Intermediate bonding and bond polarity

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Intermediate bonding and bond polarity
Date
Lesson Outcomes
Task 1 I can explain the meaning of the term electronegativity as applied to atoms in a covalent bond (GRADE C)
Task 2 I can recall that ionic and covalent bonding are the extremes of a continuum of bonding type and explain this in terms of electronegativity differences leading to bond polarity in bonds and molecules, and to ionic bonding if the electronegativity is large enough I can distinguish between polar bonds and polar molecules and be able to predict whether or not a given molecule is likely to be polar (GRADE B)
Task 3 I can carry out experiments to determine the effect of an electrostatic force on jets of liquids and use the results to determine whether the molecules are polar or non-polar (GRADE A)
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BIG picture

• What skills will you be developing this lesson?
• ICT
• Numeracy
• Literacy
• Team work
• Self management
• Creative thinking
• Independent enquiry
• Participation
• Reflection
• How is this lesson relevant to every day life?
  (WRL/CIT)

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Is the pair of electron shared equally by the two
H atoms?
What is the type of bonding in hydrogen?
HH
How will the electrons be shared in HCl?
In a hydrogen molecule, one pair of electrons is
shared between the two hydrogen atoms. The
attraction between the nuclei and electrons is
same from each atom as they both are same. The
pair of electrons in the bond is equally shared.
These type of molecules are called NON-POLAR.
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HCl
In a Hydrogen chloride molecule, one pair of
electrons is shared between the hydrogen and
chlorine atom. The attraction between the nuclei
and electrons is different from each atom as they
both are different. The pair of electrons in the
bond is unequally shared. These type of molecules
are called POLAR.
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How do we know if the molecule is Polar or
Non-polar?

• We can predict the relative attracting powers of
  the nuclei of atoms of different elements by
  using the concept of electronegativity.
• What is Electronegativity?

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Electronegativity
Electronegativity is a measure of how strongly an
atom or atoms attracts the bonding electrons in a
covalent bond. The higher the electronegativity,
the stronger an atom's attraction for bonding
electrons.
We use the Pauling Scale of Electronegativities. I
t has no units and runs between 0 4.
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Pauling Scale of Electronegativities
Write any three trends that you see.
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Electronegativity Trends
What about the electronegativity of noble gases?
They have zero electronegativity as they rarely
form covalent bonds and they have no electron
affinity.
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1) Of the commonly occurring elements, this makes
_________ the most electronegative and
______________the least.
2) Atoms of the same electronegativity joined by
a covalent bond give an electronegativity
difference of zero - i.e. H2 2.1 - 2.1 0. This
indicates a normal nonpolar covalent bond.
Calculate the electronegativity difference
between C and H. How can we use the
electronegativity difference to calculate the
percentage of ionic and covalent bonding (Bond
character)?
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How to calculate the Bond character?
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59 ionic character and 41 covalent character
100 covalent character
89 ionic character and 11 covalent character
COVALENT
IONIC
POLAR
What is the relation between electronegativity
difference and percentage ionic character?
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Questions?
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Answers
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Polar covalent bond
Non-Polar covalent bond
Polar covalent bond
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Polarisation of Ionic bond

• Ionic bonds can be polarised by the cations
  strongly attracting the outer shell electrons of
  the anion.

• What does the polarising power of a cation depend
  on?
• Its charge density, which depends on the size of
  the ions and its charge.
• Smaller the ionic radius/greater the charge the
  greater the charge density.
• For an anion, larger the radius, more easily
  polarised it is.

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Questions
CLUE Ionic compounds tend to have high melting
and boiling points and dissolve in polar solvents
only.
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Answers
1 HF as there biggest difference in
electronegativity. 2
3 Difference in electronegativity between lithium
and iodine is 1.5 (2.5 - 1.0). From Table 2.1.1,
percentage ionic character 43. So possibly
lower melting and boiling points, and soluble in
an organic solvent.
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Task 1 Review
Go back to your lesson outcome grid and fill out
the How I did and the Targets column.
Lesson Outcomes How I did Targets
Task 1 Grade C Met? Partly met? Not met? How can I improve on task 1?
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What is a dipole?

• Dipole a molecule (or part of a molecule) with
  a positive end and a negative end.
• E.g. HCl molecules have a dipole.
• When a molecule has a dipole it is polarised.

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Dipole Moment

• Polarity of a molecule is measured as Dipole
  moment.
• It is defined as the product magnitude of charge
  on the atoms and the distance between the two
  bonded atoms. Its common unit is debye D.
• Direction of the polar bond in a molecule
• Arrow points toward the more electronegative atom.

• Is it possible to have several polar bonds in a
  molecule?
• Does this make the whole molecule polar?

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Determining Molecular Polarity

• Nonpolar Molecules
• Dipole moments are symmetrical and cancel out.

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Determining Molecular Polarity

• Polar Molecules
• Dipole moments are asymmetrical and dont cancel .

Resultant dipole
Because water is an angular molecule the dipoles
do not cancel and a resultant dipole moment is
produced.
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Molecules with 3 Atoms
Even though the C-O bond is polar, the bonds
cancel each other out because the molecule is
linear the dipole moments are equal and in
opposite directions.Therefore CO2 is non-polar.
CO2
The dipole moment between H-C points in the
direction of C. The dipole moment points between
C-N points in the direction of the N. Therefore
the dipole vectors are additive and HCN is polar
HCN
SO2
SO2 is a polar molecule because the S-O dipole
Moments dont cancel each other out due to the
angle
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Determining Molecular Polarity

• Therefore, polar molecules have...
• asymmetrical shape (lone pairs) or
• asymmetrical atoms

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Molecules with 4 Atoms
Non-polar
CCl4 is non-polar
Polar
CHCl3 is polar
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dipole moment polar molecule
dipole moment polar molecule
no dipole moment nonpolar molecule
no dipole moment nonpolar molecule
10.2
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Polar or non-polar?
?
?
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Task 2 Review
Go back to your lesson outcome grid and fill out
the How I did and the Targets column.
Lesson Outcomes How I did Targets
Task 2 Grade B Met? Partly met? Not met? How can I improve on task 2?
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Experiment

• To carry out experiments to determine the effect
  of an electrostatic force on jets of liquids and
  use the results to determine whether the
  molecules are polar or non-polar
• Equipment chemicals
• Burette with filling funnel
• Glass rod and dry piece of cloth
• Beaker, 600 cm3, or large diameter container
• A range of organic hydrocarbons, alcohols and
  ketones for which hazard information is
  available,
• 50 cm3 each

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Procedure
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New Information for Task 3
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Observations and interpretations

1. What happens to the jet of your liquid?
2. What happens with other liquids?
3. Why does this happen?

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Task 3 Review
Go back to your lesson outcome grid and fill out
the How I did and the Targets column.
Lesson Outcomes How I did Targets
Task 3 Grade A Met? Partly met? Not met? How can I improve on task 3?
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Review
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Homework

• Homework task
• Due date
• Criteria for Grade C
• Criteria for Grade B
• Criteria for Grade A/A

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Information

• Properties of Covalent compounds
• http//misterguch.brinkster.net/covalentcompounds.
  html
• Properties of Ionic compounds
• http//misterguch.brinkster.net/ionic.html

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Polar Bonds
..
F
N
O
H
Cl
H
H
H
B
H
H
F
F
Polar
Polar
Nonpolar
Polar
Cl
Cl
F
F
F
H
Cl
C
C
Xe
Cl
F
F
Cl
H
F
F
Cl
H
Polar
Nonpolar
Nonpolar
Polar
A molecule has a zero dipole moment because their
dipoles cancel one another.
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