Chemistry - EDEXCEL IGCSE - Chemistry of the elements (part 2)

CHEMISTRY OF THE ELEMENTS (PART 2)

Properties of CO₂:

How to collect CO₂:

1. Calcium carbonate (marble chips) placed in a flat-bottom flask
2. Hydrochloric acid poured through a thistle funnel
3. CO₂ collected through upward displacement of air as CO₂ is more dense than air

Thermal decomposition of metal carbonates:

Copper (II) carbonate (green powder) to Copper oxide (black powder):

CuCo₃(s) —> CuO + CO₂

Carbon dioxide is a colourless and odourless gas that is denser the air and slightly soluble int water. It is used in carbonated drinks. It is more soluble in higher pressures so when you open the can, as the pressure falls the gas bubbles out. It is also used in fire extinguishers as it is more dense than oxygen and so prevents oxygen from reaching, thereby stopping the fire.

Acid rain is formed when acidic air pollutants such as sulfur dioxide and nitrogen dioxide dissolve in rain water. This increases the rate of erosion to marbles and limestone, destroys marine life and causes stunted growth as they important nutrients.

Hydrogen and Water:

1. Magnesium + Hydrochloric acid —> Magnesium chloride + Hydrogen

    Magnesium + Sulphuric acid —> Magnesium sulphate + Hydrogen

     Mg + 2HCl —> MgCl₂ + H₂

     Mg + H₂SO₄ —> MgSO₄ + H₂

      Rapid effervescence (gas releasing), exothermic, H₂ released, acidic solution left

2.  Aluminium + Hydrochloric acid —> Aluminium chloride + Hydrogen 

     Aluminium + Sulphuric acid —> Aluminium Sulphate + Hydrogen

     2Al + 6HCl —> 2AlCl₃ + 3H₂

     2Al + 3H₂SO₄ —> Al₂(SO₄)₃ + 3H₂

     Effervescence, H₂ released

3. Same occurs with zinc only slower

4. Copper does not react with acid as it is less reactive than hydrogen meaning it cannot displace     it.

Combustion of Hydrogen:

2H₂ + O₂ —> 2H₂O

Anhydrous Copper (II) Sulphate (white powder) reacts with water to create Hydrous Copper (II) Sulphate (blue powder). This is one of the tests for water.

The only water to test if water is pure, however, is to see if it boils at 100°C and freezes at 0°C.

Reactivity series:

Potassium - K

Sodium - Na

Lithium - Li

Calcium - Ca

Magnesium - Mg

Aluminium - Al

Carbon - C

Zinc - Zn

Iron - Fe

Tin - Sn

Lead - Pb

Hydrogen - H

Copper - Cu

Mercury - Hg

Silver - Ag

Gold - Au

Platinum - Pt

The order of the reactivity series is deduced on how violently they react with water and dilute acid. This can be seen from the speed bubbles are produced. If it does not react with cold water then it’s below magnesium. If it reacts in acid when warmed then it is above hydrogen.

A more reactive substance will displace a lesser one in a displacement reaction. this can be used to deduce the order using metal oxides with metals and metal salts with metals.

Redox reactions:

Oxidation - the gaining of oxygen, or the loss of an electron

Reduction - the losing of oxygen, or the gain of an electron

Redox - a reaction involving both oxidation and reduction 

e.g. Mg + CuO —> MgO + Cu

Oxidising agent - something that causes oxidation. CuO, in this case, is the oxidising agent.

Reducing agent - something that causes reduction. Mg in this case.

Rusting:

Iron rusts in the presence of oxygen and water. It may be sped up by electrolytes such as salts.

To prevent rusting, you can:

1. Coat the iron with paint, oil, grease or plastic to prevent oxygen or water from reaching the iron.
2. Galvanising - covering the iron with a layer of zinc. This means that even if a hole is created, the iron would not be touched the zinc would have to corrode first since zinc is more reactive.
3. Sacrificial Protection - this is the same as galvanising except with other metals such as aluminium.

Testing for ions and gases:

Flame test:

Place a small amount of the metal on a wire and burn using a bunsen burner. The colour of the flame should indicate what metal it is.

Lithium - red

Sodium - orange

Potassium - lilac

Calcium - brick - red

Using sodium hydroxide:

Add some sodium hydroxide to the solution. The colour of the solution formed should indicate what the substance.

Copper - blue gelatinous precipitate

Iron (III) - orange-brown gelatinous precipitate

Iron (II) - green gelatinous precipitate

Ammonium - solution does not have colour, a smell of ammonia, gas turns damp litmus paper blue

The above were all tests for cations (positive ions). The following are for anions (negative ions).

Testing for chlorides bromides and iodides:

1. Add nitric acid to remove other substances apart from the substance you are testing for.
2. Add silver nitrate
3. Chlorides - white precipitate

      Bromides - cream precipitate

      Iodides - yellow precipitate

Testing for sulphates:

1. Add HCl to remove other substance such as carbonates
2. Add barium chloride
3. Sulphates - white precipitate

Testing for carbonate:

1. Add nitric acid
2. If CO₂ is formed then carbonate is present

The following are tests for gases…

1. Hydrogen - squeaky pop heard when a it splint is placed in the gas
2. Oxygen - relights a glowing splint 
3. CO₂ - turns limewater from colourless to cloudy
4. Ammonia - turns damp litmus paper blue
5. Chlorine - turns damp litmus paper red