acids and bases

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acids and bases

• Joan-Ramon Ivern

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Acids

• One important group of chemicals is called acids.
• Have you ever heard this word? Acid
• When? Where? What do they do?

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Some ideas

• - stomach acidity
• - lemon juice
• - vinegar
• - the liquid of a battery

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• - The acids in the laboratory
• o Hydrochloric acid HCl
• o Nitric acid HNO3
• o Sulphuric acid H2SO4
• o Ethanoic acid CH3COOH
• o Citric acid

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• They are all liquids. In fact they are solutions
  of pure compounds in water.
• Most of them must be handled carefully,
  especially the concentrated acids, because they
  are corrosive. They can eat away metals, skin and
  cloth.

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• But some acids are not so corrosive, they are the
  weak acids.
• - Ethanoic acid (acetic acid). Its found in
  vinegar.
• - Citric acid. Its found in lemon juice.
• - Carbonic acid. Its found in gaseous drinks

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• You can know if something is acid by its effect
  on litmus. It can be used as a solution or on
  paper.
• If it turns red the substance is an acid.

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Bases

• What do we do against stomach acidity?
• - We take baking soda, bicarbonate of soda,
  sodium hydrogen carbonate.
• What do we do against a bee sting?
• - We rub on ammonia lotion or calamine lotion
  which contains zinc carbonate.
• Ant stings and nettle stings contain methanoic
  acid.

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• Any substance that neutralize an acid is called a
  base.
• The sodium hydrogen carbonate, zinc carbonate,
  ammonia are bases.
• This group of chemicals also affects litmus, but
  in a different way to acids. They turn litmus
  blue.

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What causes acidity?

• What makes that acids act alike are hydrogen
  ions.
• Acids are solutions of pure compounds in water.
  The pure compounds are molecular. But in water,
  the molecules break up to form ions. They always
  give positive hydrogen ions. For example, in
  hydrochloric acid
• HCl(aq) ? H(aq)  Cl-(aq)

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The more H ions there are in a solution, the
more acidic it is.
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• Bases are substances that remove H ions of the
  solution.
• B(aq)  H(aq) ? BH(aq)

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• The molecules of water (one into 555 milions)
  also break off giving H ions and OH- ions in
  equal number. It is said that water is neutral
• H2O ? H(aq)  OH-(aq) 

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The pH scale

• The strength of an acid is measured in a scale of
  numbers called the pH scale. The numbers go from
  0 to 14.
• Pure water is neutral and has a pH of 7 in the
  middle of the pH scale.

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• Strong acids leave a great number of H(aq) ions.
• Weak acids leave a little number of H(aq) ions.
• The numbers from 0 to 7 correspond to acids. The
  less is the number stronger is the acid.

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• A solution of a base in water will remove some H
  ions and its concentration will decrease and the
  pH will be bigger than 7.
• B(aq)  H(aq) à BH(aq)
• The strength of bases increases from 7 to 14 in
  the pH scale.

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• You can find the pH of any solution by using
  universal indicator.
• Universal indicator is a mixture of dyes.
• It goes a different colour at different pH, from
• red (pH 0),
• yellow pH 4-5,
• green (pH 7),
• greenish-blue (pH 8-9),
• blue (pH 10-11),
• violet (pH 12-14).

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• The solutions with a pH between 7 and 14 are
  called alkaline solutions.
• And an important group of bases receive the name
  of alkalis.

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• Most pure alkalis are solids. But they are
  usually used in laboratory as aqueous solutions.
  The main ones are
• - sodium hydroxide NaOH(aq)
• - potassium hydroxide KOH(aq)
• - calcium hydroxide Ca(OH)2(aq)
• - ammonia NH3(aq). Pure NH3 is a gas.

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A little of history of the definition of ACID and
BASE.

• First of all we can see that the ion H is the
  nucleus of the hydrogen atom, that's a proton.
• In water solution a proton, H, cannot exist in
  isolation and is solvated. In water it exists as
  H3O.
• The dissociation of ethanoic acid (acetic acid)
  should be written as
• CH3COOH H2O ? CH3COO- H3O

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• However, for convenience, we will usually write
  it as
• CH3COOH lt----gt CH3COO- H
• Similarly, for bases
• NH3 H2O NH4 HO-
• will usually be written as
• NH3 H ? NH4

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Arrehnius defined

• ACID as a substance that leaves H ions in water
  solution.
• HA(aq) ? H(aq)  A-(aq)
• BASE as a substance that leaves OH- ions in water
  solution.
• BOH ? B OH-

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Broensted and Lorry defined

• ACID as a substance that leaves H ions in water
  solution. It is a proton donor.
• HA ? H    A-
• BASE as a substance that removes H ions from
  water solution. It is a proton acceptor.
• B H ? BH

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Lewis defined

• ACID as a substance that accepts an electron
  pair. It's an electron pair acceptor.
• BASE as a substance has an electron pair to
  donate. It's an electron pair donor.
• NH3 H NH4
• Base acid