CHEM 113 1nd EditionExam # 3 Study Guide Lectures: 18-27Exam #3: Sections 18.3-19.2 (excluding 18.6 & 18.8)Lecture 1818.3: Proton Transfer and the Bronsted-Lowry Acid-Base Definition- An acid is a proton donor, any species that donates an H+ ion.o An acid must contain H in its formula- A base is a proton acceptor, any species that accepts and H+ iono A base must contain a lone pair of electrons to bond to H+- An acid-base reaction is a proton-transfer process- Every acid has a conjugate base and every base has a conjugate acido The conjugate base has one fewer H and one more minus charge than the acid.o The conjugate acid has one more H and one fewer minus charge than the base. - Relative Acid-Base Strength and the Net Direction of Reactiono The net direction of an acid-base reaction depends on the relative acid and base strengths.o 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.o Competition for the Proton When looking at the bases in these situations, you can think of it as a competition for the proton between the two bases When strong acids dissolve, they completely transfer the proton to the base forming the conjugate base and acid.- For strong acids, although there is and equilibrium arrow, the net direction is very far to the right and the reaction is essentially complete.- In fact, the only acidic species that remains in strong-acid solutions is H3O+ The weaker an acid, the stronger the conjugate base, meaning the acid can’t give up its proton very readily because it’s conjugate base it holding it too strongly.Lecture 20 (Lecture 19 was just a review for the previous exam)18.4: Solving Problems Involving Weak-Acid Equilibria- There are two types of equilibrium problems involving weak acids and their conjugate bases.o 1). Where we are given equilibrium concentrations and we are asked to find Ka Very unlikely that this type of question will be on exam because it’s very elementary.o 2). Where we are given Ka and some concentrations and we are asked to find other equilibrium concentrations- Steps for solving this type of problemo 1. Write a balanced equation.o 2. Write an expression for Kao 3. Define x as the change in concentration that occurs during the reaction.o 4. Construct a reaction table in terms of xo 5. Make assumptions that simplify the calculationo 6. Substitute values into the Ka expression and solve for xo 7. Check the assumptions are justified *This is a very important concept to understand. These problems are also called ICE table problems and are applicable to various types of situations- The assumptionso [H3O+] from the autoionization of H2O is negligible. It is so much smaller than the [H3O+] from the dissociation of an acid with a H+ that we can neglect it in these problems.o A weak acid has small Ka and its dissociation is negligible Therefore, it dissociates to such a small extent that we can neglect the change in its concentration to find its equilibrium concentration.- The effect of concentration on the extent of acid dissociationoo As the initial concentration decreases, the percent dissociation of the acid increases- The Behavior of Polyprotic Acidso A polyprotic acid is an acid with more than one ionizable proton. This means that each dissociation step has a different value for Ka.- In general, these Ka values tend to decrease with each step.Lecture 2118.5: Weak Bases and Their Relation to Weak Acids - The Bronsted-Lowry concept expands the definition so that: to accept a proton, a base needs only a lone electron pair- The base-ionization constant can be given byoo This is basically the same K equation we have been using ([products]/[reactants]) where the reaction is the base being dissolved in water. - Molecules as Weak Bases: Ammonia and the Amineso Ammonia and the Amines have a defining structural feature that is a lone pair of electrons that can bind the proton donated by the acid.o Just remember that ammonia and the other compounds on page 741, are weak acids and that they have a lone pair that binds to H+- Anions of Weak Acids as Weak Baseso Anions of weak acids can also act as weak bases- The Relationship Between Ka and Kb of a Conjugate Acid-Base Pairo The sum of the two dissociation reactions is the autoionization of watero Recall for a reaction that is the sum of two or more reactions, the overall equilibrium constant is the product of the individual equilibrium constants. Therefore the relationship between Ka and Kb is oo This relationship is true for any conjugate acid-base pairo So if you know the overall K value and the K value for an acid, you can find the K value for its conjugate baseLecture 2218.7: Acid-base Properties of Salt Solutions- When a salt dissolves, one or both of its ions may react with water and affect the pH of the solution.- Salts that yield neutral solutionso A salt consisting of the anion of a strong acid and the cation of a strong base yields a neutral solution because the ions do not react with the water.- Salts that yield acidic solutionso A salt consisting of the cation of a weak base and the anion of a strong acid yieldsan acidic solution because the cation acts as a weak acid and the anion does not rect.- Salts that yield basic solutionso A salt that consists of the anion of a weak acid and the cation of a strong base yields a basic solution.- Salts of Weakly Acidic Cations and Weakly Basic Anionso If both the acid and the base are weak, then the overall acidity of the solution depends on the relative acid strength (Ka) and the base strength (Kb) of the separated ions.Lecture 2518.9: Electron-Pair Donation and the Lewis Acid-base Definition- The Lewis acid-base definition sayso A base is any species that donates an electron pair to form a bond A Lewis base must have a lone pair of electrons to donateo An acid is any species that accepts an electron pair to form a bond A Lewis acid must have a vacant orbital to accept a lone pair and form a new bond- Although this doesn’t expand the base definition, it does allow more substances to be called acidso Things that don’t have H in their formula but still accept an electron pair can be considered an acidLecture 2419.1: Equilibria of Acid-Base Buffers- An acid-base buffer is a solution that lessens the impact of an external force. o They usually consist of a conjugate acid-base pair