Module 2: Chemical Species and Formulae

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Definitions

An element is a substance that connot be broken down into simpler substances and where all the atoms have the same number of protons

A chemical species is the smallest particle of a substance, e.g. atoms, molecules, ions and molecular ions

Metal elements are atoms

Non-metal elements or non-metal compounds are molecules

An ion is a particle with either a positive charge (cation) or negative charge (anion)

Molecular ions are charged chemical species that contain two or more elements

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Ions and State Symbols

Metal elements form cations, where the positive charge equals the group number

Non-metal elements usually form anions, where the negative charge equals 8 - group number

All ionic compounds of the nitrate ion dissolve in water

All acids release aqueous hydrogen ions

(s): solid

(l): liquid

(aq): aqueous (in solution)

(g): gas

All ionic compounds and metals are solid at room temperature, with the exception of mercury which is a liquid

The only liquid non-metal element at room temperature is bromine

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Qualitative Analysis of Ions

Carbonate ions                    CO32- reacts with H+ (aq) from acids
                                                to produce CO2 (g)

bubblesof a colourless gas are produced
CO32-  +  2 H+ (aq) -->
  CO2 (g)  +  H2O (l)

Aqueous sulfate ions        SO42- (aq) reacts with Ba2+ (aq) from BaCl2 (aq)
                                                to produce a precipitate of BaSO4 (s)

a white precipitate is produced
Ba2+ (aq)  +  SO42- (aq)  -->
  BaSO4 (s)  white solid

Aqueous chloride ions      Cl- (aq) reacts with Ag+ (aq) from AgNO3 (aq)
                                                to produce a white precipitate of AgCl (s)

a white precipitate is produced which dissolves in dilute NH3 (aq)

Ag+ (aq)  +  Cl- (aq)  -->  AgCl (s)  white solid

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Qualitative Analysis of Ions (continued)

aqueous bromide ions     Br- (aq) reacts with Ag+ (aq) from AgNO3 (aq)
                                                to produce a cream precipitate of AgBr (s)

a cream precipitate is produced which does not dissolve in dilute NH3 (aq)
but does dissolve in concentrated NH3 (aq)

Ag+ (aq)  +  Br- (aq)  -->  AgBr (s)  cream solid

aqueous iodide ions         I- (aq) reacts with Ag+ (aq) from AgNO3 (aq)
                                                to produce a yellow precipitate of AgI (s)

a yellow precipitate is produced which does not dissolve  in either dilute  NH3(aq) or concentrated NH3 (aq)

Ag+ (aq)  +  I- (aq)  -->  AgI (s)  yellow solid

ammonium ions                  NH4+ reacts with warm NaOH (aq) to produce NH3 (g)

colourless gas produced which turns damp red litmus paper blue (ammonia is an alkaline gas).

NH4+  +  OH- (aq)  -->  NH3 (g)  +  H2O (l)

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