Sixth form chemists are starting to use their knowledge of structure and bonding to explain the properties of various materials including melting points, solubility in water and electrical conductivity in various materials.They had to discuss the changes observed when sulphur is heated.
Tony Peek took a rather boring animoto from an earlier post and produced a vibrant remix with cool sounds and sound science.
….melts to form a runny yellow liquid at 113 degrees celcius
Continued heating leads to a highly viscous substance that cannot be poured from the test tube.
with further heating the deep red viscous form of sulphur becomes more fluid. At its boiling point, 445 degrees celcius, the molten sulphur can be poured out of the test tube.
Boiling sulphur quenched in cold water produces a different form (allotrope) called plastic sulphur because of its elastic nature.
Different forms of the same element are called allotropes. Diamond and graphite are allotropes of the element carbon.
Produce a powerpoint presentation a short video or poster to explain the changes in sulphur observed as it is heated up to its boiling point. Think about the particles that are present at each stage. What are the forces between them and how does this affect their properties (in particular viscosity) Here are some key points.
Covalent bonds between atoms are strong ( water doesn’t break down into hydrogen and oxygen when you heat it to make a cup of coffee before school). It takeas a lot of energy to break covalent bonds
The forces beween molecules are comparatively weak ( they are called Van der Waals forces)
Van der Waals bonds are elcctrostatic in nature. Molecules can have a permanent dipole. All molecules have transient dipoles that are continually switching on and off
The cumulative effect of transient dipoles increases with molar mass (the size of molecules).
Covalent bonds within molecules (inter) are strong. If some of these bonds are polar the molecule can have a permanent dipole. This influences the size of the Van der Waals forces between molecules (intra).
Van der Waals forces and permanent dipoles.
The steps involved in deciding whether a molecule has a permanent dipole include:
Deciding if there is a significant electronegativity difference between atoms in the covalent bonds. Are there polar bonds in the molecule?
From the Lewis diagram predict the shape of the molecule.
Is the shape symetrical with a zero vector addition of dipoles (non polar molecule)
Is the shape asymetrical with a non zero vector addition of dipoles ( polar molecule)