O Level Chemistry Questions: Energy Changes and/or Periodic Table

08 Jul

Posted by: sean in: Chemistry Coaching, Chemistry Notes & Tips

Hi everyone,

One of my GCE O Level Chemistry Sec 4 student from Anglo Chinese School (ACS) sent emailed me a challenging O Level Chemistry Multiple Choice Question (MCQ) and i would like to share it with you.

At first look, it seems to be a very challenging tricky question since you would not know which chapter / topic does the question belongs to. I was contemplating between using essential concepts from either ENERGY CHANGES (Exothermic & Endothermic) or TRENDS IN PERIODIC TABLE to answer the question.

Let’s take a look at the question:

Question 1:

Hi sean, I came across this question and I would like to know the answer.

Which pair of elements, described by their proton number, will react together most exothermically?

A. 1 and 8
B. 3 and 10
C. 9 and 19
D. 12 and 16

Suggested answer and reasoning by my student as follows (i always encourage my student to analyze the question for possible answers and propose it, before asking me for an answer, cos that is how they learn effectively):

I know for sure that B is wrong since proton number 10 is for an unreactive noble gas. My guess is 12 and 16, because both are 2+ and 2-, so the overall charge is greater than the rest.

Which is the correct answer? And on what basis do I use to determine the correct answer?

So all Chemistry students out there (be it GCE O Levels, GCE A Levels, IGCSE, IB and IP Chemistry), what is your take?

Feel free to give your answers & reasoning in the “LEAVE A REPLY” section below. I would love to hear from you.

PS: I will come in at later stage to give my suggested answer and reasoning

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5 Responses

Kwee
13|Jul|2009 1

You can’t tell if proton number 12 (magnesium) with 16 (sulfur) actually gives an exothermic reaction; even then, 1 (hydrogen) with 8 (oxygen) is a combustion reaction, hence it is highly exothermic. Combustion reactions are highly exothermic, so option A is most likely the answer.

Reply
sania Reply:
August 22nd, 2009 at 8:09 pm
the cOrrect answer is C. i.e 9[fluorine] with 19[potassium]..coz both the elements in their series are at the tOp like fluorine [the most reactive halogen] and potassium the most reactive in metal reactive series!

Reply
sania
21|Aug|2009 2

although i`m not that gud at this subject,but still i wanna give it a try!
accordin to me,its option C. i.e 19[Potassium] with 9[Fluorine],as potassium lies on
the tOp position in the reactivity series and therefore is most reactive metal,and with
it fluorine is also the most reactive non-metal in group VII,the reaction b/w an alkali metal and halogen is very active as both elements are on their top positions.Ionic Bond will be formed
b/w the two which`l b quick and more exothermic

it can`t be magnesium and sulphur because cOmparatively these elements are less active
than 9 n 19!

this is what i think is right!

Reply
Al NASAR AHMED
29|Aug|2009 3

I’m confiused.First explain me that does amount of heat produced in a reaction
depends upon the reactants valency or reactivity.

Reply
Razia Alex
16|Oct|2009 4

Options C & D are between metal and a non-metal where transfer of electrons take place. B a reaction cannot take place one of them is an inert gas. But A is between non metals and electron affinity is very high for these two elements therefore exothermic

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