Natural Approach to Chemistry Chapter 4 Physical and Chemical Change

1. Natural Approach to Chemistry Chapter 4 Physical and Chemical Change 4.1 Understanding Chemical Changes Frame 2-41 4.2 Chemical Reactions Frames 42-82 4.3 Chemical Reactions in the lab Frames 83-109

2. CHAPTER 4 Physical and Chemical Change 4.1 Understanding Chemical Changes

3. 4.1 Understanding Chemical Changes H2O(s) H2O(l) Is it still the same substance? Have the physical properties changed?

4. 4.1 Understanding Chemical Changes H2O(s) H2O(l) ? Can water go back to being ice?

5. 4.1 Understanding Chemical Changes In the presence of a flame: 2C20H42(s) + 61O2(g) 42H2O(g) + 40CO2(g) Is it still the same substance? Have the physical properties changed?

6. 4.1 Understanding Chemical Changes If the flame cools down: Does the candle come back? ? 2C20H42(s) + 61O2(g) 42H2O(g) + 40CO2(g)

7. 4.1 Understanding Chemical Changes If the flame cools down: The candle does not come back by itself. X 2C20H42(s) + 61O2(g) 42H2O(g) + 40CO2(g) In this chemical reaction, the atoms were rearranged. This change is irreversible.

8. 4.1 Understanding Chemical Changes In a physical change: - the molecules are rearranged - intermolecular forces are broken

9. 4.1 Understanding Chemical Changes • In a physical change: • - the molecules are rearranged • intermolecular forces are broken • interatomic forces are not broken In a chemical change: - the atoms are rearranged - interatomic forces are broken

10. 4.1 Understanding Chemical Changes Interatomic forces. Intermolecular forces. Intermolecular forces are much weaker than interatomic forces.

11. 4.1 Understanding Chemical Changes Physical or chemical change?

12. 4.1 Understanding Chemical Changes Physicalor chemical change?

13. 4.1 Understanding Chemical Changes Physical or chemical change?

14. 4.1 Understanding Chemical Changes Physicalor chemical change?

15. H H O O H H O O H H H O H H 4.1 Understanding Chemical Changes H H O2 (oxygen) H2 (hydrogen) H2O (water) Can we obtain any arrangement of atoms? H4O (?) H3O (?)

16. H H O O O 4.1 Understanding Chemical Changes H H O2 (oxygen) H2 (hydrogen) H2O (water) Can we obtain any arrangement of atoms? NO:each type of atom allows only certain chemical bonds to be formed This is due to the structure of the atom

17. . 4.1 Understanding Chemical Changes Structure of the Atom An atom is not a hard ball. 99.8% of the mass of the atom is in the nucleus!

18. . . . 4.1 Understanding Chemical Changes The number of electrons corresponds to the atomic number of the element, as shown in the periodic table.

19. 4.1 Understanding Chemical Changes Electrons have a negative electric charge. .

20. 4.1 Understanding Chemical Changes Electrons have a negative electric charge. . If negative charges repel each other, how does an atom stay together?

21. 4.1 Understanding Chemical Changes Electrons have a negative electric charge. Protons in the nucleus have a positive electric charge. Proton: A tiny particle in the nucleus that has a positive charge. Neutral: Having zero total electric charge.

22. 4.1 Understanding Chemical Changes Why don’t the negative electrons “fall” into the positive nucleus?

23. 4.1 Understanding Chemical Changes Why don’t the negative electrons “fall” into the positive nucleus? Because the electrons have energy and momentum - The same reason why the Earth does not fall into the sun, but gravitates around it instead.

24. 4.1 Understanding Chemical Changes Electrons are responsible for bond formation. Electrons can be: - +

25. 4.1 Understanding Chemical Changes In ionic compounds, electrons are NOT SHARED but TRANSFERRED. +1 -1 Cl Na Sodium ion Chloride ion 1 electron Ionic bond Cl Na Sodium atom Chlorine atom

26. 4.1 Understanding Chemical Changes In ionic compounds, electrons are NOT SHARED but TRANSFERRED. Magnesium chloride (MgCl2) is also an ionic compound. 1 electron 1 electron +2 -1 Mg -1 Chloride ion Cl Cl Chloride ion Magnesium ion Ionic bond Ionic bond Chloride atom Chloride atom Mg Cl Cl Magnesium atom

27. 4.1 Understanding Chemical Changes Covalent bonds A chemical bond is formed by sharing or transferring electrons. There are two covalent bonds in a water molecule. Covalent bond: A chemical bond that consists of two shared electrons. Molecule: A neutral group of atoms bonded together by covalent bonds.

28. 4.1 Understanding Chemical Changes Multiple bonds Single bonds Double bond Triple bond Some elements can share multiple electrons with the same atom.

29. 4.1 Understanding Chemical Changes Chemical bonds form because there is an advantage in energy. Lower energy = more stable

30. 4.1 Understanding Chemical Changes Chemical bonds form because there is an advantage in energy. Lower energy = more stable Enthalpy of formation ∆Hf (kJ/mole)

31. 4.1 Understanding Chemical Changes All chemical reactions involve three key components: Reactants • Reactants

32. 4.1 Understanding Chemical Changes All chemical reactions involve three key components: Products Reactants • Reactants • Products

33. 4.1 Understanding Chemical Changes All chemical reactions involve three key components: Products Reactants • Reactants • Products • Energy (in or out) Energy

34. 4.1 Understanding Chemical Changes Electrons are responsible for bonding among atoms. Some elements are never found in pure form in nature: Ex: Lithium (Li), sodium (Na) Some elements are always found in pure form in nature: Ex: Helium (He), argon (Ar) He

35. 4.1 Understanding Chemical Changes Some elements are more likely to react than others. They are morereactive. Reactivity: The tendency of elements to form chemical bonds.

36. 4.1 Understanding Chemical Changes Electrons make bonds. Electrons can be:

37. 4.1 Understanding Chemical Changes Can we predict if a compound is ionic or covalent?

38. 4.1 Understanding Chemical Changes Can we predict if a compound is ionic or covalent? Yes! Metal Nonmetal In an ionic compound, one atom is a nonmetal, and one atom is a metal

39. 4.1 Understanding Chemical Changes Can we predict if a compound is ionic or covalent? Yes! Nonmetal Nonmetal In a covalent compound, both atoms are nonmetals.

40. 4.1 Understanding Chemical Changes Use the periodic table to help determine whether a compound is ionic or molecular.

41. 4.1 Understanding Chemical Changes Is the compound CF4 ionic or molecular?

42. 4.1 Understanding Chemical Changes Is the compound CF4 ionic or molecular? Both nonmetals

43. CHAPTER 4 Physical and Chemical Change 4.2 Chemical Reactions

44. 4.2 Chemical Reactions Electrolysis ? Experimental setup: An electric current transfers a large amount of energy to water. ? Observations: The mass of water decreases over time. The volume of gas increases.

45. 4.2 Chemical Reactions Electrolysis ? Experimental setup: 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.

46. 4.2 Chemical Reactions Electrolysis ? Hypothesis: 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.

47. 4.2 Chemical Reactions Electrolysis ? Hypothesis: 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.

48. 4.2 Chemical Reactions Electrolysis Clues: One gas burns. One gas causes a flame to get brighter. The gases come from water. ? ?

49. 4.2 Chemical Reactions Electrolysis Clues: One gas burns. One gas causes a flame to get brighter. The gases come from water. +

50. 4.2 Chemical Reactions 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.