1. A simple chemical cell can be set up using copper and zinc electrodes. Here, we’ll show you how it works.

2. We’ll construct this cell. We start with two empty containers… containers

3. We place a strip of copper metal in one of them. We call this a copper electrode. copper electrode

4. A metal is composed of neutral atoms. We’ll show (click) a few neutral copper atoms on this electrode. Cu copper electrode

5. In the other container, we’ll place a piece of zinc metal, which we call the zinc electrode. Cu zinc electrode

6. We’ll show a few neutral zinc atoms (click) on this electrode. Zn Cu zinc electrode

7. We obtain an ammeter. A ammeter measures the rate of flow of electrons, or current. Zn Cu ammeter A

8. We’ll attach the ammeter to each electrode using conducting wires. Zn Cu + – + – A wires

9. wires are composed of neutral atoms, which have the same number of protons as electrons. Zn Cu Wires are composed of neutral atoms + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

10. Because protons are positive, we’ll represent protons in the wires by positive charges Zn Cu protons + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

11. And electrons are negative, so we’ll represent electrons by negative charges. Zn Cu electrons + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

12. Protons in all materials are in fixed positions in the nuclei of atoms, so they don’t move in the wires. Zn Cu protons are in fixed positions + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

13. But in metals, electrons can move easily Zn Cu + + ++++++++++++++++ + + – – – – – – ++ ––––––– ––––––––– A electrons

14. In the container with the copper electrode, we add some copper(II) nitrate solution Zn Cu Cu(NO 3 ) 2 + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

15. Copper(II) nitrate consists of (click) copper 2 plus ions and (click) nitrate ions. These ions are free to move around in the solution. Zn Cu Cu(NO 3 ) 2 + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

16. The three copper 2+ ions we’ve represented here, have total charge of positive 6. Zn Cu + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A +6

17. And the six nitrate ions we’ve represented have a total charge of negative 6, so at this point, charges are balanced in this solution. Zn Cu + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A –6

18. In the beaker with the zinc electrode, we add zinc nitrate solution. Zn Cu Cu(NO 3 ) 2 Zn(NO 3 ) 2 + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

19. Zinc nitrate solutions consists of (click) zinc 2 plus ions and (click) nitrate ions Zn Cu Cu(NO 3 ) 2 Zn(NO 3 ) 2 + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

20. The zinc 2+ ion we’ve represented here has a charge of positive 2. Zn Cu Cu(NO 3 ) 2 Zn(NO 3 ) 2 + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A +2

21. And the two nitrate ions we’ve represented have total charge of negative 2. So at this point, charges are also balanced in this solution. Zn Cu Cu(NO 3 ) 2 Zn(NO 3 ) 2 + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A –2

22. Between the two containers we add (click) an inverted U tube with a solution of a salt like potassium nitrate, KNO 3. Zn Cu Cu(NO 3 ) 2 Zn(NO 3 ) 2 KNO 3 + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

23. We call this a salt bridge. Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 KNO 3 + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

24. KNO3 solution consists of K plus and NO3 minus ions. Like all ions in solutions, these ions are free to move. Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ KNO 3 + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

25. Now we’ll focus on the zinc electrode and have a look at one of the zinc atoms. Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– – + – + – A

26. – This zinc atom loses two electrons and changes into a zinc 2 plus ion, as shown by the equation Zn gives Zn 2+ plus 2 electrons. This ion then (click) leaves the metal and is dissolved in the solution. Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – A – Zn – Zn  Zn 2+ + 2e –

27. – – – The electrons supplied by the zinc push the other electrons through the wire (click twice while watching) Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – A Zn  Zn 2+ + 2e –

28. – – Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Zn  Zn 2+ + 2e –

29. – Meanwhile, over at the copper electrode, a copper 2+ ion moves to the surface of the electrode. Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – – – – A Zn  Zn 2+ + 2e –

30. – The two extra electrons on the copper electrode (click), move onto the Copper 2+ ion Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A – – Zn  Zn 2+ + 2e –

31. – The copper 2+ ion gains these electrons and changes into a neutral copper atom, as shown by the equation Cu 2+ plus 2 electrons forms Cu. Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A – Cu – Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e –

32. – This whole process keeps repeating itself, causing electrons to continuously move through the ammeter. Zn Cu salt bridge Cu(NO 3 ) 2 Zn(NO 3 ) 2 K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e –

33. – Now, we’ll focus on the ions in the container with the copper(II) nitrate solution Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – Cu(NO 3 ) 2

34. – In our example, one copper(II) ion was used up. Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – Cu(NO 3 ) 2 used up

35. – The two copper ions that now remain, have a total charge of positive 4. Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – +4

36. – And the six nitrate ions have total charge of negative 6. Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – –6

37. – So there is an excess of negative charge in the solution in the container on the left. Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – excess of negative charge

38. – Now, we’ll focus on the container on the right, with the zinc nitrate solution. Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – excess of negative charge Zn(NO 3 ) 2

39. – In our example, one zinc 2+ ion was produced… Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – excess of negative charge Zn(NO 3 ) 2 produced

40. – The two zinc 2+ ions that are now present, have total charge of positive 4 Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – excess of negative charge +4

41. – And the two nitrate ions have a total charge of negative 2. Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – excess of negative charge –2

42. – So there is an excess of positive charge in the solution in the container on the right. Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – excess of negative charge excess of positive charge

43. – In order to balance charges, positive ions tend to move (click) toward the left through the salt bridge, away from the side with excess positive charge and toward the side with excess negative charge. Zn Cu salt bridge K+K+ K+K+ K+K+ K+K+ K+K+ + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – excess of negative charge excess of positive charge K+K+

44. – And negative ions tend to (click) move toward the container on the right, away from the side with excess negative charge and toward the side with excess positive charge. Zn Cu salt bridge + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – excess of negative charge excess of positive charge K+K+ K+K+ K+K+ K+K+ K+K+ K+K+

45. – So the salt bridge is an important part of a chemical cell. It allows positive and negative ions to move through it so that the charges in the solutions remain balanced. Without a salt bridge, a chemical cell would not work. Zn Cu salt bridge + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – A Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Allows ions to move so charges are balanced

46. – If we were to replace the ammeter with a voltmeter, Zn Cu salt bridge + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – V Cu K+K+ K+K+ K+K+ K+K+ K+K+ K+K+

47. – Under what are called standard conditions, this cell would initially have a voltage of 1.1 volts. Zn Cu salt bridge + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – V Cu K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Voltage = 1.1 volts

48. – As this cell operates (click), zinc atoms will continue to dissolve to form zinc ions as they lose electrons… Zn Cu salt bridge + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – V Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – K+K+ K+K+ K+K+ K+K+ K+K+ K+K+

49. – And (click) copper 2+ ions will continue to gain electrons as they form copper atoms. Zn Cu salt bridge + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – V Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – K+K+ K+K+ K+K+ K+K+ K+K+ K+K+

50. – As zinc atoms on the zinc electrode dissolve, Zn Cu salt bridge + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – V Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Zinc atoms are dissolved

51. – Zn Cu salt bridge + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – V Cu Cu 2+ + 2e –  Cu Zn  Zn 2+ + 2e – K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Zinc atoms are dissolved And copper 2+ ions in the copper(II) nitrate solution are used up, the voltage supplied by this cell will gradually decrease, Cu 2+ ions are used up

52. – Zn Cu salt bridge + + ++++++++++++++++ + + – – ––––––– – – – ––––––– + – + – – – – – V Cu K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Zinc atoms are dissolved and will over time, drop to zero. Cu 2+ ions are used up Voltage = 0 volts