1. Physical and Chemical Change
CHAPTER 4
Physical and Chemical Change
4.2 Chemical Reactions

2. Electrolysis ? ? Experimental setup: Observations:
An electric current transfers a large amount of energy to water. ?
Observations:
The mass of water decreases over time.
The volume of gas increases.

3. Electrolysis ? ? Experimental setup: Observations: Hypothesis:
An electric current transfers a large amount of energy to water. ?
Observations:
The mass of water decreases over time.
The volume of gas increases.
Hypothesis:
The gas produced is water vapor.

4. Electrolysis ? ? Hypothesis: Observations on the gases:
The gas produced is water vapor. ?
Observations on the gases:
1. When cooled, the gas does not condense into liquid water.
2. One of the gases burns.
3. One of the gases causes a flame to get brighter.

5. Electrolysis ? ? Hypothesis: Observations on the gases: Conclusion:
The gas produced is water vapor. ?
Observations on the gases:
1. When cooled, the gas does not condense into liquid water.
2. One of the gases burns.
3. One of the gases causes a flame to get brighter.
Conclusion:
The gas produced is not water vapor.

6. Electrolysis ? ? Clues: One gas burns.
One gas causes a flame to get brighter.
The gases come from water. ? ?

7. Electrolysis Clues: One gas burns.
One gas causes a flame to get brighter.
The gases come from water. +

8. The energy from the electric current was high enough to cause a chemical change through a chemical reaction. +
chemical reaction: a process that rearranges the atoms in any substance(s) to produce one or more different substances.
chemical change: a result of chemical reaction.

9. +
A chemical equation

10. The chemical equation oxygen oxygen hydrogen hydrogen
How many oxygen atoms are present on each side? What about hydrogen atoms?
hydrogen
hydrogen

11. How can we make both sides the same number of each atom?
The chemical equation ! 1 2
oxygen
oxygen
This equation is not balanced. 2 2
hydrogen
hydrogen
How can we make both sides the same number of each atom?

12. The chemical equation coefficients oxygen oxygen hydrogen hydrogen
Count again the number of oxygen and hydrogen atoms on each side, considering the coefficients.
hydrogen
hydrogen

13. The chemical equation 2 2 coefficients oxygen oxygen 4 4 hydrogen

14. The chemical equation 2H2O(l) 2H2(g) + O2(g) Reaction arrow Reactants
Products
2H2O(l) H2(g) O2(g)
Coefficients
tell you how many of each molecule participate in the reaction.
(No coefficient appears when it equals 1)

15. The chemical equation Rules of balancing a chemical equation:
1. Determine if the equation is balanced.
Count the number of each kind of atom on either side of the arrow.
2. If it is not balanced, use coefficients.
Ex: H2O becomes 2H2O
3. Do not change the subscripts inside the molecular formulas.
Ex: H2O cannot become H3O
4. Check that the equation is balanced.
Count the number of each kind of atom on either side of the arrow with the new coefficients.

16. Is it balanced?
Reactants
Products
Iron (Fe)
Oxygen (O)

17. Is it balanced?
Reactants
Products
Iron (Fe) 1 2
Oxygen (O) 3

18. Not balanced
Reactants
Products
Iron (Fe) 1 2
Oxygen (O) 3

19. The chemical equation Rules of balancing a chemical equation:
1. Determine if the equation is balanced.
Count the number of each kind of atom on either side of the arrow.
2. If it is not balanced, use coefficients.
Ex: H2O becomes 2H2O
3. Do not change the subscripts inside the molecular formulas.
Ex: H2O cannot become H3O
4. Check that the equation is balanced.
Count the number of each kind of atom on either side of the arrow with the new coefficients.

20. Add coefficients ? ? ?
Reactants
Products
Iron (Fe)
Oxygen (O)

21. 4 3 2
Reactants
Products
Iron (Fe) 4
Oxygen (O) 6
Balanced!

22. a sugar
(glucose)
gases
a gas
Photosynthesis is the basis of most life on Earth.

23. Verify that the equation is balanced
Reactants
Products
Carbon (C)
Oxygen (O)
Hydrogen (H)
Verify that the equation is balanced

24. Number of atoms or moles
Verify that the equation is balanced
Reactants
Products
Carbon (C) 6
Oxygen (O) 18
Hydrogen (H) 12
Number of atoms or moles

25. Use the formula mass to convert from moles to grams.

26. 108 g + 264 g 372 g 180 g + 192 g 372 g Total mass of reactants
Total mass of products
108 g
+ 264 g
372 g
180 g
+ 192 g
372 g

27. Conservation of mass 108 g + 264 g 372 g 180 g + 192 g 372 g
Total mass of reactants
Total mass of products
108 g
+ 264 g
372 g
180 g
+ 192 g
372 g
Conservation of mass

28. Total mass of reactants = Total mass of products
conservation of mass: law that states that, in any chemical reaction, the total mass remains the same.
Total mass of reactants = Total mass of products

29. All chemical reactions involve three key components:
Reactants
Products
Energy
Reactants
Products
Energy (in or out)

30. Energy “in” Energy as a reactant.
reactants
products
An endothermic reaction requires an input of energy.

31. Energy “out” Energy as a product.
reactants
products
An exothermic reaction releases energy.

32. Energy is absorbed
Energy is released

33. First law of thermodynamics:
Energy can neither be created nor destroyed.
Energy is absorbed
Are these violations of the first law?
Energy is released

34. First law of thermodynamics:
Energy can neither be created nor destroyed.

35. What is missing? Photosynthesis Respiration
Mixing water and CO2 doesn’t make
sugar and O2 because energy input is needed.
Respiration
Energy input is
not needed
but just mixing sugar and O2 doesn’t make
water and CO2.

36. What is missing? A candle burning gives off heat.
It is an exothermic reaction.
However, the candle does not spontaneously light itself up.

37. What is missing? A candle burning gives off heat.
It is an exothermic reaction.
However, the candle does not spontaneously light itself up.

38. Activation energy
C6H12O O H2O + 6CO ,800,000J

39. Activation energy
C6H12O O H2O + 6CO ,800,000J

40. Activation energy
C6H12O O H2O + 6CO ,800,000J

41. Activation energy C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J
an energy barrier

42. Chemical equations must be balanced so that mass is conserved
conservation of mass: law that states that, in any chemical reaction, the total mass remains the same.
In an exothermic reaction energy is released.
In an endothermic reaction energy is absorbed.