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Transcript
Intermolecular Forces The strength of the intermolecular forces determines the physical properties of the substance: - weaker than covalent bonds, but can be stronger than ionic bonds - the temperature at which a liquid boils/solid melts reflects the kinetic energy needed to overcome the attractive intermolecular forces - intermolecular bonds are broken when a molecular compound melts and boils 1. Hydrogen bonding -water is a highly polar molecule: large electronegativity difference and shape (bent) boiling points -since oxygen has the higher electronegativity, the electrons spend more time around oxygen than they do around hydrogen - the dipoles created by O-H in water are attracted to opposite charges creating hydrogen bonds (1, right side): the strongest of all intermolecular forces - as a consequence, water is capable of dissociating some ionic compounds 2) van der Waals forces -weak forces of attraction between molecules a) dipole–dipole force (DDF) -occurs between polar molecules, such as hydrogen chloride, HCl -the slightly positive end of one hydrogen chloride molecule is attracted to the slightly negative end of a neighbouring hydrogen chloride molecule b) London dispersion force (LDF) - an intermolecular force of attraction that forms between atoms of neighbouring molecules as a result of a temporary imbalance in the position of the atoms’ electrons -forms between all molecules, polar and nonpolar - the side of the atoms with more electrons develops a temporary negative charge, and the side with fewer electrons develops a temporary positive charge; if same happens to neighbouring molecule they attract each other - since electrons move quickly, the dipole lasts for only a fraction of a second! physical change: involves intermolecular bonds and therefore changes of state (s, l, g) chemical change: involves intramolecular bonds and leads to formation of new products! TYPES OF CHEMICAL REACTIONS 1. Synthesis Reactions (combination/addition reaction) -the combination of two or more simple substances to form a more complex substance A+B --> AB ex. hydrogen gas and oxygen gas react to produce water 2. Decomposition Reactions -the breakdown of large, more complex molecules or ionic compounds into smaller and simpler entities AB --> A + B ex. hydrogen peroxide (H2O2 )decomposes to produce oxygen gas and water 3. Single Displacement Reactions -reaction of an element and a compound to produce a new compound AB + C --> AC + B ex. solid zinc in lead (II) nitrate solution will give you solid lead and aqueous zinc nitrate 4. Double Displacement Reactions - a chemical reaction in which two compounds in aqueous solution react to form two new compounds AB + CD --> AD + CB ex. aqueous silver nitrate + aqueous calcium chloride produce solid silver chromate + aqueous calcium nitrate