last time remem
I've finished chapter 18 environmental
chemistry and today I will solve a
problem
end of chapter problems from chapter 18
and we will continue to chapter 21 which
is the nuclear chemistry so we've tried
to finish it this week
and the last week next week actually we
will cover chapter 24 part of the
chapter 24 mostly the biochemistry part
of that chapter and then that would be
all for the semester and the final exam
will be made of questions multiple
choice type and fill-in-the-blank type
that means you have to know the
definitions of certain terms so you can
answer those fill in the blank part will
just give you the definitions and leave
in a sentence one or two words blank and
you filled out now the multiple choice
type questions will be again two
different types one will be mostly
verbal type of multiple-choice questions
which you just have to circle the
correct answer out of five answers and
the second type of multiple-choice
questions will be made of more numerical
calculation type problems that means you
have to solve the problems we have to
calculate and find the result but those
again will be more simpler than
definitely the one we had in the midterm
exams short type of problems if you know
that you should be able to calculate it
there and find the answer and circle the
answer but again for those
multiple-choice questions which requires
calculations as he we will ask you to
show your work in the space given for
the question
very shortly very briefly how you have
we have to see how you got that answer
that you circled out of five different
answers in fact there will usually be a
number right Numerical questions so to
get full credit which means that those
questions which will require
calculations will weigh more than those
questions will be only verbal type and
to get full credit for those calculation
type multiple-choice questions we have
to show your work briefly very briefly
how you got it when we look at it we
have to see that you actually got the
answer yourself not just guessed it
throw it there so if you simply leave if
you show no work nothing then some
credits will be taken off from that
question you can't get a full credit
even though you got the correct answer
actually circle the correct answer you
have to show your work but if you don't
really circle the wrong answer then we
will not look at you what you did as a
matter of fact yes
midterms are finished next semester
that's that I don't know because I won't
be probably given this course next
semester anyhow I mean I can't tell you
exactly what will be the plan for the
next semester
this may be evaluated but the next
semester we're probably more like we
have done this so it might be again I
don't know yes
any questions calculations calculator
just like in the midterm exam you have
to have your calculator but not those
programmable one please don't bring
those programmable type calculators
which is little computers in fact so it
has to be just a simple calculator which
you should be able to multiply divide
take the logarithms square cubes and
things like that actually the exam it
will be I guess 19th maybe this second
Monday off the final week the final
stars at one Monday I guess it's what
maybe twelve anyhow
following Monday that's what I don't I
mean okay again all chapters we have
covered will be included in the final
exam remember as I said there may be
more questions from the chapters that we
have covered after the second midterm
exam but there will be questions from
all those chapters starting from chapter
5 and actually the previous chapter
stochiometry we expect that you know the
Sociology on gas
things and so on okay now let's come
back to this problem of chapter 18 and
close this chapter 18 now see again it
says the concentration of water in
stratosphere is about 5 ppm very very
low actually remember that store the
sphere goes from 10 to 50 kilometers
somewhere about there it undergoes a
photodissociation
according to this again where the energy
comes from radiations right high-energy
radiation is coming from the Sun and
this is the photo dissociation reaction
for the dissociation reaction now you
have to be very careful again like when
you write down this photo dissociation
reactions for example in the exam or
somewhere if you just put here some
signs like plus or minus that is wrong
right that would be wrong you have to be
very careful you see again it's a
neutral hydrogen not h+ h+ and hydrogen
neutral hydrogen are quite different and
this is hydroxyl group not hydroxide ion
that means no charge on it so what
really happens that as you see again the
bond dissociation take place like the
shared electrons are equally left on the
two atoms so the species the products
are not ions but neutral species now it
says write out the lewis structure for
both products and the reactants now
lewis structure for both products and
reactants water simple again we know
that this is the Lewis structure of
water and again we have to show these
lone pair electrons on the hydrogen
right hydrogen and the lewis structure
for hydrogen is simple electron and then
the lewis structure for the hydroxide
will be simply o h
and like this now these type of
compounds which have unpaired electrons
are called radicals so these are simply
called radicals and radicals are always
more reactive than the non radicals so
they are very reactive we have to
remember that so again it says here
using table
88 for calculate the wave
Waveland required to cause this
dissociation now that means the
electromagnetic radiation wavelength to
cause the dissociation now to do that we
should simply find out what is the
energy required for this reaction to
take place this reaction will be
endothermic reaction right endothermic
reaction that means we have to give
energy for this reaction to take place
now to find out how much energy required
for this we can calculate it by using
the bond energies right bond energies
actually what we have done here we have
simply broke one hydrogen oxygen bond
right there and from that table we can
simply find that bond energy and that
bond energy turns out to be H Oh bond
energy from the table is given as 463
kilojoules per mole right so now we will
simply calculate what is the wavelength
of the radiation which will cause that
photodissociation
reactions now this is again we have done
that remember the energy of the photon
let's say would be equals to H nu and
that's equal to HC
/ landau right we use that now we have
to put the values what we simply will
calculate here lambda which land off the
electromagnetic radiation which is
capable of breaking that bond between
the oxygen and hydrogen and water
molecule right now that is this energy
the energy of the photon is over here so
we can simply rearrange this again as
follow since we're looking for the
wavelength of the electromagnetic
radiations lambda would be equal to H
times C this is the Planck's constant
and this is the speed of light remember
divided by energy now let's put the
values there the H is constant which is
six point six two times ten to the minus
34 joules seconds and C is the speed of
light which is three point zero times 10
to the 8 meters per second right these
are the constants that we would simply
use how about the energy that we will
put here the energy would be equal to
that but since this simply says per mole
we have to convert it to per water
molecule per molecule to do that we will
just divide that by Avogadro's number
right
simple Avogadro's number and that means
that e here that we will put would be
equal to 463
this is kilojoules it's better to use
joules in fact make it Jews 10 to the
three jewels and then this is divided by
Avogadro's number 6.02 times 10 to the
23 23 now you can work out the unit's
here yeah if you work out the units
joules will cancel out joules and this
second will cancel out that second the
only things will left here
meters that means that the wavelength
will be given will be will come out from
here as meters in units of meters clear
have to be very careful about those very
simple but just careful as a matter of
fact so if I do this calculation it
turns out to be this is equals to two
point five eight times ten to the minus
seven meters right and if I convert it
to nanometers this will simply be 258
nanometers
what range is this UV right UV
radiations actually quite energetic
radiations this is how we simply
calculate the wavelength of the
radiation which have enough energy to
dissociate that that means that the
wavelength of the photon should be lower
than that value right lower the
wavelength higher the energy should be
lower than 258 nanometers now the third
it says the hydroxyl radicals Oh H can
react with ozone given the following
reaction it said I told it's a radical
reactive species and the reactions with
the ozone are given their hydroxyl group
reacts with the ozone to give H 2h o -
it's not
it looks like waters not water but just
h o 2 plus o 2 and then that's the first
reaction and the second reaction is that
H OH - which is an intermediate reacts
with another oxygen radical and then we
have simply hydroxide and the o2 there
it says what overall reaction results
from these two we will just sum them up
right so
when we sum up those two reactions let's
say see there the sum will give you this
OHS on and plus oxygen gas and that goes
to two oxygen molecule gas again like
this this is the overall if you just add
them up actually those will cancel out
the hydroxide and so on it says what is
the catalyzed in this overall reaction
remember the catalyzed enters the
reaction and comes out from the reaction
it doesn't really appear in the overall
reaction equations and the same is true
for the hydroxide ion here hydroxide
radical here so hydroxide is a catalyzed
you see again the hydroxide simply helps
to dissociate or increases the rate of
the reaction between the OHS on and the
oxygen atom to give you oxygen molecule
that means it helps the depletion of the
ozone layer right Finan off the ozone
layer it simply is a catalyzed okay this
is the problem that I went I wanted to
go through in chapter 18
now chapter 21 is about nuclear
chemistry now so far we simply have
talked about the chemical reactions now
the chemical reaction simply involves
the valence electrons of the atom right
and some of the chemical reactions are
exothermic some of the chemical
reactions are endothermic it simply
involves breaking off the bond and
making of bond between the atoms and so
on but there are also nuclear reactions
now the nuclear reactions are the
reactions which only involves the
nucleus of the atom and what we have in
the nucleus we have protons and neutrons
in the nucleus right and that's all so
the nuclear reaction involves a proton
and a neutron off the nucleus so when
you actually change the number of
protons for example you change the
identity of the elements for example you
change the goal to some other elements
for example or uranium to lead or
something else carbon to nitrogen and so
on
remember in chemical reactions the
chemical identity of the atoms never
ever changes
but in nuclear reactions the chemical
identity of the atom changes because the
reactions are taking place only or
indeed the nuclear involves the nucleus
so the nuclear reactions actually are
not affected by the environment by the
pressure by the temperature or in which
compound the atom is it really doesn't
matter in which compound the nucleus is
the reaction nuclear reactions are
totally independent of those effects as
a matter of fact now what is the
difference main difference between the
chemical reaction and the nuclear
reactions the energy involved in the
nuclear reactions are
compared to the chemical reactions we
simply any chemical reactions we talk
about 100 kilojoules 200 kilojoules 500
kilojoules and so on nuclear reactions
the energy values are at least thousand
or million times greater than the
chemical reactions know at least
thousand times greater than the
chemicals so there is a huge amount of
energy is associated with these nuclear
reactions now the question is how can we
use those nuclear reaction energies as a
metaphor or can we use it or are those
nuclear energy reactions actually are
harmful to the environment or live in
orgasm and so on so in this chapter we
will more less talk about those in
nuclear chemistry let's go through these
slides and we will see again 'harry says
the energy chemical versus nuclear as I
said chemical energy is associated with
making and breaking a chemical bonds as
we said nuclear energy is enormous in
comparison as I said at least thousand
times at least actually and nuclear
energy is due to the changes in the
nucleus of the atoms
changing them into different atoms so
you change the carbon to nitrogen right
or nitrogen to oxygens or some other
thing some other elements actually now
13% of the worldwide energy uses comes
from the nuclear energy again remember I
said that they have to be you know some
way to be able to utilize this huge
amount of energy involved in nuclear
reactions nuclear reactions now if you
look at again remember the energy that
is used from nuclear reactions are
coming from the nuclear power plants
these are the power plants that produces
electricity by using nuclear regular
active actually actually they're
substances as fuel as fuel and we will
see that now if you look at again in
general here the worldwide this is the
worldwide here
the energy that is used comes almost 41%
from the coal and then 22% from gas and
hydroelectric about 16% nuclear overall
is 13% 13% and then we have simply the
others here oil and others renewable
sources these may be the renewable
sources so you see again there is quite
a bit of the nuclear energy is used on
earth somehow we utilize it now this is
overall for the whole world actually but
if you look at the countries like France
United States and China now in France
the nuclear energy total I should
probably say let's put it this way this
this is actually the electrical energy
that is produced by using these fuels in
France for example you see again 77
percent of the electrical energy use in
friends comes from the nuclear power
plants 77 percent almost the highest and
then again here in the United States the
that is about 19 percent nuclear is
about 19 percent now in China it is much
lower I guess it's about 2 percent but
it is also increasing this gives you
sort of a perspective how the nuclear
energy is used overall world and
different countries as a matter of fact
now let's look at the nucleus again when
we say nucleus we simply mean the proton
and neutrons of the atom right and
they're right here in the center of the
atom actually and when we give a simple
foreign element like this remember this
simply shows the mass number we call
that that's the number of neutrons a
number of protons the sum of number of
neutrons and some of the protons now
this is the atomic number which is the
number of protons here again remember
that the nucleus is composed of two type
of
Rohan's nucleons protons and neutrons
actually in general the particles that
is in the nucleus we call them nucleons
maybe the protons or neutrons both
protons and neutrons are called nucleons
the number of protons is the atomic
number we know the number of protons and
neutrons together is the batch number
remember when if the identity of the
atomists change that means we have to
change the atomic number right when the
atomic number changes the identity of
the atom element actually changes we
becomes a different element as a matter
of fact
okay now the isotopes remember that the
mass of the atoms of one element are not
all the same they are different and
these are called the isotopes of the
element and why they are different
because different number of neutrons
that present in the nucleus right
different number of nucleons
for example uranium these are simply the
isotopes that we write and pronounce for
example uranium 234 is the uranium
isotope with the mass number 234 uranium
235 again the mass number 235 uranium
238 and so on remember these are natural
isotopes and these natural isotopes has
abundances in nature right natural
abundance we call that for example in
uranium 238 99.3% so that simply means
that the natural uranium are mostly
uranium 238 isotopes consists of 238
isotopes then we all have 235 which is
about point seven percent much less than
that
now this 234 is much less than these
even not simply recorded here given
there but uranium 234 press R is present
as an isotope but the abundance is much
lower than the other two as a matter of
fact
now some just like in chemical reactions
in fact some compounds are stable some
compounds are not stable if a compound
is not stable they decomposes right and
we explain that by the number of valence
electrons and so in structures and so on
off the compound now in this case also
we can imagine that some of the nuclei
I'm not stable right if they are not
stable what will happen they will simply
go enter in nuclear reaction they will
decompose actually or they will simply
emit some particles to reach to more
stable state more stable state so these
type of isotopes or Radio isotopes I
should say are called radioactive
radioisotopes and they are simply called
radio nuclides in fact we refer to these
as radio nuclei that means radioactive
nuclides nuclei is again an atom with a
specified number of proton and a neutron
for example if you consider the carbon
six six six protons six neutrons there
is a nucleoid of carbon in fact now we
can also have another nuclei of carbon
like six protons and seven neutrons
carbon 13 right it's another nuclei so
these are called radio nuclides again
the ones that are not stable that means
they decompose or in other words they
enter nuclear reactions nuclear
reactions there are several ways of
radio nuclides can decay into a
different nuclei right different nuclei
this doesn't mean a different element
but mostly nuclear reaction results
change in the identity of the reactant
element reactant element end up with a
different element or new nucleus of a
different elements now we use nuclear
equations to show
these nuclear reactions that are cure
just like chemical reactions nuclear
reactions are also described by an
equation and we call that nuclear
equations just like chemical equations
and as you can imagine just like we do
in chemical reaction equations we has to
be balanced and these also has to be
balanced now let's see how we write
those nuclear reaction equations
actually here now the equations those
nuclear equations nuclear reaction
equations in chemical equation atoms in
charge need to be balanced remember for
the chemical equations we do that in
nuclear equations atomic number and the
mass number need to balance that means
we have to be careful about the atomic
numbers and the mass numbers of the
nuclides that enter a chemically
reaction and comes out from the nuclear
reactions and this is a way of balancing
like charge on an atomic scale like
atomic number and mass in fact just like
balance in the chemical equation it just
tried to tells you that now what kind of
radiation emitted by radio nuclei that
means radioactive nuclides what kind of
particles emitted in these nuclear
reactions one type is called
alpha particle represented by this alpha
sign here and then the beta particles
and gamma particles and usually we call
this alpha particles we call beta as
beta ray or beta emission gamma is
always called gamma rays now let's look
at the properties of these particles or
emitted I should say radiations and
alpha particle has a two plus charge and
a mass of six 64 times 10 to the minus
24 gram per gram
and relative penetrating power it says
one that is we will talk about that
simply penetrations is not that much
nature of the radiation what is actually
alpha particle alpha particle is
actually is a helium nucleus helium
nucleus and and that is what helium four
nucleus that means the mass number four
atomic number two but remember that
alpha particle normally helium contains
two protons and two electrons right but
when we think about the alpha particle
and as a as that as helium that simply
means alpha particle equals two helium 2
plus no electrons we just consider the
nucleus has two protons and two neutrons
that's it right that's what we mean by
the alpha particles or its charge now
beta particles or beta emission you see
again the charge is minus one and the
mass is nine eleven times 10 to minus 28
what is this this is electron so
actually beta particles are electrons
just the electrons and relative
penetrating power is 100 that means they
penetrate the substances the matter
almost hundred times more than the alpha
particles and they are electrons they
are nothing but just electrons simple
electrons now we also have gamma rays
now the gamma rays have no charge no
mass much more penetration penetrating
power and these are actually high-energy
photons very short wavelength in a
photons what does that means they carry
out huge amount of energy they are the
most energetic electromagnetic
radiations and very very harmful
actually because they carry a lot of
energy actually if an alpha if a gamma
in fact hits a molecule it will
definitely break the chemical bonds
there enough energy to break
any chemical bonds as a matter of fact
so now let's continue here types of
radioactive decay now we classify the
decays radioactive decays again as
follow now decay simply means that a
radioactive nuclide that means which is
not stable this integrates simply
decomposes or gives particles like that
it is it that means in other words in
these decay reactions there's only one
reactant in nuclear reaction one
reactant simply right so that's those
are simply called decays not stable
nucleus and it goes to more stable
nuclei or emits particle in fact to the
other atoms as a matter of fact now
alpha decay that simply means that the
the radiation given off from the
reactant nuclei parent nuclei you can
call that alpha particles and those are
alpha particles are two plus charge
helium's actually right ions and then
beta decay that means just given
electron but this electron comes from
the nucleus of the atom
not the electrons outside the nucleus
that we have talked about so far not
those valence electron core electrons
that comes out from the nucleus in fact
right so gives electron and the gamma
radiation that's a very high-energy
photons again and positron emission this
is now just like the same as beta decay
actually this is called beta plus
emission and it is the same as electron
in mess wise but the charge is plus one
instead of minus one right these are
called
positrons actually and they are called
the positron emission now electron
capture
this is again another special type of
decay that means that the nucleus the
nucleus captures an electron where the
electron comes from now these are the
core electrons of the atom outside right
so one of the atom in fact comes out
comes in to the nucleus nucleus captures
that and as you can imagine what will
happen if you simply nucleus captures an
electron mostly that electron will go
and combine with a proton and convert
that proton to a neutron that means it
will change the identity of the element
again this electron captures now here
are the the alpha decay is given here
like this and the beta decay and
positron decays and electron captures
now what we have to be careful here is
this all of these reactions for example
alpha decay this reaction nuclear
reaction and beta decay another one here
has to be balanced and remember how we
balance the nuclear reactions the mass
numbers on the left must be equal to the
mass number on the right right so the
mass number is left upper again for
example here a would be equal to simply
a minus four plus four right and then Z
will be equal to Z of the products that
comes out now when we think about the
helium the mass number is four alpha
particles actually the helium 2 plus and
the atomic number is two right now when
we consider an electron this is the
electron right or beta emission actually
and we have the mass number zero but the
atomic number Z is minus one and the
four positron emission mass number zero
again atomic number plus one so we have
to be careful about those two
in a gamma radiation actually both mass
and charge zero we don't have to
consider that as we balance the nuclear
reactions actually okay now here is the
example alpha decay for example one
alpha decay over decay is the loss of an
alpha particle helium-4 nucleus now
remember that this is very important it
is not a helium atom it is nearly new
quiesce what does that means the atom
has two electron normally like and the
helium nucleus is just two protons two
neutrons so the charge is two plus here
as a matter of fact we have to remember
that so this is the decay nuclear
reaction in fact balanced it has to be
balanced let's check that some of the
mass numbers 234 plus for 238 and equals
to the mass number of the reactant 238
here only one and the atomic number here
z2 plus 90 92 this is 92 so this is a
balanced nuclear reaction equations
balanced nuclear reaction equations and
we show it here how we carry out that
now beta decay is a loss of beta
particle what is that means that simply
means loss of an electron right beta
particle and again it says a high-speed
electron emitted by the nucleus that
comes out from the nucleus not the
outside electrons and here is the
iodine-131
goes to xenon 54 131 and the beta
particle and the beta particle is
electrons simply again the balance here
will be done by adding the mass numbers
of the products which is 131 +0 131
equals 131 and 54 minus 1 equal to 53
that's equal to 53 so this is again a
balanced nuclear chemical equations
again now gamma emission gamma emission
is the loss of
gamma ray which is high-energy radiation
that almost always accompanies the loss
of a nuclear particle that means another
beta alpha particle comes out in
addition to that gamma radiations also
comes out from there
now again gamma radiation says the gamma
contains a lot of energy it is a kind of
energy carrier right it has a lot of
energy carries out the energy actually
the unstable high-energy radionuclide
goes into more stable radio nuclides and
it gives off energy during the nuclear
reactions now positron emission and this
is again the same as the electron we
said positron but plus charge you can
think about this as plus charge electron
the matter is exactly the same and we
consider the mass number a zero here and
again the carbon 11 goes to boron 11 and
gives off a positron emission as a
positron again we see the mass numbers
balanced and the the atomic numbers are
also balanced of the nuclear a
nuclearized and here again how we do
that we carry on that now as a result
you see again when you and nuclei gives
a positron the number of proton
decreases by one what does that means
you can imagine or visualize that
positron emission is achieved by a
proton given a positron right electron
is a positive charge in the nucleus and
that proton when it gives it positron a
positive charge it is converted to a
neutron right converted to a neutron as
we have seen here converted to neutron
again in this case we have six neutrons
here we have five neutrons again very
very simple now how about electron
capture as I said the electron capture
is taking place by capturing an electron
outside
the nucleus outside of the nucleus in
maybe the core electrons or what valence
electrons
mostly the core electrons in fact and
here is again nuclear reaction equation
for the electron capture and this is
review diem again 81 37 you see it
captures an electron and turns out to
Krypton and the Krypton has 36 proton
and the rubidium has 37 proton what is
that simply means when you take capture
and electron that electron combines with
a proton and converted to a neutron in
this case also the number of protons
decreased by 1 by the electron capture
again now these are the source of some
nuclear particles as we see this is the
positron beta particles coming from the
iodine which is a radioactive nuclide -
xenon 131 1 again you see again the
identity of the element changes iodine
is changed to the xenon and if positrons
actually emission will be like
converting a proton to a neutron or we
can write it down as P goes to n plus a
positron and a positron emission and the
number of proton decreases in a nucleus
by 1 number of neutron increases by 1
what happens with the electron capture
again we can visualize that electron
capture turns out converts one proton to
a neutron and again the proton number
decreased and the neutron number
increase you see again in both cases
electron capture and positron emission
the atomic number decreases by 1 and the
number of neutron increases by 1 again
now here's a question what we said that
the the nuclei that are not stay
enter a nuclear reactions they decay
actually there turns out to be
radioactive why because they somehow
wants to give their excess energy out
and go to a more stable States more
stable state like this now can we simply
find out which nuclides will be
radioactive which nucleus will be not
ready active right yeah can we simply
develop a criteria
this is talks about that again again any
atom with more than one proton will have
repulsion between the protons in the
nucleus so if we consider the energies
interactions involved in the nucleus
right
we have protons they're positively
charged protons and we have neutral
simply particles neutrons right you can
imagine that the proton and proton will
repel each other right will repel each
other but when you have the proton and
proton very close to each other actually
there is actually these short distance
forces attractive forces comes into the
picture and even though they repel the
positive past while we repel each other
but there is also an attraction between
those protons but in nucleus except the
hydrogen hydrogen has only one proton
right in all other nuclei you have
proton and neutrons so somehow the
presence of neutrons you can think about
that sort of counteracts the repulsion
of the protons somehow enters between
those protons and decreases the
repulsions and make the nucleus more
stable and as a matter of fact so let's
look at strong nuclear forces helps keep
the nucleus together as I said these are
called short distance forces you know
short range forces neutrons play a key
role
stabilizing the eclairs again as I said
so the ratio of the neutrons to the
protons is an important factor this is
it that simply means if you have too
much protons compared to the neutrons
that nuclear ID will not be stable it
will be radioactive while on the other
side if you have too much neutrons and
less protons compared to protons that
will also be not very stable right will
be radioactive so what is the the ideal
ratio of the protons and the neutrons
for any cried to be stable not really
active or for a nuclei to be not stable
radioactive again so let's let's look at
that
here is the neutron proton ratio graph
and we stop here and continue the next
hour
let's have 10 minutes break and come
back quickly again