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Chapter 16: Equilibrium in Acid-Base Systems

Chapter 16: Equilibrium in Acid-Base Systems. 16.2b: Predicting Acid-Base Reactions. Predicting Rxtn. Equilibria. B-L theory does not explain why certain protons are attracted more than others we must look at relative strengths of acids and bases

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Chapter 16: Equilibrium in Acid-Base Systems

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  1. Chapter 16: Equilibrium in Acid-Base Systems 16.2b: Predicting Acid-Base Reactions

  2. Predicting Rxtn. Equilibria • B-L theory does not explain why certain protons are attracted more than others • we must look at relative strengths of acids and bases • both collision-reaction and Bronsted-Lowry theory deal with proton transfer • we use a combination of concepts to help us predict which acid-base rxtn. is most dominant and observable in a system where multiple reactions can be taking place

  3. Collision-rxtn theory in an A-B system collisions of all entities are always happening however the only significant rxtn is that between the SA and SB other proton transfer has a negligible effect on the system Bronsted-Lowry theory a proton will only transfer if an acid collides with a base that is a stronger proton attractor a proton can transfer multiple times until with the strongest base once the transfer from SA to SB has occurred, proton remains Predicting Rxtn. Equilibria con’t

  4. H3O+ is strongest acid entity in aqueous sol’n, all SA are written as H3O+ OH- is strongest base entity in aqueous sol’n, exceptions are ionic oxides where the oxide ion can be written as OH- (K2O) no entity can react as a base if it is a weaker base than water, conj. base of SA are not bases A-B system eqtn. represents a single proton transfer between entities in a 1:1 mole ratio the SA and SB are both present and reacting in approx. equal concentrations Guide for predicting reaction Restrictions for proton transfer in aqueous solution Assume unless otherwise informed refer to acid-base strength table in data booklet and LSM 16.2a summary 2 for 5 step method

  5. Relative equilibrium positions +

  6. Relative equilibrium positions +

  7. Example • Predict the products and direction in which the rxtn. between sulphate ions and acetic acid will proceed. SO4 2- + CH3COOH <---> ? + ? Sol’n 1)List all entities SO4 2- , CH3COOH , H20 2)Label as B-L acid/base according to A-B strength table SO4 2- , CH3COOH , H20 B A A,B 3)Identify SA and SB present CH3COOH -- SA SO4 2- -- SB

  8. Example con’t 4)Write eqtn. showing transfer of one proton from SA to SB and predict conjugates SO4 2- + CH3COOH <---> HSO4- + CH3COO- SB SA CA CB 5)Predict approx. position of the equilibrium -since SO42- is above CH3COOH, the equilibrium is to the left and the reactants are favoured - <50% is used over the equilibrium arrows <50% SO4 2- + CH3COOH <----> HSO4- + CH3COO-

  9. Homework • Textbook p.731 #9,12,13,15 p. 735 #3,4 • LSM 16.2a summary 2

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