November 24: Metallic Bonds & Lab Design Review

November 24: Metallic Bonds & Lab Design Review

In class we finished up on metallic bonds, reviewing what we knew and adding a bit more.

A quick question sheet was handed out, referring to: melting points, ductility, malleability and conductivity (questions below).

Questions:

1. Discuss the melting point of Mg (which is 6500oC) with Na (which is 97oC).
2. Explain why Li has a melting point of 180oC, but K has a melting point of 63oC.
3. Describe why Copper allows to be ductile, malleable and a good conductor.

Additional Information:

Alloy Structure Diagram

Steel: is a mixture of Fe and C. Adding Cr makes stainless steel (used in cutlery), in addition adding Ti (and other metals like V) makes titanium steel (used in armour plating and aircrafts).

Brass: is an alloy of Co and Zi (used in taps and musical instruments). Bronze is an alloy of Co and Sn (used in statues). Cupro-nickel is used in 'silver' coins. All of these are corrosion resistant.

We also covered Metallic Characteristics, meaning the set of chemical properties associated with elements that are metals. These chemical properties result from how readily metals lose their electrons. Metallic characters includes the ability to be reduced, formation of ionic chlorides and basic oxides, and the ability to displace hydrogen from dilute acids.



Relating to metallic characteristics, we covered trends and influences for certain switches between metals and non-metals. 

-------------------------------------------------------------------------------------

Lab Design Review:

In class as well as going over metallic bonds, we also used class to go through the IB1 Chemistry Analysis and Evaluation rubric. We covered systematic errors, conclusions and data.

Systematic Erros:

1. Folding of filter paper/surface area
2. Time
3. Wind draft
4. Room temp.
5. length of run/scales
6. Elastic band coverage

Conclusion:

To understand the conclusion and receive full marks you need to understand the differences between: interpreted conclusion, the actual conclusion and the contextual conclusion. The main difference is what you make your conclusion upon. For example the interpreted conclusion should be done on your general data, where as the actual conclusion should focus on processed data. Finally the Contextual conclusion should be according to the theory of the experiment.

Data:

For the data recordings, the main emphasis was placed on "keeping units and uncertainties" correct and according to their relevant points of interest.

Also you shouldn't forget to include qualitative and quantitative observations.

***

November 14: Intermolecular Forces

In  class, we learned about 4.4- Intermolecular Forces 

The Physical Properties of molecular substances result from different types of forces between their molecules

Our key understandings for this topic were:

        Intermolecular forces include London (dispersion) forces, dipole-dipole forces and hydrogen bonding
    The relative strengths of these interactions are London (dispersion) forces < dipole-dipole forces < hydrogen bonds << covalent bond

London (dispersion) forces: 

  These are the weakest form of intermolecular force        
       Arise due to electrons constantly moving around → electrons are unevenly spread
    This produces temporary instantaneous dipoles
      An instantaneous dipole can induce another dipole in a neighboring particle 
    This results in a weak attraction between the two particles
      Their strength increases with increasing molecular mass

Dipole-Dipole Forces:

                   Still relatively weak, but the attraction is stronger than London (dispersion) forces

                     Occurs when polar molecules are attracted to each other by electrostatic forces                         
                     Permanent dipole: when one end of the molecule is electron deficient with a partial positive charge (δ+), while the other end is electron rich with a partial negative charge (δ–) 
                     Dipole-dipole forces occur when the opposite charges on neighboring molecules    attract each other

Hydrogen Bonding:

                  
 Hydrogen bonds are the strongest form of intermolecular attraction 
 Occurs when hydrogen is (covalently) bonded directly to a highly electronegative element, such as fluorine, oxygen or nitrogen

 

This causes boiling points of substances that contain them to be higher than what would be expected from their molar mass

Physical Properties → related to bonding type:

Melting/Boiling Points 
-       The melting & boiling points (of simple covalent molecules) depend on the type of forces of attraction between the molecules:

Hydrogen bonding > Dipole-dipole > London (dispersion) forces

  Solubility
-       ‘Like dissolves like’
-       Polar substances tend to dissolve in polar solvents
-       Non-polar substances tend to dissolve in non-polar solvents

Ex: Water H₂O (which shows hydrogen bonding) will be good for dissolving Ethanol C₂H₅OH (which also shows hydrogen bonding), but it won’t dissolve Hexane (only London forces) → oil & water won’t mix

  Volatility
-       The weaker the intermolecular forces, the more volatile the substance

  Electrical conductivity
-       For conductivity to occur the substance must possess electrons or ions that are free to move