Acids and Bases. Feb 28 4:40 PM

Transcription

1 Acids and Bases H O s O Cl H O O H H N H Na O H H Feb 28 4:40 PM Properties of Acids 1. Taste sour 2. Conduct electrical current 3. Liberate H 2 gas when reacted with a metal. 4. Cause certain dyes to change color (Litmus, an indicator, turns from blue to red.) 5. Have a ph less than 7 (0 7) Ex: Fruits with citric acid Vinegar with acetic acid Rain water with sulfuric acid Soda with carbonic and phosphoric acid Feb 23 9:31 AM 1

2 Properties of a Base (sometimes called alkaline) 1. Taste bitter 2. Conduct electrical current 3. Feel Slippery 4. Cause certain dyes to change color (Litmus, an indicator, turns from red to blue) 5. Have ph more that 7 (7 14) Ex: Drano contains sodium hydroxide Cleaners contain ammonia Milk of magnesia contains magnesium hydroxide Feb 23 9:32 AM Acids and Bases Arrehenius Acid Definition: Substances that will dissolve in water to give a solution that will: give H + ions Strong Acid: HCl (aq) Weak Acid: HC 2 H 3 O 2 (aq) [H 3 O + hydronium] Feb 20 11:25 AM 2

3 Arrehenius Base Definition: Things that will dissolve in water to give a solution that will: give OH Examples: Strong Base: NaOH (s) Weak Base: K 2 O (s) Feb 20 11:34 AM Both Acids and Bases Lose their Properties when reacted together Neutralization Why? HA + MOH HOH + MA Apr 20 1:03 PM 3

4 Is there a problem with the Arrhenius' Definition? 1. Only aqueous solutions 2. What about ammonia? 3. H + is not stable in water solutions. 4. What about amphoteric substances? Apr 20 1:03 PM Amphoteric substance whose solution can have both acidic & basic properties. Ex. H 2 O, HSO 4, HPO 4 2, H 2 PO 4 H 2 O + H 2 O Self ionization of water = equilibrium. The reaction does not go to completion; Reactants partially dissociate (break apart). How much/how quickly they dissociate depends on if they are weak or strong! Key: the stronger an acid/base is, the more it dissociates. Feb 23 10:12 AM 4

5 Brønsted Lowry Definition: Acid: substance that can donate a proton (H + ) Base: substance that can accept a proton (H + ) Example: HCl (l) + H 2 O (l) HC 2 H 3 O 2(l) + H 2 O (l) Feb 20 11:38 AM Brønsted Lowry Acid Base Pairs: Conjugate Acid: what the base use to be plus a H + Conjugate Base: what the acid use to be without the H + Example: HCO 3 (aq) + H 2 O (l) HCl (aq) + H 2 O (l) NH 3(aq) + H 2 O (l) H 3 PO 4(aq) + H 2 O (l) Feb 20 11:38 AM 5

6 Strength vs. Concentration Strength How easy an A/B will break apart into its ions. (Things that won t break apart into ions are molecular compounds because they have covalent bonds.) Concentration Molarity Feb 23 11:48 AM Examples of Strong Acids (The Big 6) HCl HBr HI H 2 SO 4 HNO 3 HClO 4 Bases (Group 1 & 2) NaOH LiOH KOH Ba(OH) 2 Mg(OH) 2 Ca(OH) 2 HCl (aq) Feb 23 11:48 AM 6

7 Examples of Weak everything else. HC 2 H 3 O 2(aq) NH 4 OH (aq) Feb 23 11:48 AM The more a substance ionizes, the better it will conduct an electrical current. Qualitatively, we use electricity to determine how well a substances dissociates in water. Apr 20 1:24 PM 7

8 STRONG ACID HBr HNO 3 HCl Sort the substances listed below into correct boxes weak acid HC 2 H 3 O 2 H 2 CO 3 H 2 S HF STRONG BASE NaOH LiOH Ba(OH) 2 KOH weak base NH 3 NaHCO 3 NaClO Al(OH) 3 Apr 24 7:16 AM You need your calculator!!! Apr 23 4:36 PM 8

9 Acidic Solutions [H + ] [OH ] Basic Solutions [H + ] [OH ] Neutral Solutions [H + ] [OH ] H 2 O (l) H + (aq) + OH (aq) The ph Scale P stands for power, H stands for [H + ]. ph is a convenient way to measure the acidity or the amount of H + ions present in a solution. More H + ions ph Less H + ions ph Apr 22 8:14 AM The poh Scale poh measures the basicity or amount of OH ions present in a solution ***ph and poh are dependent on both the concentration & strength. *We will only consider strong acids & strong bases (completely dissociated). 1M HCl = 1M H + ions Apr 23 4:23 PM 9

10 Review of logs (base 10). A log is simply the power of 10 necessary to equal a given number log 100 = 10 2 = Feb 28 7:52 PM Summary of equations to use ph = log [H + ] poh = log [OH ] [H + ] =10 ph [OH ]=10 poh ph + poh = 14 [H + ][OH ] = 1 x Apr 23 4:23 PM 10

11 Ex. 1 What is the ph of a solution whose [H + ] = 2.50 x M? Ex.2 What is the poh of this solution? Ex 3 Calculate the [OH ] for this solution. Is it Acidic or Basic? Feb 23 10:18 AM Quantitatively, the Equilibrium Constant, K eq, is used to determine how well a substance dissociates in water. Definitions: a reaction that is complete (100% ionization) a reaction that only takes place partially (< 100% ionization) [ ] the concentration of the compound (measured using molarity) K eq the equilibrium constant for an reaction at equilibrium Apr 20 1:27 PM 11

12 K eq the equilibrium constant for an reaction at equilibrium For aa + bb cc + dd *** The larger the K, the stronger the substance. Apr 20 1:27 PM 6 Special K s K c = Concentration (M) {water is excluded} K p = pressure (gases only atm) K sp = Solubility Product (slightly soluble solids) {solids are excluded } K a = Acids {water is excluded} K b = Bases {water is excluded} K w = water 1.0 X M Ex. Write the appropriate K expression for the following BCE s. a) PbI 2 (s) Pb +2 (aq) + 2I (aq) b) H 2 O (l) H + (aq) + OH (aq) c) SO 2(g) + O 2(g) SO 3(g) Apr 20 1:30 PM 12

13 General Acid Equation HA + H 2 O --> A - +H 3 O + <-- K a = General Base Equation A - + H 2 O --> HA + OH - <-- K b = Apr 17 3:24 PM 4 Steps for solving equilibrium problems: 1. Write a BCE. 2. Write the equilibrium expression. (Solids or water in the liquid phase can be eliminated.) 3. If the equilibrium concentrations of everything are known...plug the numbers into the K eq expression...if not continue to step Setup an ICE Table to determine the equilibrium concentration and return to step 3. Initial concentration Change in concentration Equilibrium concentration Apr 20 1:31 PM 13

14 Ex. Aspirin is a weak acid. A chemist mixes moles of aspirin with water to make 1 L of solution. At equilibrium the H 3 O + concentration was found to be M. What is the K a for aspirin? Apr 22 8:16 AM Ex. The K a of acetic acid is 1.75 x 10 5, find the equilibrium concentration of hydronium ions in a 1.00 M solution of acetic acid. Apr 17 3:48 PM 14

15 Ex. How many moles of sulfuric acid would you require to neutralize 0.50 mol of sodium hydroxide? Apr 29 8:28 AM Titrations lab neutralization reactions used to determine ph, poh, [H + ],[OH ] or the molar mass of acids and bases. Find Moles Required to Neutralize a Substance. 1. Write a Balance Chemical Equation 2. Take the moles of the substance that you have and calculate moles of the other substance that you need to neutralize it by using the mole ratio. Apr 23 5:26 PM 15

16 Tools & Calculations for Acid Base Titrations Burette graduated glass tube with a stopcock used to deliver a solution of known concentration. Final Volume Initial Volume = Volume Used Standard Solution the solution of known concentration and known volume used. Pipet a calibrated glass tube used to measure out exactly 0.5mL, 1mL, 10mL, 20mL, 25mL solutions Solution of Unknown Concentration substance being titrated. Erlenmeyer Flask contains substance being titrated Indicator changes color to indicate the end of the titration. If a indicator is used to show the ph is 7 (neutral), at this point the moles of acid = moles of base. This is called the equivalence point and the end of our titration. *** We will use Phenolphthalein. Apr 29 8:30 AM Step 1. Calculate moles of acid or base. Use the known molarity and volume of the standard solution to determine the # of moles of the standard solution reacted. Step 2. Find the moles required to neutralize the substance. For 1:1 mole ratios, at the equivalence point the moles of acid will equal moles of base. Step 3 Use the moles to determine ph, poh, [H + ],[OH ] or the molar mass. Apr 29 8:32 AM 16

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18 Definitions Salts Ionic compounds w/o H + ions or OH ions Hydrolysis Reaction with water (opposite of neutralization) ***May be acidic or basic or neutral Feb 24 9:04 AM 4 Situations Salts made from 1. The cation of a strong base & the anion of a weak acid. Ex. NaC 2 H 3 O 2 + H 2 O NaOH + HC 2 H 3 O 2 Ionic strong base weak acid Will this salt be acidic, basic or neutral? Show with a net ionic equation. Feb 24 9:04 AM 18

19 2. The cation of a weak base & the anion of a strong acid. Ex. AlCl 3 + 3H 2 O Al(OH) 3 + 3HCl 3. The cation of a strong base & the anion of a strong acid Ex. NaCl + H 2 O HCl + NaOH Feb 24 9:14 AM 4. The cation of a weak base & the anion of a weak acid **Compare K a of cation to K b of anion K a > K b = acidic K b > K a = basic K a = K b = neutral Feb 24 9:16 AM 19