Chem 1AL Review Session 1: 1/31/2013 Moles Since atoms and molecules are very small we need units to easily describe large number of particles. This unit is known as the: Mole = 6.02 x 1023 particles (atoms, molecules, ions, etc.) - Mole is abbIf you want to learn more check out biology 2401
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reviated as mol 1 mole of water (H2O) = 6.02 x 1023 molecules of water 1 mole of Iron (Fe) = 6.02 x 1023 atoms of Iron 1 mole of hydroxide (OH-) = 6.02 x 1023 ions of hydroxide Molecular weight of an atom, molecule, etc. can be given in terms of grams/mole (abbreviated as g/mol) - This tells us how much 6.02 x 1023 atoms, molecules, etc. of that substance would weigh in grams. - This can be calculated by summing the atomic weights for each element found on the periodic table. Example: What is the molecular weight of water (H2O)? Molecular weight of O = 16.00 g/mol Molecular weight of H = 1.008 g/mol Water is made up of 2 H, and 1 O so the molecular weight is equal to: MW H2O = 2(1.008 g/mol) + 1(16.00 g/mol) = 18.016 g/mol Molarity Molarity = # moles solute/ Liters of solution - Way of measuring the concentration of a solutionChem 1AL Review Session 1: 1/31/2013 - Unlike other measurements of concentration like grams of solute/L of solution, molarity tells us about the number of atoms/molecules in solution - It is abbreviated as M (or mol/L) Example: What’s the molarity of a solution of 1.00 g of NaCl in 0.500 L H2O? Molecular Weight NaCl = 58.44 g/mol _1.00 g_ x _1 mol_ = _1.00 _ x _1 mol_ = _(1.00)(1 mol)_ = 0.0342 M 0.500L 58.44 g 0.500L 58.44 (0.500L)(58.44) Types of Bonding Ionic Bonding - Usually between a metal and a non-metal - One atom (the non-metal) takes an electron from the other (metal) - This causes one atom to be positively charged and the other to be negatively charged o Atoms or molecules with a charge are known as ions - A strong interaction forms based on the opposite charges attracting, thus making the ionic bond. Covalent Bonding - Usually between two nonmetals - Bond formed by atoms sharing electrons. - No positively charged and negatively charged ions formed since electrons are shared o However small partial positive and negative charges can formChem 1AL Review Session 1: 1/31/2013 Electronegativity - Some elements have a stronger affinity for electrons than others o This property is called electronegativity and these elements are called more electronegative. - A general trend exists for this property o As you go from the bottom left of the periodic table to the top right electronegativity increased - Most extreme case of this is ionic bonding. - This can still have an impact on covalent bonding δ δδ indicates that it is a partial charge. - In the O—H bond, oxygen is more electronegative that hydrogen is, causing there to be a partial negative charge on oxygen and a partial positive charge on hydrogen since oxygen has more of the electrons. - This is called a dipole or a polar bond (since two poles are formed by it having a positive end and a negative end.) - Nonpolar bonds occur between: o Two of the same atom (same electronegativity so electrons are equally shared) o Different atoms with similar electronegativity - A good example of this is the C—H bond which has no significant dipole since carbon and hydrogen have very similar electronegativities. Intermolecular Forces - Bonds are forces that hold molecules together - Weaker forces exist which cause molecules to interact with and stick to each other. o These allow solid and liquid states to exist. Interactions between the Same Kind of Molecules Dipole-dipole interactions - Interactions between polar molecules (ie. Molecules with dipoles) - Partial positive and negative charges are attracted to each other causes the molecules to be drawn together. δ δ Chem 1AL Review Session 1: 1/31/2013 δ δ This molecule is known as formaldehyde Subset of dipole-dipole interactions is Hydrogen Bonding: o Strongest type of dipole-dipole interaction o Only happens between H—N, H—O, and H—F bonds ▪ Large difference in electronegativity between the two atoms causes a more powerful dipole δ δ δ δ δ δδ δ δ Water is a great example of hydrogen bonding. Many of waters important properties are due to hydrogen bonding. This includes the fact that it is a liquid of room temperature, even though it is a very small and light molecule. Chem 1AL Review Session 1: 1/31/2013 London Dispersion Forces - Interaction between nonpolar atoms and molecules o Just because something in non-polar doesn’t mean it won’t interact with other atoms and molecules - Instantaneous dipoles can form which induce dipoles in nearby atoms and molecules. (A) δ δ (B) δδδ δ δ δ (C)o Since electrons randomly travel around the nucleus, you may end up with more electrons on one side of the nucleus than the other.(B) This causes an instantaneous dipole to form where there is a partial negative and positive side. This in turn induces dipoles in nearby atoms as electrons in other atoms are attracted/repelled by the dipole (C). - Size matters o Larger molecules/atoms have less tightly held electrons which can form instantaneous and induced dipoles more easily. - Molecular shape matters o Atoms need to be in close proximity for London Dispersion Forces to occur. o Shapes that allow for easier contact will have stronger London Dispersion Forces. Chem 1AL Review Session 1: 1/31/2013 Interactions Between Different Types of Molecules Ion-dipole interaction - Between an ion and a polar molecule. δ δδ - Strongest intermolecular force Ion-induced dipole interaction - Charged species can induce dipoles in nonpolar species. Dipole – Induced Dipole interaction - Polar molecules induce dipoles in non-polar species - Weaker than a dipole-dipole interaction Chem 1AL Review Session 1: 1/31/2013 Line-Angle Formula - Really popular method of drawing molecules. - Carbon must have four bonds. - Place where lines begin and end = carbons; you’re only drawing the bonds in between them. - Hydrogens on carbons are implied, any carbon with fewer than four bonds is assumed to be bound to as many hydrogens as needed to reach four bonds. - Any non-carbon or hydrogen atoms are written out, as are any hydrogen atoms attached to them. Here we go from writing all the elements to line angle formulas.Chem 1AL Review Session 1: 1/31/2013 Polymers - Molecules with a structure composed of repeating until called monomers - polyvinyl alcohol Copolymers - Polymers made up of more than one monomer - Sometimes arranged in a simple ABAB pattern - Sometimes are branched - Guar gum is a good example of a copolymer. Cross-linking - Bonds that link one polymer chain to another - Crosslinking causes chains to lose some of their flexibility. o You could see knitting as an analogy for this process. The individual polymer chains are like the individual thread that go into making a scarf, they are very flexible and can be wrapped and tied in a variety of different ways. However when you knit them together into a scarf, you have different properties and greatly reduced flexibility. How do we make crosslinks and what do they look like? - In lab we used Borax to form various types of cross-linking interactions.Chem 1AL Review Session 1: 1/31/2013 o Borax forms in solution. - This interacts with polymer chains: All covalent linkages Mixed covalent and hydrogen bonding linkages - It’s also possible to have interactions with the polymer chains that are all hydrogen bonding. - The more crosslinking the more strands are linked together and the less flexibility they have. - However in lab you have seen that not all polymers crosslink or have the same results when they do crosslink.