Hi everyone, I am Hendra Wong, A & O Level Chemistry Tutor at Bright Culture. Yesterday I ate cheese and bacon. Why cheese and bacon? Because Cheese starts with ?C ?, and Bacon starts with ?B ?. ?C ? and ?B ? sound like Chemical Bonding.
This past few weeks, the J1 one students have been starting Chemical Bonding in school.
They complain to me, wow, Hendra! Why are the notes so thick and scary? So different from Sec 3 ? But don’t worry. Sorry about the joke being a bit lame, now we continue. This very thick Chemical Bonding chapter, we can summarize in three different parts.
Part one, the small stuff, like sigma and pi bond,are those elements which can have more than 8 or less than 8 valence electrons dative bonding and dimer.
Part two, those molecular shapes, when you start drawing the dot and cross diagram, after that, you identify the number of bond pairs and lone pairs of electrons and thus you know the molecular shape and the bond angle, right? That’s part two.
Part three is about comparing and explaining the physical properties such as melting and boiling points, including those id-id,pd-pd and hydrogen bonding and so on.
Part three: Comparison and explanation of physical properties of bonds.
Today,for this short video, we do part three, about id-id, pd-pd and hydrogen (cut out chemical, it s supposed to be hydrogen) bonding. Okay, it s like this, if someone asked you, which has a higher melting and boiling point, NaCl, or HCl?
How do we handle that question? Step 1: Identify whether these two substances belong to ionic compound, simple molecular, giant molecular structure, or ionic.
Step 1: Identify which group the substances belong to.
For these two, NaCl belongs to ionic, and then HCl belongs to simple molecular, right? Thus we conclude that NaCl has higher melting and boiling point. That s point number 1.
Point number 2, you need to know the full explanation, then look at your notes and memorize them.
Example number 2: Someone asks you which one has a higher boiling point, HCl or N2? Guess. It s the same step. Step 1: These two molecules both belong to simple molecular structure.
So, we go to step 2. Step 2 will be,identify whether they belong to id-id,pd-pd or hydrogen bonding. HCl belongs to pd-pd. After that, N2 belongs to id-id. So, we conclude that HCl has the higher boiling point.
Next, example number 3. Which one has higher boiling point, HF, or HCl? Same thing again, these two molecules both belong to simple molecular structure.
So we go to step 2. HCl belongs to pd-pd, but HF belongs to hydrogen bonding. So, we conclude that HF has higher boiling point. So far so good.
Next example is someone asking you, which has a higher boiling point, HF or ammonia? Same as before. Both belong to simple molecular, and both them are about hydrogen bonding.
So, what to compare, if both are about hydrogen bonding?
So, move on to Step 3.Which one is more electronegative, fluorine or nitrogen? Fluorine, right? Yes, thus we conclude that HF has a higher boiling point than ammonia.
Next example, which one has a higher boiling point, water, H2O, or HF? This one is a bit tricky. Both of them are about hydrogen bonding, and fluorine is more electronegative than oxygen.
However, by date, water has a higher boiling point. How come? This is because H2O forms more hydrogen bonds on average per molecule, so H2O has 2 hydrogen atoms and 2 lone pairs of electrons, compared to HF, which can only form one hydrogen bond on average.
Why is it like that? To understand that, draw the dot-and-cross diagram for H2O and HF, then you will observe something. You will know that H2O molecules have two hydrogen atoms, and then two lone pairs of electrons. But then for HF, you have only one hydrogen atom, and three lone pairs of electrons. That s why water has a higher boiling point.
We still have more examples to compare melting and boiling points but it will be too long, so we ll stop here for today first. Good luck.
I am Hendra Wong,O & A Level Chemistry Tutor at Bright Culture.
Visit www.bright-culture.com/a-level-chemistry-tuition for A Levels Chemistry Tuition in Singapore
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