Covalent bond

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Covalent bond

• Water molecule

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Covalent bonds

• Covalent bonds are found in covalent compounds,
  ie compounds that are formed by sharing
  electrons.
• There are different kinds of covalent bonds,
  such as Van Der Waals , polar covalent bonds or
  hydrogen bonding.

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Water molecule
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Hydrogen bonding

• It is a bond that is formed between a H and
  another electronegative atom such as O, F, or Cl.
• It is a very strong bond so it changes the
  properties of the compound, example is the water
  molecule.

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Water molecule
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Water molecule

• The symbols delta and delta- are used to
  indicate partial charges.   Oxygen, because of
  its high electronegativity, attracts the
  electrons away from the hydrogen atoms, resulting
  in a partial negative charge on the oxygen and a
  partial positive charge on each of the
  hydrogens.  The possibility of hydrogen bonds
  (H-bonds) is a consequence of partial charges. 

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H- bonding
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Hydrogen-Oxygen Bonding

• Covalent bonds can also have partial charges when
  the atoms involved have different
  electronegativities. Water is perhaps the most
  obvious example of a molecule with partial
  charges. 

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Water unusual properties














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Ice lighter than water
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Water special properties

• Ice lighter than water reserves animal and
  plant life in frozen lakes,seas and rivers.
• Water is used as the main solvent since it can
  dilute many different substances.
• Water has a high boiling point so provides a
  stable environment that is essential for life.

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Chemical reactions
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Chemical reactions
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Types of chemical reactions

• Six basic types
• Combustion
• Synthesis
• Decompositions
• Displacement
• Double displacement
• Acid base
• Redox

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Combustion

• 1) Combustion A combustion reaction is when
  oxygen combines with another compound to form
  water and carbon dioxide. These reactions are
  exothermic, meaning they produce heat. An example
  of this kind of reaction is the burning of
  napthalene
• C10H8 12 O2 ---gt 10 CO2 4 H2O

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Synthesis

• 2) Synthesis A synthesis reaction is when two or
  more simple compounds combine to form a more
  complicated one. These reactions come in the
  general form of
• A B ---gt AB
• One example of a synthesis reaction is the
  combination of iron and sulfur to form iron (II)
  sulfide
• 8 Fe S8 ---gt 8 FeS

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Decomposition

• 3) Decomposition A decomposition reaction is the
  opposite of a synthesis reaction - a complex
  molecule breaks down to make simpler ones. These
  reactions come in the general form
• AB ---gt A B
• One example of a decomposition reaction is the
  electrolysis of water to make oxygen and hydrogen
  gas
• 2 H2O ---gt 2 H2 O2

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Single displacement

• 4) Single displacement This is when one element
  trades places with another element in a compound.
  These reactions come in the general form of
• A BC ---gt AC B
• One example of a single displacement reaction is
  when magnesium replaces hydrogen in water to make
  magnesium hydroxide and hydrogen gas
• Mg 2 H2O ---gt Mg(OH)2 H2

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Double displacement

• 5) Double displacement This is when the anions
  and cations of two different molecules switch
  places, forming two entirely different compounds.
  These reactions are in the general form
• AB CD ---gt AD CB
• One example of a double displacement reaction is
  the reaction of lead (II) nitrate with potassium
  iodide to form lead (II) iodide and potassium
  nitrate
• Pb(NO3)2 2 KI ---gt PbI2 2 KNO3

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Acid-base

• 6) Acid-base This is a special kind of double
  displacement reaction that takes place when an
  acid and base react with each other.
• The H ion in the acid reacts with the OH- ion in
  the base, causing the formation of water.
  Generally, the product of this reaction is salt
  and water
• HA BOH ---gt H2O BA
• One example of an acid-base reaction is the
  reaction of hydrochloric acid (HCl) with sodium
  hydroxide (NaOH)
• HCl NaOH ---gt NaCl H2O
• Also called neutralization reaction 

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Redox Reactions (Oxidation - Reduction
Reactions)

• Redox reactions involve the loss or gain of
  electrons.
• Oxidation loss of electrons. All metals loose
  electrons to become cations.
• Reduction gain of electrons. All non-metals
  loose electrons to become anions.
• Example Ca O 2 ---? 2 CaO
• In these reactions, the valency or the oxidation
  number of the reactants change.

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Redox reactions

• Ca O 2 ---? 2 CaO
• Can split the reaction into two half equations
• First half equation
• Ca ----? Ca 2 2e- Loss of
  electrons is the oxidation part.
• Second half equation
• O 2e- ---? 2O2- Gain of electrons
  is the reduction part.

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Balancing redox reactions

• Ca ----? Ca 2 2e-
• O 2e- ---? 2O2- this is not correct
  because oxygen is diatomic so we should write
• O2 4e- ----? 2 O 2-. THIS IS CORRECT.
• So now we need to have 4e- not two, therefore we
  go back to the first half equation and adjust
• 2 Ca ----? 2 Ca 2 4e-
• O2 4e- ---? 2 O 2-
  add
• 2 Ca O2 ---? 2 CaO note that
  electrons are cancelled.

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Redox transfer of electrons

• Redox reactions then, involve the transfer of
  electrons from one reactant to another... When
  there is oxidation, there is also reduction.
• This is like the ionic bonding reactions that we
  learned earlier.
• These are reactions between metals and non-metals
  

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Redox and oxidation number

• For Example Zn 2HCl -gt Zn2 H2 2Cl-
• In this reaction we are interested in
• Zn 2H -gt Zn2 H2
• Zn is oxidised to Zn2 (loses 2 electrons)
• So for Zn2 the O.N. increases from 0 to 2
• Therefore oxidation loss of electrons OR
  increase in oxidation number.

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Redox and O.N.

• Zn 2HCl -gt Zn2 H2 2Cl-
• H is reduced to H2 (gains 2 electrons)
• H has an oxidation number (valency) of 1, and
  is reduced to an oxidation number (valency) of 0.
  
• So for H the Oxidation number is reduced.
• Therefore reduction gain of electrons or
  decrease in O.N.

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Redox and O.N.

• Zn 2HCl -gt Zn2 H2 2Cl-
• In this reaction we are not interested in Cl
  because on the left the O.N, for Cl is -1 and on
  the right is also -1.
• Therefore the chlorine is NOT changed in its ion
  state, so is not oxidised or reduced

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Redox and O.N.

• So oxidation increases the oxidation number
  (valency).
• So, reduction decreases the oxidation number
  (valency).

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Redox
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Redox reactions

• Reaction between Al and O2 , Na and O2 ,and Al
  and Cl2 .

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Magic words for Redox

• OIL RIG
• OIL oxidation is loss of electrons
• RIG reduction is gain of electrons

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Importance of Redox

• Six key elements make up 95 of all living
  organisms carbon (C), hydrogen (H), oxygen (O),
  nitrogen (N), phosphorus (P) and sulfur (S).
  There are also a number of other elements
  important to living organisms such as potassium
  (K), calcium (Ca), and magnesium (Mg). Chemical
  oxidation and reduction reactions involving these
  key elements They are crucial for life because
  they link the chemical, biotic, and geologic
  systems together. These reduction-oxidation
  (redox) reactions are usually mediated by
  organisms and especially bacteria, who gain
  energy from the exchanges of electrons.

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Redox reaction

• A simple demonstration of a redox reaction
  involves placing a solid piece of copper wire in
  a silver nitrate solution. Within minutes the
  wire begins to look fuzzy or furry, as small
  silver crystals begin to form on the wire.
  Meanwhile, the originally clear silver nitrate
  solution begins to take on a pale bluish tint.
  Furthermore, if the crystals are shaken off of
  the wire we see that the wire partially
  disintegrated.
• The overall equation for our demonstration
  describes the events
• Cu(s) 2AgNO3 (aq) ? Cu(NO3)2 (aq) 2 Ag(s)

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Redox
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Oxidation old definition

• The term oxidation originally referred to
  substances combining with oxygen, as happens when
  an iron bar rusts or a campfire log burns. We
  often refer to these two examples as corrosion
  and combustion.

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Redox remember

• Oxidationthe loss of electrons OIL
• OIL Oxidation Is Loss
• Reductionthe gaining of electrons RIG
• RIG Reduction Is Gain

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Battery

• A battery is a device that converts chemical
  energy directly to electrical energy. It consists
  of one or more voltaic cells. Each voltaic cell
  consists of two half cells connected in series by
  a conductive electrolyte. One half-cell is the
  negative electrode (the anode) and the other is
  the positive electrode (the cathode). In the
  redox reaction that powers the battery, reduction
  occurs in the cathode, while oxidation occurs in
  the anode. The electrodes do not touch each other
  but are electrically connected by the
  electrolyte, which can be either solid or liquid.
  In many cells, the materials are enclosed in a
  container, and a separator, which is porous to
  the electrolyte, which prevents the electrodes
  from coming into contact.

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Zn battery
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Zn battery

• In a dry cell, the outer zinc container is the
  anode (-). The zinc is oxidised according to the
  following half-equation.
• Zn(s) ? Zn2(aq) 2 e- A graphite rod surrounded
  by a powder containing manganese(IV) oxide is the
  cathode(). The manganese dioxide is mixed with
  carbon powder to increase the conductivity of the
  cathode mixture. The cathode reaction is as
  follows
• 2MnO2(s) 2 H(aq) 2 e- ? Mn2O3(s) H2O(l)
  The H comes from the NH4(aq)
• NH4(aq) ? H(aq) NH3(aq) and the NH3 combines
  with the Zn2.

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Oxidation number (ON)

• The oxidation number is a number identical with
  the valency but with a sign, expressing the
  charge on the ion in question when formed from
  the neutral atom.
• Thus, the oxidation number of chlorine in
  hydrochloric acid HCl is -1, while it is 1 in
  hypochlorous acid (HClO)
• Similarly we can say that the oxidation number of
  chlorine in chloric acid (HClO3) is 5, and in
  perchloric acid (HClO4) 7.

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Calculation of O.N.

• What are the O.N. In the following??
• MgO2
• FeCl3, FeCl2,
• KMnO4, MnO4- , MnO, MnO2
• NO, NO2, HNO3, NO3- ,NH3
• Note that some elements have a constant O.N.
• Other elements ,have a variable O.N.
• In molecules sum of O.N. Is zero

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Radioactivity
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Radioactivity definition

• It is the spontaneous breakdown of nuclei to form
  smaller and more stable atoms, release particles
  and energy.
• Radioactivity is a natural phenomenon and it
  occurs because some nuclei are very large.
• Usually atoms with atomic number bigger than 80
  are radioactive.
• They contain many protons and neutrons

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Radioactivity



Radiation Relative Relative charge mass Nature Penetration Deflection by electric field
Alpha particle 2 4 2 protons and 2 neutrons (He2 ion) Stopped by a few sheets of paper Low f
Beta particle 1 TSfff Electron Stopped by a few mm of plastic or aluminium High ,
Gamma rays 0 0 Electromagnetic radiation of shorter wavelength than X-rays Stopped by a few cm of lead Nil
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Half- life of different isotopes




Isotope Half-life
Carbon- 1 4 5700 years
Uranium-238 4.5 X 10' years
Iodine- 135 6.5 hours
Strontium-90 28 years
Uranium-235 7.13 X 10s years
Polonium-212 3 X I07 seconds
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Radioactive decay
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Half - life

• Definition It is the time taken for the
  radioactivity or the mass of a radioactive
  isotope to be reduced by half.
• For example C-14 is radioactive and has a
  half-life of 5700 years. If have 100Kgs of
  Carbon today in 11400 years how much carbon will
  I have left??
• Answer 25Kgs.

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Radioactive isotopes

• Stable have a very long half - life, e.g.
  uranium
• Unstable Have a very short half - life ,e.g.
  pollonium

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Nuclear equations
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Alpha particle

• This is a helium atom
• When an atom breaks down by alpha decay
• Its atomic number is reduce by 2
• Its RAM is reduced by 4.
• Uranium
• Polonium
• radon

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Alpha particle...

• Alpha particles are made of 2 protons and 2
  neutrons.
• This means that they have a charge of 2, and a
  mass of 4We can write them as , or, because
  they're the same as a helium nucleus,
• Alpha particles are relatively slow and heavy.
• They have a low penetrating power - you can stop
  them with just a sheet of paper.
• Because they have a large charge, alpha particles
  ionise other atoms strongly.

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Alpha decay
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Beta decay

Beta particles have a charge of minus 1, and a
mass of about 1/2000th of a proton. This means
that beta particles are the same as an electron.
We can write them as or as . They
are fast, and light. Beta particles have a
medium penetrating power - they are stopped by a
sheet of aluminium or plastics such as
perspex. Beta particles ionise atoms that they
pass, but not as strongly as alpha particles do
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Beta decay
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Exercise

• Complete and balance the following nuclear
  equations

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Gamma rays

• Gamma rays are waves, not particles. This means
  that they have no mass and no charge. So we
  sometimes write .
• Gamma rays have a high penetrating power - it
  takes a thick sheet of metal such as lead, or
  concrete to reduce them significantly.
• Gamma rays do not directly ionise other atoms,
  although they may cause atoms to emit other
  particles which will then cause ionisation.

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Properties




Type of Radiation Alpha particle Beta particle Gamma ray
Symbol or or or (can look different,depends on the font)
Mass (atomic mass units) 4 1/2000 0
Charge 2 -1 0
Speed slow fast very fast (speed of light)
Ionising ability high medium 0
Penetrating power low medium high
Stopped by paper aluminium lead



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Penetration
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Penetration
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Uses of radioactivity

• Nuclear energy
• Carbon dating in archaelogy
• Medical applications
• Cancer treatment
• Industry
• Sterilization of food products- stop growth of
  bacteria

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Carbon dating

• Animals and plants have a known proportion of
  Carbon-14 (a radioisotope of Carbon) in their
  tissues.
• When they die they stop taking Carbon in, then
  the amount of Carbon-14 goes down at a known rate
  (Carbon-14 has a half-life of 5700 years).
• The age of the ancient organic materials can be
  found by measuring the amount of Carbon-14 that
  is left.

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Medical applications

• The most common tracer is called Technetium-99
  and is very safe because it only emits gamma rays
  and doesn't cause much ionisation.
• Radioisotopes can be used for medical purposes,
  such as checking for a blocked kidney. To do
  this a small amount of Iodine-123 is injected
  into the patient, after 5 minutes 2 Geiger
  counters are placed over the kidneys.

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Cancer treatment

• Because Gamma rays can kill living cells, they
  are used to kill cancer cells without having to
  resort to difficult surgery. This is called
  "Radiotherapy", and works because cancer cells
  can't repair themselves when damaged by gamma
  rays, as healthy cells can.
• It's vital to get the dose correct - too much and
  you'll damage too many healthy cells, too little
  and you won't stop the cancer from spreading in
  time.

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Detect leaking pipes

• Also radioisotopes are used in industry, to
  detect leaking pipes. To do this, a small amount
  is injected into the pipe. It is then detected
  with a GM counter above ground

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Sterilising

• Even after it has been packaged, gamma rays can
  be used to kill bacteria, mould and insects in
  food.
• This process prolongs the shelf-life of the food,
  but sometimes changes the taste.
• Gamma rays are also used to sterilise hospital
  equipment, especially plastic syringes that would
  be damaged if heated

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Control of thickness

• In paper mills, the thickness of the paper can be
  controlled by measuring how much beta radiation
  passes through the paper to a Geiger counter.
• The counter controls the pressure of the rollers
  to give the correct thickness. With paper, or
  plastic, or aluminium foil, b rays are used,
  because a will not go through the paper.
• We choose a source with a long half-life so that
  it does not need to be replaced often.

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Control of thickness
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Nuclear reactor

• All nuclear reactors now in operation use nuclear
  fission (link to advantages of nuclear energy).
  Nuclear fission is the process where the nucleus
  (hence nuclear energy) of a heavy,
  fissionable atom is split. Enormous amounts of
  energy are released in this process.
• An atoms nucleus can only split if it is
  fissionable. Only the nuclear isotopes Uranium
  235 (U235), Plutonium 239 and Uranium 238 are of
  this type. Only U235 occurs naturally.

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Nuclear reactor
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Diagram of a Nuclear Power Plant

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Geiger Muller counter
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Geiger-Müller

• Geiger counters are used to detect radiation
  usually gamma and beta radiation, but certain
  models can also detect alpha radiation.
• The sensor is a Geiger-Müller tube, an inert
  gas-filled tube (usually helium, with halogens
  added) that briefly conducts electricity when a
  particle or photon of radiation temporarily makes
  the gas conductive.

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Units

• The activity of a source is measured in
  Becquerels (Bq), One Becquerel is one decay per
  second.
• The amount of radiation that your cells absorb is
  measured in grays (Gy),One gray is one Joule of
  energy absorbed by 1kg of your body. This is the
  dose you receive.
• Counts per minute (cpm).

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Carbon dating problem

• In an old tomb anarchaelogist found a piece of
  wood which measured 20 cpm of radioactivity.
• Estimate the age of the tomb provided that new
  wood gives 80 cpm. The half life for C-14 is 5700
  years.

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Uses of radioactivity
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Atomic bomb

• Nuclear fission - You can split the nucleus of an
  atom into two smaller fragments with a neutron.
  This method usually involves isotopes of uranium
  (uranium-235, uranium-233) or plutonium-239.
• Nuclear fusion -You can bring two smaller atoms,
  usually hydrogen or hydrogen isotopes (deuterium,
  tritium), together to form a larger one (helium
  or helium isotopes) this is how the sun produces
  energy
• In either process, fission or fusion, large
  amounts of heat energy and radiation are given
  off.

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Atomic bomb