Chemistry I Notes Unit 10: Acids and Bases

Transcription

1 Chemistry I Notes Unit 10: Acids and Bases Acids 1. Sour taste. 2. Acids change the color of acid- base indicators (turn blue litmus red). 3. Some acids react with active metals and release hydrogen gas, H2. 4. Acids react with bases to produce salts and water. 5. Acids conduct electric current. Acid Nomenclature A binary acid is an acid that contains two elements: hydrogen and one of the more electronegative elements. HF, HCl, HBr, and HI Binary Acid Nomenclature 1. The name of a binary acid begins with the prefix hydro-. 2. The root of the name of the second element follows this prefix. 3. The name then ends with the suffix ic Acid Nomenclature An oxyacid is an acid that is composed of hydrogen, oxygen, and a third element, usually a nonmetal. HNO3, H2SO4 The names of oxyacids follow a pattern. Strength of Acids Bases If ion ends in ate it is an ic acid. If ion ends in ite it is an ous acid. A strong acid is one that ionizes (breaks apart) completely in aqueous solution. a strong acid is a strong electrolyte HClO4, HCl, HNO3 A weak acid releases few hydrogen ions in aqueous solution (not all the molecules of a weak acid break apart in solution). Weak acids are weak electrolytes hydronium ions (H3O ) and some anions HCN acetic acid 1. Taste bitter 2. Bases change the color of acid- base indicators (turn red litmus blue). 3. Dilute solutions of bases feel slippery. 4. Bases react with acids to produce salts and water. 5. Bases conduct electric current.

2 Arrhenius Acids and Bases An Arrhenius acid is a compound that increases the concentration of hydrogen ions, H, in solution. An Arrhenius base is a substance that increases the concentration of hydroxide ions, OH, in solution. Brønsted- Lowry Acids and Bases A Brønsted- Lowry acid is a molecule or ion that is a proton donor. HCl NH NH Cl 3 4 In this equation HCl is giving away a proton (hydrogen), (donating hydrogen to become Cl- ) A Brønsted- Lowry base is a molecule or ion that is a proton acceptor. HCl NH NH Cl 3 4 In this equation NH3 is gaining a proton (hydrogen), (accepting a hydrogen to become NH4) In a Brønsted- Lowry acid- base reaction, protons are transferred from one reactant (the acid) to another (the base). Conjugate Acid substance formed after a proton (H ) has been added. Conjugate Base substance formed after a proton (H ) has been lost. Brønsted- Lowry acid- base reactions involve two acid- base pairs, known as conjugate acid- base pairs. HF( aq ) H2O( l ) F ( aq ) H3O ( aq) acid base conjugate base conjugate acid Monoprotic and Polyprotic Acids A monoprotic acid is an acid that can donate only one proton (hydrogen ion). HClO4, HCl, HNO3 A polyprotic acid is an acid that can donate more than one proton (hydrogen). H2SO4, H3PO4 Polyprotic Acids A diprotic acid is the type of polyprotic acid that can donate two protons. H2SO4

3 A triprotic acid is the type of polyprotic acid that can donate three protons. H3PO4 Lewis Acids and Bases A Lewis acid is an atom, ion, or molecule that accepts an electron pair to form a covalent bond. A Lewis base is an atom, ion, or molecule that donates an electron pair to form a covalent bond. The Lewis definition is the broadest of the three acid definitions. Summary of Acid- Base Theories Amphoteric Compounds Any species that can react as either an acid or a base is described as amphoteric. Amphoteric compounds are sometimes called amphiprotic. In the equation shown below, water (H2O) is acting as a base by accepting a proton (a hydrogen ion) to become H3O. H 2 SO 4 (aq) H 2 O(l) H 3 O (aq) HSO 4 (aq) In the equation shown below, water (H2O) is acting as an acid by donation a proton (a hydrogen ion) to become OH NH ( g ) H O( l ) NH ( aq ) OH ( aq)

4 Neutralization Reactions Neutralization is the reaction of hydronium ions and hydroxide ions to form water molecules. A salt is an ionic compound composed of a cation from a base and an anion from an acid. In the reaction shown below, hydrochloric acid reacts with sodium hydroxide (a base) to produce a salt (NaCl) and water. HCl(aq) NaOH(aq) NaCl(aq) H 2 O(l) ph ph measurement of the acidity or alkalinity of a substance. ph scale scale ranging from acidic; the lower the ph, the stronger the acid 7.0 neutral basic (alkaline); the higher the ph, the stronger the base Calculating ph ph = - log[h ] ph Values of Some Common Materials Indicators Acid- base indicators are compounds whose colors are sensitive to ph. Transition interval the ph range over which an indicator changes color. The point in a titration at which an indicator changes color is called the end point of the indicator.

5 Titration Titration is the controlled addition of a solution of known concentration with a measured amount of a solution of unknown concentration. The point at which the two solutions used in a titration are present in chemically equal amounts is the equivalence point. Buffer chemical compound that resists changes in ph. Buffer solutions are necessary to keep the correct ph for enzymes in many organisms to work. Many enzymes work only under certain conditions; if the ph moves outside of a narrow range, the enzymes slow or stop working and can break down. A buffer is present in blood plasma, to maintain a ph between 7.35 and 7.45.