C4: Chemical Changes

Metal Ores

  • Metals are found in costal areas

  • Depending on the reactivity of the metal, they are often chemically combined with another element such as oxygen

  • We must separate them in order to obtain the pure metal

  • Reactivity of Metals

Acids and Bases

  • A base is a substance that will neutralise an acid. An alkali is a soluble base.

  • Not all bases are soluble

HCl

Hydrochloric acid

Chloride

HNO3

Nitric acid

Nitrate

H2PO4

Sulphuric acid

Sulphate

H3PO4

Phosphoric acid

Phosphate

  • Acid + Base —> Salt + water

  • Acid + Metal —> Salt + Hydrogen

  • Acid + Any Metal Carbonate —> Salt + carbon dioxide + water

  • e.g. Hydrochloric Acid + Sodium Hydroxide —> Sodium Chloride + Water

  • e.g. Nitric Acid + Calcium Carbonate + Calcium Nitrate + Carbon Dioxide + Water

Required Salt Practical

  • Independent variable - the one you change

  • Dependent variable - the one you measure / record

  • Control variables - the ones you keep the same - make sure test is valid

Method

  1. Measure 20ml of 0.5M HCl into a measuring cylinder and pour into a beaker

  2. Weigh 5g of Sodium Oxide into the beaker and stir until no more can be dissolved

  3. Heat gently with a Bunsen burner, then remove solution

  4. Filter solution into an evaporating dish and place back on the heat

  5. Heat solution until 2/3 has evaporated

  6. Remove dish from heat and wait until it has evaporated at room temp

  7. Dry crystals with a paper towel

Strong and Weak Acids

  • The formation of the H+ ions is the reason something will be acidic

  • pH = potential of H+ the H+ ions only form when the acid is dissolved in water, called an aqueous solution

  • e.g. HCl —> H+ + Cl

  • The formation of OH- ions in water is the reason something will be alkaline

  • The OH- ions only form when the base is dissolved in water, called an aqueous solution

  • e.g. H+ + OH- —> H2O

Electrolysis

  • Breaks down ionic compound (salts) into their elements using electricity

  • The solution need to be molten so the ion can move to the electrodes

  • Ions must be free to move, for current to flow

  • Electrical current = movement of charge

  • Positive electrode = Anode

  • Negative electrode = Cathode

  • Positive Anode Negative Is Cathode - PANIC

Diagram

  • During electrolysis of molten ionic compounds, the metal element is formed at the cathode and the non metal at the anode

Aluminium Extract

  • Aluminium is found naturally within the ore bauxite

  • Pure Al2O3 (alumina) is purified from bauxite and melted (with cryolite)for electrolysis

  • The melting point of alumina is 2070oC. The temperature and energy cost is too high

  • Alumina is mixed with cryolite (a mineral) which reduces the melting point of the mixture and Al2O3. Lowering the temperature (and energy requirements) for ions to move.

Electrolysis of Solutions

  • During electrolysis of solution (aq), a small percent of the the H2O molecules break up into ions: H2O —> H+ + OH-

  • These ions could be discharged (a process where ion gain or lose electrons to form neutral atom or molecules) in electrolysis of ionic compound dissolve in water.

Cathode (negative)

The ions of the least reactive element will be discharged

Anode (positive)

If the negative ion is a Halide (Cl, Br, I) it is discharged

If any other anion is present, O2 gas will be produced from the OH- ions

The half equation: 4OH- —> O2 + 2H2O + 4e-

Electrolysis Required Practical

  • Independent variable - different metals ions in the electrolyte e.g. copper chloride and sodium chloride

  • Dependent variable - products of electrolysis using observations and test for gases

  • Control Variables - volume and concentration