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CGB IGCSE Chemistry

Acids and Alkalis

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Acids and Alkalis

 

Acids

 

Acids have an excess on H+ ions, the more H+ ions the more acidic the solution.

 

Acids turn universal indicator yellow, orange or red

Acids turn blue litmus paper red

Acids turn phenolphthalein solution clear

 

Acids have a pH numbers less than 7

 

All acids are solutions of pure compounds in water.  Solutions of non-metal oxides are acidic

            SO2 sulphur dioxide

            CO2 carbon dioxide

            P2O5 phosphorus (V) oxide

 

Some common acids

            Mineral Acids

 

Hydrochloric Acid HCl strong acid

            Sulphuric Acid H2SO4 strong acid

            Nitric Acid HNO3 strong acid

            Phosphoric Acid H3PO4 strong acid           

            Carbonic Acid H2CO3 weak acid

 

            Organic Acids

 

Citric Acid C6H8O7 weak acid

            Ethanoic Acid CH3COOH weak acid

Methanoic Acid HCOOH weak acid

            Lactic Acid CH3CH(OH) weak acid

                                           |

                                          COOH      

 

Acids react with metals to form salts

            A salt is a compound made from acid when a metal takes the place of the hydrogen in the acid.

 

            Acid + metal Salt + Hydrogen

           

2HCl(aq) + Zn(s) ZnCl2(aq) + H2(g)

            H2SO4(aq) + Mg(s) MgSO4(aq) + H2(g)

            2HNO3(aq) + 2Na(s) 2NaNO3(aq) + H2(g)

            2H3PO4(aq) + 3Ca(s) Ca3(PO4)2(aq) + 3H2(g)

            3H2CO3(aq) + 2Al(s) Al2(CO3)3(aq) + 3H2(g)

 

The salt made depends on the acid

           

            HCl chlorides

            H2SO4 sulphates

            HNO3 nitrates

            H3PO4 phosphates

            H2CO3 carbonates

           

Bases/Alkalis

 

Alkaline solutions have an excess of OH- ions.  The more OH- ions the more alkaline the solution.

 

Alkaline solutions turn universal indicator blue, purple and violet

Alkaline solutions turn red litmus paper blue

Alkaline solutions turn phenolphthalein pink

Alkaline solutions turn methyl orange yellow

 

Alkaline solutions have a pH greater than 7

 

Metal oxides dissolve in water to form alkaline solutions

 

            Na2O Sodium oxide

            MgO Magnesium oxide

            CaO Calcium oxide

 

So what is the difference between alkalis and bases?

 

            Alkalis are substances that dissolve in water to give a pH greater than 7

            Alkalis turn red litmus paper blue

            Alkalis neutralize acids to form a solution with a pH 7.

 

            Bases are metal oxides and metal hydroxides which are insoluble in water.

            Bases neutralize acids to form a solution with a pH 7.

 

            Therefore Alkalis are a subset of a group of chemicals that can neutralize acids.

 

            Alkalis are soluble bases.

 

            All alkalis are bases, but not all bases are alkalis.

 

Common Alkalis

 

            NaOH sodium hydroxide strong alkali

            KOH potassium hydroxide strong alkali

            Ca(OH)2 calcium hydroxide (limewater) strong alkali

            NH3(aq) or NH3OH ammonia solution or ammonium hydroxide weak alkali

 

Common Bases

 

            CaO calcium oxide strong base

            MgO magnesium oxide strong base

 

 

Acids react with bases to form salts + water

 

            This is a neutralization reaction.

 

            acid + base salt + water

 

            HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

            HCl(aq) + KOH(aq) KCl(aq) + H2O(l)

            2HCl(aq) + MgO(aq) MgCl2(aq) + H2O(l)

            2HCl(aq) + CuO(aq) CuCl2(aq) + H2O(l)

 

            HNO3(aq) + NaOH(aq) NaNO3(aq) + H2O(l)

            HNO3(aq) + KOH(aq) KNO3(aq) + H2O(l)

            2HNO3(aq) + MgO(aq) Mg(NO3)2(aq) + H2O(l)

            2HNO3(aq) + CuO(aq) Cu(NO3)2(aq) + H2O(l)

 

            H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l)

            H2SO4(aq) + 2KOH(aq) K2SO4(aq) + 2H2O(l)

            H2SO4(aq) + MgO(aq) MgSO4(aq) + H2O(l)

            H2SO4(aq) + CuO(aq) CuSO4(aq) + H2O(l)

 

H3PO4(aq) + 3NaOH(aq) Na3PO4(aq) + 3H2O(l)

            H3PO4(aq) + 3KOH(aq) K3PO4(aq) + 3H2O(l)

            2H3PO4(aq) + 3MgO(aq) Mg3(PO4)2(aq) + 3H2O(l)

            2H3PO4(aq) + 3CuO(aq) Cu3(PO4)2(aq) + 3H2O(l)

 

Acids react with carbonates to give salt, water and carbon dioxide

 

            Acid + metal carbonate salt + water + carbon dioxide

 

            2HCl(aq) + CaCO3(s) CaCl2(aq) + H2O(l) + CO2(g)

            H2SO4(aq) + Na2CO3(s) Na2SO4(aq) + H2O(l) + CO2(g)

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