Things To Do Now: 1 Unit 13: Acids & Bases Lesson 13.1: Properties of Arrhenius Acids & Bases 2 What s an Acid? What s a Base? They ve been known for a long time. They were unexplained for a long time. They are very useful in our everyday lives. They do not, mostly, pose a health concern. We will now learn about them 3 The Arrhenius Definition Proposed by Svante Arrhenius Acids: Substances that dissociate to produce H + ions in aqueous solution. Bases: Substances that dissociate to produce OH - ions in aqueous solution Acids, Bases & Salts are Electrolytes (conduct electricity when dissolved in water) Svante Arrhenius (1859 1927) 4 1
Acids Acids are the only molecules that dissociate into ions when dissolved in water: Step 1: The acid dissociates Step 2: The H + (proton) associates with a H 2 O molecule, producing an H 3 O + (hydronium) ion. Acids How do you know if a water solution of a chemical is an acid? The formula of the chemical usually starts with Hydrogen (inorganic acid) HCl, H 2 SO 4, HNO 3, etc. (but not water!) Or end with COOH (organic acid) HCOOH, CH 3 COOH, C 3 H 5 COOH 5 6 Dissociate these acids! HCl (1 step): H 2 SO 4 (2 steps) : H 3 PO 4 (3 steps) : Properties of Acids 1. Acidic solutions conduct electricity. 2. Dilute acids taste sour (NOTE: Don t taste acids in lab.) 3. Acids react with carbonates to form CO 2, salt and water: NaHCO 3 (s) + HC 2 H 3 O 2 (aq) CO 2 (g) + H 2 O (l) + NaC 2 H 3 O 2 (aq) 4. Acids are formed by reacting gaseous oxides with water: CO 2 (g) + H 2 O (l) H 2 CO 3 (aq) 1. This is where acid precipitation comes from. 7 8 2
Properties of Acids 5. Acids corrode (oxidize) active metals. The loose protons (H + ) will grab electrons from more active metals and leave the solution as hydrogen gas (H 2 ). Reference Table J has a list of active metals. Any metal more active than H 2 - located above H2 in Table J, will be oxidized by an Arrhenius acid. 9 6. Acids Affect Indicators, by changing their color Blue litmus paper turns red in contact with an acid (and red paper stays red). Phenolphthalein turns colorless in contact with an acid. Properties of Acids 7. Acids have a ph less than ph 7 is a scale that measures how acidic or basic a solution is. Each decrease of 1 on the ph scale reflects a 10 times more acidic and is a 10X increase in the [H 3 O + ] 8.Acids Neutralize Bases HCl + NaOH NaCl + H 2 O -Neutralization reactions ALWAYS produce a salt (which is an ionic compound) and water. -Of course, it takes the right proportion of acid and base to produce a the salt 11 3
Naming Acids (Review) Common acids are listed on Reference Table K. Binary acids: hydro-anion-ic acid Ternary Acids: Polyatomic anion ends in -ate or -ide: Polyatomic ion-ic acid Polyatomic ion ends in ite: Polyatomic ion-ous acid Strong Weak 13 Name These Acids HCl (aq) HBr (aq) H 2 S (aq) H 3 N (aq) HNO 3 (aq) HNO 2 (aq) HClO 3 (aq) HClO 2 (aq) HCN (aq) Hydrochloric acid Hydrobromic acid Hydrosulfuric acid Hyrdronitric acid Nitric acid Nitrous acid Chloric acid Chlorous acid Cyanic acid 14 Formulate these Acids Bases Hydroflouric acid Hydrophosphoric acid Hypochlorous acid Oxalic acid Sulfuric acid Sulfurous acid HF (aq) H 3 P (aq) HClO (aq) H 2 C 2 O 4 (aq) H 2 SO 4 (aq) H 2 SO 3 (aq) Bases dissociate to produce OH - (hydroxide) ions in solution. Common Bases are listed On Reference Table L: Strong 15 Weak 16 4
Examples of Bases (metallic hydroxides) Sodium hydroxide, NaOH (lye for drain cleaner; soap) Potassium hydroxide, KOH (alkaline batteries) Magnesium hydroxide, Mg(OH) 2 (Milk of Magnesia) Calcium hydroxide, Ca(OH) 2 (lime; masonry) Properties of Bases: 1. Conduct electricity in solution. 2. Bases taste bitter (DO NOT TASTE IN LAB!) 3. Bases fill Slippery (DO NOT TRY THIS EITHER) 4. Formed when group 1 & 2 metals or metallic oxides react with H 2 O: Mg (s) + H 2 O (l) Mg(OH) 2 (aq) + H 2 (g) MgO (s) + H 2 O (l) Mg(OH) 2 (aq) 5. ph greater than 7 6. Bases change the color of Indicators. 18 Bases Affect Indicators Red litmus paper turns blue in contact with a base (and blue paper stays blue). Phenolphthalein turns pink in a base. Bases have a ph greater than 7 Each increase of 1 on the ph scale reflects a 10 times more basic and is a 10X decrease in the [H 3 O + ] 5
7. Bases hydrolyze fats (are caustic ) Bases cut through grease. The base breaks the bonds between the carbons in the fatty grease molecules. The product of this reaction is a soap. You can make soap from any fat & a strong base. Also: Ammonia (a base) will react with Bleach to produce Chlorine gas (and 2 other explosive substances), which will kill you so don t do that. Bacon fat soap (why not?) 21 Why Do Acids & Bases Burn Skin? The reactive ions in acids and bases are what cause them to burn flesh. The higher the concentration of hydronium ions in acid or hydroxide ions in base, the faster and worse the burn will be. A concentrated base is just as bad for you as a concentrated acid is. 22 How to tell if a solution is an acid or a base? 1. ph probes: The electrical conductivity of the solution is detected by the probe. 2. Indicators: Indicators change colors over different ph ranges. Common indicators are listed on Reference Table M A solution yields the following results when tested with various indicators: Methyl Orange = yellow Phenolphthalein = clear Bromcresol Green = blue Thymol Blue = yellow What could the ph be: a) 2.8 b) 6.5 c) 8.5 d) 4.8 23 24 6
Acid-Base Theories Svante Arrhenius He was a Swedish chemist (1859-1927), and a Nobel prize winner in chemistry (1903) one of the first chemists to explain the chemical theory of the behavior of acids and bases 1. Arrhenius Definition - 1887 Acids produce hydrogen ions (H 1+ ) in aqueous solution (HCl H 1+ + Cl 1- ) Bases produce hydroxide ions (OH 1- ) when dissolved in water. (NaOH Na 1+ + OH 1- ) Limited to aqueous solutions. Only one kind of base (hydroxides) NH 3 (ammonia) could not be an Arrhenius base: no OH 1- released. Svante Arrhenius (1859-1927) 7
Arrhenius Ain t The End! Is Ammonia (NH 3 ) an acid or a base? It s a base! That s weird. Arrhenius Base: Produces OH - in solution Where is Ammonia s OH -? Need a new definition of base. The Bronstead-Lowry Theory Redefines Acid & Base: Acid- Donates a proton (H + ) Base- Accepts a proton (H + ) If the Arrhenius definition is a pine, the Bronstead-Lowry definition is all trees. Johannes Nicolaus Bronstead (1879 1947) Thomas Martin Lowry (1874 1936) 29 30 We can Now Explain Why Ammonia is a Base Ammonia can be explained as a base by using Brønsted-Lowry: NH 3(aq) + H 2 O (l) NH 4 1+ (aq) + OH 1- (aq) Ammonia is the hydrogen ion acceptor (base), and water is the hydrogen ion donor (acid). This causes the OH 1- concentration to be greater than in pure water, and the ammonia solution is basic Acids and bases come in pairs A conjugate base is the remainder of the original acid, after it donates it s hydrogen ion A conjugate acid is the particle formed when the original base gains a hydrogen ion Thus, a conjugate acid-base pair is related by the loss or gain of a single hydrogen ion. Chemical Indicators? They are weak acids or bases that have a different color from their original acid and base 8
Acids and bases come in pairs General equation is: HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) Acid + Base Conjugate acid + Conjugate base NH 3 + H 2 O NH 4 1+ + OH 1- base acid c.a. c.b. HCl + H 2 O H 3 O 1+ + Cl 1- acid base c.a. c.b. Amphoteric a substance that can act as both an acid and base- as water shows Acid, Base, or Amphoteric? HCl Cl - H 2 SO 4 SO 4-2 HSO 4 - H 2 S S -2 HS - Acid Base Acid Base Amphoteric Acid Base Amphoteric 34 The Coordinate Covalent Bond A substance doesn t need to have a negative charge to be a base. All it needs to do it accept a proton. This is what happens with ammonia: This kind of bond is called a coordinate covalent bond (1 atom supplies both e - in the bond) Conjugate Acid-Base Pairs Redux A strong acid will have a very weak conjugate base. A strong base will have a very weak conjugate acid. Go to Honors Reference Table C if you need to know these things. Amphoteric substances can act as acids or bases, depending on the reaction. 35 36 9
Identifying the conjugates 1. There will only be one acid and base on each side of the equation. 2. The conjugates will look like each other, just differing by one H + Things Get Even Weirder Bronstead-Lowry is not the most expansive definition of acids and bases. Gilbert Newton Lewis (of Lewis dot diagrams) took a shot: Lewis Acids: Donate a pair of e- Lewis Bases: Accept a pair of e- If Arrhenius is a pine, and BL is trees, Lewis is all vegitations. Some folks have suggested that anything that donates an e- should be considered an acid and anything that accepts one should be a base. Only problem- that s every element in the universe! 37 38 Gilbert Lewis (1875-1946) Any Questions? What now? 40 10