Chapters 21 (Radioactivity) and 25 (Organic)

Transcription

1 Chapters 21 (Radioactivity) and 25 (Organic)

2 Radioactivity to emit radiation Nuclear reactions change an element into a new element!! Lots of energy involved! Unlike a chemical reaction because we are doing more than rearranging we CHANGE the identity. Change in the atom s nucleus. UNSTABLE nuclei are unhappy and lose energy by emitting radiation radioactive decay. They form STABLE atoms of a different element.

3 Radioisotopes Isotopes of atoms with unstable nuclei. Undergo radioactive decay to attain stability. Emit 4 types of radiation 0 alpha,, ( 2 He) positron +1 e 0 4 beta,, ( -1 e) gamma, and electron capture, ( -1 e) 0

4 What are the charges on radioactive particles?

5 Types of Radiation Alpha radiation - 2 protons and two neutrons (no electrons!) 4 He 2+ or 2

6 Alpha decay U He Th Ra He Rn Cm He Pu

7 Types of Radiation Beta radiation negatively charged beta particles Unstable neutron turns into a proton and ejects 1 electron e - or

8 Types of Radiation Gamma radiation emits gamma rays, high energy photon that has no mass nor charge. Gamma rays almost always accompany alpha and beta radiation and account for the energy lost in the nucleus. Usually omitted from nuclear equations U 2He 90Th + + 2

9 Penetrating Power of Radiation

10 Penetrating Power Least Alpha particles most mass and charge. Isotopic mass 4 He 2+ 2 Beta particles less mass (only the mass of an electron) and a neg charge. Gamma rays have no mass and no charge. Most

11 Positron Emission Positron has same mass as an electron but an opposite charge. Carbon -11 decays by positron emission: 11 C 11 B + 0 e Emission of a positron converts a proton to a neutron, and decreases the atomic # by 1

12 Electron Capture Nucleus captures an electron from the surrounding cloud. 11 C 11 B + 0 e Atomic number decreases by 1

13 In the Nucleus Radioactive decay transmutation Atomic # is altered = identity of element changed Nucleons Strong nuclear force between all nucleons. Repulsive force between 2 protons (electrostatic). Neutron attraction have to overcome the repulsive forces as atomic # increases we need more neutrons to stabilize the nucleus!!!

14 Decay Practice decay 238 Pu 94 4 He U 92 Thorium-229 is used to increase the lifetime of fluorescent bulbs. What type of decay occurs when thorium-229 decays to form radium-225? Write out the nuclear equation. 229 Th 90 Write a balanced nuclear equation for the decay shown on the right. Identify A and B 212 Bi Tl 81 4 He Tl A Pb B 82 4 He 2 Bismuth Ra 88 A B

15 Nuclear Reactions Induced Transmutation FORCE an element to change its identity by bombarding it with radioactive particles! 4 He N O H 1 Particle accelerators move particles at extremely high speeds to overcome repulsive forces.

16 Nuclear Fission Splitting of a nucleus into fragments - LARGE release of energy! Large atoms want to be smaller for stability (Atomic # larger than 60 = large atoms) Kr - 92 neutron U-235 U-236 Ba - 141

17 Neutrons Perpetuate Fission

18 Critical Mass Fissionable material must have sufficient mass to split Not massive enough subcritical no chain reaction Extremely massive violent nuclear reaction

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20 Violent Nuclear Reactions Subcritical masses that get together to form supercritical mass. Equal to 20,000 tons of TNT

21 Nuclear Reactors Reactor core controlled by Cd or B to absorb neutrons Assignment : Look up Chernobyl Nuclear Power Plant!

22 Nuclear Fusion Bind low atomic mass (less than 60 because 60 is ideal) to form more stable atom. Combination of nuclei called fusion. How the sun works H + 2 energy + 4 He 2 Need very high energy to initiate and sustain.

23 Orgo

24 Properties of Alkanes Only van der Waals force: London force. Boiling point increases with length of chain.

25 Naming Prefixes # Carbons Prefix Name Formula 1 Meth Methane CH 4 2 Eth Ethane CH 3 CH 3 3 Prop Propane CH 3 CH 2 H 3 4 But Butane CH 3 CH 2 CH 2 CH 3 5 Pent Pentane CH 3 CH 2 CH 2 CH 2 CH 3 6 Hex Hexane And 7 Hept Heptane So 8 Oct Octane On 9 Non Nonane 10 Dec decane

26 Isomers Have same molecular formulas, but atoms are bonded in different order.

27 Practice Problem 1998 D Answer using appropriate chemical principles. Dimethyl ether, H 3 C-O-CH 3, is not very soluble in water. Draw a structural isomer of dimethyl ether that is much more soluble in water and explain the basis of its increased water solubility.

28 AP Practice The structural formula of one isomer of pentane is shown below. Draw the structural formulas for the other two isomers of pentane. Be sure to include all atoms of hydrogen and carbon in your structures.

29 Alkanes Only single bonds. Saturated hydrocarbons. Saturated with hydrogens.

30 Organic Nomenclature Three parts to a compound name: Base: Tells how many carbons are in the longest continuous chain.

31 Organic Nomenclature Three parts to a compound name: Base: Tells how many carbons are in the longest continuous chain. Suffix: Tells what type of compound it is.

32 Organic Nomenclature Three parts to a compound name: Base: Tells how many carbons are in the longest continuous chain. Suffix: Tells what type of compound it is. Prefix: Tells what groups are attached to chain.

33 To Name a Compound 1. Find the longest chain in the molecule. 2. Number the chain from the end nearest the first substituent encountered. 3. List the substituents as a prefix along with the number(s) of the carbon(s) to which they are attached.

34 To Name a Compound If there is more than one type of substituent in the molecule, list them alphabetically.

35 Naming! Give the systematic name for the following alkane: 3,4-dimethylheptane. 2,4-dimethylpentane Write the condensed structural formula for 3-ethyl-2-methylpentane.

36 Cycloalkanes Carbon can also form ringed structures. Five- and six-membered rings are most stable. Can take on conformation in which angles are very close to tetrahedral angle. Smaller rings are quite strained.

37 Alkenes Contain at least one carbon carbon double bond. Unsaturated. Have fewer than maximum number of hydrogens.

38 Structure of Alkenes Unlike alkanes, alkenes cannot rotate freely about the double bond. Side-to-side overlap makes this impossible without breaking -bond.

39 Structure of Alkenes This creates geometric isomers, which differ from each other in the spatial arrangement of groups about the double bond.

40 Alkynes Contain at least one carbon carbon triple bond. Carbons in triple bond sp-hybridized and have linear geometry. Also unsaturated.

41 Nomenclature of Alkynes 4-methyl-2-pentyne Analogous to naming of alkenes. Suffix is -yne rather than ene.

42 Aromatic Hydrocarbons Cyclic hydrocarbons. p-orbital on each atom. Molecule is planar. Odd number of electron pairs in -system.

43 Aromatic Nomenclature Many aromatic hydrocarbons are known by their common names.

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46 Carbonyl Compounds Contain C O double bond. Include many classes of compounds.

47 Practice 1981 Assume that you have two different gases that you know are not cyclic (i.e. not ring) compounds, each with the following elementary analysis: C = 85.7%, H = 14.3%. Each gas has a molecular weight of 56.1 g/mol. (a) What is the molecular formula for the compounds? (b) Draw the structural formulas for the four possible noncyclic isomers with this molecular formula. (c) In the presence of an appropriate catalyst, both gases add hydrogen. The hydrogenated products are identical, their molecular weight is 58. Which of the structures you drew to answer (b) can definitely be eliminated on the basis of this additional information?

48 Compound Name Compound Formula H vap (kj mol -1 ) Propane CH 3 CH 2 CH Propanone CH 3 COCH (a) For propanone, 1-propanol CH 3 CH 2 CH 2 OH 47.3 (i) draw the complete structural formula (showing all atoms and bonds and (ii) predict the approximate carbonto-carbon-to-carbon bond angle. (b) For each pair of compounds below, explain why they do not have the same value for their standard heat of vaporization, H vap. (i) Propane and propanone (ii) Propanone and 1-propanol (c) Draw the complete structural formula for an isomer of the molecule you drew in, part (a) (i). (d) Given the structural formula for propyne (i) indicate the hybridization of the carbon atom indicated by the arrow in the structure above; (ii) indicate the total number of sigma (s) bands and the total number of pi (π) bonds in the molecule

49 1981 Answer: (a) molecular formula: C 4 H 8 (b) (c) The last formula in Part (b) can be eliminated (2-methyl-1-propene).