Department of Chemistry CHEM1010 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:

CHEM1010/General Chemistry _________________________________________ Chapter 7. (L27)-Acids and Bases Today’s Outline..Review: How to write acid and base molecules in molecular formula..Review: Common acids and bases; strong acids and strong bases; weak acids and weak bases..Review: Acidic and basic anhydrides..Acidity..pH scale to measure acidity

Chapter 7. (L27)-Acids and Bases How to write acid and base molecules in molecular formula..Acids: We commonly put H or proton on the left and the anion on the right (for inorganic acids). Example: HCl, H 2 SO 4, HNO 3..Bases: We commonly put OH on the right and the cation on the left (for inorganic bases). Example: NaOH, Ca(OH) 2

Chapter 7. (L27)-Acids and Bases Co mmon acids..Common acids (see Table 7.1 for more details): NameMolecular FormulaStrength hydrochloric acidHClstrong sulfuric acidH 2 SO 4 strong nitric acidHNO 3 strong phosphoric acidH 3 PO 4 moderate hydrogen sulfate HSO 4 - moderate carbonic acid H 2 CO 3 weak acetic acidCH 3 COOHweak lactic acidCH 3 CHOHCOOHweak boric acid H 3 BO 3 very weak hydrocyanic acidHCNvery weak

Chapter 7. (L27)-Acids and Bases Co mmon bases..Common acids (see Table 7.2 for more details): NameMolecular FormulaStrength sodium hydroxideNaOHstrong potassium hydroxideKOHstrong lithium hydroxideLiOHstrong calcium hydroxideCa(OH) 2 strong* magnesium hydroxideMg(OH) 2 strong* ammoniaNH 3 weak *very low solubility although classified as a strong base

Chapter 7. (L27)-Acids and Bases Acidic and basic anhydrides, nonmetal oxides: Acidic anhydrides Nonmetal oxides can react with water to form acids. These nonmetal oxides are called acidic anhydrides. Example: SO 3 + H 2 O = H 2 SO 4 SO 2 + H 2 O = H 2 SO 3 CO 2 + H 2 O = H 2 CO 3 General form: nonmetal oxide + H 2 O  acid anhydride means without water.

Chapter 7. (L27)-Acids and Bases Acidic and basic anhydrides, metal oxides: Basic anhydrides M etal oxides can react with water to form bases. These metal oxides are called b asic anhydrides. Example: CaO + H 2 O = Ca(OH ) 2 B aO + H 2 O = Ba(OH ) 2 Li 2 O + H 2 O = 2LiOH General form: metal oxide + H 2 O  base

Chapter 7. (L27)-Acids and Bases Acidity of acids Question: How to know how much acid is present in a water solution? In other words, how to know how much protons, H +, are present (acid is a molecule that can dissociate to give H + in a water solution) So, we need a quantity or parameter to tell us about the acidity of a solution.

Chapter 7. (L27)-Acids and Bases Acidity of acids One way to express acidity can be concentration of H + : Molarity Example: [H + ] = 1 M or mole/L [H + ] = M or mole/L [H + ] = M or mole/L [H + ] = M or mole/L

Chapter 7. (L27)-Acids and Bases Acidity of acids However, in natural environments and biological bodies, the acidity is commonly quite small. Example: Lake, river water [H + ] = from M to M Clean rain water [H + ] = from M to 0.001M Blood [H + ] = ~ M

Chapter 7. (L27)-Acids and Bases Acidity of acids In science, we like to use small numbers and values to express quantities Example: We use mole to represent 6.022×10 23 molecules Question, can we have some way to express those very small values of acidity?

Chapter 7. (L27)-Acids and Bases Acidity of acids: pH scale In chemistry, we use pH scale to express very small values of acidity. Definition of pH: pH = -log[H + ] where the unit of [H + ] is M or mole/L pH value has no unit common pH range: 1-14 pH scale was first proposed in 1909 by a Danish biochemist, S.P. L. Sorensen.

Chapter 7. (L27)-Acids and Bases Acidity of acids: pH scale Express acidity in pH scale Examples: Lake water: [H + ] = from M to M pH = -log( ) = -log (10 -7 ) = -(-7) = 7 pH = -log( ) = -log (10 -8 ) = -(-8) = 8 Clean rain water: [H + ] = from M to M pH = -log( ) = -log (10 -5 ) = -(-5) = 5 pH = -log(0.001) = -log (10 -6 ) = -(-6) = 6 Blood: [H + ] = ~ M pH = -log( ) = -log (10 -7 ) = -(-7) = 7

Chapter 7. (L27)-Acids and Bases Acidity of acids: pH scale Express acidity in pH scale: A special feature Because the definition of pH scale, the higher the concentration of H +, the lower the values of pH; So, the direction pH and concentration of H + change is just opposite Example: [H + ] = M < 0.001M but, correspondingly, pH = 5 > 3

Chapter 7. (L27)-Acids and Bases Acidity of acids: pH scale Convert H + concentration to pH scale: Example: [H + ] = M pH = -log( ) = -log (10 -7 ) = -(-7) = 7 General equation: pH = -log (10 -x ) = -log(-x) = x

Chapter 7. (L27)-Acids and Bases Acidity of acids: pH scale Convert pH scale to H + concentration: Example: pH = 7 [H + ] = M General equation: pH = x = -log (10 -x ) [H + ] = 10 -x M

Chapter 7. (L27)-Acids and Bases Acidity of acids: pH scale Use pH scale to define acidic solution and basic solution: Acidic solutions: pH < 7 Basic solutions: pH > 7 This definition is based on the pH of pure water, which is 7 (6.9975) at 25 ºC. So, -if an aqueous solution has the acidity higher than pure water, then its pH is lower than water’s pH, or 7 -if an aqueous solution has the acidity lower than pure water, then its pH is higher than 7

Chapter 7. (L27)-Acids and Bases pH values of some common solutions SolutionpH value Lemon juice2.1 Vinegar (4%)2.5 Soda pop Rainwater (thunderstorm) Milk Saliva Urine Pure water at 25 ºC7.0 Blood7.4 Fresh egg white Washing soda12.0

Chapter 7. (L27)-Acids and Bases Quiz Time Which of the following is the definition of pH scale? (a) pH = log [H + ]; (b) pH = [H + ]; (c) pH = -[H + ]; (d) pH = -log[H + ].

Chapter 7. (L27)-Acids and Bases Quiz Time If a solution has a pH of 4, then its concentration of H + is (a) [H + ] = 0.4 M; (b) [H + ] = 4 M; (c) [H + ] = M = M; (d) [H + ] = M = M.

Chapter 7. (L27)-Acids and Bases Quiz Time If a solution has a concentration of H + being M, then its pH is (a) pH = 1; (b) pH = M; (c) pH = -5; (d) pH = 5.

Chapter 7. (L27)-Acids and Bases Quiz Time If a solution has a concentration of H + being 10 -6, then its pH is (a) pH = 10; (b) pH = 0.6; (c) pH = -6; (d) pH = 6.