In the previous post, i have mentioned that Energy Changes is one of the chemistry topic that many students find problems scoring in Chemistry Exams.
This is not a memorising topic (in fact most chemistry topics does not require memorising) but one that requires students to understand the concepts very well and then to apply it on exam questions.
Let’s check out an example that i have given to my student:
As you can see, students tend to choose “A” or “Burning Methane” as an answer – which is the Incorrect Answer!
Common Conceptual Mistakes by students:
They argue that in Burning (= Combustion Process), it should be Endothermic. They insists by giving an example such as:
“Mr Chua. Just like burning of a piece of paper, you first need to light a match-stick or start a lighter in orde to ignite the paper for it to burn. From here you can see that heat energy is taken in from the paper in order to burn. This confirm that the process is Endothermic! as per how we define it.”
Sounds logical perhaps, you might think???
This is where the students got missed up, and how examiners are ready to mark you down! They have been waiting for you to fall into this conceptual mistake.
Examiner’s Remarks:
During the burning of a piece of paper, it is true that we need to light a match-stick in order to ignite the piece of paper and cause it to burn. But students need to realise that whether a process is Endothermic or Exothermic – we are really talking about the NETT reaction / process.
Let me use some values to make more sense to it.
Assume the heat energy provided by match-stick is 10 kJ. Once it ignite the piece of paper and cause it to burn, more i mean MORE heat energy is given out to the surrounding, perhaps in the range of 100 – 1000kJ. So if you look at the NETT heat energy transfer,
Heat Energy Given Out (by paper) >> Heat Energy Taken In (by paper)
As such, Combustion or Burning is an Exothermic Process.
PS: Hope you learn something from the above example. Do drop me a comment. I would love to hear from you.
Related Articles:
- O Level Chemistry: Energy Changes (Exo/Endo) & Bond Energy
- O Level Chemistry: Energy Changes – Exothermic & Endothermic Reactions
- O Level Chemistry: Energy Changes (Endothermic & Exothermic Reactions)
- O Level Chemistry: Energy Changes (Exo/Endo) & Bond Energy
- Mole Calculations – No. 1 Killer in O Level Chemistry?
I suppose there’s no formation of bonds? It’s just gaining of electrons.
Reduction of silver ions to silver atoms in photography could be a redox reaction? Thus while gaining electrons is exothermic, the anion that the silver cation is bound to will lose electrons, giving an endothermic ‘side’ to the reaction.
Since silver has a comparatively larger atomic size, the outer electrons are further away from the attractive force of the nucleus thus lesser energy is released when the silver ion gains an electron.
Since overall change in enthalpy = Endothermic + Exothermic, since the negative exothermic value is relatively smaller, and the endothermic relatively larger, the overall reaction tends to be exothermic.
Not sure if I’m right though… haha
Why is B the answer anyway. Isnt gaining electron bond forming = exothermic?