i) Electrolysis

i) Electrolysis

1.48 understand that an electric current is a flow of electrons or ions
An electric current is a flow of electrons. In order for a substance to conduct electricity, it must have charged particles that are free to move around, and as electrons and ions both fit this description, they can conduct electricity. 

1.49 understand why covalent compounds do not conduct electricity
Covalent compounds do not contain charged particles as they share electrons instead of losing or gaining them. Therefore, they do not have the mobile charged particles they need in order for an electric current to be present, so they do not conduct electricity. 

1.50 understand why ionic compounds conduct electricity only when molten or in

solution
Ionic compounds do not conduct electricity when solid as they are not free to move around. However, they do conduct electricity when molten or in solution as the ions are free to move around and able to move to the oppositely charged electrode. 

1.51 describe experiments to distinguish between electrolytes and non electrolytes
1)Set up an electric circuit with an LED and a break in the wire.
2) Put both ends of wire into a solution/molten substance.  

Then, if the LED lights up there is a current flowing as this will only occur if the solution is conducting, so it must be an electrolyte. On the other hand, if the LED does not light up then this means that there is no current flowing and the solution is not conducting electricity, so it must be a non electrolyte.

1.52 understand that electrolysis involves the formation of new substances when
ionic compounds conduct electricity

In order for electrolysis to occur, ionic compounds must conduct electricity. The positively charged ions (cations) will move to the negative electrode (cathode) and the negatively charged ions (anions) will move to the positive electrode (anode.) Having moved to their respective electrodes, the ions will then become atoms as electrons will be lost at the cathode and gained at the anode, reverting them back to molecules and therefore forming new substances. 

1.54 describe experiments to investigate electrolysis, using inert electrodes, of aqueous solutions such as sodium chloride, copper (II) sulfate and dilute sulfuric acid and predict the products.

• An inert electrode is one that helps the electrolysis but is not used up in the reaction itself. 
• In aqueous solutions, there are H+ (hydrogen) and OH- (hydroxide) ions present from the water, as well as the ions from the ionic compound. 
• At the cathode, if H+ ions and metal ions are present, hydrogen gas will be produced if the metal ions are more reactive than the H+ ions. If the H+ ions are more reactive than the metal ions, then a solid layer of the pure metal will be produced instead.
• At the anode, if OH- and halide ions (Cl-, Br-, I-) are present, molecules of chlorine, bromine or iodine will be formed. If there are no halide ions present, then oxygen will be formed. 

For example, a solution of sodium chloride (NaCl) contains four different ions: Na+ and Cl- from the ionic compound and OH- and H+ ions from the water. 

• At the cathode, negatively charged ions lose electrons. These negatively charged ions are the sodium ions, which lose electrons that are given to the hydrogen ions. Therefore, the sodium (metal) is the more reactive of the two and following the rule above, this means that we can expect hydrogen gas to be produced. 
• At the anode, positively charged electrons gain ions. These positively charged ions are the hydroxide ions, which gain electrons as the chloride ions lose them. As there are both OH- ions and halide ions (Cl- in this case) present, then we can expect chlorine gas to be produced at the anode. 

1.55 write ionic half-equations representing the reactions at the electrodes during electrolysis

A ionic half-equation shows you what happens at one of the electrodes during electrolysis. A half-equation should be balanced by adding or taking away the number of electrons equal to the number of charges on the ions in the equation. 

So, Pb2+ 2e- →  Pb (electrons are represented by 'e' in the equation.) 

This is the ionic half-equation that shows what happens at the cathode when molten lead bromide (PbBr2) is electrolysed. The positive Pb+ ions are attracted to the negative cathode., where a lead ion accepts two electrons to become a lead ion. This will form molten lead that will sink to the bottom.

The equation for the reaction at the anode = 2Br- → Br2 + 2e-

This negative Br- ions are attracted to the positive anode, where two bromide ions lose one electron each and become a bromine molecule. This causes brown bromide gas to form at the top of the anode. 

1.56 recall that one faraday represents one mole of electrons 

96500 coulombs is one faraday. One faraday contains one mole of electrons.

1.57 calculate the amounts of the products of the electrolysis of molten salts and aqueous solutions

Some molten lead (II) chloride (PbCl2) is electrolysed for 20 minutes. The current flowing is 5 amps. Find the mass of the lead produced. 

1) Write out the ionic half-equation: Pb2+ +2e-  → Pb 

2) Calculate the number of faradays: Coloumbs = current (amps) x time (s) 

                                                                            = 5 x (20x60)  (20 x 60 to find number of time in seconds overall)

                                                                             = 6000 coloumbs 

 Number of faradays = coloumbs / 96000 (96000 coloumbs = one faraday)

                                = 0.0625 faradays

3) Calculate the number of moles of lead produced. To do this, you must divide the number of faradays by the number of electrons in the half-equation.) 

So, faradays / number of electrons = 0.0625/2

                                                           = 0.02125 moles of lead atoms. 

4) Substitute the Mr values from the periodic table to work out the mass of solid lead produced. 

Mass of lead = Mr x No. of moles 

                      = 207 x 0.03125 

                      = 6.5g (1d.p.)