What you need to be able to do and understand:• State that electrolysis is the chemical • Relate the products of electrolysis to effect of electricity on ionic the electrolyte and electrodes compounds, causing them to break up used, exemplified by the specific into simpler substances, usually examples in the Core together with elements. aqueous copper(II) sulfate using• Use the terms carbon electrodes and using copper electrode, electrolyte, anode and electrodes (as used in the refining of cathode. copper).• Describe electrolysis in terms of the • Describe the electroplating of ions present and the reactions at the metals, using laboratory apparatus. electrodes. • Predict the products of the electrolysis• Describe the electrode of a specified binary compound in the products, using inert electrodes, in the molten state, or in aqueous solution. electrolysis of: • Describe, in outline, the chemistry of • molten lead(II) bromide the manufacture of: • aqueous copper chloride, • aluminium from pure aluminium oxide in • dilute sulfuric acid. molten cryolite,• State and use the general principle • chlorine, hydrogen and sodium that metals or hydrogen are formed at hydroxide from concentrated aqueous the negative electrode (cathode), and sodium chloride. that nonmetals (other than hydrogen) are formed at the positive electrode (anode).
Before we begin with this unit we need a quickreview of electricity, conductors and insulators.Electricity is a stream of electrons in a circuit.Insulators are substances that resist a flow of electronsConductors are substances that allow the flow of electrons.The only solids that conduct are metals and graphite. To review all these we are going to practice with this animation – the circuit construction kit
So then… how do batteries work andWhat is electrolysis?Ionic substances do not conduct when solid (solidlead). But they do conduct when melted ordissolved in water – and they decompose at thesame time. This is the basis of how batteries canpower things (make electrons move). Ionic substances have IONS (something that has a charge) and electricity is the movement of charged particles- specifically electrons.Decomposition brought about by electricity is called electrolysis. Inother words, electrolysis is the chemical effect of electricity on ioniccompounds, causing them to break up into simpler substances, usuallyelements.
Electrolysis: breaking down To start here are a few animations –by electricity some fun and some serious A simple animation to show A bit on the fun side the electrolysis of water Any liquid that contains ions will conduct electricity. Now think about this… what conducts electricity better – fresh or salt water? How your car batteries work
Anatomy of a battery Flow of electronsElectronsflow fromthepositiveterminal The negative The positive endto the end is the is the anode - itnegative cathode - it connects to theterminal connects to the positive end of negative end of the battery the battery Flow of electronsThe solution inside a batteryis called the electrolyte Remember PA These rods are called electrodes Positive Anode
The electrolysis of molten lead bromide The rods are graphite Flow of electrons (carbon). They are inert (remain unchanged)At the anode (+) the Br-ions give up electrons.You can see red-brown At the cathode (-) the Pb2+bromine gas bubbles. ions accept electrons. TheBromine is a non-metal cathode looks like it is growing as lead collects on it. Lead is a metal Lead(II)bromide is the electrolyte (the solution). It has Pb2+ and Br- ions. Here the ions move to the electrode of opposite charge
What is the result? Lead(II)bromide has decomposed. The chemical reaction is: lead(II)bromide lead bromide PbBr2(aq) Pb(l) Br2(g)Things to remember:• Electrons carry current through the wires and electrodes, but the ions carry it through the electrolyte (liquid) This pattern is the SAME for all other molten ionic compounds of two elements.Electrolysis decomposes ionic compounds into its elements, giving the metal at the cathode, and the non-metal at the anode.
Let’s work together to fill in the blanks for the electrolysis of aqueous copper(II)chloride The rods are graphite Flow of electrons (carbon). They are inert (remain unchanged)At the ________ (+)the ______ ions_________ electrons. At the _________ (-) the __________ is a ions ________ electrons. _______________ is a ________ ________________ is the electrolyte (the solution). It has ______ and ______ ions. Here the ions move to the electrode of __________charge
Let’s work together to fill in the blanks for the electrolysis of aqueous copper(II)chloride The rods are graphite Flow of electrons (carbon). They are inert (remain unchanged)At the anode (+) the Cl-ions give up electrons.You can see chlorine gas At the cathode (-) the Cu2+ ionsbubbles. Chlorine is a accept electrons. The cathodenon-metal looks like it is growing as copper collects on it. Copper is a metal Copper(II)chloride is the electrolyte (the solution). It has Cu2+ and Cl- ions. Here the ions move to the electrode of opposite charge
What is the result? Copper(II)chloride has decomposed. The chemical reaction is: copper(II)chloride copper chloride CuCl2(aq) Cu(s) Cl2(g) Now on your own draw a diagram and write the equation for the electrolysis of dilute sulfuric acid (H2SO4). This will make you think…
Electrolysis of dilute dilute sulfuric acid (H2SO4).At the anode the concentration ofhydroxyl ions is too low to maintaina reaction and the sulphate ions arenot oxidized but remain on insolution at the end. Water moleculesmust be the species reacting at theanode.At the anode (+) the OH- ions give At the cathode (-) the H+ ionsup electrons. You can see oxygen accept electrons. Hydrogengas bubbles. Oxygen is a non-metal gas is given off. Hydrogen acts as a metalSulfuric acid is the electrolyte (the solution). It has H+ and SO42- ions. BUT…water is present as well. It has H+ and OH- ions. Here the ions move to theelectrode of opposite charge
What is the result? The chemical reactions are:Sulfuric acid is a strong electrolyte is fully dissociated in aqueous solution. H2SO4(aq) H+ SO42-Water is a weak electrolyte and is only slightly dissociated. H2O (aq) H+ OH-During electrolysis, the hydrogen ions migrates towards the cathode, and aredischarged there (i.e. they gain an electron and are converted to hydrogen gas). 4H+ 4e- 2H2(g)At the anode the concentration of hydroxyl ions is too low to maintain a reactionand the sulphate ions are not oxidized but remain on in solution at the end.Water molecules must be the species reacting at the anode. 2H2O (aq) O2(g) 4H+ 4e-
What are the rules for the electrolysis of a solution?At the cathode(-), either a metal or hydrogen forms.• The more reactive an element, the more it likes to ‘exist’ as ions. So if a metal is more reactive than hydrogen its ions STAY in solution (it does not form a product) and instead hydrogen gas is produced.• But if the metal is less reactive than hydrogen, the metal forms.At the anode(+), a non-metal other than hydrogen forms.• If it is a concentrated solution of a halide (something that has Cl-, Br- , I-… ions), then the respective halogen forms.• But if the halide solution is dilute, or there is no halogen (i.e. the sulfuric acid example), oxygen forms. A non-metal A metal or hydrogen forms here forms here
Now knowing the general rules determine whatforms at the anode and cathode when a solutionof copper(II)sulfate is the electrolyte andgraphite are the electrodes. At the anode (+) the OH- ions give up electrons. You can see oxygen gas bubbles. Oxygen is a non-metal. At the cathode (-) the Cu2+Copper(II)sulfate is the electrolyte. It has ions accept electrons.Cu2+, SO42-, H+, and OH- ions. The blue colour Copper coats theof the solution fades as copper ions are electrode. Copper is adischarged. metal. You may be asked to write half-equations for the reactions at the electrodes. 1. Name the ions present, and then the products 2. Write each half-equation correctly while adding symbols for the statesDo this for the abovereactionsAt the anode: 4OH-(aq) → 2H2O(l) + O2(g) + 4e-At the cathode: 2Cu2+(aq) + 4e- → 2Cu(s)
Now apply the same rules but instead of an .inert graphite electrode we can use copperelectrodes and copper(II)sulfate. At the anode (+), the anode dissolves, giving copper ions in the solution. These move to the cathode, to form copper. So copper moves from the anode to the cathode. The colour of the At the cathode (-) the Cu2+ ions solution does not fade. accept electrons. Copper still Write the half-reactions coats the electrode.At the anode: Cu(s) → Cu2+(aq) + 2e-At the cathode: Cu2+(aq) + 2e- → Cu(s)This method is the basis for two veryimportant uses of electrolysis: refiningcopper and electroplating.When you refine copper, the anode is impurecopper, while the cathode is pure. The copperin the anode dissolves and then builds up onthe cathode while the impurities drop to thefloor of the container.
This animation is about a Galvanic cell. You don’t need to know about Galvanic cells but it’s a good animation on what is going on at the molecular level at the anode and cathode when you have an active metal as the electrode. http://www.mhhe.com/physsci/ch emistry/essentialchemistry/flash/g alvan5.swfA battery is an example of aGalvanic cell. It is anelectrochemical cell in whicha spontaneous reactiongenerates a flow of current.
Electroplating means using electricity to coat one metal with another.There are many uses of this including making cheap jewellery look moreexpensive by coating it with a thin layer of sliver or gold. To electroplate: At the cathode: the object to be electroplated At the anode: metal X (the metal you want to coat the object with) Electrolyte: an aqueous solution of compound X This animation is excellent forAnode – the metal Cathode – viewing electroplatingwe want to coat the objectthe object with This is a good video on gold plating
Chemistry in action: How do you extract aluminum? Steps in aluminum extraction: 1. Mine bauxite 2. Purify bauxite to get aluminum oxide (alumina). 3. Alumina is mixed with cryolite (sodium fluoride and aluminum fluoride). This isThis video talks about done because you need a lowerthe whole process. temperature to melt the aluminum this way. (908°C vs 2040°C) 4. This mixture is then electrolysed to form aluminum metal (at the cathode) and oxygen gas is formed at the anode, where it reacts with the carbon in the anode to give carbon dioxide gas. This one is more specific onAnother good animation the electrolysis.on the electrolysis
Cryolite (added to Bauxite (mainly Oxygen and reduce melting point) in aluminum oxide) in carbon dioxide out Al3+ move towards the O2- move cathode towards the anode Graphite anodes Graphite accept cathode gives electrons, reacts electrons, to with O2 to form Al3+ to form Al(l) CO2 Write the half-reactionsAt the anode (2 reactions): Write the overall reaction:6O2-(l) → 3O2(g) + 12e- & C(s) + O2(g) → CO2(g)At the cathode: 4Al3+(l) +12e- → 4Al(l) 2Al2O3(l) → 4Al(s) + 3O2(g)
Chemistry in action #2:Electrolysis of concentrated sodiumchloride (brine, NaCl), to makeH2(g), Cl2(g) and NaOH(l) A short video At the anode (+) the Cl- ions give up electrons. You can see chlorine gas bubbles.Sodium chloride is the electrolyte. It hasNa+, Cl-, H+ and OH- ions. Here the ions At the cathode (-) the H+move to the electrode of opposite charge. ions accept electrons.When the hydrogen and chlorine bubble Hydrogen gas is given off.off, Na+ and OH- ions are left behind, so asolution of sodium hydroxide (NaOH) isformed.What are the half-reactions?At the anode: 2Cl-(aq) → Cl2(g) + 2e-At the cathode: 2H+(aq) +2e- → H2(g)