Chemistry of Acids and Bases

Chemistry of Acids and Bases - Chemistry 112 Laboratory:...

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Chemistry of Acids and Bases T he chemistry of acids and bases is an area of fundamental importance in chemistry. In this experiment you will study acid-base equilibria by deter- mining the pH of a number of acids and bases and their mixtures and by per- forming a pH titration. In particular, the aims of this experiment are: 1. To demonstrate that only a very small concentration of H 3 O + and OH - are generated in solutions of weak acids and bases, respectively. 2. To demonstrate the acid-base properties of salts of weak acids and bases. 3. To prove the buffering action of a solution of a weak acid and its salt or of a weak base and its salt. 4. To demonstrate the pH changes occurring in the course of the titration of a weak polyprotic acid with a strong base and how to determine the values for the acid. 5. To demonstrate the connection between the pH of a solution and the color of an acid-base indicator. Introduction to Acids and Bases 1. Acidic and Basic Substances Acids are molecules or ions that act as proton (H + ) donors. As illustrated in Figure 1 , the acid can be a neutral molecule, a cation, or an anion. The species that results from the loss of the H + ion by the acid is called its conjugate base. Thus, the conjugate base will always be one unit more negative in its charge than the acid. Acid Conjugate Base + H + Bases are the opposite of acids: bases are proton acceptors. This means that the conjugate acid of a base will be more positively charged than the base. Chemistry 112 Laboratory: Chemistry of Acids & Bases Page 43 This experiment is based on experiments done in general chem- istry at the University of Auckland in Auckland, New Zealand. We thank Dr. Sheila Woodgate for sharing this information. A complete discussion of acid-base chemistry is given in Chapters 17 and 18 of Chemistry & Chemical Reactivity. CH 3 CO 2 HH 2 ON H 4 + H 3 O + H 2 PO 4 - ACID acetic acid water ammonium ion hydronium ion dihydrogen phosphate ion BASE 3 2 - acetate ion OH - hydroxide ion NH 3 ammonia H 2 O water HPO 4 2- hydrogen phosphate ion +H + -H + +H + -H + +H + -H + +H + -H + +H + -H + Neutral Acid Cationic Acid Anionic Acid Anionic Base Neutral Base Dianion Base Figure 1 Some repre- sentative acids and bases. NOTE: If you download this experiment from the Net, the double arrows font used for an equilibrium process does not print.
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If we expand the view of acids in Figure 1 , you will notice that the H atom lost as an H + ion is often attached to an electronegative atom such as oxygen or a halogen. We can also notice that, in –OH acids, the –OH group is often attached to an atom that is also double-bonded to another atom such as O. These observations are true for acids such as acetic acid and nitric acid. In cationic acids, the acidic hydrogen atom is often bonded to a positively charged nitrogen or oxygen. Indeed, a very common class of cationic acids is represented by ones of the type R 3 NH + , where R is an organic group and/or an H atom.
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Chemistry of Acids and Bases - Chemistry 112 Laboratory:...

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