The Periodic Table - groups 2 & 7

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Ionisation Energy

The first ionisation energy is the energy change when one mole of atoms of an element in the gaseous state loses one electron each to form one mole of positively charged ions

The second ionisation energy is when one mole of gaseous single positively charged ions loses one mole of electrons to become double positively charged ions. 

Group 2 of the periodic table is Alkaline earth metals.

There is a big increase between the second and the third ionisaition energies. 

Elements readily lose two electrons but the large third ionisation energy makes it impossible for them to lose a third. 

The have 2 electrons in the s shell, so losing 2 gives them a noble gas configuration. 

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Flame Test

Particularly useful for identifying metals from the s block - groups 1 & 2.

METHOD

Add a few drops of concentrated hydrochloric acid to a samle on a watch glass.

A clean platinum wire is dipped into the mixture and then held over a hot bunsen burner. 

(http://farm3.staticflickr.com/2044/2239548959_f216be63c1.jpg)

More accurate test - Flame photometer. Measures wavelengths quantitatively. 

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Reactions of group 2 metals

OXYGEN

Forms solid metal oxides. 

Reactivity of group 2 metals with oxygen increases down the group. 

CHLORINE

Form solid metal chlorides.

Reactivity of group 2 metals increases down the group, although not as clearly as with oxygen.

WATER

Form oxides or hydroxides.

Reactivity of group 2 metals increases down the group.

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Reactions of group 2 oxides

WATER

Beryllium does not react with water, whole magnesium oxide reacts only slightly. 

Slaking lime - cold water is added to calcium oxide to form calcium hydroxide.

DILUTE HYDROCHLORIC ACID AND NITRIC ACID

React with dilute hydrochloric acid to form chloride salt and water.

React with nitric acid to form nitrate salt and water.

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Reactions of group 2 hydroxides

DILUTE HYDROCHLORIC ACID AND NITRIC ACID

React with dilute hydrochloric acid to form chloride salt and water.

React with nitric acid to form nitrate salt and water.

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Solubility of group 2 compounds

All group 2 nitrates and chlorides are soluble.

Group 2 salts, (anion charge of -2), sulphates, are largley insoluble. 

Solubility of salts decreases down the groups as the atomic number and ionic size of metal increases. 

Insolubility of Barium sulfate is used in laboratory tests for sulphate ions. 

Dilute hydrochloric acid is added to the sample. 

Barium chloride or nitrate is added.

If a white precipitate is formed a salt is present. 

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Volumetric Analysis

Most common method is titration. 

Concentration of an unknown solution can be found by reacting it with a standard substance. 

A standard solution is one whose concentration is known and does not change over time. 

The volumes of two solutions which exactly react with each other are measured. The concentration of one can be calculated if the concentration of the other is known. 

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Precision, Accuracy and Reliability

Something can be precise, but not accurate.

The same volume is deliveredeach time, but not the volume it claims to have delivered. 

The reliability depends upon errors particular to that experiment and the skill of the experimenter. 

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Thermal Stability of Carbonates and Nitrates

  • Thermal stability increases down the group

Carbonates and Nitrates are large and can be made unstable by the presence of a positively charged ion.

The cation polarises the anion, distorting it. Greater distortion, less stability.

  • Group 1 & 2 compounds are less thermally stable than group 1 compounds

The greater the charge on the cation, the greater the distortion and the less stable

Group 2 have a +2 charge, compared to group 1, which has a +1 charge, so group 2 are less stable than group 1. 

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Testing Thermal Stability

Nitrates:

Measuring how long it takes until oxygen is produced. 

How long it takes until a brown gas is produced (NO2) - toxic.

Carbonates:

How long it takes until carbon dioxide is produced. 

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Group 7, The Halogens

Halogens is greek, and means 'salt producing'.

Non-metallic and exist in diatomic molecules.

All very reactive and are strong oxidising agents. 

Atomic number increases down the group, so they become less reactive, less volatile and darker in colour. 

Solubility of Halogens decreases down the group. 

Halogens are non-polar so are more soluble in hydrocarbon solvents, than in water. 

OXIDATION WITH METALS

Become reduced to negative halide ions. 

OXIDATION WITH NON-METALS

Achieve noble gas configuration through covalent bonding. 

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Disproportionation

All reactions with chlorine, bromine and iodine, with alkali, are disproportionation reactions. 

Products depend on the temperature inwhich it takes place.

With chlorine, decomposition occurs very slowly at 15 degrees, but very rapidly at 70 degrees. 

With bromine, both occur rapidly at 15 degrees. It must be lower than 0 degrees to prevent decomposition occuring. 

Both reactions occur rapidly at 0 degrees with iodine. 

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Reactions of hydrogens and halides

Hydrogen halides result from the reaction of the halides with hydrogen. 

Potassium halides will react with another halogen in a displacement (redox) reaction

Concentrated sulfuric acid reacts with a halide to form a hydrogen halide

TESTING FOR SILVER HALIDES:

Dilute nitric acid is added to the unknown halide solution - to prevent the precipitation of other salts

Silver nitrate solution is then added

 Cl- gives a white precipitate is formed

Br- gives a cream precipitate

I- gives a yellow precipitate

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Reactions of hydrogens and halides

Hydrogen halides result from the reaction of the halides with hydrogen. 

Potassium halides will react with another halogen in a displacement (redox) reaction

Concentrated sulfuric acid reacts with a halide to form a hydrogen halide

TESTING FOR SILVER HALIDES:

Dilute nitric acid is added to the unknown halide solution - to prevent the precipitation of other salts

Silver nitrate solution is then added

 Cl- gives a white precipitate is formed

Br- gives a cream precipitate

I- gives a yellow precipitate

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