AQUEOUS SOLUTIONS -An aqueous solution is any solution in which water (H2O) is the solvent -Water needed by all known forms of life -an active participant and facilitator of biochemical processes that are responsible for life -water is often called the universal solvent -the structures of the molecules on which life is based, proteins, nucleic acids, lipids, and complex carbohydrates, result directly from their interactions with their aqueous environment -the combination of solvent properties responsible for intramolecular and intermolecular associations of these substances is peculiar to water; no other solvent even resembles water in this respect -Water is the largest component of cellsâ€™ interiors, and chemical reactions in the cell take place in water
The glass in the picture above appears to be floating on water. How can a solid float on water? Actually, nothing is floating on the water; rather this picture depicts some of the most unique properties of water. What appears to be a glass floating on water is really water splashing upward in response to an item being dropped into the water. Waterâ€™s unique properties of adhesion, surface tension, and its viscosity are what contribute to this illusionary picture.
Why Is an Aqueous Solution Important?
-In an aqueous solution where water is the solvent, the solute to be dissolved by the water has fewer particles in it, making the particles move in random motion. -Pure water has a low concentration of ions and therefore does not conduct electricity. -When a solute dissociates in water and forms an electrolyte, then the solution is a good conductor of electricity. -Solutes that dissociate in water and forms ions are electrolytes. -Strong acids and bases in an aqueous solution form a strong electrolyte, which can dissolve completely as a soluble item. -Weak electrolytes do not completely dissociate and are usually weak acids and bases. -Since strong electrolytes supply ions to the solution, strong electrolytes create aqueous solutions that are more conductive of electricity.
What Is the Difference Between a Liquid and an Aqueous Solution? -A liquid has free flowing particles, meaning that is has a definite volume, but doesn't have a definite shape. To be considered a liquid, all of the following properties have to be met: -They have to be almost incompressible. Their value only decreases slightly under pressure. -Liquid densities are affected by pressure but change very slightly when pressure is added. -Liquids always take the shape of any type of container they are in. -Liquids have surface tension causing wetting. -All the particles in a liquid have greater freedom to move about than in a solid state.
What is the difference between a solution and an aqueous solution? The main and basic difference between an aqueous solution and liquids is that a liquid is a state of matter which has some typical characteristics which distinguishes it from other states of matter, i.e., solids and gases; whereas an aqueous solution is a solution where the solvent is water. Is aqueous and liquid the same thing? 'Aqueous' in a chemical equation indicates that the material dispersed in water (a solution in water). For example, when we pour sodium hydroxide solution into dilute hydrochloric acid, we get an aqueous solution of sodium chloride. Liquid is used to indicate formation of liquid substances in a reaction.
Structure of the Water Molecule Polarity and Hydrogen Bonds
Structure of the Water Molecule H + H + O Ă H2O (chemical formula)
O Structural formula
Water Molecules are Polar The Polar Nature of Water -Each water molecule consists of two hydrogen atoms bonded to an oxygen atom in a bent (Vshaped) structure. -Because the oxygen atom attracts electrons more strongly than the hydrogen atoms do, the oxygen atom is partially negatively charged (2Î´âˆ’; blue) and the hydrogen atoms are partially positively charged (Î´+; red). -For the molecule to have a net charge of zero, the partial negative charge on oxygen must be twice as large as the partial positive charge on each hydrogen.
How is a Water Molecule Polar? â€˘ The oxygen atom attracts the shared electrons more strongly than the hydrogen atoms do. â€˘ As a result, the electrons spend more time near the oxygen atom than they do near the hydrogen atoms. H+
How Do Water Molecules Bond With Each Other? • Hydrogen bonds form between neighboring water molecules • Because of the asymmetric charge distribution in the water molecule, adjacent water molecules are held together by attractive electrostatic (δ+…δ−) interactions between the partially negatively charged oxygen atom of one molecule and the partially positively charged hydrogen atoms of adjacent molecules • The positive H of one molecule “sticks” to the negative O of another molecule.
ยงThey do not share electrons, so they are weaker than covalent bonds. ยงThey easily break and form again
States of Water â€˘ Water is the only substance found on Earth in all three states (phases): 1. Liquid
2. Solid (Ice)
Water molecules are constantly moving
3. Gas (Steam or Vapor)
Temperature increase = Increase in movement vapor
How Water Recycles Hydrological Cycle
Evaporation—Liquid water on Earth’s surface changes into water vapour in the atmosphere. Sublimation—Snow or ice on Earth’s surface changes directly into water vapour in the atmosphere. Transpiration—Plants give off liquid water, most of which evaporates into the atmosphere. Condensation—Water vapour in the atmosphere changes to liquid water droplets, forming clouds or fog. Precipitation—Water droplets in clouds are pulled to Earth’s surface by gravity, forming rain, snow, or other type of falling moisture.
Properties of Water -water is tasteless, odourless, and transparent-in small quantities, it is also colourless
-Hydrogen bonds are responsible for several important properties of water • Polarity • Cohesion (surface tension)-the attraction among molecules of a substance – makes water “stick” together
• Adhesion (universal solvent)-the attraction among molecules of DIFFERENT substances – makes water “stick” to other materials
• High Specific Heat (water plays a very important role in temperature regulation) • High Heat of Vaporization (high boiling point-liquid water turns into water vapor at a higher temperature) • Density – greatest at 4oC, less dense as a solid (solid below 0°C ) • Universal solvent of life-water has the ability to dissolve both bases and acids, so it is called a universal solvent
Polarity -A difference in electrical charge between different parts of a molecule is called polarity. A polar molecule is a molecule in which part of the molecule is positively charged and part of the molecule is negatively charged (Fig.1). Figure 1. Water Molecule. This diagram shows the slightly positive (hydrogen) and slightly negative (oxygen) parts of a water molecule.
Figure 2. This model is an atomic diagram of water, showing the two hydrogen atoms and oxygen atom in the centre. The protons (red) are in the centre (nucleus) of each atom, and the electrons (light blue) circle each nucleus.
-The model in Figure 2 shows the arrangement of oxygen and hydrogen atoms in a water molecule. A water molecule has a bent or angular (non-linear) shape, with an angle of about 105Â°. -The nucleus of the oxygen atom attracts electrons more strongly than do the hydrogen nuclei. As a result, the middle part of the molecule near oxygen has a negative charge, and the other parts of the molecule have a positive charge. -In essence, the electrons are "pulled" toward the nucleus of the oxygen atom and away from the hydrogen atom nuclei. Water is a polar molecule, with an unequal distribution of charge throughout the molecule.
-Water is a strongly polar solvent, and polar solvents are better at dissolving polar solutes. -Many organic compounds and other important biochemical are polar, so they dissolve well in water. -Waterâ€™s polarity gives it the ability to dissolve both ionic compounds and other polar molecules. Polar Ex: The liquid part of your blood, called plasma, is about 95% water. The solvent in plasma is water and all the dissolved substances are the solutes: sugars and proteins etc..
Nonpolar Ex: Fats and oils rarely dissolve in water. Fats and oils are nonpolar so they do not have charged regions so they are not attracted to polar molecules.
Cohesion -hydrogen bonds hold the substance together, a phenomenon called cohesion • Cohesion is responsible for the transport of the water column in plants (surface tension) • Cohesion among water molecules plays a key role in the transport of water against gravity in plants • Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels. (capillary action)
Surface tension (cohesion/polar) The water's surface (left, dyed red) is curved down because water has greater adhesion than cohesion. The surface of the mercury (right) is curved up because mercury has greater cohesion than adhesion.
Droplets of dew-drops of dew cling to a spider web in this picture. Can you think of other examples of water forming drops? (Hint: What happens when rain falls on a newly waxed car?)
Surface tension cont…. -Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. -Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. Water behaves as if covered by an invisible film.
Helps insects (water striders) walk across water without breaking the surface
Water’s surface tension comes from hydrogen bond’s that cause water molecules to stick together.
Solvency -Water is one of the most common ingredients in solutions.
-A solution is a homogeneous mixture composed of two or more substances. -The dissolved substance in a solution is called the solute. The substance in which it is dissolved is called the solvent. An example of a solution in which water is the solvent is salt water. In this solution, a solid— sodium chloride (NaCl)—is the solute.
•Attraction between two different substances. •Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and cotton. •Capillary action-water molecules will “tow” each other along when in a thin glass tube. •Example: Water is moved through small blood vessels in animals through capillary action also, transpiration process which plants and trees remove water from the soil, and paper towels soak up water.
Adhesion-is the bonding of a water molecule to another substance • Attraction between two different substances. • Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and cotton. (Fig. 1) • Capillary action-water molecules will “tow” each other along when in a thin glass tube. (Fig. 2) • Water's surface tension is so strong that, as water is pulled upward along the straw's walls, the water in between tends to be pulled upward also. The downward pull of gravity prevents the central water from rising quite as high as the water which is adhered to the straw, so the result is a meniscus, as shown in a graduated cylinder below. (Fig.3)
(Fig. 1) Capillary action occurring in conjunction with adhesion and surface tension
(Fig. 2) Adhesion pulls water up the sides of the straw
Thermal Properties of Water Water has a high heat capacity
-water can absorb or release large amounts of energy in the form of heat while only slightly changing its temperature. -Energy must be absorbed to break hydrogen bonds, and energy is released as heat when hydrogen bonds form. -The energy that water initially absorbs breaks hydrogen bonds between molecules. -Only after these hydrogen bonds are broken does the energy begin to increase the motion of the water molecules, which raises the temperature of the water. -When the temperature of water drops, hydrogen bonds reform, which releases a large amount of energy in the form of heat. -Temperature is a measure of the average kinetic energy (energy of motion) of particles in a sample of matter. This physical property can determine the rate and extent to which chemical reactions can occur within living systems.
Heat Capacity Because of the multiple hydrogen bonds between water molecules, it takes a large amount of heat energy to cause those molecules to move faster and raise the temperature of the water. Waterâ€™s heat capacity, the amount of heat energy required to increase its temperature, is relatively high. Large bodies of water, such as oceans and lakes, can absorb large amounts of heat with only small changes in temperature. This protects organisms living within from drastic changes in temperature. At the cellular level, water absorbs the heat produced by cell processes, regulating the temperature of the cell. For instance, our fish in the pond is indeed happy because the heat capacity of the water in his pond above means the temperature of the water will stay relatively the same from day to night. He doesn't have to worry about either turning on his air conditioner or putting on his woolen flipper gloves.
High Specific Heat Water has a high specific heat
-it takes a lot of energy to raise or lower the temperature of water. -Specific heat is a measure of how much energy it takes to raise the temperature of a substance. -It is the amount of energy (in joules) needed to raise the temperature of 1 gram of the substance by 1 째C. -the specific heat capacity of water being 1 cal/(g째C). The specific heat capacity of water is much higher than that of other common substances. For the sake of comparison, the specific heat capacity of oil is about 0.5 cal/(g째C) and the specific heat capacity of aluminum is about 0.2 cal/(g째C). This means that it takes a lot more heat to raise the temperature of water compared to the amount of heat it would take to raise the temperature of oil or aluminum. -Water helps to buffer temperature changes because of its relatively high specific heat capacity (the heat required to raise 1 kg of water by 1 oC). This is reflected in the unusually high boiling and melting points of water.
High Heat of Vaporization • Amount of energy to convert 1g of a substance from a liquid to a gas • In order for water to evaporate, hydrogen bonds must be broken. • As water evaporates, it removes a lot of heat with it. Water's heat of vaporization is 540 cal/g. In order for water to evaporate, each gram must GAIN 540 calories (temperature doesn’t change --- 100oC). As water evaporates, it removes a lot of heat with it (cooling effect).
High Heat of Vaporization….cont..
• Water vapor forms a kind of global ‘‘blanket” which helps to keep the Earth warm. • Heat radiated from the sun warmed surface of the earth is absorbed and held by the vapor.
High Heat of Vaporizationâ€Ś.cont..
-Water is less dense as a solid due to its hydrogen bonds It also has relatively large enthalpy of vaporisation (heat energy required to convert a liquid to a gas) and enthalpy of fusion (heat energy required to convert a solid to a liquid)
Compare: How are hydrogen bonds similar to ionic bonds?
Homeostasis • Ability to maintain a steady state despite changing conditions • Helps maintain a constant physiological condition of cells, and organisms’ global ecosystems • Water is important to this process because: a. Makes a good insulator b. Resists temperature change c. Universal solvent d. Coolant e. Ice protects against temperature extremes (insulates frozen lakes)
WATER AND LIFE -Water is essential to important chemical reactions in organisms
-Molecules of water are involved in many chemical reactions necessary to sustain life. -Most cells are surrounded by water -Living organisms are composed of about 60-70 percent water (not counting water in body fat). -Water’s ability to dissolve most biologically significant compounds—from inorganic salts to large organic molecules— makes it a vital solvent inside organisms and cells. -Water is an essential part of most metabolic processes within organisms. Metabolism is the sum total of all body reactions, including those that build up molecules and /or store energy (anabolic reactions) and those that break down molecules releasing energy(catabolic reactions). In anabolic reactions, water is generally removed from small molecules in order to make larger molecules. In catabolic reactions, water is used to break bonds in larger molecules in order to make smaller molecules. Water is central to two related, fundamental metabolic reactions in organisms: photosynthesis and cellular respiration. All organisms depend directly or indirectly on these two reactions. In photosynthesis, cells metabolically use the energy in sunlight to change water and carbon dioxide into glucose (C6H12O6) and oxygen (O2). Anabolic reaction, represented by the chemical equation: 6 CO2 + 6 H2O + energy → C6H12O6 + 6 O2. In cellular respiration, cells metabolically transfer chemical energy from glucose (by breaking it down)to ATP (a usable energy-rich molecule) in the presence of oxygen and release energy, water, and carbon dioxide. Catabolic reaction, represented by the chemical equation: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy
Lesson Summary • Most of Earth’s water is salt water in the oceans. Less than 3% is freshwater. • Water molecules are polar, so they form hydrogen bonds. This gives water unique properties, such as a polarity, solvency, cohesion, adhesion, high specific heat, and the ability to be a buffering agent. • A solution is a homogeneous mixture in which a solute dissolves in a solvent. Water is a very common solvent, especially in organisms. Water is known as the universal solvent. • Water has high surface tension because of extensive hydrogen bonding. • The capillary action property of water action is important in moving water upwards through small spaces. Plants depend on capillary action to move water upward from the roots to the leaves. In the soil, capillary action also tends to move water upward between the soil particles. Water is moved through small blood vessels in animals through capillary action also. • The ion concentration of neutral, pure water gives water a pH of 7 and sets the standard for defining acids and bases. Acids have a pH lower than 7, and bases have a pH higher than 7. • Water is involved in most biochemical reactions; such as, photosynthesis and cellular respiration. Therefore, water is essential to life.
WATER ACTS LIKE A BUFFER -Acids are substances that release hydrogen ions (H+) into solution. For example HCl, or hydrochloric acid, is a compound formed by ionic bonds. When you drop it in water, the H+ and Cl- come apart, because water is polar and will attack charged ions. -As a result, a whole bunch of H+ ions are released into solution -Bases, on the other hand, are substances that will bind to the free hydrogen ions (H+) -NaOH is an example of a base. -When you drop NaOH in water, the Na+ ions become separated from the hydroxide ions (OH-). -To summarize, acids release a bunch of H+ ions into solution, and bases mop them up like they're Swiffer. The last thing to mention before we come back to buffers is pH. The pH scale goes from 1 to 14 and is the way we measure how acidic a solution is, which has to do with how many hydrogen ions are in solution. Pure water has a pH of 7, which is neutral, and this indicates that it has exactly the same number of H+ (hydrogen) ions as OH- (hydronium) ions floating around in solution. Two things to remember: 1. If there are relatively more H+ ions, the pH goes down, increasing the acidity. More H+, more acidic, lower pH. 2. If there are fewer H+ ions, the pH increases, increasing the basicity. Less H+, more basic, higher pH.
Some compounds form acids or bases. • An acid releases a hydrogen ion when it dissolves in water. • high H+ concentration • pH less than 7
stomach acid pH between 1 and 3
• A base removes hydrogen ions from a solution. – low H+ concentration – pH greater than 7 bile pH between 8 and 9
â€˘ A neutral solution has a pH of 7. pure water pH 7
Apply: Cells have higher H+ concentrations than blood. Which has a higher pH? Why?
Biology Connection and pH • Most organism’s (including humans) need to keep their pH within a very small range (around 7). pH can be regulated by buffers, compounds that can bind to an H+ ion when the H+ concentration increases and can release H+ ions when the H+ concentration decreases. • Exceptions: A few organisms thrive in very acidic environments: azalea’s prefer soil around a 4.5 pH and microorganisms called Picrophilus survive best at extremely acidic pH (0.7).
pH • If you want to mathematically express the concentration of the hydrogen ions found in a solution, you would refer to the solution’s “pH”.
pH of Solutions • If acid is added to water, the concentration of hydronium increases and pH decreases • If base is added to water, the concentration of hydronium decreases (ion product of water) and the pH increases • Addition of MORE acid vs. addition of a STRONGER acid
Why is pH important to living organisms?
• pH affects solubility of many substances. • pH affects structure and function of most proteins - including enzymes. • All our cells function within a certain pH range. • If the fluids bathing those cells is “off” these cells won’t function at max capacity. • Other than just cells, our blood needs to be at a certain pH.
But… The chemical reactions of life constantly produce acids and bases within cells. These have a tendency to throw off the pH values. We need some sort of mechanism to minimize how much the pH is altered.
pH • pH is commonly expressed as –log[H+] • Pure water has [H+]=10-7 and thus pH=7. • Acids have a high [H+] and thus a low pH. • Bases have a low [H+] and thus a high pH. Bases contribute –OH ions when they dissociate. These bind to the H+ ions produced when water dissociates. Thus, these OH ions “suck up” the H+ ions in solution, reducing their concentration. NaOH with a pH of 12.0 contributes so many –OH ions that almost all the H+ ions are bound into water molecules, reducing the free H+ (and hydronium) ion concentration to 1 x 10-12 (1,000,000,000,000 = 1/trillion)
pH Acid Acetic Acetic Acetic Hydrochloric Hydrochloric Hydrochloric Sulfuric Sulfuric Sulfuric
Normality N 0.1 N 0.01 N N 0.1 N 0.01 N N 0.1 N 0.01 N
pH 2.4 2.9 3.4 0.1 1.1 2.0 0.3 1.2 2.1
How do normality and molarity relate to pH??
Molarity is the fractions of a mole in solution; normality is a measure of the concentration of reactive groups which may affect pH.
Acids and Bases -A solution is a mixture of two or more substances that has the same composition throughout -Some solutions are acids and some are bases -In pure water (such as distilled water), a tiny fraction of water molecules naturally breaks down to form ions. The products of this reaction are a hydronium ion (H3O+) and a hydroxide ion (OH-).
Acidity and pH
Acidity refers to the hydronium ion concentration of a solution. It is measured by pH. -In pure water, the hydronium ion concentration is very low. This gives water a pH of 7.
Acids -If a solution has a higher concentration of hydronium ions and lower pH than pure water, it is called an acid and has a pH lower than 7 -As the hydronium ion concentration increases, the pH value decreases. Therefore, the more acidic a solution is, the lower its pH value is -Stronger acids can be harmful to organisms. Strong acids can also damage materials, even hard materials such as glass -Acids turn blue litmus paper red.
Bases -If a solution has a lower concentration of hydronium ions and higher pH than pure water, it is called a base and has a pH higher than 7. -Strong bases can harm organisms and damage materials -Bases, like acids, can be identified with litmus paper -Bases turn red litmus paper blue.
READING ASSIGNMENT: 1. 2.
Aqueous Solution-chemical characteristics of water, acid, base, buffer, pH, pKa etc. Thermodynamics in biochemistry