acids and bases. why ‘acids and bases’? scientists like to classify things and donating them as...
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ACIDS and BASES
Why ‘acids and bases’?
•Scientists like to classify things and donating them as acids or bases is one way to do that.
•If a solution is not an acid or a base, it is half way between the two and known as a neutral solution.
What are some properties of Acids?
•The word acid comes from the Latin word acere, which means "sour." All acids taste sour. •Well known from ancient times were vinegar, sour milk and lemon juice. •By the 1884s acids defined by Svante Arrhenius (Sweden), as a material that can release hydrogen ions (H+).•Acids turn litmus paper red.•Acids are corrosive.
Some Common Acids
Name Chemical formula Uses
Hydrochloric acid
HCl Cleaning brickwork
Sulfuric acid H2SO4 Car batteries
Nitric acid HNO3 Fertiliser and dyes
Acetic acid CH3COOH Vinegar
Carbonic acid H2CO3Carbonated drinks
Citric acid C6H8O7Citrus fruits
Lactic acid C3H6O3Milk products
What are some properties of Bases?
•All bases taste bitter. •Mustard tastes bitter. Many medicines such as cough syrup taste bitter. Cough syrups are advertised as having a great taste. This is because sweeteners are added in order to cover the bitterness of the active ingredient in cough syrup.•Many bases are cleaning products and they have a slippery feel to them.•Bases release OH- ions into solution.•Bases turn litmus blue.•Bases are corrosive.
Some Common Bases
Name Chemical Formula Uses
Sodium hydroxide NaOH Soap-making, oven cleaners
Potassium hydroxide KOH Soft soap making
Calcium hydroxide Ca(OH)2Cement, plaster
Ammonium hydroxideNH4OH Household cleaners
Magnesium hydroxideMg(OH)2
Indigestion cleaners
Sodium carbonateNa2CO3
Glass-making
Sodium hydrogen carbonate NaHCO3
Baking soda
The pH of a solution
•This scale is logarithmic : It means that every time you go down 1 level on the scale, the solution is 10x stronger.•For example an acid of pH 1 is 10 times more acidic than an acid with pH 2 and 100 times more acidic than an acid of pH 3.•It also means that a base of pH 11 is 10 times more basic (stronger basicity) than a base of pH 10 and 100 times more basic than a base of pH 9.
70 14
More acid, strong acid
Less acid,Weak acid
Less basic,Weak base
More basic, strong baseNeutral
The pH scale enables us to classify acids and bases
The pH scale
pH Example
Acids
0 HCl : Hydrochloric acid
1 Stomach acid
2 Lemon juice
3 Vinegar
4 Soda, tomato juice
5 Black coffee
6 Milk, rain water
Neutral 7 Pure water
Bases
8 Egg whites
9 Baking Soda
10 Limewater / antacid
11 Ammonia
12 Mineral Lime - Ca(OH)2
13 Drano®
14 NaOH : sodium hydroxide
10X less acidic
100x more acidic
Indicators
•Indicators are substances that undergo colour changes in either acids or bases or in both
•Because of these properties, scientists can identify whether a solution is an acid or a base by noting the colour change
•Some popular indicators are: phenolpthalene, litmus paper, universal, methyl blue
Indicators
70 14
More acid, strong acid
Less acid,Weak acid
Less basic,Weak base
More basic, strong baseNeutral
What does a neutral solution look like?
H HO
HO
-H
++
+-
+-
+-
+-
•The glass of pure water is neutral (pH=7). •There are as many “H+” ions as there are “OH-” ions.•When a water molecule splits into hydrogen and hydroxide ions it is said to be dissociated.
Water molecules in a glass jar
Some water molecules have a natural tendency to split into OH- and H+ ions (see inside red oval shape)
What does an acid look like in solution?
Add some acid to the water
H+
OH-
The acid dissociates leaving more H+ than OH-. The liquid is therefore Acidic
H+OH-
H+ H+
H+
Hydrochloric Acid (HCl)
H+ H+
H+
Cl- Cl-
Cl-
Start with a neutral solution of water
What does a base look like in solution?
H+
OH-
Base: Sodium Hydroxide(NaOH)
The base dissociates leaving more OH- than H+ : the liquid is therefore Basic
Add some base to the water
OH- OH-
OH-
Na+ Na+
Na+
H+OH-
OH-
OH-
OH-
Start with a neutral solution of water
•When there are as many OH- as H+, the solution becomes neutral as the acid has been neutralised by the base.
Base: NaOH
OH- OH-
OH-
Na+ Na+
Na+
H+OH-
H+H+
H+
Cl-
Cl-
Cl- H+
OH-
OH-
OH-
OH-
H+
H+
H+
Na+
Na+
Na+
Cl-
Cl-
Cl-
Neutralisation
Start with an acid solution i.e. HCl: more H+ than OH-
Add a base i.e. NaOH
The OH- from the base associates with the H+ from the dissociated acid
•When there are as many H+ as OH-, the solution becomes neutral as the base has been neutralised by the acid.
Acid: HCl
H+OH-
OH-
OH-
OH-
Na+
Na+
Na+
H+
OH-
OH-
OH-
OH-
H+
H+
H+
Na+
Na+
Na+
H+ H+
H+
Cl-
Cl-
Cl-
Neutralisation
Start with a Basic solution i.e.NaOH: more OH- than H+
Add some acid i.e.HCl
The H+ from the acid associates with the OH- from the dissociated base
Some Common Salts
Acid + Base Salt + Water
Base Acid Salt Use
Sodium hydroxide
Hydrochloric acid
Sodium chloride
Table salt
Potassium hydroxide
Nitric acid Potassium nitrate
Explosives, fertilisers
Calcium hydroxide
Sulfuric acid
Calcium sulfate
Plaster
Salts
What is a salt?A salt is an ionic compound made up of a metal and a non metal
Non metal
Metal
H+
Cl-
Cl-
H+
H+Cl- H H
HydrogenGas
Fe+
Metal and acid reactions
Fe
e-
e-
e- e-
e-e-
e- e-
e-
A metal lattice of iron with many electrons
HCl is added and dissociated
The H+ ions join to form Hydrogen gas
The chloride joins with the metal to make iron chloride
Overall reaction for metals and acids:
Metal + Acid Hydrogen + Salt
Metal and acid reactionsMetal + Acid Hydrogen + Salt
•When sulfuric acid forms a salt it is a sulfate salt
•When nitric acid forms a salt it is a nitrate salt
•When acetic acid forms a salt it is an acetate salt
•When hydrochloric acid forms a salt it is a chloride salt
Sulfuric acid + magnesium Magnesium sulfate + hydrogen
Nitric acid + zinc Zinc nitrate + hydrogen
Acetic acid + potassium Potassium acetate + hydrogen
Hydrochloric acid + sodium Sodium chloride + hydrogen
ACIDS and BASESThe End