1. CHAPTER 15
Electro-chemistry
15.1 Electrochemistry and Electricity

2. The lemon battery
In lab investigation 15A, we saw that a lemon can cause an LED light up
Magnesium ribbon
Copper ribbon
LED
Electrochemistry describes the relationship between chemical reactions and electricity

3. 2Fe(s) + 3O2(g) → Fe2O3(s) Fe Fe3+ Rusting is an oxidation process
The integrity of iron (Fe) is compromised as it loses 3 electrons to become Fe3+ Fe
Fe3+
loses 3 electrons

4. Molecules in the body can also be damaged by losing electrons

5. Free radicals can cause a lot of harm to the body
Molecules in the body can also be damaged by losing electrons
Free radicals can cause a lot of harm to the body
free radical: molecules or atoms that are highly reactive owing to their having one or more unpaired valence electrons.

6. Antioxidants protect our body by deactivating free radicals
Molecules in the body can also be damaged by losing electrons
Free radicals can cause a lot of harm to the body
Antioxidants protect our body by deactivating free radicals
antioxidant: a molecule that reacts easily with a free radical by donating an electron.

7. Free radicals can cause a lot of harm to the body
Molecules in the body can also be damaged by losing electrons
Free radicals can cause a lot of harm to the body
Antioxidants protect our body by deactivating free radicals
antioxidant: a molecule that reacts easily with a free radical by donating an electron.
We are encouraged to consume foods that have a high antioxidant content

8. What do we know about electrons?
Electron lost
Electron gained

9. What do we know about electrons?
protons
neutrons
electrons
Electrons are found in the atom
They have a negative charge
This charge is counterbalanced by protons
Protons have a positive charge

10. The unit of charge is called the coulomb (C)
What do we know about electrons?
protons
neutrons
electrons
The unit of charge is called the coulomb (C)

11. Electric current in metals
Copper wire
In a metal like copper (Cu) some electrons are “free” to move around
Ordinarily the motion of the electrons is random and in all directions – for example, the same number go left as go right – leaving zero average motion.
On average, every volume always contains the same number of positive and negative charges, even when the negative charges are moving.

12. Electric current in metals
When a battery is attached, the free electrons acquire an average motion in one direction.

13. When a battery is attached, the free electrons acquire an average motion in one direction.
electric current: the directed flow of electric charge.
ampere: the SI unit of electrical current.

14. Electric current in solutions
NaCl crystal
In an electrolyte solution such as a solution containing NaCl, positive and negative ions are present: Na+ and Cl–.
electric current: the directed flow of electric charge.

15. Electric current in solutions
NaCl crystal
In an electrolyte solution such as a solution containing NaCl, positive and negative ions are present: Na+ and Cl–.
Na+ and Cl– are free to move around.
Movement of either (or both) positive and/or negative ions may create an electric current.
electric current: the directed flow of electric charge.

16. The lemon battery
In the lemon battery, magnesium chemically reacts with citric acid and loses electrons.
Magnesium
Citric acid

17. Copper
Electron flow
LED
In the lemon battery, magnesium chemically reacts with citric acid and loses electrons.
These electrons (and their charges) flow through the LED then back into the lemon via the copper metal.

18. The whole system is charge neutral
Electric charge, like mass, is a fundamental property of matter, and it is conserved.
The number of positive charges is equal to the number of negative charges.
The whole system is charge neutral
electroneutrality: the property of charges to balance each other, resulting in a charge neutral system.

19. If there are no mobile electric charges, then no electric current can flow.
This is why a sugar solution cannot carry any current.
electric current: the directed flow of electric charge.
ampere: the SI unit of electrical current.

20. Voltage What is voltage? Is there a current if the voltage is zero?
You measured the voltage in the lemon battery.
What is voltage?
Is there a current if the voltage is zero?

21. Voltage
Water flows only if there is a height or pressure difference between two points

22. Voltage
Water flows only if there is a height or pressure difference between two points
Electrons flow only if there is an electrical pressure difference between two points

23. What is voltage?
Voltage is the difference in electrical pressure or electrical potential energy between two objects
voltage: the electrical potential difference between two objects.

24. Voltage No
If there is not difference in electrical energy (V = 0) there is no current flow
Is there a current if the voltage is zero?
Except in a superconductor.

25. Voltage
If a charge of 1 C flows from a low voltage to a voltage 1 V higher, the charge will gain 1 joule of energy

26. Voltage
If a charge of 1 C flows from a low voltage to a voltage 1 V higher, the charge will gain 1 joule of energy
If a charge of 1 C flows from a high voltage to a voltage 1 V lower, the charge will lose 1 joule of energy

27. Resistance
When electrons flow in solid copper, they collide with the fixed copper ions.
This resistance limits how much current flows for any given voltage
Ohm’s law:

28. An electric current can only occur when:
- mobile electric charges are present and
- there is a difference in electrical potential energy (non-zero voltage)
Ohm’s law: