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What determines if a molecular substance is a solid, a liquid or a gas? clue: think between molecules!

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Presentation on theme: "What determines if a molecular substance is a solid, a liquid or a gas? clue: think between molecules!"— Presentation transcript:

1 What determines if a molecular substance is a solid, a liquid or a gas? clue: think between molecules!

2 intermolecular forces What happens to covalently bonded molecules as they change state? gasliquidsolid When a solid melts or a liquid boils the energy put in is being used to overcome the INTERMOLECULAR forces.

3 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.

4 Permanent Dipoles These occur when a two atoms sharing a covalent bond have substantially different electronegativity. HCl has a permanent dipole as Cl has a much higher electronegativity than H. Molecules with a permanent dipole are described as polar molecules.

5 Bonded atoms have similar electronegativity. Electron cloud evenly distributed; no dipole Cl

6 At some instant, more of the electron cloud happens to be at one end of the molecule than the other, electron density is constantly changing.

7 Cl At some instant, more of the electron cloud happens to be at one end of the molecule than the other; molecule has a dipole.

8 Instantaneous dipoles… If this happens near other molecules it can cause induced dipoles

9 Induced dipoles The permanent dipole of a molecule can induce a dipole in an unpolarised molecule. A dipole can also be induced by the effects of an instantaneous dipole.

10 Dipoles & Intermolecular Forces  All intermolecular forces arise from the attraction between dipoles.  There are three types of attraction.  permanent dipole-permanent dipole.  permanent dipole-induced dipole.  instantaneous dipole-induced dipole.

11 Instantaneous dipole – induced dipole Much weaker than other types of intermolecular forces. Occur between all molecules. noble gases alkanesThey can be observed in most easily in noble gases and alkanes. Xe This atom has an instant dipole This atom is unpolarised, at the moment! Xe The electrons are repelled and an induced dipole occurs

12 Factors affecting instantaneous-induced dipole intermolecular forces? Atom or molecule size. Molecular shape.

13 Atom or molecule size Larger molecules (or isolated atoms) will have more electrons, hence the strength of the dipole interaction with be greater.

14 Molecular shape In straight chain alkanes there more contacts between the atoms of different molecules. Therefore more opportunities for induced dipoles to occur. This is the reason straight chain alkanes have higher boiling points than their branched isomers.

15 Polar molecules? Decide if these molecules are polar or not. and WHY they might be.

16 Bond polarity v. Molecular polarity  A polar molecule is a molecule which has a permanent dipole.  The spatial arrangement of polar covalent bonds is an important factor in determining if a molecule is polar or not.

17 Permanent dipoles–permanent dipoles Present in polar molecular substances Act as additional electrostatic forces of attraction between these molecules. Stronger than instantaneous dipole – induced dipole interactions for molecules of equivalent size.

18 Dipole - dipole interactions There are also permanent dipole – induced dipole forces AND instantaneous dipole – induced dipole forces as well. Overall; more attraction than repulsion, this is one reason why the molecules stay as a liquid.

19 Effects of permanent dipole interactions If we have two substances, one polar and one non-polar, whose molecules have the same mass (and so approximately the same number of electrons), the polar substance will have a higher boiling point than the non-polar substance.


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