eelctrolysis

Electrolysis

The decomposition/splitting of an ionic compound when molten or in an aqueous solution by passing an electric current through it

Electric current

Causes a chemical reaction that breaks down the ionic compound.

Electrolyte

an ionic compound that conducts electricity and undergoes electrolysis when it is molten or in solution

Electrodes

Rods that carry electric current to and from the electrolyte,point where the electric curretn enters an dleaves the electrolytic cell..attached to positive terminal of battery is called the anode when attached to the negative terminal of the battery is called cathode.

Positive ions

attracted to cathode.

Negative ions

attracted to anode.

Inert electrode

Unreactive that carry the current but other than that are unchanged and won't that won't interfere with the electrolysis. Suppleirs and acceptors of electrons only.

Brine

Concentrated aqueous solution of sodium chloride, obtained from seams of rock salt underground.

Electrolysis of brine produces...

Chlorine, hydrogen, sodium hydroxide.

overall reaction of brine

2NaCl + 2H2O --->2NaOH + Cl2 + H2

what si the anode and cathode made of in electrolysis of brine?

titanium anode and nickel cathode

what si produced at the anode and cathode in electrolysis of brine?

Cathode: hydrogen 2H+ +2e- -->H2 (reduction) Anode: chlorine gas 2Cl- ---> Cl2 +2e- (oxidation)

Brine is electrolysed in...

in a membrane/diaphragm cell made of pores/asbestos to let ions through but jeep gases apart

At the anode...

Ions lose electrons.

cations

positive ions

At the cathode...

Ions gain electrons.

Half-equations

Ionic equations that only show what is happening at only one of the electrodes.

Define a conductor?

A material that allows heat and electricity to pass through it.

Define an insulator?

A material that does not allow heat or electrons to move through it easily.

non-electrolytes are ......

Liquids that do not conduct electricity. These are covalent compounds. When you try to electrolyse them tehre wouldnt eb a current flow bcs there arent any ions so it wont be decomposed by electricity

An anion is ..........

A negatively charged ion

What forms at the cathode?

either a metal or hydrogen

what forms at the anode

a non metal other than hydrogen

what is brine

Concentrated sodium chloride solution

what forms at the cathode in electrolysis of lead (II) bromdie?

lead II ions Pb2+ + 2e- ---> Pb

what forms at the anode in electrolysis of lead (II) bromide?

bromine-red/brown gas appears 2Br- -->Br2+2e-

Does oxidation occur at the anode or cathode?

Oxidation occurs at the anode. Oxidation is the loss of electrons.

where is aluminium extracted from? and its impurities

extracted form the ore bauxite which contains aluminium oxide.Main impurity of bauxite is Iron (III) oxide which colours the ore red

how to purify ore of bauxite to extract aluminium oxide?

First purify ore by crushing and then mixing with sodium hydroxide. Aluminium oxide reacts with the sodium hydroxide to form sodium aluminate solution. Al2O3 + 2NaOH-->2NaAlO2+H2O The iron III oxide and other impurities insoluble and can be seperated by filteration. Sodium aluminate is hten heated to produce pure aluminium oxide

Why is a mixture of aluminium oxide and cryolite used, rather than aluminium oxide alone?

Before electrolysis the aluminium oxide is dissolved in molten cryolite to reduce the working/meltign temperature and to improve electrical conductivity of aluminium oxide. This makes process mroe economically and energetically sustainable+ reduces soem energy costs

Why during the extraction of aluminium from aluminium oxide, do the anodes need to be replaced during ?

Since the electrodes are made of carbon/graphite. At high temperature the oxygen produced reacts with carbon/graphtie anode formign cabron dioxide gas. Anode gets used up and has to be repalced periodically. C+ O2--> CO2

describe extraction of aluminium from alumina:

Aluminium oxide dissolved in molten cryolite to reduce the working/meltign temperature and to improve electrical conductivity of electrolye. The cathode and anode are made out of carbon/graphite. The anode gets used up and has to be replased periodically. Cathode: Al3+ + 3e- -->Al aluminium metal formed and is drained out Anode: 2O2- -->O2+4e- oxygen formed whihc reacts with carbon/graphite forming carbon dioxide

During electrolysis what do you understand by the term 'discharge'?

loses its charge, becomes neutral

Which cation (metal or hydrogen) gets discharged during electrolysis of an aqueous solution? +fromula

the least reactive if CU,Ag,Au present it is released otherwise hydrogen produces 2H+ + 2e- --->H2

Which anion (OH- or halide) gets discharged during electrolysis of an aqueous solution? +formula

usually the halide ion (unless very dilute) if not oxygen 4OH- --->O2+2H2O+4e- or

2H2O —> 4H+ + O2 + 4e-

What forms at the anode

a non metal other than hydrogen

What is brine

Concentrated sodium chloride solution

Give 2 uses for hydrogen

To manufacture margarine In the Haber process

Give 2 uses for sodium hydroxide

• Used to make soap

• Manufacture bleach (with chlorine)

Explain the method of Electrolysis of Molten Compounds/SALTS e.g: Lead (II) Bromide:

Method: • Add Lead (II) Bromide into a beaker and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge.

• Add two graphite rods as the electrodes and connect this to a power pack or battery.

• Turn on power pack or battery and allow electrolysis to take place.

• Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off.

• Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode.

What are the rules for Electrolysis of Aqueous Solutions:(2)

Aqueous solutions will always have water (H2O). • H+ and OH- ions from the water are involved as well.

What happens at the anode in Electrolysis of Aqueous Solutions

Positive Electrode (anode) • OH- ions and non-metal ions attracted to positive electrode.

• Either OH- or non-metal ions will lose electrons and oxygen gas or gas of non- metal in question is released E.g. Chlorine, Bromine, Nitrogen.

• The product formed depends on which ion loses electrons more readily, with the more reactive ion remaining in solution. A reactivity series of anions is shown below:

o More reactive halides eg chlorine,bromine or iodine

What happens at the cathode in Electrolysis of Aqueous Solutions?

• H+ and metal ions attracted to the negative electrode but only one will gain electrons.

• Either hydrogen or metal will be produced.

Concentrated and dilute solutions

• Concentrated and dilute solutions of the same compound give different products.

• For anions, the more concentrated ion will tend to get discharged over a more dilute ion.

What is the method of the Electrolysis of Aqueous Solutions

What are the reactions at the electrodes? See notes!!!!

Method: • Add aqueous solution into a beaker.

• Add two Graphite rods as the electrodes and connect this to a power pack or battery.

• Turn on power pack or battery and allow electrolysis to take place.

NNB!!! Predict the products of electrolysis of a specified halide in dilute or concentrated aqueous solution

Electrolysis of halide solutions • We have seen that cations that are lower down on the reactivity series tend to be discharged in preference to more reactive cations.

• The same occurs for anions which can be arranged in order of ease of discharge: o More reactive SO42- → NO3- → OH- → Cl- → Br- → I- Less reactive

• E.g: in a concentrated aqueous solution of barium chloride, the Cl- ions are discharged more readily than the OH- ions, so chlorine gas is produced at the anode.

• If the solution is dilute however only the OH- Ion is discharged and so oxygen would be formed.

NNB!!! Describe the transfer of charge during electrolysis to include:

• the movement of electrons in the metallic conductor

• the removal or addition of electrons from the external circuit at the electrodes

• the movement of ions in the electrolyte

Draw a diagram to help!!!!

Transfer of charge • During electrolysis the electrons move from the power supply towards the cathode.

• Positive ions within the electrolyte move towards the negatively charged electrode which is the cathode.

• Here they accept electrons from the cathode and either a metal or hydrogen gas is produced.

• Negative ions within the electrolyte move towards the positively charged electrode which is the anode.

• If the anode is inert (such as graphite or platinum), the ions lose electrons to the anode and form a non-metal or oxygen gas.

• If the anode is a reactive metal, then the metal atoms of the anode lose electrons and go into solution as ions, thinning the anode.

What halogen gases are:

• red-brown

• yellow-green

• pale yellow).

bromine is red-brown, chlorine is yellow-green fluorine is pale yellow

The student records his observations of electrolysing a pure sample of molten lead(ii) bromide:

small blob of silvery liquid forms at one electrode

brown substance forms at the other electrode

the lamps stops working soon after the teacher stops heating the lead

which statement is correct a) brown substance is bromide b)the products are both elements c) the silvery liquid forms at pos. electrode d) the silvery liquid is molten lead(ii) bromide

a) bromine NOT bromide is formed b) Correct c) silvery liquid is a metal and will form pos ions, thus attrated to neg electrode d) this is an element not a compound

why does the lamps stop working soon after the teacher stops heating the lead?

the ions are no longer free to move.

A diagram shows the diaphragm cell used in the electrolysis of concentrated sodium chloride solution NaCl(aq)

identify gas A(pos elec), gas B( neg elec)and solution C

Gas A-Chlorine Gas B -Hydrogen Solution C- Sodium hydroxide

Sodium is manufactured by the electrolysis of molten sodium , NaCl(i).

Sodium is produced at neg. electrode and chlorine(pos electrode).

Why does sodium chloride have to be molten before it conducts electricity?

the ions need to be free to move

Write the ionic half equation for the formation of chlorine from chlorine ions

Na+. +e-. ---- Na

2Cl-. -2e- ---Cl2

2Cl-. ----Cl2+2e-

The electrolysis of sodium solution is an industrial process. Why do chloride ions move to the positive electrode?

they are oppositley (neg) charged thus attracted to pos. electrode.

Sodium chloride solution contains two types of positive ions, sodium and hydrogen ions. Why is hydrogen produced at neg electrode and not sodium?

Hydrogen is LESS reactive than sodium

Solution x is alkaline. Which ion makes solution x alkaline?

oh-

In an electrolysis experiment, a solution contained a mixture of ionic compounds.

Name the particles which carry electric current through a) metal wires b) solution

a) delocalised electrons b) ions

how can the amount of metal deposited at the cathode be increaseed?

increase electrons availble fro reduction by: -increasing current-increase amount of charge so more metal ions reduced in teh same time -increasing time-more metal ions can be reduced

how to set up an electrochmcial cell

• An electrochemcial cell is a circuit made up of the andoe,cathode,eelctrolyte, a power source and the wires that conenct the two eelctrodes. Heres how you set up an electrochemcial cell for an aqueous solution:

  1) Get two inert electrodes eg graphite or platinum electrodes

  2) Clean the surfaces of the electrodes usign some emery paper (or sandpaper)

  3) Be careful not to touch the surfaces of the electrodes with your hands-you could transfer grease back onto the strips

  4) Place both electrodes into a beaker filled with your electrolyte

  5) Connect the electrodes to a power supply using crocodile clips and wires. when you turn the power supply on, ac urrent will flow throguht eh cell.

electrolysis of dilute sulfuric acid using inert electrodes?

Cathode: hydrogen 2H+ + 2e- —> H2

Andoe: sufate ions and hydroxide ions from water arrive. Oxygen produced form oxidation of hydroxide ions from the water.

4OH- —> 2H2O + O2 + 4e-

twice as much hydrogen is produced as oxygen. for every 4 electrons that flwo aroudn the circuit you would get 1 molecule of oxygen. But 4 electrons would produce 2 moelcules of hydrogen.

When we do thsi experiemnt the amount fo hydrogen we obtain is mroe than twice as mcuh oxygen as oxygen si mroe solubel in water. We can carry out electrolysis experiemnt for a few minutes frist in order to satureate the wate rwith oxygen and then start to colelct the gases

reduction suign carbon with metal oxide

When you heat a metal oxide with carbon, the carbon removes the oxygen.

Metal oxide + carbon 🡪 metal + carbon dioxide

Lead oxide + carbon 🡪 lead + carbon dioxide

This is called : REDUCTION