CAMBRIDGE-IGCSE CHAPTER 6- ELECTROCHEMISTRY

Material about electrochemistry for upper secondary level. The focus of this PPT is electrolysis. The voltaic cell will be explained in different PPT. It contains the explanation about electrochemistry such as electrolysis. It also described the reaction happen in electrochemistry. One of the application of electrolysis is electroplating. The process can be used to increase the appearance of metal such as gold or silver.

1. CHAPTER 6 ELECTROCHEMISTRY

2. LEARNING OBJECTIVES Describe metals as electrical conductors and non- metalic materials as non-conducting insulators. Define electrolysis and identify the components of an electrolytic cell. Describe and predict the electrolysis products of binary compounds in the molten state. Describe the electrolysis of concentrated sodium chloride solution and dilute sulfuric acid using inert electrodes. Describe how electroplate a metal object. State that hydrogen-oxygen fuel cell generate electricity.

3. ELECTRICITY: a stream of electrons. Electric current is the flow of charged particles. This usually refers to electrons , but it could also mean the flow of ions. CONDUCTOR: material that allow electricity to pass through or allow electrons to flow. INSULATOR: material that do not allow electricity to pass through or do not allow (inhibit) electrons to flow.

4. CONDUCTIVITY OF IONIC COMPOUNDS Ionic compounds can conduct electricity in the  molten  state or in  solution  as they have ions that can move and carry charge. They cannot conduct electricity in the solid state as the ions are in fixed positions within the lattice and are unable to move.

5. CONDUCTIVITY IN LIQUIDS Any liquids that contains ions will conduct electricity. Because the ions are free to move . The process in which an electric currents flows through a liquid compound or solution is called electrolysis . Electrolysis : breaking down by electricity. Electrolysis in liquid compound or solutions produces a chemical reaction .

6. Testing substances to see if they conduct

7. ELECTROLYSIS ELECTROLYTES : a chemical substance which contains free ions and carries electric current and is the substance that you are going to split into ions using electrolysis. NON-ELECTROLYTES : liquid that do not conduct electricity. ELECTRODE  is a rod of metal or graphite through which an electric current flows into or out of an electrolyte. ANODE  is the positive electrode of an electrolysis cell. ANION  is a negatively charged ion that is attracted to the anode. CATHODE  is the negative electrode of an electrolysis cell. CATION  is a positively charged ion that is attracted to the cathode.

8. Electrolysis works using a practical setup, like the one below. This is called an electrolytic cell. 

9. The molten lead bromide contains lead ions ( Pb 2+) and bromide ions ( Br 2 - ). This shows what happens when the switch is closed:

10. The result is that lead(II) bromide has decomposed: Lead(II) bromide → lead + bromine PbBr 2 (l) → Pb (l) + Br 2 (g) Note that: Electrons carry the current through the wires and electrodes. But the ions carry it through the liquid. The graphite electrodes are inert. They carry the current into the liquid, but remain unchanged.

11. Differences between conductivity in solids and electrolytes. CONDUCTIVITY IN METALS Electrons flow A property of elements (metals, and carbon as graphite) and alloys Take place in solids and liquids. No chemical change takes place. ELECTROLYTIC CONDUCTIVITY Ions flow A property of ionic compounds Takes place in liquids and solutions (not solids) Chemical decomposition takes place.

12. PRODUCTS OF ELECTROLYSIS 6.2

13. The electrolysis of other molten compound Electrolysis breaks the molten ionic compound down to its elements, giving the metal at the cathode and the non-metal at the anode.

14. The electrolysis of aqueous solutions Electrolysis can also be carried out on solutions of ionic compounds in water, because the ions in solutions are free to move. But the result may be different than molten compound because the water itself produces ions. The water splits into ions H 2 O(l) → H + ( aq ) + OH - ( aq ) These ions also take part in the electrolysis

15. 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 and hydrogen bubbles off. But if the metal is less reactive than hydrogen, metal forms.

16. At the anode (+), a non-metal other than hydrogen forms . If it is a concentrated solutions of a halide ( a compound containing Cl - , Br - , or I - ions), then chlorine, bromine, or iodine form. But if the halide solutions is dilute, there is no halide, oxygen forms.

17. What happens in the molten l ead bromide?

18. For a concentrated solution of sodium chloride When the hydrogen and chlorine bubble off, Na + and OH - ions are left behind, so a solution of sodium hydroxide is formed.

19. For a dilute solution of sodium chloride When the hydrogen and oxygen bubble off, the Na + and Cl - ions are left behind. So a solution of sodium chloride are formed. The overall result is that water has been decomposed.

20. ELECTROPLATING Electroplating is the process of using electrodeposition to coat an object in a layer of metal. An unreactive metal can be coated on the surface of cathode by electrolysis. The basic rules for electroplating: The object to be plated must be made the cathode. The electrolyte must be a solution of a salt of metal to be plated on the object. The anode is made of a strip of metal to be plated on the object.

21. HYDROGEN FUEL CELL 6.3

22. HYDROGEN-OXYGEN FUEL CELL A fuel is a substance which releases energy when burned Hydrogen is used as a fuel in rocket engines and in fuel cells to power some cars A fuel cell is an electrochemical cell in which a fuel  donates  electrons at one electrode and oxygen  gains  electrons at the other electrode H 2  → 2H +  + 2e –   O 2  + 4e –  → 2O 2–   The  hydrogen-oxygen  fuel cell produces electricity by combining both elements, releasing energy and water The overall equation for the reaction within a hydrogen fuel cell is: hydrogen   +   oxygen   →   water 2H 2  + O 2  → 2H 2 O The diagram below shows the setup of a hydrogen fuel cell  The air entering provides the oxygen The fuel entering is hydrogen The only chemical product made is  water 

23. Advantages & disadvantages of hydrogen fuel cells Extended tier only Hydrogen-oxygen fuel cells are becoming more common in the automotive industry to replace petrol or diesel engines. Advantages Hydrogen can be produced from water so the process is  renewable They  do not produce any   pollution : the only product is water whereas petrol engines produce carbon dioxide, and oxides of nitrogen They release  more   energy  per kilogram than either petrol or diesel No power is lost in transmission as there are no moving parts, unlike an internal combustion engine Quieter so less noise pollution compared to a petrol engine

24. Disadvantages Hydrogen obtained by methods that involve: The combustion of fossil fuels releases carbon dioxide and other pollutants into the atmosphere The electrolysis of water requires large amounts of electricity to produce Materials used in producing fuel cells are  expensive Hydrogen is more difficult and expensive to store compared to petrol as it is very flammable and easily explodes when under pressure  Fuel cells are affected by  low  temperatures, becoming less efficient There are only a small number of hydrogen filling stations across the country