Cooking_Lect1_section2.pdf

Kitchen Chemistry Lecture 1 1 PROCESS OF COOKING Cooking is the process by which the ingredient molecules are transformed into the molecular structure of the final recipe Mixture of Mixture of Product Ingredient Molecules Molec...

Kitchen Chemistry Lecture 1 1 PROCESS OF COOKING Cooking is the process by which the ingredient molecules are transformed into the molecular structure of the final recipe Mixture of Mixture of Product Ingredient Molecules Molecules 2 3  “ALL COOKING IS MOLECULAR” Perception of food = Texture + Flavour Flavour = Taste + Aroma Food = Texture Molecules + Flavour Molecules Nutrient labels show protein, carbohydrates and fats in the food  Important from a caloric standpoint  fat contributes 9 Calories/gram, while protein and carbohydrates contribute 4 Calories/gram Source: Brenner, M et. al. (2020). Science and Cooking: Physics Meets Food, from Homemade to Haute Cuisine. 4  WATER IS A CRITICAL TEXTURE MOLECULE Texture molecules: Proteins, Carbohydrates, Fats And Water Source: Brenner, M et. al. (2020). Science and Cooking: Physics Meets Food, from Homemade to Haute Cuisine. 5  WATER IS A CRITICAL TEXTURE MOLECULE Texture molecules: Proteins, Carbohydrates, Fats And Water Laws that govern the heating of water govern the heating of food Water surrounds all texture and flavour molecules All chemical reactions due to cooking occurs in aqueous medium Source: Brenner, M et. al. (2020). Science and Cooking: Physics Meets Food, from Homemade to Haute Cuisine. 6  WHAT ARE WE DISCUSSING TODAY? 1. Why is water so special?  Polar and non-polar molecules  Intermolecular interactions  Properties of water and life as we know it 2. Cooking using heat (and water)  Heat transfer mechanism  Thermal properties of water and how we manipulate them 7  VARIATION IN ELECTRONEGATIVITY The degree of ionic character of a bond depends on the differences in the abilities of the two atoms to attract the electrons they share: the greater the difference, the more ionic is the bond between them. Electronegativity  refers to the ability of an atom in a chemical bond (with another atom) to attract the shared electrons. 8  VARIATION IN POLARITY OF THE BOND Electronegativity is a measure of the pulling power of an atom on the electrons in a bond. A polar covalent bond is a bond between two atoms with partial electric charges arising from their difference in electronegativity. The presence of partial charges gives rise to an electric dipole moment. 9  ELECTRONEGATIVITY : PERIODIC TRENDS Pauling Scale Source: T. Brown et. Al, Chemistry: The Central Science, Pearson, 2018 10 ELECTRONEGATIVITY AND BOND POLARITY Source: T. Brown et. Al, Chemistry: The Central Science, Pearson, 2018 11 12 13 Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 14 Depends on molecular shape Source: M. Silberberg et.al., Chemistry: The Molecular Nature of Matter and Change, McGraw Hill, 2018 15 STRUCTURE OF WATER 16 WHAT IS SPECIAL ABOUT WATER? 17 AND WHY IS WATER SPECIAL? 18 INTERMOLECULAR FORCES AKA NON-COVALENT INTERACTIONS Important because they determine: 1. Solubility 2. Special properties of water including thermal properties 3. Structure of Biological Macromolecules (and other polymers)  Intramolecular Forces 19 WEAK AND STRONG BONDS Source: P.W. Atkins et. Al, Chemical Principles: The Quest for Insight, WH Freeman, 2013 20  Molecular Potential Energy Curve STRENGTH OF INTERMOLECULAR FORCES Responsible for the existence of different phases (states of matter) A phase is a form of matter that is uniform throughout in both chemical composition and physical state. Solids and liquids  condensed phases Responsible for biomolecular structure Responsible for solubility Molecular potential Energy curve: Shallower minimum; intermolecular forces are also electrostatic in nature but weaker than bonding forces When no bond forms, minimum reached when molecules are much further apart Source: P.W. Atkins et. Al, Chemical Principles: The Quest for Insight, WH Freeman, 2013 The macroscopic properties of substances such as melting points and boiling points, which we can observe and measure, arise from the microscopic interactions between the particles that make up those substances. 21 INTERMOLECULAR INTERACTIONS 22 SOLUTION FORMATION Three kinds of intermolecular interactions are involved in solution formation: 1. Solute–solute interactions between solute particles must be overcome to disperse the solute particles through the solvent. 2. Solvent–solvent interactions between solvent particles must be overcome to make room for the solute particles in the solvent. 3. Solvent–solute interactions between the solvent and solute particles occur as the particles mix. For solution to form: Solute-Solvent ≥ Solvent-Solvent or Solute-Solute 23 SUGAR AND SALT Dissolve in water Conducting vs Non-conducting Water molecules are dipoles 1. NaCl: ion-dipole interactions 2. Sucrose: dipole-dipole interactions 24  SOLUBILITY IN WATER: HYDRATION Ion-Dipole interaction Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c The sodium cation (Na+) is surrounded by a cloud of water molecules that are oriented to present their slightly negative oxygens toward the positively charged sodium The chloride anion (CI–) is surrounded by a cloud of water molecules that are oriented to present their slightly positive hydrogens toward the negatively charged chloride 25  INTERACTIONS AMONG POLAR MOLECULES Type I: Ion-Dipole Interactions: an attractive force between an ion and a molecule that has a permanent dipole moment. Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c sphere of hydration the cluster of water molecules surrounding an ion in aqueous solution; the general term applied to such a cluster forming in any solvent is sphere of solvation. 26  INTERACTIONS AMONG POLAR MOLECULES Each hydrated Na ion and hydrated Cl ion is surrounded by six water molecules that create an inner hydration sphere. Water molecules in the inner hydration sphere (basically closest to the ion) are oriented in a fixed way  oxygen atoms (negative poles) directed towards cation, hydrogen atoms (positive poles) directed towards anions. Number of water molecules depends on size of the ion: typically 6 but can range from 4 to 9 Molecules further away from the ions are more randomly oriented  Outer Hydration Sphere Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 27  INTERACTIONS AMONG POLAR MOLECULES Outer Hydration Sphere  Dipole- Dipole Interactions Type II: Dipole-Dipole Interactions: electrostatic attractions between molecules that have permanent dipole moments (polar molecules) Dipole–dipole interactions are not as strong as ion– dipole interactions because dipole–dipole interactions involve only partial charges, caused by unequal sharing of electrons within the molecule. Ion involved in an ion–dipole interaction has lost or gained one or more electrons, and it has a full positive or negative charge. Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 28  DIPOLE-DIPOLE FORCES Source: M. Silberberg et.al., Chemistry: The Molecular Nature of Matter and Change, McGraw Hill, 2018 29  SOLUBILITY IN WATER: HYDRATION Dipole-Dipole interaction Source: Husband, Tom. "The sweet science of candymaking." ChemMatters 10.11 (2014): 5-8. 30 SOLUBILITY IN WATER Dipole-Induced Dipole interaction 31  DIPOLE MOMENTS AND STRENGTH OF INTERACTIONS Permanent dipole moments are experimentally measured values (units of Debyes)  that define the polarity of molecules Dimethyl ether, CH3OCH3, and acetone, CH3C(O)CH3 , have similar formulas and molar masses. However, their dipole moments are quite different: 1.30 D for dimethyl ether and 2.88 D for acetone. Predict which compound has the higher boiling point and explain why Why specify Molar Mass? Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 32 Why specify Molar Mass? 33  INTERACTIONS AMONG ALL MOLECULES All atoms and molecules  negatively charged and positively charge particles Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 34  INTERACTIONS AMONG ALL MOLECULES All atoms and molecules  negatively charged and positively charge particles When atoms approach each other, they interact in ways that are similar to the electrostatic interactions involved in covalent bond formation.  Nucleus attracted to other atom’s electrons & vice versa  Electrons around each atom get distributed unevenly, producing temporary induced dipoles of partial electrical charge Dispersion Forces/Van der Waals Forces: Interactions between temporary induced dipoles  AKA London Forces  Momentary uneven distribution Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 35 Source: T. Brown et. Al, Chemistry: The Central Science, Pearson, 2018 36  INTERACTIONS AMONG ALL MOLECULES  All atoms and molecules have electrons  all experience dispersion forces  Why does mass matter?  Larger atoms Electrons held less tightly (distance +screening of nuclear charge Greater polarizability Stronger temporary dipoles and stronger intermolecular attractions  Polarizability increases as the number of electrons in an atom or molecule increases. The strength of dispersion forces therefore tends to increase with increasing atomic or molecular size.  Because molecular size and mass generally parallel each other, dispersion forces tend to increase in strength with increasing molecular weight. Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 37  MOLAR MASS AND BOILING POINTS OF HALOGENS Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 38  MOLAR MASS AND BOILING POINTS OF STRAIGHT CHAIN ALKANES Form “hydrophobic interactions” Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 39  SHAPE AND STRENGTH OF DISPERSION FORCES Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c Source: M. Silberberg et.al., Chemistry: The Molecular Nature of Matter and Change, McGraw Hill, 2018 40  The greater the forces of attraction the higher the boiling point or the greater the polarity the higher the boiling point. Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 41 HYDROGEN BOND: STRONGEST DIPOLE-DIPOLE INTERACTION Occurs between a hydrogen atom bonded to a small, highly electronegative element (O, N, F) and an atom of oxygen or nitrogen in another molecule. 42  HYDROGEN BOND: STRONGEST DIPOLE-DIPOLE INTERACTION Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 43 Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 44 INTERMOLECULAR INTERACTIONS 45 Source: M. Silberberg et.al., Chemistry: The Molecular Nature of Matter and Change, McGraw Hill, 2018 46 Source: M. Silberberg et.al., Chemistry: The Molecular Nature of Matter and Change, McGraw Hill, 2018 47 Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 48 QUESTION Dimethyl ether (C2H6O) has a molar mass of 46.07 g/mol and a boiling point of - 24.9°C. Ethanol (C2H6O) has the same formula and therefore the same molar mass, but a boiling point of 78.5°C. Explain this difference in boiling points considering the structures shown below. 49 QUESTION Isopropanol (molar mass 60.10 g/mol), the familiar rubbing alcohol in your medicine cabinet, boils at 82°C. Ethylene glycol, used as automotive antifreeze, has almost the same molar mass (62.07 g/mol) but boils at 196°C. Why do these substances (shown in the Figure below) have such different boiling points? 50 QUESTION Explain the variation in boiling point in this table 51  SOLUBILITY OF ORGANIC COMPOUNDS Most prevalent bonds in organic molecules are carbon– carbon and carbon– hydrogen, which are not polar Ascorbic Acid Overall polarity of organic molecules is often low, which makes them generally soluble in nonpolar solvents and Glucose not very soluble in water: Hydrophobic Organic molecules that are soluble in polar solvents are those that have polar groups on the molecule surface, such as glucose and ascorbic acid: Hydrophilic Organic molecules that have a long, nonpolar part bonded to a polar, ionic part such as sterate ion function as surfactants and are used in soaps and detergents Sodium stearate 52  SOLUBILITY OF ORGANIC COMPOUNDS Source: T. Brown et. Al, Chemistry: The Central Science, Pearson, 2018 53  Substances with similar intermolecular attractive forces tend to be soluble in one another. “Like Dissolves Like” Source: T. Brown et. Al, Chemistry: The Central Science, Pearson, 2018 54  QUESTION: IS THIS DISSOLUTION? Source: T. Brown et. Al, Chemistry: The Central Science, Pearson, 2018 55 SOLUBILITY The solubility of a particular solute in a particular solvent is the maximum amount of the solute that can dissolve in a given amount of the solvent at a specified temperature, assuming that excess solute is present. Dynamic Equilibrium Rate of the forward process = Rate of the backward process Solution is saturated 56  VITAMINS: FAT SOLUBLE OR WATER SOLUBLE Source: T. Brown et. Al, Chemistry: The Central Science, Pearson, 2018 Vitamins have unique chemical structures that affect their solubilities in different parts of the human body. Vitamin C and the B vitamins are soluble in water, for example, whereas vitamins A, D, E, and K are soluble in nonpolar solvents and in fatty tissue (which is nonpolar). Because of their water solubility, vitamins B and C are not stored to any appreciable extent in the body, and so foods containing these vitamins should be included in the daily diet. In contrast, the fat-soluble vitamins are stored in sufficient quantities to keep vitamin-deficiency diseases from appearing even after a person has subsisted for a long period on a vitamin-deficient diet. 57  MORE GASES ARE DISSOLVED IN WATER AT LOWER TEMPERATURES At lower temperatures, molecules of gas dissolved in water have less kinetic energy, and dipole–induced dipole interactions between solute and solvent molecules keep more gas dissolved. At higher temperatures, molecules have more kinetic energy, which overcomes solute– solvent interactions and reduces solubility. Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 58  PRESSURE AFFECTS SOLUBILITY OF GASES IN WATER (AND OTHER SOLVENTS) The solubility of a gas in a liquid solvent increases in direct proportion to the partial pressure of the gas above the solution Henry’s law As pressure increase, the rate at which gas molecules strike the liquid surface and enter the solution phase increases Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 59  WHY DOES CO2 BUBBLE OUT WHEN A SODA IS OPENED? CO2 bubbles out of solution when a carbonated beverage is opened because the CO2 partial pressure above the solution is reduced. 60  WHY DO MOUNTAIN CLIMBERS NEED SUPPLEMENTAL OXYGEN AT HIGH ALTITUDES? concentration of dissolved oxygen in blood is proportional to the partial pressure of oxygen in the air we inhale and thus is proportional to atmospheric pressure. The oxygen transport system in our bodies must adjust to accommodate local atmospheric conditions. The amount of oxygen in the blood is related to both the concentration of hemoglobin and the fraction of the hemoglobin sites that contain oxygen as the blood leaves the lungs. This saturation of binding sites depends on the partial pressure of oxygen, as well as on proper lung function. For most people, breathing air with PO2. 0.11 atm results in nearly 100% saturation of hemoglobin binding sites. If PO2 decreases to about 0.066 atm (as it does on high mountains), the percent saturation decreases to about 80%. Over several weeks, the body responds to lower oxygen partial pressures by producing more red blood cells and more hemoglobin. This increase in the concentration of hemoglobin, and therefore in the number of O2 binding sites, compensates for the lower partial pressure of O2. Even though less than 100% of the oxygen binding sites are saturated, the actual number of oxygen binding sites has increased because of the production of more hemoglobin molecules, and therefore the same amount of O2 is delivered to tissues. Some endurance athletes try to capitalize on these physiological effects by using a technique known as “live high, train low.” They acclimate to higher altitudes, typically 61 defined as any elevation above 1500 meters (5000 ft), to bring about the physiological changes, but they continue to train at lower elevations. 61 BLOOD GASES AND DEEP-SEA DIVING Gas solubility increases as pressure increases. So solubility greater deeper in the ocean Divers who use compressed gases must be concerned about the solubility of the gases in their blood. 1. Bubbles form if the ascend too fast: decompression sickness or “the bends” 2. Helium often used instead of Nitrogen and Oxygen mixture since N2 more soluble: 95% He + 5% O2 has same O2 partial pressure as air at 1 atm 3. Too high oxygen partial pressue and urge to breathe reduced and cause CO2 poisoning 62 VISCOSITY OF A LIQUID The resistance of a liquid to flow is called viscosity. The greater a liquid’s viscosity, the more slowly it flows. Viscosity can be measured by timing how long it takes a certain amount of the liquid to flow through a thin vertical tube. Viscosity can also be determined by measuring the rate at which steel balls fall through the liquid. The balls fall more slowly as the viscosity increases. The viscosity of a liquid is related to how easily its molecules flow past one another. It depends on the attractive forces between molecules and on whether the shapes and flexibility of the molecules are such that they tend to become entangled (for example, long molecules can become tangled like spaghetti) 63  SURFACE TENSION OF WATER: INTERMOLECULAR INTERACTIONS A molecule within the bulk of a liquid experiences attractions to neighboring molecules in all directions, but since these average out to zero, there is no net force on the molecule. For a molecule that finds itself at the surface, the situation is quite different; it experiences forces only sideways and downward, and this is what creates the stretched-membrane effect. Insect can walk on water, water strider, uses the surface tension of water Liquid forms spherical droplets: to minimise surface area, have as few molecules on surface as possible. 64  SURFACE TENSION OF WATER: INTERMOLECULAR INTERACTIONS High surface tension of water: due to strong hydrogen bonding forces Surface tension is the energy/force required to increase the surface area of a liquid by a unit amount.  Surface tension of water at 20 °C is 7.29 ×10-2 J/m2, which means that an energy of 7.29 × 10-2 must be supplied to increase the surface area of water by 1 m2  Or, 0.0729 N/m  Surface tension of olive oil 0.0320N/m Source: T. Brown et. Al, Chemistry: The Central Science, Pearson, 2018 A molecule within the bulk of a liquid experiences attractions to neighboring molecules in all directions, but since these average out to zero, there is no net force on the molecule. For a molecule that finds itself at the surface, the situation is quite different; it experiences forces only sideways and downward, and this is what creates the stretched-membrane effect. Insect can walk on water, water strider, uses the surface tension of water Liquid forms spherical droplets: to minimise surface area, have as few molecules on surface as possible. 65  SURFACE PROPERTIES OF WATER High surface tension and high capillarity. When a small-diameter glass tube, or capillary, is placed in water, water rises in the tube. The rise of liquids up very narrow tubes is called capillary action. The adhesive forces between the liquid and the walls of the tube tend to increase the surface area of the liquid. The surface tension of the liquid tends to reduce the area, thereby pulling the liquid up the tube. The liquid climbs until the force of gravity on the liquid balances the adhesive and cohesive forces. Source: T. Gilbert et. Al, Chemistry: The Science in Context, WW Norton, 2018c 66 DUE TO SURFACE PROPERTIES OF WATER High surface tension keeps plant debris resting on a pond surface, providing shelter and nutrients for fish and insects. High capillarity means water rises through the tiny spaces between soil particles, so plant roots can absorb deep groundwater during dry periods. 67 SOLVENT PROPERTIES OF WATER 1. It dissolves ionic compounds through ion-dipole forces that separate the ions from the solid and keep them in solution∙ 2. It dissolves polar nonionic substances, such as ethanol (CH3CH2OH) and glucose (C6H12O6), by H bonding. ∙ 3. It dissolves nonpolar atmospheric gases to a limited extent through dipole– induced dipole and dispersion forces. 68 DUE TO SOLVENT PROPERTIES Water is the environmental and biological solvent, forming the complex solutions we know as oceans, lakes, and cellular fluid. Aquatic animals could not survive without dissolved O2, nor could aquatic plants without dissolved CO2. Tiny marine animals form coral reefs made of carbonates from dissolved CO2 and HCO3−. Life began in a “primordial soup,” an aqueous mixture of simple molecules from which emerged larger molecules capable of self-sustaining reactions. From a chemical point of view, all organisms, from bacteria to humans, are highly organized systems of membranes enclosing and compartmentalizing complex aqueous solutions. 69 EXPANSION OF WATER ON FREEZING 70 WATER AND ICE: HYDROGEN BONDING NETWORK 71 LOW DENSITY OF SOLID WATER (ICE) In the solid state, the tetrahedral arrangement of H-bonded water molecules leads to the hexagonal, open structure of ice The large spaces within ice make the solid less dense than the liquid Liquid water is most dense (1.000 g/mL) at around 4°C (3.98°C). 72  DUE TO LOW DENSITY OF ICE Surface ice of lakes. When the surface of a lake freezes in winter, the ice floats. If the solid were denser than the liquid, as is true for nearly every other substance, the surface water would freeze and sink until the entire lake was solid. Aquatic life would not survive from year to year. ∙ 73  DUE TO LOW DENSITY OF ICE Nutrient turnover. As lake water becomes colder in early winter, it becomes denser before it freezes. Similarly, in spring, less dense ice thaws to form more dense water before the water expands. During both of these seasonal density changes, the top layer of water reaches maximum density first and sinks. The next layer of water rises because it is slightly less dense, reaches 4°C, and likewise sinks. This alternation of sinking and rising distributes nutrients and dissolved oxygen. ∙ 74  DUE TO LOW DENSITY OF ICE Soil formation. When rain fills crevices in rocks and freezes, an outward force is applied that is relieved when the ice melts. In time, this repeated freeze-thaw stress cracks the rock. Over eons, this effect helps produce sand and soil. 75 Source: M. Silberberg et.al., Chemistry: The Molecular Nature of Matter and Change, McGraw Hill, 2018 76 READINGS Section 12.3 (Types of Intermolecular Forces) from Silberberg Section 12.4 Properties of the Liquid State from Silberberg Section 12.5 The Uniqueness of Water from Silberberg Section 8.3 Polar bonds from Gilbert Chapter 10 (Intermolecular Forces) from Gilbert  Skip Section on Clausius-Clapeyron Equation and 10.7 on Phase diagrams. 77