Chapter 3 Reading Notes
Chapter 3 Reading Notes CHMY 321-001
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This 4 page Class Notes was uploaded by Rebeka Jones on Monday September 19, 2016. The Class Notes belongs to CHMY 321-001 at Montana State University taught by Holmgren, Steven in Fall 2016. Since its upload, it has received 5 views.
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Date Created: 09/19/16
Chapter 3 An ionic reaction is a reaction in which ions participate as reactants, intermediates, or products The definition of a Bronsted-Lowery acids and bases is based on the transfer of a proton (H+) An acid is a proton donor A bas is a proton acceptor The products of a proton transfer reaction are all conjugate based and the conjugate acid -conjugate base: what remains of the acid *water can be an acid or a base All reactions are accomplished via a flow of electron density. The flow of electron density is illustrated with curved arrows. Even though they look just like the arrows used for resonance structures these ones actually represent the movement of electrons. The arrows show the reaction mechanism – they show how the reaction occurs in reference to the electrons *always involves at least two arrows There are two ways to predicts when a proton transfer reaction will occur. -via quantitative approach (comparing pka values) -via a qualitative approach (analyzing the structures of the acids) equilibrium – when there is no longer an observable change in the concentrations of reactants and products. Because when acid-base reactions are carried out in dilute aqueous solution, the concentration of water is fairly constant we can remove it from the Keq equation. This gives the Ka ▯ ▯ ???? ▯ [???? ] ???????? = [????????] Ka = strength of acid Strong acid 10 10 Weak acid 10 -50 Pka is an easier way to deal with this Pka = -logKa Pka values normally range from -10 to 50 -strong acids will have low pka values -each unit represents an order of magnitude *stronger acids create weaker bases We can also use pka values to predict where equilibrium will be Equilibrium will always favor formation of the weak er acid In order to compare acids without the use of pka values we have to look at the conjugate base of each acid. 2 - - If A is very stable (weak base) than HA must be a strong acid. If A is very unstable (strong base) the HA must be a weak acid. Electronegative atoms stabilize a negative charge, Because carbon is not very electronegative it doesn’t stabilize negative charges well. *by determining the more stable conjugate base we can identify the stronger acid 1. Which atom bears the charge a. Comparing the atoms that bear a negative charge in the conjugate base b. To determine which of these is more stable we must consider where these elements are on the periodic table i. When two atoms are in the same row, electronegativity is dominant ii. When two atoms are in the same column size is dominant 2. Resonance a. If your negative charge is on the same atom check for resonance. When there is resonance on one of the atoms the charge is more stabilized therefore that atom is more acidic. i. These compounds are called carboxylic acids 3. Induction a. When factor 1 and 2 do not show a difference look for difference in the other atoms in the molecules. i. For example, if one molecule has 3 Cl atoms while the other has none. Cl has an induction effect and therefore stabilizes the molecule. 4. Orbitals 3 a. The differences before will not explain the differences in acidity between identified protons on an atom i. The answer comes from looking at the hybridization state of the orbitals that hold the charge 1. The closer the electrons are to the nucleus the more stable it is. a. Sp-hybridized is more stable than sp - 2 hybridized The equilibrium will always favor the more stable negative charge. If a base stronger than HO is dissolved in water, the base reacts with water to produce hydroxide. This is called the leveling affect. In order to work with bases that are stronger than hydroxide, a solvent other than water must be employed. In order to work with an amide ion as a base, we used liquid ammonia (NH ) as a solvent. It continues from there. 3 Negatively charged bases are always accompanied by positively charged ions, called cations. A Lewis acid is defined as an electron acceptor A Lewis base is defined as an electron donor 4
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