Acids and Bases

Acids “ A substance that can dissolve in water form hydronium ions (H 3 O + )” 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 FormulaNameUse H 2 SO 4 Sulfuric acidCar batteries HClHydrochloric acidStomach acid HNO 3 Nitric acidMaking explosives CH 3 COOHAcetic acidVinegar C6H8O7C6H8O7 Citric acidIn citrus fruits Tartaric acidIn grapes Carbonic acidsFizzy drinks Lactic acidMakes yoghurt DNAGenetic code Formic acidIn ant/bee stings

Common Bases FormulaNameUse NaOHSodium hydroxideOven cleaner NH 3 AmmoniaHousehold cleaner NaHCO 3 Sodium bicarbonateBaking Ca(OH) 2 Calcium hydroxideFertiliser

Explaining Acids & Bases The special properties of acids are due to hydronium ions H 3 O + in aqueous solutions. HCl + H 2 O  H 3 O + + Cl - Whereas basic solutions contain hydroxide ions: NaOH + H 2 O  Na + + OH - NH 3 + H 2 O  NH 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 H 3 O + ions easily –Eg HCl becomes mostly H 3 O + ( & not much HCl) Weak Acids don’t become H 3 O + ions easily –Eg Acetic acid stays mostly acetic acid ( & not much H 3 O + ) (H 3 O + 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 NH 3 stays mostly NH 3 ( & 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 H 2 O. 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. Disappearing+ink

Disappearing Ink - Questions 1)What pH is the ammonia? 2)Why does the “ink” disappear?

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) = 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 H 3 O + there is A pH of 3 has 10 times the H 3 O + of a pH of 4 This is why strong acids have a low pH (they have loads of H 3 O +)

pH of Everyday Substances SubstancepH Blood7.4 Lemon2 Jam4 Cabbage5 Wine3 Egg8 Coke2.9

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: ColourRedOrangeYellowGreenBluePurple pH

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 + H 2 O 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 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: HNO 3 + K  KNO 3 + H 2 H 2 SO 4 + Mg  MgSO 4 + H 2 HCl + Na  NaCl + H 2 pH becomes neutral! Naming salts: 1st part from the metal, 2 nd part from the acid ending Test for H 2 : pop! test

Acid & Carbonate Reactions (a type of acid & base reaction) Acid + Carbonate  Salt + Carbon dioxide + water 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 + MgCO 3  MgCl 2 + CO 2 + H 2 O H 2 SO 4 + PbCO 3  PbSO 4 + CO 2 + H 2 O Naming salts: 1st part from the metal, 2 nd part from the acid ending pH becomes neutral! Test for CO 2 : bubble through limewater (cloudy = CO 2 ) or lit match extinguishes

?What is this slide? Al 2 (CO 3 ) 3 + 6HNO 3  2Al(NO 3 ) 3 + 3H 2 CO 3 Al 2 (CO 3 ) 3 + 3H 2 SO 4  Al 2 (SO 4 ) 3 + 3CO 2 + 3H 2 O

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 HNO 3 become H 2 NO 3 as that’s not nitric acid anymore! 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 NO 3 imbalance. 5 The green two fixes NO 3, 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! HNO 3 + K  KNO 3 + H 2 2HNO 3 + K  KNO 3 + H 2 2HNO 3 + K  2KNO 3 + H 2 2HNO 3 + 2K  2KNO 3 + H 2

Try balancing these: The reaction of sodium in water (makes sodium hydroxide and hydrogen gas) Na + H 2 O -> NaOH + H 2

Baking Soda + Vinegar Word: Acetic acid + sodium bicarbonate  sodium acetate + water + carbon dioxide Symbols: CH 3 COOH + NaHCO 3  NaCOOH + H 2 O + CO 2 Balanced: CH 3 COOH + 2NaHCO 3  2NaCOOH + H 2 O + 2CO 2

Sherbet (an acid / base reaction) Recipe 1T raro 1t citric acid 1t tartaric acid 1t sodium bicarbonate (baking soda) 1C icing sugar Mix, serve Recipe Using Formulae 1T raro 1t ____________ 1t C 4 H 6 O 6 1t ____________ 1C ____________ Mix, serve T = tablespoon, t = teaspoon, C = cup

Sherbet Questions 1)Rewrite recipe using chemical formulae 2)Explain why you can safely eat the acids 3)Describe what happens in your mouth 4)What is the name of this type of reaction 5)Why don’t the acids/base react as soon as they are mixed in the cup? 6)Why is so much sugar needed? 7)*Write a word equation for the reaction 8)**Write a chemical equation for the reaction 9)***Write a balanced chemical equation

Sherbet Questions 1). 2)They are weak acids 3)The acids and base react, fizzing 4)Neutralisation 5)When dry they can’t react, need water (from saliva) 6)Because the acids taste sour 7)Citric Acid + Tartaric Acid + Baking Soda  Carbon Dioxide + Water + Sodium citrate + Sodium tartate 8)C 6 H 8 O 7 + ??? + H 2 O + NaHCO 3  CO 2 + H 2 O + Na??? + Na ??? 9).