Acids and Bases

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 AcidsFormula Name Use

H2SO4 Sulfuric acid Car batteries

HCl Hydrochloric acid Stomach acid

HNO3 Nitric acid Making explosives

CH3COOH Acetic acid Vinegar

C6H8O7 Citric acid In citrus fruits

Tartaric acid In grapes

Carbonic acids Fizzy drinks

Lactic acid Makes yoghurt

DNA Genetic code

Formic acid In ant/bee stings

Common BasesFormula Name Use

NaOH Sodium hydroxide Oven cleaner

NH3 Ammonia Household cleaner

NaHCO3 Sodium bicarbonate Baking

Ca(OH)2 Calcium hydroxide Fertiliser

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+ + OH-

NH3 + 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 InkDone 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 - Questions1) 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?

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 pH

Blood 7.4

Lemon 2

Jam 4

Cabbage 5

Wine 3

Egg 8

Coke 2.9

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 IndicatorBackground: The purple pigment found in purple cabbage is one example of a substance that changes colour in solutions of different pHAim: 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 purple3. While heating test the supplied acids/bases with pH

paper to find their pH4. 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

Nitric acid + potassium potassium nitrate + hydrogen gas

• More ExamplesSulfuric 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

pH becomes neutral!

Naming salts: 1st part from the metal, 2nd part from the acid ending

Test for H2: pop! test

Acid & Carbonate Reactions(a type of acid & base reaction)

Acid + Carbonate Salt + Carbon dioxide + water

•ExampleNitric acid + calcium carbonate calcium nitrate + carbon dioxide + water

•More examplesSulfuric acid + copper carbonate copper sulfate + carbon dioxide + water

•Chemical equation examples:HCl + MgCO3 MgCl2 + CO2 + H2O

H2SO4 + PbCO3 PbSO4 + CO2 + H2O

Naming salts: 1st part from the metal, 2nd part from the acid ending

pH becomes neutral!

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 Equations1 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!

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.

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!

HNO3 + K KNO3 + H2

2HNO3 + K KNO3 + H2

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

Recipe Using Formulae• 1T raro• 1t ____________

• 1t C4H6O6

• 1t ____________

• 1C ____________• Mix, serve

T = tablespoon, t = teaspoon, C = cup

Sherbet Questions

1) Rewrite recipe using chemical formulae2) Explain why you can safely eat the acids3) Describe what happens in your mouth4) What is the name of this type of reaction5) 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 reaction8) **Write a chemical equation for the reaction9) ***Write a balanced chemical equation

Sherbet Questions

1) .2) They are weak acids3) The acids and base react, fizzing4) Neutralisation5) When dry they can’t react, need water (from saliva)6) Because the acids taste sour7) Citric Acid + Tartaric Acid + Baking Soda Carbon

Dioxide + Water + Sodium citrate + Sodium tartate

8) C6H8O7 + ??? + H2O + NaHCO3 CO2 + H2O + Na??? + Na ???

9) .