Announcements

1. Announcements • Wednesday second lab (VSPR) • Molecular model kit, building compounds and molecules • Prelab is due at the beginning (objective, summary of procedure in your own words and prelab questions)

2. Review Molecular Shape • A molecular formula shows the number and identity • of all of the atoms in a compound, but not which • atoms are bonded to each other (H2O). • A Lewis structure shows the connectivity between • atoms, as well as the location of all bonding and • nonbonding valence electrons. • Use the VSEPR theory to determine the shape.

3. Review Lewis Structures HOW TO Draw a Lewis Structure H For CH3Cl: 4 bonds x 2e− = 8 e− H C Cl + 3 lone pairs x 2e− = 6 e− H 14 e− 8 e− on Cl 2 e− on each H All valence e− have been used. • If all valence electrons are used and an atom still does not have an octet, proceed to Step [4]. Use multiple bonds to fill octets when needed. Step [4]

4. Multiple Bonds Review Lewis Structures Change one lone pair into one bonding pair of e–, forming a double bond. Step [4] H C C H H C C H H H H H Answer Each C now has an octet.

5. Review Exceptions to the Octet Rule • Most of the common elements generally follow the octet rule. • H is a notable exception, because it needs only 2 e−in bonding. • Elements in group 3A do not have enough valence e− to form an octet in a neutral molecule. F F B F only 6 e− on B

6. Review Exceptions to the Octet Rule • Elements in the third row have empty d orbitals available to accept electrons. • Thus, elements such as P and S may have more than 8 e−around them. O O HO P OH HO S OH O OH 12 e− on S 10 e− on P

7. Review VSEPR theory • VSEPR • (V ) Valence • (S) Shell • (E) Electron • (P) Pair • (R) Repulsion The idea of VSEPR is that the valence electron pairs surrounding an atom mutually repel each other, and will therefore adopt an arrangement that minimizes this repulsion, thus determining the molecular geometry.

8. Review Two Groups Around an Atom • Any atom surrounded by only two groups is linearand has a bond angle of 180o. • An example is CO2: • Ignore multiple bonds in predicting geometry.Count only atoms and lone pairs.

9. Review Three Groups Around an Atom • Any atom surrounded by three groups is trigonal planar and has bond angles of 120o. • An example is H2CO:

10. Review Four Groups Around an Atom • Any atom surrounded by four groups is tetrahedral and has bond angles of 109.5o. • An example is CH4:

11. Review Four Groups Around an Atom • If the four groups around the atom include one lone pair, the geometry is a trigonal pyramid with bond angles of ~109.5o. • An example is NH3:

12. Review Four Groups Around an Atom • If the four groups around the atom include two lone pairs, the geometry is bent and the bond angle is 105o (i.e., close to 109.5o). • An example is H2O:

13. Review Molecular Shape

14. Review Electronegativity and Bond Polarity • Electronegativity is a measure of an atom’s attraction for e− in a bond. • It tells how much a particular atom “wants”e−.

15. Review Electronegativity and Bond Polarity • If the electronegativities of two bonded atoms are equal or similar, the bond is nonpolar. • The electronsin the bond are being shared equallybetween the two atoms.

16. Review Electronegativity and Bond Polarity • Bonding between atoms with different electro- negativities yields a polar covalent bond or dipole. • The electrons in the bond are unequally shared between the C and the O. • e− are pulled toward O, the more electronegative element; this is indicated by the symbol δ−. • e− are pulled away from C, the less electronegative element; this is indicated by the symbolδ+.

17. Review Electronegativity and Bond Polarity

18. Review Polarity of Molecules The classification of a molecule as polar or nonpolar depends on: • The polarity of the individual bonds • The overall shape of the molecule Nonpolar molecules generally have: • No polar bonds • Individual bond dipoles that cancel Polar molecules generally have: • Only one polar bond • Individual bond dipoles that do not cancel

19. Review Polarity of Molecules

20. Question 4.89 (page 121) • Answer the following questions about the molecule Cl2O • a. How many valence electrons does Cl2O contain? • 7x2 + 6 = 20 • b. draw a valid Lewis structure O Cl Cl 8 e− on Cl 8 e− on Cl c. label all polar bonds O Cl Cl

21. Question 4.89 continues • d. What is the shape around the O atom? • Two bonds and tow lone pairs of electrons (the shape is bent) • e. Is Cl2O a polar molecule? Explain. The compound is polar since the two bond dipoles do not cancel.

22. Lab 2: VSPER • Objectives: • To systematically determine the shapes of molecules using VSEPR theory. • To learn determination of molecular polarity based on shape and bond polarity

23. Construct molecule using the model kit Lowes structure Space filling Balls and sticks Lowes structure Balls and sticks

24. Chapter 5 chemical reaction Physical change take place without changing the identity of matter. Examples: freezing, melting, or evaporation of a substance Do you remember physical vs chemical change from chapter 1 Chemical change are always accompanied by a change in the identity of matter (composition). Examples: burning of paper and the fizzing of a mixture of vinegar and baking soda

25. 5.1 Introduction to Chemical Reactions General Features of Physical and Chemical Changes • A physical change alters the physical state of a substance without changing its composition. Ice melting and liquid water evaporating

26. General features of physical and chemical changes • A chemical change (a chemical reaction) converts • one substance into another. • Chemical reactions involve: • Breaking bonds in the reactants (starting materials) • Forming new bonds in the products

27. Example: treating wounds Treating wounds with hydrogen peroxide (H2O2) The enzyme catalase in red blood converts (H2O2)to water and oxygen gas, which appears as visible white foam on the bloody surface.

28. Example: • A chemical reaction: CH4 and O2 CO2 and H2O

29. Problem 5.1 (page 125) • Use the molecular art to identify the process as a chemical reaction or physical change and explain your choice? The process is a chemical reaction because the reactants contain two gray spheres joined (indicating H2) and two red spheres joined (indicating O2), while the product (H2O) contains a red sphere joined to two gray spheres (indicating O–H bonds).

30. Your Turn! Problem 5.2 (page 125) • Use the molecular art to identify the process as a chemical reaction or physical change and explain your choice? The process is a physical change (freezing) since the particles in the reactants are the same as the particles in the products.

31. Writing Chemical Equations A chemical equation uses chemical formulas and othersymbols showing what reactants are the starting materials in a reaction and what products are formed. • The reactants are written on the left. • The products are written on the right. • Coefficients show the number of molecules of a given element or compound that react or are formed.

32. Writing Chemical Equations • The law of conservation of mass states that atoms cannot be created or destroyed in a chemical reaction. • Coefficients are used to balance an equation. • A balanced equation has the same number of atoms of each element on both sides of the equation.

33. Writing Chemical Equations

34. Problem 5.3 (page 128) • Label the reactants and products and indicate how many atoms of each type lf element are present on each side of the following equation. • a. 2H2O2 (aq) 2H2O(l) + O2 (g) • Reactants products • 4H and 4O 4H and 4O • b. 2C8H18+25O2 16CO2 + 18H2O • Reactants products • 16C, 36H, 50O 16C, 36H, 50O • c. 2Na3PO4(aq) + 3MgCl2 (aq) Mg3(PO4)2 (s) + 6NaCl(aq) • Reactants products • 6Na, 2P, 8O,3Mg, 6Cl 6Na, 2P, 8O,3Mg, 6Cl

35. Problem 5.5 (page 128) • Write the chemical equation from the following description of a reaction: one molecule of gaseous methane (CH4) is heated with four molecules of gaseous chlorine (Cl2), forming one molecule of liquid carbon tetrachloride (CCl4) and four molecules of gaseous hydrogen chloride (HCl) • CH4 (g) + 4Cl2(g)∆ CCl4(l)+ 4HCl(g)

36. 5.2 Balancing Chemical Equations HOW TO Balance a Chemical Equation Write a balanced chemical equation for the reaction of propane (C3H8) with oxygen (O2) to form carbon dioxide (CO2) and water (H2O). Example Write the equation with the correct formulas. Step [1] C3H8 + O2 CO2 + H2O • The subscripts in a formula can never be changed to balance an equation, because changing a subscript changes the identity of a compound.

37. 5.2 Balancing Chemical Equations HOW TO Balance a Chemical Equation Balance the equation with coefficients one element at a time. Step [2] • Balance the C’s first: • Balance the H’s next:

38. 5.2 Balancing Chemical Equations HOW TO Balance a Chemical Equation Balance the equation with coefficients one element at a time. Step [2] • Finally, balance the O’s:

39. 5.2 Balancing Chemical Equations HOW TO Balance a Chemical Equation Check to make sure that the smallest set of whole numbers is used. Step [3]

40. Problem 5.6 (page 130) • Write a balanced equation for the reaction. • a. H2 + O2 H2O • 2H, 2O 2H, 1O • Need to have 2O on product side, so multiply H2O by 2 • H2 + O2 2 H2O • 2H, 2O 4H, 2O • Need to have 4H to be on the reactant side, so multiply H2 by 2 • 2H2 + O2 2 H2O • 4H, 2O 4H, 2O

41. Problem 5.6 (page 130) • b. NO + O2 NO2 • 1N, 3O 1N, 2O • Need to have 3O on product side, so multiply NO2 by 2 • NO + O2 2NO2 • 1N, 3O 2N, 4O • Need to have 2N to be on the reactant side, so multiply NO by 2 • 2NO + O2 2NO2 • 2N, 4O 2N, 4O

42. Problem 5.6 (page 130) • c. CH4 + Cl2 CH2Cl2 + HCl • 1C, 4H, 2Cl 1C, 3H, 3Cl • Need to have 4H on product side, so multiply HCl by 2 • CH4 + Cl2 CH2Cl2 + 2HCl • 1C, 4H, 2Cl 1C, 4H, 4Cl • Need to have 4Cl to be on the reactant side, so multiply Cl2 by 2 • CH4 + 2Cl2 CH2Cl2 + 2HCl • 1C, 4H, 4Cl 1C, 4H, 4Cl

43. Chemistry of an automobile airbag A sever crash trigger an electric sensor sodium azide (NaH3) to ignite and convert it to sodium (Na) and nitrogen gas (N2) Reaction is not balanced

44. Balance the airbag reaction