Chapter 16: Equilibrium in Chapter 16: Equilibrium in Acid-Base SystemsAcid-Base Systems

16.2b: Predicting Acid-Base Reactions16.2b: Predicting Acid-Base Reactions

Predicting Rxtn. EquilibriaPredicting Rxtn. Equilibria B-L theory does not explain why certain B-L theory does not explain why certain

protons are attracted more than othersprotons are attracted more than others we must look at relative strengths of acids we must look at relative strengths of acids

and basesand bases both collision-reaction and Bronsted-Lowry both collision-reaction and Bronsted-Lowry

theory deal with proton transfertheory deal with proton transfer we use a combination of concepts to help us we use a combination of concepts to help us

predict which acid-base rxtn. is most predict which acid-base rxtn. is most dominant and observable in a system where dominant and observable in a system where multiple reactions can be taking placemultiple reactions can be taking place

Predicting Rxtn. Equilibria Predicting Rxtn. Equilibria con’tcon’t

Collision-rxtn theoryCollision-rxtn theory in an A-B system in an A-B system

collisions of all entities collisions of all entities are always happeningare always happening

however the only however the only significant rxtn is that significant rxtn is that between the SA and SBbetween the SA and SB

other proton transfer other proton transfer has a negligible effect has a negligible effect on the systemon the system

Bronsted-Lowry theoryBronsted-Lowry theory a proton will only transfer if a proton will only transfer if

an acid collides with a an acid collides with a base that is a stronger base that is a stronger proton attractorproton attractor

a proton can transfer a proton can transfer multiple times until with the multiple times until with the strongest basestrongest base

once the transfer from SA once the transfer from SA to SB has occurred, proton to SB has occurred, proton remainsremains

Guide for predicting reactionGuide for predicting reaction

HH33OO+ + is strongest acid entity is strongest acid entity in aqueous sol’n, all SA are in aqueous sol’n, all SA are written as Hwritten as H33OO+ +

OHOH- - is strongest base entity is strongest base entity in aqueous sol’n, in aqueous sol’n, exceptions are ionic oxides exceptions are ionic oxides where the oxide ion can be where the oxide ion can be written as OHwritten as OH- - (K(K22O)O)

no entity can react as a no entity can react as a base if it is a weaker base base if it is a weaker base than water, conj. base of than water, conj. base of SA are not basesSA are not bases

A-B system eqtn. represents A-B system eqtn. represents a single proton transfer a single proton transfer between entities in a 1:1 between entities in a 1:1 mole ratiomole ratio

the SA and SB are both the SA and SB are both present and reacting in present and reacting in approx. equal concentrationsapprox. equal concentrations

Assume unless otherwise informedRestrictions for proton transfer in aqueous solution

refer to acid-base strength table in data booklet and LSM 16.2a summary 2 for 5 step method

Relative equilibrium positionsRelative equilibrium positions

SB

>50%<--->

SA

Products favoured

+

Relative equilibrium positionsRelative equilibrium positions

SA

<50%<--->

SB

Rectants favoured

+

ExampleExample Predict the products and direction in which the rxtn. Predict the products and direction in which the rxtn.

between between sulphate ions and acetic acid sulphate ions and acetic acid will proceed.will proceed.

SO4 2- + CH3COOH <---> ? + ?

1)List all entities SO4

2- , CH3COOH , H202)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

Sol’n

Example con’tExample con’t4)Write eqtn. showing transfer of one proton from SA 4)Write eqtn. showing transfer of one proton from SA to SB and predict conjugatesto SB and predict conjugates

SO4 2- + CH3COOH <---> HSO4

- + CH3COO-

SB SA CA CB

5)Predict approx. position of the equilibrium-since SO4

2- is above CH3COOH, the equilibrium is to the left and the reactants are favoured

- <50% is used over the equilibrium arrows

SO4 2- + CH3COOH <----> HSO4

- + CH3COO-<50%

HomeworkHomework

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