientists were able to get Rubippy to work, or created a viable alternative, what would we do about the explosion potential of using Hydrogen Fuel?
Good question! Would you believe that it is possible to do the combustion of hydrogen without letting the oxygen and hydrogen come in contact? This can happen in a FUEL CELL. A fuel cell is like a battery - It utilizes a chemical reaction to produce electricity. A drawing of a hydrogen-oxygen fuel cell is shown below:
The kind of fuel cell shown here are routinely used in the space program. If this technology ever becomes viably available to the common person, the estimated cost of a fuel-cell hydrogen powered car would be less than half that of your current gas-mobile. In addition, it would be simpler, require less maintenance, and be environmentally friendly!
Water is Weird !?
Chemically speaking, water is very weird.
It doesn't behave at all like it should. Let's consider somethings you know:
Ice Floats. That's not weird... is it?
That's very weird. The solid state of most things are much denser than the liquid state and therefore sink. Usually what happens when a solid is formed is that the molecules become more tightly packed together. When things melt, the molecules move apart and get liquid. But water is weird - the solid state is less dense than the liquid. To understand why we'll have to take a close-up look at the molecular arrangement of solid water (ice) and liquid water. The Structure of Ice
Water boils at 100°C
and freezes at 0°C.
That's certainly not unusual.
Oh, Yes it is! Did you know that the Celcius temperature scale was based on the two physical changes of water? That wasn't done because water has typical chemical behavior, only because water is a familiar substance. You may have realized too that the boiling point of water is not always "100 °C". Ever read Cake Mix Directions? They give different directions for "High Altitudes". That's because many physical changes depend on pressure. The boiling point of water depends on the pressure of the air around it:
Let's compare the boiling of water with some other chemically similar substances. Water is way out of line! It boils at an extremely high temperature for its size. Why? Because of the extensive network of Hydrogen bonds. Those H-bonds are cohesive forces - they want to hold the water molecules together - and there are a lot of them! The process of boiling requires that the molecules come apart: a process that takes a lot more energy than expected. What's unusual about the freezing point?
The freezing point is much higher than expected again because of the hydrogen bonding. To get the water molecules to undergo the transition from liquid to solid is relatively easy. Liquid water has only 15 percent more H-bonds than solid water. How am I affected by these temperature - phase relationships?
If water were "normal", it would be a gas at room temperature. No lakes, no rain, no body fluids!
Is there anything else?
Another result of the Hydrogen bonding network is that water has a very high Specific Heat. This is like the baked potato effect. Once heated, water takes a very long time to cool off. Or in reverse, it takes a lot of heat to make water hot. Compare the specific heat of water to some other common substances:
You've noticed and used water's high heat capacity yourself.
Lake Effect Snow
Coolant for engines
The Chemistry of Water
The polarity of water
Water has a simple molecular structure. It is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Oxygen also has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom. Oxygen is an "electronegative" or electron "loving" atom compared with hydrogen. Water is a "polar" molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge ( ) near the oxygen atom due the unshared pairs of electrons, and partial positive charges ( ) near the hydrogen atoms. An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond as shown in the illustration. The ability of ions and other molecules to dissolve in water is due to polarity. For example, in the illustration below sodium chloride is shown in its crystalline form and dissolved in water. Many other unique properties of water are due to the hydrogen bonds. For example, ice floats because hydrogen bonds hold water molecules further apart in a solid than in a liquid, where there is one less hydrogen bond per molecule. The unique physical properties, including a high heat of vaporization, strong surface tension, high specific heat, and nearly universal solvent properties of water are also due to hydrogen bonding. The hydrophobic effect, or the exclusion of compounds containing carbon and hydrogen (non-polar compounds) is another unique property of water caused by the hydrogen bonds. The hydrophobic effect is particularly important in the formation of cell membranes. The best description is to say that water "squeezes" non-polar molecules together. Acids and Bases, Ionization of Water
• Acid release H+
• Bases accept H+
We define the pH of a solution as the negative logarithm of the hydrogen ion concentration. • at pH 7.0, a solution is neutral
• at lower pH (1-6), a solution is acidic
• at higher pH (8-14), a solution is basic
States of Water
Adhesion and Cohesion
The States of Water
Water has three states. Below freezing water is a solid (ice or snowflakes), between freezing and boiling water is a liquid, and above its boiling point water is a gas.
There are words scientists use to describe water changing from one state to another. Water changing from solid to liquid is said to be melting. When it changes from liquid to gas it is evaporating. Water changing from gas to liquid is called condensation (An example is the 'dew' that forms on the outside of a glass of cold soda). Frost formation is when water changes from gas directly to solid form. When water changes directly from solid to gas the process is called sublimation. Gas Liquid Solid
Most liquids contract (get smaller) when they get colder. Water is different. Water contracts until it reaches 4 C then it expands until it is solid. Solid water is less dense that liquid water because of this. If water worked like other liquids, then there would be no such thing as an ice berg, the ice in your soft drink would sink to the bottom of the glass, and ponds would freeze from the bottom up!
Water is found on Earth in all three forms. This is because Earth is a very special planet with just the right range of temperatures and air pressures. Earth is said to be at the triple point for water.
Adhesion and Cohesion
Water is attracted to other water. This is called cohesion. Water can also be attracted to other materials. This is called adhesion. The oxygen end of water has a negative charge and the hydrogen end has a positive charge. The hydrogens of one water molecule are attracted to the oxygen from other water molecules. This attractive force is what gives water its cohesive and adhesive properties.
Surface tension is the name we give to the cohesion of water molecules at the surface of a body of water. Try this at home: place a drop of water onto a piece of wax paper. Look closely at the drop. What shape is it? Why do you think it is this shape?
What is happening? Water is not attracted to wax paper (there is no adhesion between the drop and the wax paper). Each molecule in the water drop is attracted to the other water molecules in the drop. This causes the water to pull itself into a shape with the smallest amount of surface area, a bead (sphere). All the water molecules on the surface of the bead are 'holding' each other together or creating surface tension.
Surface tension allows water striders to 'skate' across the top of a pond. You can experiment with surface tension. Try floating a pin or a paperclip on the top if a glass of water.
A metal pin or paper clip is heavier than water, but because of the surface tension the water is able to hold up the metal.
Surface tension is not the force that keeps boats floating. If you want to know why a boat floats look here: Why do boats float?
Surface tension is related to the cohesive properties of water. Capillary action however, is related to the adhesive properties of water. You can see capillary action 'in action' by placing a straw into a glass of water. The water 'climbs' up the straw. What is happening is that the water molecules are attracted to the straw molecules. When one water molecule moves closer to a the straw molecules the other water molecules (which are cohesively attracted to that water molecule) also move up into the straw. Capillary action is limited by gravity and the size of the straw. The thinner the straw or tube the higher up capillary action will pull the water (Can you make up an experiment to test this?).
Plants take advantage of capillary action to pull water from the into themselves. From the roots water is drawn through the plant by another force, transpiration. Click here for more information about transpiration.
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