BASCHEM LQ1 PDF Chemistry Past Paper

BASCHEM_LQ1 CHAPTER 9 LEWIS DOT SYMBOLS ❖ Gilbert Newton Lewis (1875-1946) American Chemist Illustrated how atoms bond to achieve stable electron configuration Maximum stabil...

BASCHEM_LQ1 CHAPTER 9 LEWIS DOT SYMBOLS ❖ Gilbert Newton Lewis (1875-1946) American Chemist Illustrated how atoms bond to achieve stable electron configuration Maximum stability results when an atom is isoelectronic with a noble gas ❖ Valence Electrons Outer shell electrons Participate in chemical bondings ❖ Lewis Dot Symbol Symbol of an element + dot for each side representing its valence electrons Put the dots on each side first before filling out the others (although this is not really followed xD (as in oxygen)) Octet Rule Exhudes stability [ 2 e- for H, 8 for others] Achieved through forming bonds BONDINGS. ❖ Ionic Bonding Transfer of electrons An electrostatic force which holds ions together (metal + non-metal) Ionic compounds can be: Anions Formed through high electron affinity a Mostly Halogens (7A) and O- Negatively charged Cations Formed through low ionization energy Positively charged ions BASCHEM_LQ1 ❖ Covalent Bonding Share of electrons A chemical bond, mostly between NM-NM, M-M Bond length Distance between the two nuclei which relates to the strength of a molecule In length triple bond < double bond < single bond In strength, the shorter the length, the stronger the bonds: triple bond > double bond > single bond ❖ Lewis Structures Uses the principle of Lewis Dot 2D representation that shows the bonding of electrons Representation of covalent bonding in which pairs are shown either as lines or dots e- per bonds are equivalent to 2 atoms BASCHEM_LQ1 ** high melting point = stronger bonds ** most covalent bonds are usually insoluble in water therefore don’t conduct electricity ** strength based on bondings Ionic > Hydrogen Bonding > dipole-dipole > London Dispersion ELECTRONEGATIVITY ❖ Electronegativity The ability of an atom to attract an electron toward itself Increases from left to right and bottom to top Transitional metals don’t follow this Electron Affinity release of energy when an electron gets added to an atom measurable highest: Cl Electronegativity relative (can only be determined by relating it to other atoms) highest: F.. BASCHEM_LQ1 ❖ Polar Covalent Bond Has greater electron density around one of two atoms Heisenberg’s Principle “Uncertainty Principle” Since we can’t pinpoint the location of an electron, we can predict the area of electron probability ❖ Non-Polar Covalent Bond ❖ Remember! Electronegativity dictates the polarity of a molecule Nonpolar < polar < ionic WRITING LEWIS STRUCTURE ❖ TIPS !! Skeletal Structure: H2O In chemical rule, the first element mentioned is usually the central atom, except H and F BASCHEM_LQ1 Hydrogen and Helium are already stable as they only need 2 e- Least electronegative atom is mostly the central atom Ex. FORMAL CHARGE ❖ Formal Charge Used when there can be two or more skeletal structures of compounds/molecules It is the difference between number of valence e- and number of e- assigned to the atom in the Lewis structure The sum of formal charges of atoms in a molecule/ion must be equal to the charge on a molecule/ion: Neutral = 0 Cation = + Anions = - BASCHEM_LQ1 Ex. Formaldehyde [H2CO]-2 ** for the ½, we can just count the bonds around the atom. ** take into consideration that if it’s FC on O then only focus on oxygen, not the whole molecule ❖ RULES !!! B is better !! RESONANCE ❖ Resonance Structure one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure In chemistry, arrows dictate the direction of the chemical reaction, as such, ⇿ means resonance in resonance experiments, it showed that all bonds have the same length, which means that this is just another point of view from the same molecule as long as the accounting is equal. Ex. , BASCHEM_LQ1 CHAPTER 10 Molecular Geometry ❖ Valence Shell Electron Pair Repulsion Theory [ VSEPR ] Electron pairs surrounding an atom want to be as far apart from each other as possible [think 3Ds] It gives rise to different molecular shapes TERMS: Molecular Group Geometry [MGG] Only concerned about the atoms attached to the central atom, but taking into account the impact of lone pairs Electron Group Geometry [EGG] concerned about both atoms attached to the central atom, and lone pairs RULES: The bond angle changes Lone pairs show stronger repulsions than those who don’t The lower the angle(?) the stronger the repulsion BASCHEM_LQ1 BASCHEM_LQ1 ❖ PREDICTING MGG Dipole Moments ❖ TERM: Electron Region Poor / Region Delta (δ) indicates partiality ❖ Behavior of Polar Molecules Positive charge attracts negative and aligns Itself BASCHEM_LQ1 ❖ Dipole Moments Add up diatomic molecules containing atoms of different elements for molecules containing three or more atoms relies on its MGG to determine dipole moments Check electronegativity! BASCHEM_LQ1 CHAPTER 11 Intermolecular Forces ❖ Intermolecular Force Condensed states of matter: Liquid and Solid Affects the behavior of solids, liquids, and gases Attractive forces between molecules Dipole-Dipole Forces Attractive forces between polar molecules Origin: electrostatic forces, the larger dipole moment, greater force In liquid, it’s not as held as tightly as solids HYDROGEN BOND Special dipole-dipole interaction ◆ Normally it increases molar mass and boiling point, however, in hydrogen bonding ◆ Between two polar molecules with [O, N, F] there must be both a hydrogen donor and an acceptor present BASCHEM_LQ1 Ion-Dipole Forces Attractive force between an ion and polar molecule Its strength depends on the charge size of the ion, the magnitude of the dipole, and the size of the molecule Charges on cations (+) are generally more concentrated as they are smaller than anions (-). Cations interact more strongly with dipoles than does an anion with a charge of the same magnitude Dispersion Force. London dispersion Attractive forces that arise as a result of temporary dipole induced in atoms/molecules Present in all bondings Polarizability The likelihood of a dipole being induced or not depends on the charge of an ion, strength of the dipole, also polarizability of the ion/molecule The ease of distortion of electron distribution in an atom BASCHEM_LQ1 ❖ Intramolecular Force Holds atoms together in a molecule Intramolecular needs more energy to melt something ❖ Phase Homogenous part of the system in contac with other parts of the system but separated from them by a well-defined boundary Phases are used in ‘change of state’ ❖ TIPS and EXERCISE Determine the structure and bond type BASCHEM_LQ1 BASCHEM_LQ1 PHASE CHANGES ❖ Liquid State Particles are close to each other = have small compressibility When heated, particles rapidly move Little thermal expansion Has many close neighbors therefore travels short distance before a collision and bouncing back in the opposite direction. PROPERTIES Surface tension. Amount of energy required to stretch or increase the surface of liquids by unit area. Stronger IMF, higher surface tension Capillary action Example of surface tension phenomena A thin film of water adheres to the wall of a glass tube. The surface tension of H2O causes the film to contract which pulls the water up the tube COHESION ◆ Intermolecular attraction between like molecules ADHESION ◆ Intermolecular attraction between unlike molecules BASCHEM_LQ1 Viscosity Measure of fluid’s resistance to flow.; the tendency of the liquid to be fluid High IMF, high viscosity ❖ Solid State Particles are held in fixed lattice positions Has dominant Cohesive Forces than dispersive forces Has a fixed shape, volume, and density Little Thermal expansion Particles may only move a small amount around fixed positions BASCHEM_LQ1 PHASE CHANGES ❖ Phase Changes States = Phase Transformation of one phase to another Physical changes that are characterized by changes in molecular order Dictated by the changes in order and affected by temperature ❖ ❖ Equilibrium Vapor Pressure. The vapor pressure is measured when a dynamic equilibrium exists between condensation and evaporation.. BASCHEM_LQ1 It is the maximum vapor pressure a liquid exerts at a given temperature As you change temperature, the equilibrium also changes ❖ Molar Heat of Vaporization The energy required to vaporize 1 mole of liquid at its boiling point Usually in KJ Directly related to the strength of IMFA that exists in liquid High IMFA, High MHV Liquids have relatively low vapor pressure, and high molar heat of vp. ❖ Boiling Point Temperature at which the equilibrium vapor pressure (lq.) = external pressure Normal Boiling Point The temperature at which liquid boils when external pressure is 1 atm Pressure changes due to altitude BASCHEM_LQ1 ❖ Solid Liquid Equilibrium Melting point of a solid or freezing point of a liquid Temperature at which both phases coexist in equilibrium ❖ Molar Heat of Fusion Energy required to melt 1 mole of solid at freezing point MHV is higher than MHF considering the same substance as: Molecules in liquid are fairly packed together ❖ Heating Curve BASCHEM_LQ1 ❖ Solid Gas Equilibrium Sublimation (s -> g), deposition (g -> s) Ex. Naphthalene, Albatross ❖ Molar Heat of Sublimation Energy required to sublime 1 mole of solid. PHASE DIAGRAMS ❖ These summarize the conditions at which a substance exists as solid, liquid, or gas. ❖ Lines are the equilibrium of two phases, which means, the two phases coexist ❖ ❖ BASCHEM_LQ1 ❖ ❖ Curve AB BASCHEM_LQ1 ❖ Curve AC ❖ ❖ Curve AD ❖ ❖ Phase Diag