Thermochemistry 1 (Intermolecular Forces) 2024 PDF

OBAFEMI AWOLOWO UNIVERSITY, ILE-IFE, NIGERIA COURSE: CHM 101 (INTRODUCTORY CHEMISTRY I) TOPIC: THERMOCHEMISTRY  Introduction Thermochemistry is the study of heat changes associated with chemical processes either at constant volume or at constant pressure. The follow...

OBAFEMI AWOLOWO UNIVERSITY, ILE-IFE, NIGERIA COURSE: CHM 101 (INTRODUCTORY CHEMISTRY I) TOPIC: THERMOCHEMISTRY  Introduction Thermochemistry is the study of heat changes associated with chemical processes either at constant volume or at constant pressure. The following sub-topics will be discussed: Balancing of intermolecular forces. Order-disorder phenomenon, Entropy, free energy, energy effect, Exothermic and Endothermic changes, Enthalpy of reaction, Hess’s law of enthalpy summation (with relevant calculations), Heats of neutralization, combination and formation, Bond dissociation energies (with relevant calculations) Free energy and spontaneous change. INTERMOLECULAR FORCES What are intermolecular forces? Recall: Intramolecular forces are those forces within the molecule that keep the molecule together. They are responsible for the formation of the stable units called molecules. These molecules are formed by sharing or transfer of electrons; for example, the bonds between the atoms. ❖ Intermolecular forces are the forces that exist between (rather than within) molecules or particles. Intermolecular forces (IMFs) are the noncovalent attractive forces between atoms, molecules, and/or ions. They hold molecules together in a substance and determine many of the physical properties of a substance. Intermolecular forces (cont’d) E.g. When water changes from solid to liquid to gas, the molecules remain intact but change of state is due to changes in forces among the molecules rather than those within the molecule. ❖The strengths of these attractive forces vary widely, though usually the IMFs between small molecules are weak compared to the intramolecular forces that hold atoms together within a molecule. ❖Bulk Properties like melting point and boiling points are a measure of how strong the attractive forces are between atoms or molecules. What Causes Intermolecular Forces? 1. Fluctuations in electron cloud in non- polar molecules (instantaneous dipole is formed) 2. Asymmetrical charge distribution due to differences in electronegativity (polar molecules). Intermolecular forces (cont’d) Let’s consider HCl molecules as an example……… HCl has permanent dipoles due to electronegativity 𝛅+ difference between 𝛅- 𝛅- 𝛅+ chlorine atoms and hydrogen atoms Intramolecular forces keep a molecule intact. Intermolecular forces hold multiple molecules together and determine many of a substance’s properties.  Balancing of Intermolecular Forces Two factors determine whether a substance is a solid, liquid or gas. 1. The kinetic energy of the particles (atoms, molecules or ions) that make up the substance. → This keeps particles moving apart. 2. The attractive intermolecular forces between particles. → This keeps particles close together. The balance between these 2 factors determines the phase of a substance. i.e. the preferred phase of substance depends on the strength of the intermolecular forces and the kinetic energy of the particles. If the K.E is > the attractive forces, a gas phase is preferred. If the K.E < the attractive forces, a liquid or solid is formed.  Balancing of Intermolecular Forces (cont’d) Transitions between solid, liquid, and gaseous states of a substance occur when conditions of temperature or pressure favour the associated changes in intermolecular forces. (Note: The space between particles in the gas phase is much greater than shown.) Source: https://cooljargon.com/ebooks/chemistry/m51077/index.cnxml.html  Balancing of Intermolecular Forces (cont’d) At low temperatures, most substances are solids (except helium, a liquid at absolute zero). As temperature increases, substances with very weak intermolecular forces become gases directly (sublimation); substances with weak interaction can become liquids as temperature increases. Note: Increase in temperature increases the Kinetic Energy of the molecules As temperature increases, all substances progress from solid to liquid to gas. Also, as bonds become more polarized, the charges on the atoms become greater, the intermolecular attractions become greater leading to higher boiling points.  TYPES OF INTERMOLECULAR FORCES. 1). Dispersion forces (also called London dispersion) Instantaneous position of an e- in a molecule makes that point of the molecule negatively charged and the rest positively charged. Dispersion is the only IMF that occur between non-polar molecules (e.g H2, CCl4, CO2..). It is the weakest IMF. It is present in all substances with electrons Dispersion forces increase with molecular weight. The greater the number of electrons in a species, the stronger the dispersion forces. E.g Halogen Molecular Boiling Melting weight Point Point F2 38.00 g/mol 85K 53K Cl2 70.91 g/mol 238K 172K Dispersion forces result from the formation of temporary dipoles, as illustrated here for two Br2 159.81g/mol 332K 226K nonpolar diatomic molecules. And sometimes, smaller molecules are gases and E.g larger molecules are solid at the same temperature 2. Dipole – induced dipole forces These forces occur between permanent dipoles and polarisable electron cloud. A dipole-induced dipole attraction is a weak attraction that occurs when a polar molecule causes a dipole to form in an atom or nonpolar molecule by disrupting the electron configuration in the non-polar species They are weak but stronger than London forces Dispersion forces and dipole-induced Other examples: carbondioxide in forces are generally referred to as van ethylene, xenon in water der Waal forces  3. Dipole – Dipole attraction (Permanent Dipoles) Dipole-dipole forces are the electrostatic force between two permanent dipole molecules. The oppositely charged ends of a polar molecule which have partial charges (due to uneven distribution of electron) on them attract each other. E. g polar molecules like HCl, CH2O, e.t.c CH2Cl2 has a higher boiling point than CF4(non-polar) even though it has a lower molar mass. The larger the dipole, the Formaldehyde has greater the dipole – dipole permanent dipoles δ- intermolecular interaction. (based on the polarized carbon- δ+ They are stronger than van der Waal forces oxygen bond) and thus shows dipole- dipole interaction.  4. Hydrogen Bonding This is a special case of dipole-dipole forces. Molecules with H covalently bonded to F, N, or O have hydrogen bonding which are very strong. F, N and O have high electronegativity values It is stronger than dispersion forces, Water, H2O has relatively high melting and boiling point while H2S molecules do not. This strong dipole-induced dipole forces and attraction between H2O requires additional energy to permanent dipoles. separate its molecules. The dipole – dipole interactions. between these bonds are unusually strong. Examples of hydrogen bonds include HF⋯HF, H2O⋯HOH, and H3N⋯HNH2, acetic acid, acetone in water, etc  Hydrogen Bonding (cont’d) Hydrogen bonding is the reason water has unusual properties. For such a small molecule (its molar mass is only 18 g/mol), H2O has relatively high melting and boiling points. Its boiling point is 373 K (100 °C), while the boiling point of a similar molecule, H2S, is 233 K (−60 °C). This is because H2O molecules experience hydrogen bonding, while H2S molecules do not. This strong attraction between H2O molecules requires additional energy to separate the molecules in the condensed phase, so its boiling point is higher than would be expected. H- bonding in water is responsible for water’s ability as a solvent, its high heat capacity and its ability to expand when freezing  5. Ion-dipole and Ion-induced dipole Ion-dipole : Force between an ion and a dipole (full charge and a partial charge). An example is seen when salt dissolves in water (since water molecules have permanent dipoles). Ion-dipole interaction increases as the charge increases and also increases with smaller size of ion involved. They are stronger than dipole-dipole forces Example: compare dissolving MgCl2 and KCl in water. Solution: For Mg2+ and K+, charge +2 is higher. Hence, ion – dipole interaction is greater when Ion–induced dipole force consists of an ion and MgCl2 is dissolved in water than when KCl is dissolved in water. a non-polar molecule interacting. Like a dipole– induced dipole force, the charge of the ion causes Exercise: 1) Contrast between KCl and CsCl dissolved in water. distortion of the electron cloud on the non-polar molecule  Practice Questions 1. What type of intermolecular force do all substances have? 2. What is necessary for a molecule to experience dipole-dipole interactions? 3. What is necessary for a molecule to experience hydrogen bonding? 4. How does varying the temperature change the preferred phase of a substance? 5. Identify the strongest intermolecular force present in each substance. He, CHCl3, HOF 6. Identify the strongest intermolecular force present in each substance. CH3OH, (CH3)2CO, N2 7. Identify the strongest intermolecular force present in each substance. HBr, C6H5NH2, CH4 8. Identify the intermolecular forces present in the following solids: (a) CH3CH2OH (b) CH3CH2CH3 (c) CH3CH2Cl 9.Arrange each of the following sets of compounds in order of increasing boiling point temperature: (a) HCl, H2O, SiH4 (b) F2, Cl2, Br2 (c) CH4, C2H6, C3H8 (d) C3H6, CH3CN, CH3OCH3, 10. O2, NO, N2, Neon and HF have approximately the same molecular masses. (a) Explain why the boiling points of Neon and HF differ. (b) Compare the change in the boiling points of Ne, Ar, Kr, and Xe with the change of the boiling points of HF, HCl, HBr, and HI, and explain the difference between the changes with increasing atomic or molecular mass. 11. On the basis of intermolecular attractions, explain the differences in the boiling points of n–butane (−1 °C) and chloroethane (12 °C), which have similar molar masses. 12. Explain why a hydrogen bond between two water molecules is weaker than a hydrogen bond between two hydrogen fluoride molecules.