Engineering Chemistry 1B 2024 Liquids PDF

DEPARTMENT OF CHEMISTRY ENGINEERING CHEMISTRY 1B 2024 LIQUIDS BY DR L.D. MTHEMBU  OUTLINE Intermolecular forces Dipole-dipole force London (dispersion) forces Hydrogen bonding Properties of liquids Vapour pressure Surface Tension (Adhesion and C...

DEPARTMENT OF CHEMISTRY ENGINEERING CHEMISTRY 1B 2024 LIQUIDS BY DR L.D. MTHEMBU  OUTLINE Intermolecular forces Dipole-dipole force London (dispersion) forces Hydrogen bonding Properties of liquids Vapour pressure Surface Tension (Adhesion and Cohesion forces) Viscosity Refractive Index A PHASE IS A HOMOGENEOUS PART OF THE SYSTEM IN CONTACT WITH OTHER PARTS OF THE SYSTEM BUT SEPARATED FROM THEM BY A WELL-DEFINED BOUNDARY. 2 Phases Solid phase - ice Liquid phase - water 1  INTERMOLECULAR FORCES Intermolecular forces are the forces of attraction or repulsion between molecules. These forces are generally weaker than intramolecular forces but play a significant role in determining the physical properties of substances, such as boiling and melting points, solubility, and vapor pressure. Examples of intermolecular forces: 1. Dipole-dipole attractions 2. London (dispersion) forces 3. Ion-Dipole Forces 4. Hydrogen bonding  INTERMOLECULAR VS INTRAMOLECULAR FORCES INTRAMOLECULAR FORCES ARE THE FORCES THAT HOLD ATOMS TOGETHER WITHIN A MOLECULE. THESE FORCES ARE MUCH STRONGER THAN INTERMOLECULAR FORCES AND DETERMINE THE CHEMICAL PROPERTIES OF A SUBSTANCE. EXAMPLES OF INTRAMOLECULAR FORCES ARE COVALENT BOND, IONIC BOND, AND METALLIC BONDS  INTERMOLECULAR ATTRACTIONS Intermolecular attractions are : (1) Dipole-dipole attractions (2) London (dispersion) forces (3) Ion-Dipole Forces (4) Hydrogen bonding. Relative strength for the different interactions  INTERMOLECULAR FORCES Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid Molecules have a permanent positive and negative charge. e.g. HCl  INTERMOLECULAR FORCES LONDON (DISPERSION) FORCES Present in all molecules, whether polar or nonpolar. Arise due to temporary dipoles created by the movement of electrons. The only type of intermolecular force in nonpolar molecules. London forces tend to increase with molecular mass. Example: Noble gases like argon (Ar), and nonpolar molecules like nitrogen (N₂).  INTERMOLECULAR FORCES Ion-Dipole Forces Ion-dipole forces are a type of intermolecular force that occurs between an ion and a polar molecule. These forces are particularly important in solutions where ionic compounds are dissolved in polar solvents, such as salt in water.  INTERMOLECULAR FORCES Hydrogen Bond A special type of dipole-dipole interaction. Occurs when hydrogen is bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F). Stronger than regular dipole-dipole forces. Example: Water (H₂O), where hydrogen bonds form between the hydrogen of one water molecule and the oxygen of another. EXAMPLE: WHAT TYPE(S) OF INTERMOLECULAR FORCES EXIST BETWEEN EACH OF THE FOLLOWING MOLECULES? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH4 CH4 is nonpolar: dispersion forces. S SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.  EXAMPLE OF INTERMOLECULAR FORCES WHAT KINDS OF INTERMOLECULAR FORCES (LONDON, DIPOLE–DIPOLE, HYDROGEN BONDING) ARE EXPECTED IN THE FOLLOWING SUBSTANCES? 1. METHANE, CH4 2. (SEE MODEL AT BELOW) 3. BUTANOL (BUTYL ALCOHOL), CH3CH2CH2CH2OH SOLUTION PROBLEM STRATEGY NOTE THAT LONDON FORCES ARE ALWAYS PRESENT. THEN ASK YOURSELF WHETHER THE MOLECULE IS POLAR— IN WHICH CASE, DIPOLE–DIPOLE FORCES EXIST. NOW NOTE WHETHER HYDROGEN BONDING IS PRESENT. (IS A HYDROGEN ATOM BONDED TO F, O, OR N?) A. METHANE IS A NONPOLAR MOLECULE. HENCE, THE ONLY INTERMOLECULAR ATTRACTIONS ARE LONDON FORCES. B. CHLOROMETHANE IS AN UNSYMMETRICAL MOLECULE WITH POLAR BONDS. THUS, WE EXPECT DIPOLE–DIPOLE FORCES, IN ADDITION TO LONDON FORCES. C. BUTANOL HAS A HYDROGEN ATOM ATTACHED TO AN OXYGEN ATOM. THEREFORE, YOU EXPECT HYDROGEN BONDING. BECAUSE THE MOLECULE IS POLAR (FROM THE OOH BOND), YOU ALSO EXPECT DIPOLE–DIPOLE FORCES. LONDON FORCES EXIST TOO, BECAUSE SUCH FORCES EXIST BETWEEN ALL MOLECULES.  LEARNING CHECK Name the intermolecular forces present in the below compounds: 1. Water 2. Methane 3. Hydrogen Chloride 4. Sodium Chloride 5. Carbon Dioxide 6. Ammonia 7. Ethanol  SOLUTION 1. Water (H₂O) - polar 5. Carbon Dioxide (CO₂) – non-polar Hydrogen Bonding London Dispersion Forces Dipole-Dipole Interactions London Dispersion Forces 6. Ammonia (NH₃) - polar Hydrogen Bonding 2. Methane (CH₄) – non-polar Dipole-Dipole Interactions London Dispersion Forces London Dispersion Forces 3. Hydrogen Chloride (HCl) - polar 7. Ethanol (C₂H₅OH) - polar Dipole-Dipole Interactions Hydrogen Bonding London Dispersion Forces Dipole-Dipole Interactions London Dispersion Forces 4.Sodium Chloride (NaCl) - ionic Ion-Dipole Forces (when dissolved in water) Ionic Bonds (in solid state) PROPERTIES OF LIQUIDS Vapour pressure Surface Tension (Adhesion and Cohesion forces) Viscosity Refractive Index  SUMMARY OF PHASE TRANSITION IN GENERAL, EACH OF THE THREE STATES OF A SUBSTANCE CAN CHANGE INTO EITHER OF THE OTHER STATES BY UNDERGOING PHASE TRANSITION. Effect of Temperature on Vapour Pressure Increased Temperature: Leads to higher kinetic energy and more molecules escaping into the vapor phase. Higher Vapor Concentration: Before equilibrium is re- established, the concentration of vapor molecules increases. New Equilibrium: A higher vapor pressure is established at the new temperature where the rates of evaporation and condensation are equal. Both concentration and kinetic anergy are proportional to temperature.  Least Order Condensation Evaporation Greatest T2 > T1 11 Order  THE BOILING POINT IS THE TEMPERATURE AT WHICH THE (EQUILIBRIUM) VAPOR PRESSURE OF A LIQUID IS EQUAL TO THE EXTERNAL PRESSURE. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm. 14 THE CRITICAL TEMPERATURE (TC) IS THE TEMPERATURE ABOVE WHICH THE GAS CANNOT BE MADE TO LIQUEFY, NO MATTER HOW GREAT THE APPLIED PRESSURE. The critical pressure is the pressure required to liquefy a substance at its critical temperature. It is the minimum pressure needed to maintain the liquid phase at the critical temperature. 15  THE EQUILIBRIUM VAPOR PRESSURE IS THE VAPOR PRESSURE MEASURED WHEN A DYNAMIC EQUILIBRIUM EXISTS BETWEEN CONDENSATION AND EVAPORATION H2O (l) H2O (g) Dynamic Equilibrium Rate of Rate of condensation = evaporation 12  H2O (S) H2O (L) The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid Freezing Melting phases coexist in equilibrium 16 PHASE DIAGRAM A PHASE DIAGRAM IS A GRAPHICAL WAY TO SUMMARIZE THE CONDITIONS UNDER WHICH THE DIFFERENT STATES OF A SUBSTANCE ARE STABLE. Triple Point: point at which the temperature and pressure allow for a substance to be a solid, liquid, and gas at the same time Critical Point: The highest point at which the temperature and pressure can allow a substance to be both a liquid and a gas (any higher of a temperature/pressure and the substance cannot exist as a liquid).  PROPERTIES OF LIQUIDS: VAPOUR PRESSURE The vapor pressure of a liquid is the partial pressure of the vapor over the liquid, measured at equilibrium at a given temperature. Determination of Vapour Pressure 1. The Dynamic Method PROPERTIES OF LIQUIDS: VAPOUR PRESSURE 2. The Static Method  PROPERTIES OF LIQUIDS: SURFACE TENSION Surface tension is defined as the force per unit length along the surface of a liquid, but it also be viewed as the amount of energy required to stretch or increase the surface of a liquid by a unit area (1 dyne cm–1 = 1 m Nm–1). Note that a molecule at the surface experiences a net force toward the interior of the liquid, whereas a molecule in the interior experiences no net force. As a result, there is a tendency for the surface area of a liquid to be reduced as much as possible. This explains why falling raindrops are nearly spherical. (The sphere has the smallest surface area for a given volume of any geometrical shape). Energy is needed to reverse the tendency toward reduction of surface area in liquids.  PROPERTIES OF LIQUIDS Determination of Surface Tension The methods commonly employed for the determination of surface tension are : 1. Capillary-rise Method PROPERTIES OF LIQUIDS: SURFACE TENSION EXAMPLE A capillary tube of internal diameter 0.21 mm is dipped into a liquid whose density is 0.79 g cm–3. The liquid rises in this capillary to a height of 6.30 cm. Calculate the surface tension of the liquid. (g = 980 cm sec–2) h = height of liquid in capillary in centimetres r = radius of capillary in centimetres d = density of liquid in g cm–1 g = acceleration due to gravity in cm sec–2 PROPERTIES OF LIQUIDS: SURFACE TENSION SOLUTION A capillary tube of internal diameter 0.21 mm is dipped into a liquid whose density is 0.79 g cm–3. The liquid rises in this capillary to a height of 6.30 cm. Calculate the surface tension of the liquid. (g = 980 cm sec–2) h = height of liquid in capillary in centimetres r = radius of capillary in centimetres d = density of liquid in g cm–1 g = acceleration due to gravity in cm sec–2 PROPERTIES OF LIQUIDS: SURFACE TENSION 2. Drop Formation Method A drop of liquid is allowed to form at the lower end of a capillary tube. The drop is supported by the upward force of surface tension acting at the outer circumference of the tube. The weight of the drop (mg) pulls it downward. When the two forces are balanced, the drop breaks. PROPERTIES OF LIQUIDS: SURFACE TENSION Cohesion is the intermolecular attraction between like molecules Adhesion is an attraction between unlike molecules (A) Capillary rise, due to the attraction of water and glass. Adhesion Hydrogen bonds between the (A) (B) water molecules and the gas are illustrated. The final water level in the capillary is a balance between the force of Cohesion gravity and the surface tension of water. (B) Depression, or lowering, of mercury level in a Water Mercury glass capillary. Unlike water, mercury is not attracted to glass.  SURFACE TENSION: INTERACTION BETWEEN LIQUID AND SOLID SURFACE Representation of the contact angle for solid surfaces: (a) Water on the hydrophilic surface (attracted to water); (b) Water in the form of a drop on the hydrophobic surface (resists water).  PROPERTIES OF LIQUIDS: VISCOSITY Viscosity is a measure of a fluid’s resistance to flow. The viscosity of a liquid can be obtained by measuring the time it takes for a given quantity to flow through a capillary tube or, alternatively, by the time it takes for a steel ball of given radius to fall through a column of liquid. SI units for viscosity is N ms-1. Similar steel balls were dropped simultaneously into two graduated cylinders, the right one containing water and the left one glycerol. A steel ball takes considerably longer to fall through a column of glycerol than through a similar column of water, because glycerol has a greater viscosity than water. PROPERTIES OF LIQUIDS: VISCOSITY EXAMPLE In an experiment with Ostwald viscometer, the times of flow of water and ethanol are 80 sec and 175 sec at 20 ºC. The density of water = 0.998 g/cm3 and that of ethanol = 0.790 g/cm3. The viscosity of water at 20ºC is 0.01008 poise. Calculate the viscosity of ethanol. t1 (water) = 80 sec d1 (water) = 0.998 g/cm3 t2 (ethanol) = 175 sec d2 (ethanol) = 0.790 g/cm3 Temperature = 20 oC Viscosity of water (20 OC) = 0.01008 poise Viscosity of ethanol?  PROPERTIES OF LIQUIDS: VISCOSITY SOLUTION In an experiment with Ostwald viscometer, the times of flow of water and ethanol are 80 sec and 175 sec at 20 ºC. The density of water = 0.998 g/cm3 and that of ethanol = 0.790 g/cm3. The viscosity of water at 20ºC is 0.01008 poise. Calculate the viscosity of ethanol. PROPERTIES OF LIQUIDS: REFRACTIVE INDEX The refractive index (n) of a substance is defined as the ratio of the velocity of light in vacuum or air, to that in the substance : n = Velocity of light in substance Velocity of light in air When a ray of light passes from air into a liquid, its direction is changed. This change of direction is called refraction. PROPERTIES OF LIQUIDS: REFRACTIVE INDEX MOLAR REFRACTION It is defined as the product of specific refraction and molecular mass. Thus molar refraction of a liquid (RM) is obtained by multiplying equation (1) by molecular mass (M) The value of molar refraction is characteristic of a substance and is temperature-independent. It can be determined by substituting the values of n, M and d PROPERTIES OF LIQUIDS: REFRACTIVE INDEX EXMPLE OF MOLAR REFRACTION CALCULATION The refractive index of carbon tetrachloride for D-line of sodium has been found to be 1.4573. Calculate its molar refraction if the density is 1.595 g/cm3. nCCl4 = 1.4573 d = 1.595 g/cm3 RM ? PROPERTIES OF LIQUIDS: REFRACTIVE INDEX SOLUTION The refractive index of carbon tetrachloride for D-line of sodium has been found to be 1.4573. Calculate its molar refraction if the density is 1.595 g/cm3. THE END