Acids and Bases General properties ACIDS • Taste sour • Turn litmus • React with active metals – Fe, Zn • • • • BASES Taste bitter Turn litmus Feel soapy or slippery (react with fats to make soap) React with acids • React with bases blue to red red to blue Acids “ A substance that can dissolve in water form hydronium ions (H3O+)” • Hydrogen is found in all acids • Can be solid , liquid or gas • Can neutralise bases (eg fizz neutralises jelly fish stings) • Corrosive, burn skin, taste sour • Can be synthetic (made from chemicals) & natural / organic (found in nature, contain carbon) • Dilute Acid a little acid, lots of water • Concentrated Acid lots of acid, a little water Bases “Substances which can form hydroxide ions (OH-) in solution” • Can be solids, liquids or gases • They neutralise acids (eg toothpaste neutralises plaque acid) • Dissolve biological material “caustic” (eg grease, dirt oven cleaner) • Bases soluble in water are called alkalis (feel soapy – turn skin oil into soap) Common Acids Formula H2SO4 HCl HNO3 CH3COOH C6H8O7 Name Sulfuric acid Hydrochloric acid Nitric acid Acetic acid Citric acid Tartaric acid Carbonic acids Lactic acid DNA Formic acid Use Car batteries Stomach acid Making explosives Vinegar In citrus fruits In grapes Fizzy drinks Makes yoghurt Genetic code In ant/bee stings Common Bases Formula NaOH NH3 NaHCO3 Ca(OH)2 Name Sodium hydroxide Ammonia Sodium bicarbonate Calcium hydroxide Use Oven cleaner Household cleaner Baking Fertiliser Definitions Arrehenius only in water Bronsted-Lowry any solvent Lewis • Acids – produce H+ • Bases - produce OH• Acids – donate H+ • Bases – accept H+ • Acids – accept e- pair • Bases – donate e- pair used in organic chemistry, wider range of substances The hydrogen ion in aqueous solution H+ + H2O H3O+ (hydronium ion) Examples Arrhenius Bronsted-Lowry Lewis HCl NaOH HCl HCN BF3 :NH3 NH3 The Bronsted-Lowry Concept Conjugate pairs HCl CH3COOH Cl- NH4+ NH3 CH3COO- HNO3 NO3- How does a conjugate pair differ? H+ transfer Neutralization In general: Acid + Base Salt + Water All neutralization reactions are double displacement reactions. HCl + NaOH NaCl + HOH HCl + Mg(OH)2 H2SO4 + NaHCO3 Explaining Acids & Bases • The special properties of acids are due to hydronium ions H3O+ in aqueous solutions. HCl + H2O H3O+ + Cl- • Whereas basic solutions contain hydroxide ions: NaOH + H2O Na+ + OHNH3 + H2O NH4+ + OH- Aqueous Solution: Particles dissolved in water Effect of Acid on Browning on Apples • How was this experiment set up? • What happened? • What do the results show? Explaining Strength of Acids & bases Original idea: acid particles burn skin because they have sharp stingy spikes. Now we know: • Strong Acids become H3O+ ions easily – Eg HCl becomes mostly H3O+ ( & not much HCl) • Weak Acids don’t become H3O+ ions easily – Eg Acetic acid stays mostly acetic acid ( & not much H3O+) (H3O+ accounts for the properties of acids) So, conversely: • Strong Bases become OH- ions easily – Eg NaOH becomes mostly OH- ( & not much NaOH) • Weak Bases don’t become OH- ions easily – Eg NH3 stays mostly NH3 ( & not much OH-) (OH- accounts for the properties of bases) Disappearing Ink Done for you… 1 Make a dilute ammonia solution by adding 4mL of concentrated ammonia to 96mL H2O. 2 Divide into 10mL samples You do… 3 Add a few drops of phenolphthalein indicator. 4 Use this to “write” on some paper with a matchstick dabbed in the solution Disappearing Ink - Questions 1) What pH is the ammonia? 2) Why does the “ink” disappear? …spray with NaOH. http://sciencewithtoys.wikispaces.com/ Disappearing+ink Disappearing Ink - Questions 1) What pH is the ammonia? 2) Why does the “ink” disappear? Does pure water conduct electrical current? Water is a very, very, very weak electrolyte. H2O H+ + OHHow are (H+) and (OH-) related? (H+)(OH-) = 10-14 For pure water: (H+) = (OH-) = 10-7M This is neutrality and at 25oC is a pH = 7. water Let’s examine the behavior of an acid, HA, in aqueous solution. HA What happens to the HA molecules in solution? 100% dissociation of HA HA H+ Strong Acid A- Would the solution be conductive ? Partial dissociation of HA HA H+ Weak Acid A- Would the solution be conductive ? HA H+ + A- HA H+ Weak Acid A- At any one time, only a fraction of the molecules are dissociated. Strong and Weak Acids/Bases Strong acids/bases – 100% dissociation into ions HCl HNO3 H2SO4 NaOH KOH Weak acids/bases – partial dissociation, both ions and molecules CH3COOH NH3 pH • Scale from 0-14 indicating acidity or basicity: 0-3 = strong acid, react fast (eg sulfuric acid) 4-6 = weak acid, react slowly (eg urine) 7 = neutral (eg pure water) 8-11 = weak base, react slowly (eg sea water) 12-14 = strong base, react fast (eg sodium hydroxide) • Most acids found in living things are weak *The truth about pH • • • • Measures the “power of hydrogen” Measures how much H3O+ there is A pH of 3 has 10 times the H3O+ of a pH of 4 This is why strong acids have a low pH (they have loads of H3O+) pH of Everyday Substances Substance Blood Lemon Jam Cabbage Wine Egg Coke pH 7.4 2 4 5 3 8 2.9 acid rain (NOx, SOx) pH of 4.2 - 4.4 in Washington DC area 0-14 scale for the chemists pH 2 3 4 5 acidic (H+) > (OH-) normal rain (CO2) pH = 5.3 – 5.7 6 7 8 neutral @ 25oC (H+) = (OH-) distilled water fish populations drop off pH < 6 and to zero pH < 5 9 10 11 basic or alkaline (H+) < (OH-) natural waters pH = 6.5 - 8.5 12 Behavior of oxides in water– Group A basic 1A amphoteric acidic 3A 4A 5A 6A 7A 2A Group B 105 107 Db Bh basic: Na2O + H2O 2NaOH (O-2 + H2O 2OH-) acidic: CO2 + H2O H2CO3 8A When life goes either way amphoteric (amphiprotic) substances Acting like a base HCO3- Acting like an acid H+ - H+ H2CO3 CO3-2 + accepts H+ donates H+ pH The biological view in the human body acidic 1 2 3 4 5 6 basic/alkaline 7 8 9 10 Tortora & Grabowski, Prin. of Anatomy & Physiology, 10th ed., Wiley (2003) 11 Does the pH influence the activity of an enzyme? Trypsin is a digestive enzyme. Where? Intestinal pH range 7.0-8.5 The amino acid glycine - amphoteric It’s an acid and a base! Gain of H+ Loss of H+ H3N+-CH2-COOH H2N-CH2-COOH2N-CH2-COOH Chime structure The amino acid glycine - Zwitterion formation Transfer of H+ from carboxylic acid group to amine group. + A dipolar ion forms. H2N-CH2-COOH Chime structure H3N+-CH2-COO- intramolecular acid-base reaction Show how water can be amphoteric. H2O + H+ - H+ Dilution water (solvent) solute moles of solute remain constant Vfinal diluted, Mfinal molesinitial = molesfinal Vinitial concentrated, Minitial adding water lowers the solute concentration Mfinal x Vfinal = Minitial x Vinitial Titration Calculation indicator HCl + NaOH NaCl + HOH A way to analyze solutions! at equivalence point: moleHCl = moleNaOH moles = M x VL Macid x Vinitial acid = Mbase x Vburet Indicators Indicators “Substances which change colour in solutions of different pH” • Sources: – Plant pigments (eg litmus comes from lichen) – Synthetic chemicals (eg bromothymol blue) • Most indicators have two colours, colour change is reversible • Universal Indicator: a mixture of indicators so that it changes colour many times at many different pHs Common Indicators (& their colour changes) • Blue litmus – Turns red in acids • Red litmus – Turns blue in bases • Phenolphthalein – Pink above pH 10.0, clear below pH 8.2 • Universal Indicator: Colour Red Orange Yellow Green Blue Purple pH 0-1 1-3 3-7 7-8 9-12 12-14 Finding the pH of soil • Method • Why care? Purple Cabbage Indicator Background: The purple pigment found in purple cabbage is one example of a substance that changes colour in solutions of different pH Aim: To investigate its use as an indicator of pH. Method: 1. Rip up a small amount of cabbage. 2. Heat in 100mL of water until water is dark purple 3. While heating test the supplied acids/bases with pH paper to find their pH 4. Add small samples (a few drops) of the supplied acids/bases to a few mL of purple water in a test tube and note the colour change. 5. Take a photo of your test tube rack clearly showing the range of colours and their pH Possible Results • cabbage Blueberry based indicator Reactions Involving Acids Acid & Base Reactions (neutralisation) • Neutralisation: when acid and base react & bring their pHs closer to 7 and a salt and water are produced. • If complete neutralisation occurs: – Sour acid taste disappears – Salty taste increases – Indicators show a pH of 7 • Reaction: General: acid + base salt + water Word: Hydrochloric acid + sodium hydroxide sodium chloride + water Symbol: HCl + NaOH NaCl + H2O • How could you prove a salt was made? • Evaporate the water off, look for crystallisation salt crystals (other chemical tests needed to prove the type of salt) Everyday Neutralisation Reactions Acid + Metal Salt + Hydrogen Gas • Example Naming salts: 1st part from the metal, 2nd part from the acid ending pH becomes neutral! Nitric acid + potassium potassium nitrate + hydrogen gas • More Examples Sulfuric acid + magnesium magnesium sulfate + hydrogen gas Hydrochloric acid + sodium sodium chloride + hydrogen gas • Chemical Equation Examples: HNO3 + K KNO3 + H2 H2SO4 + Mg MgSO4 + H2 HCl + Na NaCl + H2 Test for H2: pop! test Acid & Carbonate Reactions (a type of acid & base reaction) Acid + Carbonate Salt + Carbon dioxide + water Naming salts: 1st part from the metal, 2nd part from the acid ending pH becomes neutral! • Example Nitric acid + calcium carbonate calcium nitrate + carbon dioxide + water • More examples Sulfuric acid + copper carbonate copper sulfate + carbon dioxide + water • Chemical equation examples: HCl + MgCO3 MgCl2 + CO2 + H2O H2SO4 + PbCO3 PbSO4 + CO2 + H2O Test for CO2: bubble through limewater (cloudy = CO2) or lit match extinguishes ?What is this slide? • Al2(CO3)3 + 6HNO3 2Al(NO3)3 + 3H2CO3 • Al2(CO3)3+ 3H2SO4 Al2(SO4)3 + 3CO2 + 3H2O Balanced Chemical Equations 1 Can’t change element type (we’re doing chemical reactions, not nuclear so same types of atoms must be on each side of the reaction) AND Can’t get something from nothing (the total numbers of atoms on each side of the reaction must be the same) 2 Problem as 2 x H on the right and 1 x H on left. Can’t make HNO3 become H2NO3 as that’s not nitric acid anymore! HNO3 + K KNO3 + H2 3 But can have extras of any whole molecule by putting numbers IN FRONT 4 For example, the red two fixes the hydrogen problem (now two on each side, but now have a NO3 imbalance. 2HNO3 + K KNO3 + H2 5 The green two fixes NO3, and introduces a K imbalance which is solved with the blue two and we now have the same number of each type of atom on both sides. The equation is balanced! 2HNO3 + K 2KNO3 + H2 2HNO3 + 2K 2KNO3 + H2 Try balancing these: The reaction of sodium in water (makes sodium hydroxide and hydrogen gas) Na + H2O -> NaOH + H2 Baking Soda + Vinegar • Word: Acetic acid + sodium bicarbonate sodium acetate + water + carbon dioxide • Symbols: CH3COOH + NaHCO3 NaCOOH + H2O + CO2 • Balanced: CH3COOH + 2NaHCO3 2NaCOOH + H2O + 2CO2 Sherbet (an acid / base reaction) Recipe • 1T raro • 1t citric acid • 1t tartaric acid • 1t sodium bicarbonate (baking soda) • 1C icing sugar • Mix, serve T = tablespoon, t = teaspoon, C = cup Recipe Using Formulae • 1T raro • 1t ____________ • 1t C4H6O6 • 1t ____________ • 1C ____________ • Mix, serve Sherbet Questions 1) 2) 3) 4) 5) 6) 7) 8) 9) Rewrite recipe using chemical formulae Explain why you can safely eat the acids Describe what happens in your mouth What is the name of this type of reaction Why don’t the acids/base react as soon as they are mixed in the cup? Why is so much sugar needed? *Write a word equation for the reaction **Write a chemical equation for the reaction ***Write a balanced chemical equation Sherbet Questions 1) 2) 3) 4) 5) 6) 7) 8) 9) . They are weak acids The acids and base react, fizzing Neutralisation When dry they can’t react, need water (from saliva) Because the acids taste sour Citric Acid + Tartaric Acid + Baking Soda Carbon Dioxide + Water + Sodium citrate + Sodium tartate C6H8O7 + ??? + H2O + NaHCO3 CO2 + H2O + Na??? + Na ??? .