Lecture #2 V2 - Water

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Biology 301 Cellular and Molecular Biology Fall 2002

Lecture # 2: Water & pH

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Overall outline  Structure of Water

 Properties of Water

 Acid-base chemistry in aqueous solutions

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Structure of Water  Water is a polar molecule  Hydrogen bonds between water molecules  Phases transitions are determined by intermolecular forces

 Molecular view of ice, water, and steam  The structure of ice

Water is a polar molecule

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Hydrogen bonds between water molecules

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Phases transitions are determined by intermolecular forces

Molecular view of ice, water, and steam

Ice

Water

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Steam

The structure of ice

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Properties of Water           

Cohesion, adhesion & surface tension Cohesive behavior leads to capillarity Some thermodynamic concepts Water’s high heat capacity moderates temperature changes The high heat of vaporization cools surfaces upon evaporation Water expands when frozen: Ice floats and frozen benzene sinks … this has profound implications for the global climate Some definitions in solution chemistry The mole concept Dissolution of salt in water A water-soluble protein

Cohesion, adhesion & surface tension

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Cohesion = phenomenon of a substance being held together by noncovalent bonds Adhesion = phenomenon of a substance being attracted to a vessel wall. (e.g. water in glass containers results in a “meniscus”) Surface tension = measure of how difficult it is to stretch or break the surface of a liquid 

Water has a greater surface tension than most liquids

Cohesive behavior leads to capillarity

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Some thermodynamic concepts Kinetic energy

Energy of motion

Heat

Total kinetic energy due to molecular motion in a body of matter

Temperature

Measure of heat intensity due to the average kinetic energy of molecules in a body of matter

Calorie

Amount of heat it takes to raise the temperature of one gram of water by one degree Celsius. Conversely, one calorie is the amount of heat released by one gram of water when it cools down by one degree Celsius. Note: The “calories” on food packages are actually kilocalories (kcal).

Kilocalorie

The amount of heat required to raise the temperature of one kilogram of water by one degree Celsius (1000 cal)

Specific heat

Amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by one degree Celsius

Specific heat of water

One calorie per gram per degree Celsius (1 cal / g / °C)

Water’s high heat capacity moderates temperature changes

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

The high heat of vaporization cools surfaces upon evaporation Vaporization

Also called evaporation = transformation of a liquid to a gas

Heat of vaporization

Quantity of heat a liquid must absorb for one gram to be converted to the gaseous state. Water has a relatively high heat of vaporization at the boiling point (540 cal/g).

Evaporative cooling

Cooling of a liquid’s surface when a liquid evaporates.

… Stabilizes temperature in aquatic ecosystems … Helps organisms from overheating by evaporative cooling

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Water expands when frozen: Ice floats and frozen benzene sinks

… this has profound implications for the global climate

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

… and is the reason behind the sinking of the Titanic

Some definitions in solution chemistry

Solution

A liquid that is a completely homogenous mixture of two or more substances

Solvent

Dissolving agent of a solution

Solute

Substance dissolved in a solution

Aqueous solution

Solution in which water is the solvent

Hydrophilic

Ionic compounds and polar compounds are, in general, water soluble

Hydrophobic

Nonpolar compounds are NOT water-soluble.

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

The mole concept Molecular weight

Sum of the weight of all atoms in a molecule (expressed in Daltons)

Mole

Amount of a substance that has a mass in grams numerically equivalent to its molecular weight in daltons

Molarity

Number of moles of solute per liter of solution

A mole of various substances Advantage of using moles: 2. Rescales weighing from daltons to grams 3. A mole of any substances has a fixed number of molecules (e.g. Avogadro’s number) 4. Allows one to combine substances in fixed ratios of molecules

Dissolution of salt in water

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

A water-soluble protein

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Acid-base chemistry in aqueous solutions  Dissociation of water  Definitions of acids & bases  Definition of pH: a logarithmic scale of [H+] concentration  pH of some aqueous solutions  Conjugate acids & bases  Strong acids & bases  Weak acids & bases  Buffers attenuate changes in pH  Indicators

Dissociation of water

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

 The reaction is reversible  At equilibrium, most of the water is not ionized

Definitions of acids & bases  Acids donate protons H+ and Bases accept protons H+.

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Definition of pH: a logarithmic scale of [H+] concentration  For very small numbers we use logarithms for convenience. Typical [H+] concentrations range from 10-14 to 10-1 molar which is simplified by converting this to a pH range of 14 to 1.

The pH of the blood  is ~ 7.4 which is close to neutral but slightly on the basic side.  Changes in the blood pH of a few tenths on either side of this value can be fatal.  Does this make sense mathematically?  On a moment­by­moment basis, how is the blood pH so tightly regulated?

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

pH of some aqueous solutions

Because pH is a logarithmic measure, apparently “small” numerical changes in pH actually correspond to very “large” changes in hydrogen ion concentration. Normal physiological pH is 7.41. Depending on the circumstances a pH below 7.00 or above 7.8 can be Lethal.

Congugate acids & bases  Strong acids have weak conjugate bases and vice versa.

Why is it not possible to look at the acid alone and say that it is a good acid?

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Strong acids & bases  Strong acids (and bases) dissociate (associate) almost completely. A full equilibrium analysis is usually not necessary since you can assume that the initial acid (or base) is converted completely to products.

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Weak acids & bases  Weak acids (and bases) dissociate (associate) only partially ... so a full equilibrium analysis necessary.

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University

Buffers attenuate changes in pH  Buffers minimize changes in pH because protons can be taken up or given up by the buffer

 The maximum buffering capacity occurs when the pH equals the pKa (when [A-] = [HA]).

Acid-base indicators  A weak acid (or base) coupled to a conjugated system. Titration of the proton leads to changes in the visual absorption spectrum (hence color changes) in the conjugated system.

Ogan Gurel, MD Biology 301 Lecture #2 Roosevelt University