1. IGCSE CHEMISTRY LESSON 4

2. Section 1 Principles of Chemistry a)States of matter b)Atoms c)Atomic structure d)Relative formula mass e)Chemical formulae and chemical equations f)Ionic compounds g)Covalent substances h)Metallic crystals i)Electrolysis

3. Lesson 4 h) Metallic crystals i) Electrolysis 1.37 describe a metal as a giant structure of positive ions surrounded by a sea of delocalised electrons. 1.38 explain the malleability and electrical conductivity of a metal in terms of its structure and bonding. 1.39 understand an electric current as a flow of electrons or ions. 1.40 understand why covalent compounds do not conduct electricity. 1.41 understand why ionic compounds conduct electricity only when molten or in solution. 1.42 describe simple experiments to distinguish between electrolytes and non-electrolytes. 1.43 recall that electrolysis involves the formation of new substances when ionic compounds conduct electricity. 1.44 describe simple experiments for the electrolysis, using inert electrodes, of molten salts such as lead (II) bromide. 1.45 write ionic half-equations representing the reactions at the electrodes during electrolysis.

4. Metallic crystals + + + + + ++ + + + + + + + + + ++ + + Metal ions Free electrons Metals have a giant structure in which electrons in the highest energy level (orbit) are free to move through the whole structure. This effectively produces a regular arrangement (lattice) of metal ions in a ‘sea of electrons’.

5. Metallic crystals + + + + + ++ + + + + + + + + + ++ + + Metal ions Free electrons These free electrons:  Hold the atoms together in a regular structure  Allow the atoms to slide over each other  Allow the metal to conduct heat and electricity.

6. Metal properties PropertyExplanation Metals are good conductors of heat and electricity Delocalised electrons can move through the lattice, carrying negative charge

7. Metal properties PropertyExplanation Metals are good conductors of heat and electricity Delocalised electrons can move through the lattice, carrying negative charge Metals are malleable (can be bent or hammered into shape) and ductile (can be drawn into a wire. Layers of positive metal ions can slide past each other without breaking metallic bonds. – there will always be delocalised electrons between the positive ions even when the position of ions is changed. Metallic bonds hold the metal ions together.

8. Lesson 4 h) Metallic crystals i) Electrolysis 1.37 describe a metal as a giant structure of positive ions surrounded by a sea of delocalised electrons. 1.38 explain the malleability and electrical conductivity of a metal in terms of its structure and bonding. 1.39 understand an electric current as a flow of electrons or ions. 1.40 understand why covalent compounds do not conduct electricity. 1.41 understand why ionic compounds conduct electricity only when molten or in solution. 1.42 describe simple experiments to distinguish between electrolytes and non-electrolytes. 1.43 recall that electrolysis involves the formation of new substances when ionic compounds conduct electricity. 1.44 describe simple experiments for the electrolysis, using inert electrodes, of molten salts such as lead (II) bromide. 1.45 write ionic half-equations representing the reactions at the electrodes during electrolysis.

9. Back to a bit of basics here – what do we mean by the term “electric current”?

10. An electric current is a flow of electrons or ions. In effect, it is a flow of charge.

11. Back to a bit of basics here – what do we mean by the term “electric current”? An electric current is a flow of electrons or ions. In effect, it is a flow of charge. So, why do covalent compounds not conduct electricity, but ionic compounds (when molten or in solution) do?

13. Remember our definition of the term “electric current”?

14. An electric current is a flow of electrons or ions. In effect, it is a flow of charge.

15. Remember our definition of the term “electric current”? An electric current is a flow of electrons or ions. In effect, it is a flow of charge.

16. Ionic compounds conduct electricity (when in solution or molten) because they contain ions.

17. Covalent compounds do not conduct electricity because they do not contain ions

18. Ionic compounds conduct electricity (when in solution or molten) because they contain ions. Covalent compounds do not conduct electricity because they do not contain ions Simple, huh?

19. So, what is electrolysis?

20. Electrolysis means “splitting up with electricity”

21. It requires an electrolyte, a liquid which will conduct electriticty

22. Electrolytes conduct electricity, non-electrolytes do not.

23. Electrolyte or non-electrolyte?

25. Electrolytes are usually free ions dissolved in water, eg. NaCl solution Cl - Na+

26. Electrolytes can also be molten ionic compounds (but this involves higher temperatures) Cl - Na+ Cl - Na+ Cl - Na+

28. Cathode (-ve)Anode (+ve) ---------------------- ---------------------- ++++++++++++++++++++++++ ++++++++++++++++++++++++

29. Cathode (-ve)Anode (+ve) ---------------------- ---------------------- ++++++++++++++++++++++++ ++++++++++++++++++++++++ The electrical supply acts like an electron pump. It takes electrons away from the +ve anode and onto the –ve cathode.

30. Cathode (-ve)Anode (+ve) ---------------------- ---------------------- ++++++++++++++++++++++++ ++++++++++++++++++++++++ The electrical supply acts like an electron pump. It takes electrons away from the +ve anode and onto the –ve cathode. Ions gain or lose electrons at the electrodes and neutral atoms and molecules are released.

31. Cathode (-ve)Anode (+ve) ---------------------- ---------------------- ++++++++++++++++++++++++ ++++++++++++++++++++++++ In sodium chloride solution (brine) there are the following ions: Na + H + Cl - OH -

32. Cathode (-ve)Anode (+ve) ---------------------- ---------------------- ++++++++++++++++++++++++ ++++++++++++++++++++++++ In sodium chloride solution (brine) there are the following ions: Na + H + Cl - OH - Look what happens when electricity flows through the solution

33. Cathode (-ve)Anode (+ve) ---------------------- ---------------------- ++++++++++++++++++++++++ ++++++++++++++++++++++++ H+ Cl- Na+

34. Cathode (-ve)Anode (+ve) ---------------------- ---------------------- ++++++++++++++++++++++++ ++++++++++++++++++++++++ H H+ H Cl Na+

35. Cathode (-ve)Anode (+ve) ---------------------- ---------------------- ++++++++++++++++++++++++ ++++++++++++++++++++++++ H H Cl Na+ Each hydrogen ion gains one electron to become a hydrogen atom. Each chlorine ion loses an electron to become a chlorine atom.

36. Cathode (-ve)Anode (+ve) ---------------------- ---------------------- ++++++++++++++++++++++++ ++++++++++++++++++++++++ H H Cl Na+ +ve ions are called cations because they are attracted to the cathode -ve ions are called anions because they are attracted to the anode

37. Half-equations These explain the reactions happening at each electrode.

38. Half-equations At the cathode (-ve) At the anode (+ve) 2H + (aq) + 2e-  H 2 (g) 2Cl - (aq)  Cl 2 (g) + 2e-

39. Lesson 4 h) Metallic crystals i) Electrolysis 1.37 describe a metal as a giant structure of positive ions surrounded by a sea of delocalised electrons. 1.38 explain the malleability and electrical conductivity of a metal in terms of its structure and bonding. 1.39 understand an electric current as a flow of electrons or ions. 1.40 understand why covalent compounds do not conduct electricity. 1.41 understand why ionic compounds conduct electricity only when molten or in solution. 1.42 describe simple experiments to distinguish between electrolytes and non-electrolytes. 1.43 recall that electrolysis involves the formation of new substances when ionic compounds conduct electricity. 1.44 describe simple experiments for the electrolysis, using inert electrodes, of molten salts such as lead (II) bromide. 1.45 write ionic half-equations representing the reactions at the electrodes during electrolysis.

40. Half-equations At the cathode (-ve) At the anode (+ve) Pb 2+ (l) + 2e-  Pb (s) 2Br - (l)  Br 2 (l) + 2e- Electrolysis of molten lead bromide

41. Other examples of electrolysis Eg. Electrolysis of copper sulphate solution At the cathode (-ve)At the anode (+ve) Electrode made of carbon Ions present: Cu 2+ H + Ions present: SO 4 2- OH - Red deposit observed on electrode Bubbles of colourless gas given off which relights a glowing splint 2Cu 2+ (aq) + 4e -  2Cu4OH - (aq)  O 2 (g) + 2H 2 O (l) + 4e -

42. Other examples of electrolysis Eg. Electrolysis of dilute sulphuric acid http://www.docbrown.info/page01/ExIndChem/ExtraElectrochem.htm

43. Other examples of electrolysis Eg. Electrolysis of dilute sulphuric acid At the cathode (-ve)At the anode (+ve) Electrode made of carbon Ions present: H + Ions present: SO 4 2- OH - Bubbles of gas (twice as much as at anode). Gas burns with a squeaky ‘pop’ Bubbles of colourless gas given off which relights a glowing splint 4H + (aq) + 4e -  2H 2 (g) 4OH - (aq)  O 2 (g) + 2H 2 O (l) + 4e -

44. End of Lesson 4 In this lesson we have covered: Metallic crystals Electrolysis