Topic 1: Metal Compounds, and introduction to inorganic chemistry

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-Thermal Decomposition

 

-The breakdown of a reactant to form two or more products under the influence of heat.

 

Nitrates

 

There are a few key points that haven’t been described in the book that HAVE come in past papers. One of them is the thermal decomposition of Sodium Nitrate (NaNO3)

 

Unlike the group 2 metals we studied in detail which decompose like this:

 

2XNO3 (s) --> 2XO (s) + 4NO2 (g) + O2 (g)

 

(NOTE: Exams usually have this equation associated with observations. Observations can be classified according to what happens to the reactants, or what products are formed. I would have the following observations:

 

1)     Nitrate melts.

2)     Formation of brown gas

 

Notice I didn’t refer to XO because that is itself the same colour as the nitrate, so I couldn’t see it easily, BUT MORE IMPORTANTLY, I didn’t refer to the Oxygen. NEVER include oxygen in observations (Such as referring to glowing splint) because that isn’t an observation, it is a test. You can’t see, or even smell oxygen)

 

Sodium, because it is more reactive and for reasons we don’t need to know at this stage, only partially decomposes:

 

2NaNO3 (s) --> 2NaNO2 (s) + O2 (g)

 

Carbonates

 

This shouldn’t be a problem. Carbonates decompose to form an oxide and carbon dioxide.

 

Sulphates

 

The book is quite vague as to which oxides of sulphur are produced. They are usually SO2 and SO3. Sulphates also leave a metal oxide when decomposed.

 

 

Hydroxides

 

This one isn’t in the book, but it has come a few times in the Examination Questions for Topic 4. Hydroxides decompose to give a meta oxide and water.

 

-Precipitation

 

Solubility rules:-

 

ALL Nitrates, group 1 and ammonium salts are soluble.

 

ALMOST ALL chlorides are soluble. Lead and silver chloride are exceptions.

 

ALMOST ALL sulphates are soluble. Lead and Barium sulphate are exceptions.

 

MOST oxides, hydroxides and carbonates are INSOLUBLE. Remember, the first row are all soluble exceptions.

 

A precipitate is a SOLID product formed from two aqueous reactants. Thus, the precipitate is insoluble. The precipitate is usually used in exams to catch students out with their state symbols. It’s (s), not (aq), AND this is extremely important in terms of ionic equations. (There is a separate page that concentrates on this on the Web). For example, here is a precipitation reaction:

 

BaCl2 (aq) + Na2SO4 (aq) -->  BaSO4 (s) + 2NaCl (aq)

 

-Redox Reactions

 

Although Redox will spring the word ‘OILRIG’ into your head, don’t get carried away. It is true that oxidation is loss of electron, whilst reduction gain of electrons, but sometimes examiners ask ‘What has been oxidised?’ and it is better to answer in terms of electron gain AS WELL as in terms of oxidation state.

 

When Fe2+ is oxidised, it loses an electron to become Fe3+. The Fe2+ ion can be written as iron (II) and Fe3+ as iron (III). You must have seen this being done before. These numbers in roman numerals are called the oxidation state. When the oxidation state increases, there is oxidation. When it goes down, there is reduction.

 

So the model answer (THIS IS ALL IN MY OPINION) is: The Fe2+ is oxidised because it has lost an electron to form Fe3+. There is also an increase of oxidation state, from (II) to (III).

 

-Making Salts

 

Here is the procedure, with precautions, because they ask about that including experimental procedure:

 

1)     Measure out 25 cm3 of your acid in a pipette.

 

2)     Calculate how much salt you need in relation to molar ratio with the acid.

 

3)     Use EXCESS (Usually 10%) salt

 

4)     Add some of the salt to the acid and boil, remove the heat once all the acid has dissolved. Stir continuously.

 

5)     Make sure it isn’t boiling, add the salt in portions, this is to prevent the acid boiling up and spitting.

 

6)     Add all the salt, until excess remains.

 

7)     Filter the mixture. The Residue is the excess salt, and the filtrate is the solution of the intended salt.

 

8)     Boil the filtrate until half way. Don’t evaporate too much, we need some waters of crystallisation for the crystals.

 

9)     Leave to cool and crystallise.

 

10) Pick out the formed crystals with tweezers and dry between filter paper.