Group 2 and Group 7

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Group 2 elements: redox reaxtions

Physical properties
They have high melting and boiling points. They are light metals with low densities. The form colourless compounds.

Reactivity of the Group 2 Elements
Reactive metals and strong reducing agents. Group 2 elements are oxidised in reactions. Each atom loses 2 electrons from its outer s sub-shell to form a +2 ion. Reactivty increases down the group, reflecting the increasing ease of losing an electron. The decrease in ionisation energies down Group 2 is an important factor.

Reaction with oxygen
Group 2 elements react vigorously with oxygen. This is a redox reaction. The product is an ionic oxide with the general formula MO, where M is the element.

Reaction with water
The Group 2 elements react with water to form hydroxides with the general formula M(OH)2. Hydrogen gas is also formed. As you move further down the group, each metal reacts more vigorously with water.  

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Group 2 compounds: reactons

Group 2 oxides and hydroxides
Group 2 oxides and hydroxides are bases. They are neutralised by acids to form a salt and water. e.g. MgO + 2HCl = MgCl2 + H2o
As these reactions take place with hydrochloric acid you will see the solid oxides or hydroxides dissolve.
Group 2 oxides react with water to form a solution of the metal hydroxides. For example, MgO + H2O = Mg(OH)2. The typical pH is 10-12
Group 2 hydroxides dissolve in water to form alkaline solutions. Ca(OH)2 + aq = Ca2+ + 2OH-.

Group 2 carbonates
The group 2 carbonates are decomposed by heat, forming the metal oxide and carbon dioxide gas. MgCO3 = MgO + CO2
This type of reaction is called thermal decomposition. The carbonates become more difficult to decompose with heat as you move down the group

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Group 2 compounds: reactions

Properties of Group 2 elements and their compounds
The trends down a group. The Group 2 elements become more reactive down the group. The Group 2 carbonates decompose at higher temperatures down the group. The hydroxides become more soluble in water, and the resulting solutions become more alkaline.  

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Group 7 elements: redox reactions

Physical properties
The halogens have low melting and boiling points. The halogens exist as diatomic molecules.
As you move down the group the number of electrons increases, leading to an increase in the van der Waals' forces between molecules. The boiling points of the halogens increase down the group. The physical states of the halogens at room temperature and pressure show the classic trend of gas to liquid to solid as you move down the group.

Halogens as oxidising agents
The halogens are the most reactive non-metals in the periodic table and are strong oxidising agents removing electrons in reactions. The oxidising power of a halogen is a measure of the strength with which a halogen atom is able to attract and capture  an electron to form a halide ion. In redox reactions each halogen atom gains one electron into a p-sub shell to form a halide ion with a 1- charge.
The halogens become less reactive down the group as their oxidising power decreases. As halogens gain an electron in their reactions, reactivity decreases down the group because the atomic radius increases, the electron shielding increase and the ability to gain an electron into the p sub-shell decreases to form a halide ion. 

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Group 7 elements: redox reactions

Redox reactions of the halogens
Each halogen is mixed with aqueous solutions of the different halides. A more-reactive halogen will oxidise and displace a halide of a less-reactive halogen. This is often called a displacement reaction.
Cl2 is pale green in both water and cyclohexane. Br2 is orange in both water and cyclohexane. Iodine is brown in water and violet in cyclohexane.

Disproportionation is a reaction which the same element is both reduced and oxidised. Small amounts of chlorine are added to drinking water to kill bacteria to make water safer to drink.  

Ionic halides typically have a halide ion, X-, with a 1- charge. In contrast to the reactive elements, halide compounds are mainly very stable. Sodium Chloride, NaCl, is the most familiar halide.

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Group 7 elements: halide tests

Using silver nitrate solution

Carrying out the test

This test has to be done in solution. If you start from a solid, it must first be dissolved in pure water.

The solution is acidified by adding dilute nitric acid. (Remember: silver nitrate + dilute nitric acid.) The nitric acid reacts with, and removes, other ions that might also give a confusing precipitate with silver nitrate.

Silver nitrate solution is then added to give:

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Group 7 elements: halide tests

ion presentobservation F- no precipitate Cl- white precipitate Br- very pale cream precipitate I- very pale yellow precipitate

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Group 7 elements: halide tests

The absence of a precipitate with fluoride ions doesn't prove anything unless you already know that you must have a halogen present and are simply trying to find out which one. All the absence of a precipitate shows is that you haven't got chloride, bromide or iodide ions present.

The chemistry of the test

The precipitates are the insoluble silver halides - silver chloride, silver bromide or silver iodide.


Silver fluoride is soluble, and so you don't get a precipitate.

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