Nuclear Chemistry s3

Nuclear Chemistry

Up to this point all combination of chemical species in a chemical reaction involved only a breaking of bonds within the molecules that were reaction to form new bonds and new molecules. The identity of the atoms participating in the reaction remained the same - so in balancing the reaction we always made sure that the number of atoms of each element on both sides of the eqn. was the same and the charge was equal as well.

In a nuclear process the identity of an atom may change.

In chemical rxns., only the outer electrons of the atoms are disturbed. Remember the electrons are the species involved in bonding . In nuclear reactions, nuclear changes occurs. If the element is changing elemental identity, then the # of protons in the nucleus must change. If the mass # changes, but the # of protons stays the same, the number of neutrons must change.

Two types of nuclear reactions

1) radioactive decay - process in which a nucleus spontaneously disintegrates, giving off radiation.

2) nuclear bombardment reactions - nuclear reaction in which a nucleus is bombarded or struck by another nucleus or nuclear particle. Here fission or fusion may occur.

An example of nuclear reactions

1) A sample of Uranium-238 decays spontaneously over a period of billions of years. After about 30 billion years it is nearly gone. Strontium -90 formed by nuclear reactions that occur in nuclear weapons testing is essentially gone after several hundred years.

2) Example of a nuclear bombardment reaction is the fusion that goes on in the sun which is essentially four protons and electrons combining to make He.

Nuclide symbols:

(# of protons + # of neutrons) mass #

Symbol or Symbol - mass #

( # of p r ot on s ) a t om i c #

Radiation - small particles or light emitted from sample Common radiation particles

1 Neutron n 0 n

1 1 Proton p 1 p or 1 H

2 2 Deuteron d 1d or 1H

4 4 Alpha  2 or 2 He

0 0 - Beta Particle  -1  -1 e

+ 0 + 0 Positron e 1 1e

0 Gamma Rays  0 

______is a fast electron

Positron is a ______. emission is the release of a helium nucleus - no electrons

(what is the charge?)

Radioactive Decay

Radioactivity exhibits 1st order kinetics, but it is really a special case. Here the half life is independent of Temperature and Pressure and all other conditions. No one has yet figured out a way to speed up or slow down radioactive decay. It is a property of the particular isotope only and it even doesn’t depend on what kind of other atoms surround the particular nucleus, that is what type of molecule the nucleus is part of.

Note that in theory it would take an infinite amount of time for the sample to completely decay, but after 10 half lives, less than 0.1% radioactivity remains.

st Half life eqns. for 1 order kinetics still apply: t1/2 = 0.693/k

Radioactive dating is based on

1) The fact that radioactive Carbon-14 is present in the atmosphere and decays by the 14 14 0 beta decay. The process is: 6C -> 7N + -1 e This has a half-life of 5730 yr.

2) To use this for dating fact that living plants use atmospheric carbon dioxide and maintain a constant C-14 level, but when they die, there is no more exchange so the concentration of the C-14 isotope, and the  decay radiation decreases according to 1st order kinetics.

Example

A piece of charcoal from the time of the formation of Crater Lake gave 7.0 disintigration of carbon-14 nuclei per minute per gram of C. Present day living plant matter gives 15.3 disintegrations per minute per gram of total carbon. Determine the age of Crater Lake.

Why is carbon-14 dating limited to less than 50,000 years

How and why does a nucleus emit?

More than ____ naturally occurring isotopes (?).

______are stable, and the rest are not!

______artificially made isotopes that are radioactive.

TO BE STABLE

Lighter elements -

Heavier elements require more ______to stabilize the nucleus?

Magic # 2, 8, 20, 28, 50, 82 and

Neutrons Protons # of stable isotopes Even Even

Even Odd

Odd Odd

No stable nuclides with atomic number greater than ___

Technetium and Promethium z = 43 and 61

If neutron to proton ratio is such that the nucleus is not stable, the nucleus emits radiation (Overhead)

If the nucleus has excess neutrons, it can stabilize by:

converting proton to electron!

Beta Emission!

In Beta emission, the Beta particles are emitted from the nucleus, and the proton stays in it.

Conversion of a neutron to a proton and emitted electron changes the identity of the atom! Why?

"Alchemy"...... Making gold out of other substances. Consider Unstable Phosphorous-32 nucleus.

When this unstable molecule is converted to sulfur-32, its stability is achieved

Emission of Beta particle - b decay - results in nucleus with same mass # but different atomic # which has increased by 1.

Balancing nuclear Eqns!

1) Sum of "mass #s" on both sides of eqn. must be the same!

2) Sum of "Atomic #" on both sides of the eqn. must be the same.

3) Remember the "mass # and atomic #. For:

 particle

 particle positron

 emission

Example: Iodine-139 is a beta emitter. What product results when it emits!

Another Example:

What nucleus could undergo  decay to the only stable nucleus of gold? Alpha emission can occur as well!

What product is formed in the alpha decay of Uranium-238?

What is the transmutation product of Polonium-210 under alpha emission?

What would happen in electron capture reaction?

Or positron emission?

Note this increases the neutron/proton ratio.

r electron capture

Capture and electron from an inner orbital

4 0 0 4 0 19 K + -1 e -> 18 Ar (Usually emits X-ray or  ray too, since the outer electron fills the void left by the inner electron

 decay decreases the neutron/proton ratio! Fusion and Fission

Nuclear Binding Energy

Puzzling fact that the mass of an atom is always less than the sum of the masses of its constituent particles.

For instance the mass of a helium-4 atom is 4.00260 amu

Mass of 2 electrons = 2 x 0.000549 amu = 0.00110 amu Mass of neutron is 1.675x10- 2 4 g so 2 x 1.00867 amu = 2.01734 amu Mass of proton is 1.673x10- 24 g so 2 x 1.00728 amu = 2.01456 amu Sum of mass = 4.03300 amu

So there is a difference of m = -0.03040 amu. This mass difference is explained by the fact that when the nucleons come close together they bind so energy must be lowered. This is related to the binding energy.

The binding energy is the energy required to break a nucleus into its individual protons and neutrons.

Example 19 The 9 F isotopes has an atomic mass of 18.9984 amu Nucleus has 9 protons and 10 neutrons, 19 nucleons proton mass - 1.007825 amu neutron mass - 1.008655 amu

9 x 1.007825 + 10 x 1.008655 amu = 19.15708 amu

1 9 This is larger than the measured mass of 9 F This is a difference between the mass of the atom and sum of the masses of the nucleons is called the mass defect.

Convert mass defect into energy according to Einstein, E = mc2 m = 18.9984 - 19.15708 = -0.1587 E = -0.1587amu x (3.00x108 m/s)2 = -1.43 x 101 6 amu m2 /s2 1 amu = 1.066 x 10- 27 kg 1 J = 1 kg m2 /s2

E = -1.43 x 1016 amu m2 /s2 x 1.066 x 10- 2 7 kg / amu = -2.37 x 10- 1 1 J This is the energy released when on Fluorine-19 nucleus is formed from 9 protons and 10 neutrons. Looking at a figure of the nuclear binding energy with respect to atomic mass indicates that a mass # of around 56 is the most stable. Isotopes with mass numbers less than 56 could combine to form a more stable nucleus and when the mass is greater than 56 the nucleus should fall apart to become more stable.

Fusion

98% of all matter in the Universe is made of hydrogen and helium

At its conception only the lightest element, hydrogen was around but later as the universe expanded, stars were born when the hydrogen clouds collapsed under gravitational forces

Figure

Hydrogen fused together in the star and formed helium. This liberates a massive amount of energy as photons.

1 4 0 4 1p -> 2He + 0

Where does this massive energy come from?

Process is called fusion - it is how the sun makes energy

Transuranium Elements - Elements with atomic numbers greater than 92

The elements are typically prepared by bombarding heavy nuclei with light ones

2 5 2 10 25 7 1 98Cf + 5 B -> 103Lr + 5 0n the new element element - 268 was prepared by bombarding Bismuth wih Cobalt.

2 0 9 5 9 26 8 1 83 Bi + 27 Co -> 110 Und + 0n Und is Unnildecinnium

Thes transuranium elements are unstable, and most have very short half lives -

2 5 7 for instance 103 Lr is 0.65 s, In 1930 some people, Lisa Meitner, Otto Hahn and Frita Strassmen and Fermi were trying to produce new transuranium elements by bombarding Uranium with neutrons.

Hahn and Co. found:

2 3 5 1 1 3 9 94 1 0 92 U + 0 n -> 56 Ba + 36 Kr + 3 0 n + 0

In this fission reaction the 23 5 U is broken down to smaller particles and energy called atomic energy.

Note that each fission of Uranium-235 produces 3 neutrons which in turn generate more fission by colliding with other Uranium-23

See figure:

If even one of these produces new fission, then the process is self propagating. If all there neutrons are allowed to produce new fission, then the rate of reaction increases constantly and eventually one gets a nuclear explosion.

Rate of fission can be controlled by putting boron control rods in the reactor to absorb neutrons!

The energy released can be used to heat water to make steam and drive turbines to get electricity - nuclear power plants

Waste - Fission products are highly radioactive themselves, with long half lives. Need to be stored for a long time. No way to speed up or slow down radioactive decay. Nuclear binding energy - energy required to break up a nucleus into its component neutrons and protons Organic Chemistry

Chemistry of compounds containing carbon:

Several million have been described so far and thousands of new ones discovered every year. What is it about carbon that makes it so unusual?

Atomic electronic configuration:

2 2 2 1s 2s 2p - Know it can make compounds like methane (CH4) and CCl4 - what shape? Lewis Structures, VSEPR

Tetrahedral - Four Equivalent bonds - so think of it in terms of valence bond theory 1) orbital on one atom comes to occupy a portion of the same region of space as an orbital on the neighboring atoms are said to overlap 2) The total number of electrons in both orbitals is no more than two.

Hybrid orbitals - used to describe bonding that is obtained by taking combinations of atomic orbitals of a particular atom so the bonds it makes are all similar.

Remember orbital diagrams in CH4

or in H2CO

or in H2C2 Classification of Organic Compounds is described by the diagram in this figure:

A series of compounds in which one compound differs from the preceding one by a -CH2 Group is called a homologous series. The alkanes constitute a homologous series. Members of a homologous series have similar chemical properties and often physical properties change in a regular way.

B.P. and M.P. increasing molecular weight

Straight chain alkanes have a single bond connecting the carbon atom backbone surrounded by hydrogen atoms. They are of formula CnH2n+2 straight chain, or normal alkanes have all carbon atoms bonded to one another to give a single chain with hydrogen filling out the four valences of each carbon atom

In addition to straight-chain alkanes, branched-chain alkanes are also possible.

isobutane has the same molecular formula (formula unit) as n-butane, C4H10, but a different structural formula Butane and isobutane are said to be structural or constitutional isomers.

Nomenclature of Alkanes

Nomenclature developed over several years as a way of understanding and classifying their structures. Nomenclature it now formulated in rules agreed upon by the IUPAC.

The first four have long established names, methane, CH4 ethane, C2H5 propane, C3H8 butane, C4H10

Higher members of the series are named from the Greek words indicating the number of carbon atoms in the molecule with the suffix “ane” added. C5H12 - straight chain called pentane C6H14 - hexane C7H16 - heptane C8H18 - octane C9H20 - nonane C10H22 - decane

For branched chain alkanes: 1) Determine the longest continuous (not necessarily straight chain of carbon atoms in the molecule. the base name of the branched-chain alkane is that if the normal alkane corresponding to the longest chain.

2) Any chin branching off the longest chain is named as an alkyl group. An alkyl group is an alkane less one hydrogen atom. When a hydrogen atom is removed from an end carbon atom of a straight alkane, the resulting alkyl group is named by changing “ane” to “yl”.

Example: H-CH3, methane -CH3, methyl

3) To completely name the branch off the main chain, we must use a number that locates that branch on the longest chain. For this purpose, you number each carbon atom on the longest chain in whichever direction gives the smaller number for the locations of all branches. Branch name and base name are written as a single word with a hyphen following the number 4) When there are more than one alkyl branch of the same kind, say two methyl groups, then this number is indicated by a Greek prefix, such as di-, tri-, or tetra- used with the name of the alkyl group. The position of each group on the longest chain is given by numbers. Note the position numbers are separated by commas and followed by a hyphen. When there are two or more different alkyl branches, the name of each branch, with its position number, precedes the base name. the branch names are placed in alphabetical order.

Examples

2,3 dimethylbutane 3-ethyl-2-methylhexane 2,2-dimethylhexane

Write condensed structural formula of 4-ethyl-3-methylheptane

3,3 dimethyloctane

Cycloalkanes - don’t follow CnH2n+2 rule since connected in a ring fashion.

Alkenes and Alkynes

Alkenes - CnH2n with just one double bond

Alkynes - CnH2n-2 with just one triple bond

Unsaturated hydrocarbons - Similar naming rules for them, except longest chain with double or triple bond

H2C=CH2 ethene - ethylene lingering old nomenclature called common name

HC≡CH ethyne - acetylene

Numbering occurs from the end nearest the multiple bond number - 1st carbon in double bond

1 2 3 4 5 CH2 = CH - CH2 - CH - CH3 CH3 4-methyl-1-pentene

Example Write the condensed structure for 2,5-dimethyl-2-heptene

Alkenes give rise to another type of isomerization called Geometric Isomers. These are isomers where the atoms are joined to one another but because there is no free rotation about the double bond. Called cis and trans isomers

cis-2-butene trans-2-butene

Different boiling points indicate different compounds

The simplest organic compounds are the hydrocarbons compounds containing only carbon and hydrogen! All other organic compounds are considered, for classification purposes to be derived from the hydrocarbons.

SubClassified into two main groups!

1. Aromatic Hydrocarbons - hydrocarbons containing benzene rings or similar strucures

2. Aliphatic Hydocarbons - all hydrocarbons that do not contain benzene rings

CH4 molecular formula H structural formula H - C -H H

Sub Sub classifications include: a) saturated hydrocarbons - a hydrocarbon where all carbon atoms are bonded to max # of hydrogen (no multiple bonds) b) unsaturated hydrocarbons - hydrocarbons with carbon-carbon double or triple bonds

Alkanes, also called paraffins - are saturated hydrocarbons with general formula CnH2n+2. For n=1 you get the formula CH4, n=2 you get C2H6 etc.

Derivatives of Hydrocarbons Certain groups of atoms in organic molecules are particularly reactive and have characteristic properties.

These are the functional groups, a reactive part of the molecule that reacts readily and predictably.

C=C bond is a functional group for example. Many functional groups include atoms with lone pairs. C=O is a functional group.

Looked at hydrocarbons and their reactions, and all other organic compounds are considered to be derivatives of hydrocarbons where one or more hydrogen atoms of a hydrocarbon have been replaced by noncarbon atoms to give a functional group.

Structure of General Compound Name of functional Group

Organic Halide

Alcohol

Aldehyde

Ketone

Carboxylic Acid

Ester Amine

Alcohols and Ethers

Alcohols and Ethers are named by IUPAC rules similarly to those for hydrocarbons, except the stem name is determined from the longest chain containing the carbon atom to which the OH is attached. the suffix for the stem name is “ol”. the position of the OH group is indicated by a number preceding the stem name. alcohols are classified by the number of ca atoms attached to the carbon atom to which the -OH group is attached.

1- butanol 2-butanol 2-methyl-2-propanol

Common names for ethers are formed from the hydrocarbon groups plus the word ether.

CH3OCH2CH2CH3 methyl propyl ether

Or by IUPAC rules, the ethers are named as “oxy” derivatives based on the longest hydrocarbon chain.

1-methoxypropane

Aldehydes and Ketones Aldehydes and ketones are compounds containing a carbonyl group. C=O

An aldehyde is a compound containing a carbonyl group with at least on H attached. common names: fomaldehyde, acetaldehyde A ketone is a compound containing a carbonyl group with two hydrocarbon groups attached to it. common names: acetone, methyl ethyl ketone based on naming the two groups on either side of the carbonyl group

IUPAC rules for naming aldehydes and ketones are similar to the rules of naming for alcohols. Find the longest carbon chain containing the carbonyl group to get the stem hydrocarbon name. Then change the “e” at the end of the hydrocarbon name to al for aldehydes or one for ketones. In the case of the aldehydes, the carbon atom of the -CHO group is always the number 1 carbon. In ketones the carbonyl group may occur in various positions in the chain, and the position of the carbonyl group is given by a number be for the stem name, like the position of the hydroxyl group in alcohols.

2 butanone methanal ethanal

Carboxylic Acid Contain the carboxyl group -COOH

The carbonyl group attached to the OH makes the proton weakly acidic.

Common names here as well since these types of compounds have been known for a long time.

IUPAC rules name these carboxylic acids like for aldehydes except that “oic” is added with the word acid on the stem name of the longest carbon chain containing the carbonyl of the carboxyl group.

acetic acid ethanoic acid

Made from carboxylic acid and alcohol have carbonyl group with oxygen, similar to carboxylic acid, but with hydrocarbon on the end instead of a proton. Nitrogen Containing Amines

Organic bases that are derived by replacing one or more hydrogen atoms of ammonia withy hydrocarbon groups.

Primary Amines Secondary Tertiary

Amides are nitrogen-containing hydrocarbons derived from the reactio of ammonia, or of a primary or secondary amine, with a carboxylic acid.

ammonia + acetic acid -> acetamide + water

Nylon is an example of a polyamide.