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 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
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://science
Disappearing+
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
Blood
Lemon
Jam
Cabbage
Wine
Egg
Coke
pH
7.4
2
4
5
3
8
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 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 ???
.