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Chem 105 notes.doc

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Yoram Apelblat

Chem 105 Chapter 16, Kinetics 16.2 Expressing the Reaction Rate If we have the chemical reaction, aA+bBcC+dD Then the reaction rate in terms of concentration can be expressed as: Rate = 1 [A] =...= 1 [D] a t d t 16.3 The Rate Law and its Components The rate law has the form: Rate=k[A] [B] n Where A and B are concentrations, k is the rate constant and is specific for the given reaction and changes with temperature. m and n are the reaction orders, if m or n are zero, then the rate doesnt depend on that chemical. The components of the rate law MUST be found by experiment, it isnt possible to use reaction stoicheometry. Reaction order technology: Chapter 18, Acid-Base Equilibria 18.1 The meaning of K : a This specific equilibrium constant highlights only the species whose concentrations change to any significant extent. It follows the general products over reactants rule. Like the other equilibrium constants, it is temperature dependant. Classifying the Relative Strengths of acids and bases: Strong Acids: there are two types that should be memorized 1. The hydrohalic acids, HCl, HBr, HI 2. Oxoacids in which the number of O atoms exceeds the number of ionisable protons by two or more, such as HNO , H S3 and2HCl4 4 Weak acids: there are four types of weak acids 1. The hydrohalic acid, HF 2. Acids where the H isnt bonded to an oxygen or a halogen, HCN and H S 2 3. Oxoacids where the number of O atoms equals or exceeds by one the number of ionizable protons, HClO, HNO , H P2 3 4 4. Carboxylic acids (general formula=RCOOH) such as CH COOH 3 2- - Strong Bases: which are water soluble compounds containing O or OH. 1. These are either group one or group two metals which are bonded to either of those two ions. *MgO and Mg(OH) are on2y slightly soluble in water, but the soluble portion dissociates completely. Weak bases: Many compounds with an electron rich nitrogen atom are weak bases. A common structural feature is that the nitrogen atom has a lone pair. 18.2 Autoionization of Water and the pH scale Autoionization is a trait given to pure water because it will (very slightly) dissociate into its ions on its own. K w1E-14 18.3 The Bronsted and Lowry Definition: The definition is that an acid is a proton donor and a base is a proton receiver. From the Bronsted and Lowry perspective, the only requirements is that one species donates, and the other receives. Relative Acid-Base Strength and the Net Direction of Reaction: A reaction proceeds to the greater extent in the direction in which a stronger acid and stronger base form a weaker acid and weaker base. If the stronger acid and base are on the right, the net direction is to the right. An acid base reaction will proceed to the right if the acid reacts with a base that is lower on the list because this combination produces a weaker conjugate base/weaker conjugate acid. A weaker acid has a stronger conjugate base, and vise versa. 18.4 Solving Problems Involving Weak-Acid Equilibria Assumptions made in these types of problems: 1. The hydronium ion concentration from the autoionization of water can be neglected because it is so small in comparison. Not only that, because of Le Chateliers Principle, as hydronium is added, it further reduces the autoionization. 2. A weak acid has a small K , taerefore it dissociates to such a small extent that we can neglect the change in its concentration when finding the equilibrium concentration. The effect of concentration on the extent of Acid Dissociation As the initial acid decreases, the percent dissociation of the acid increases, but dont confuse the concentration of the dissociated acid with the percentage. The concentration is much lower, but the ratio increases as the initial conc decreases. The behaviour of polyprotic acids Each time a proton is donated, there is a new K . as a polyprotic acid donates each proton, it becomes a weaker and weaker acid. 18.5 Weak Bases and their Relation to Weak Acids In problems involving weak bases, they can be treated the same way as weak acids. The same assumptions and processes can be applied to these types of questions. For weak acids and weak bases: If in initial concentration divided by the Ka or Kb is less that 400, the assumption is correct, but if it is greater than 400, the quadratic equation can be used. The relationship between Ka and Kb of a conjugate pair When the two dissociation reactions are summed together, we get the autoionization of water. For a acid base pair: Ka*Kb=Kw
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