Secondary School Chemistry - Qualitative Analysis Flashcard

Happy New Year!

With the new year ahead, I hope to make your studies more effective, and allow all to make good use of pockets of time in your revision.

As such, I am creating apps on Google Play, where the notes are converted into small bite- size flashcards, where you can download and revise them on the GO.




The first one created is the Qualitative Analysis flashcard, available at Google Play. Click here to go to download.






Safety First

With the new school year starting in just a week's time, I would like to share this video, emphasising the importance of safety first.






I really appreciate the CSB board (U.S chemical safety board), where they share detailed findings and learnings from incidents. It's always better to learn from others' lessons than ours.

While this video is related to school demonstration, it really also applies to students who are doing the laboratory experiments. We may sometimes be playful, sometimes be stressed to do things fast, but always think when you are in the laboratory, particularly during the handling chemicals and hot surfaces. What are the risks involved, why should you be doing/ why should you not be doing something, what are the potential consequences, and finally, always remember your personal protective equipment (e.g. safety googles).

The safety precautions --- you memorise them as part of your exams, but they are not just for exams, more importantly, it's for your own safety.



Degree of Unsaturation: Worked Solution - Organic Chemistry Quiz 4

This post is the worked solution for organic chemistry Quiz 4 which talks about the degree of unsaturation of organic compounds. If you have not tried this question, you may like to visit this post first.

Saturated vs Unsaturated Compounds

From the alkanes and alkenes chapter, we learn that alkanes have only single bonds and are saturated compounds. On the other hand, alkenes have double bonds, and are unsaturated compounds. A common test for saturation (or to distinguish alkanes from alkenes) is to add aqueous bromine (in the dark). Alkenes will turn reddish brown bromine colorless. For alkanes, bromine will remain reddish brown.

This video from youtube is a good illustration of the test:



Formula of Alkane and Alkene

In O Levels, they generally tell you that alkane has a general molecular formula of CnH2n+2, while alkene has a general molecular formula of CnH2n. Note that the alkene general molecular formula of CnH2n only applies to straight chain alkene with just one double bond. What happens when there is more than one double bond?

Every additional double bond you have will decrease the number of hydrogen atoms by 2.

Worked Solutions to Quiz 4

For a straight chain alkane molecule with 20 carbon atoms, we would expect 42 hydrogen atoms (using the formula of CnH2n+2. For every double bond you have, the number of hydrogen atoms will decrease by 2. Since there are 30 hydrogen atoms in this hydrocarbon, we will expect 6 double bonds.

Number of double bonds = (42 - 30)/2 = 6

One mole of double bond will react with one mole of bromine. With 6 double bonds, it means 6 moles of bromine.



Organic Chemistry - Quiz 4

This quiz for this week is on organic chemistry. You may like to revise the chapter on alkanes and alkenes, as well as the chapter on alcohols, carboxylic acids, esters and macromolecules first before attempting the question. 





A straight chain hydrocarbon has a molecular formula of C20H30. How many moles of bromine will react with one mole of this compound?

a. 4
b. 5
c. 6
d. 7



Hint: This question is all about looking at the degree of unsaturation in organic compounds. Why is the general formula for alkane CnH2n+2, while that for an alkene (with a single double bond) is CnH2n?

Full worked solution for this quiz can be found here.

Exams are round the corner

The end of the year signifies major examinations before the holidays. Are you ready for it?

October till mid November is the period where the GCE Chemistry (and of course other subjects too), which is just on an average, a month from now.

Are you ready?