Chem Syllabus Screwed. How to go about telling BOS? (1 Viewer)

[munkaii]

It's a minority of the Prelim Chemistry Syllabus but the Syllabus i have got from my teacher (i think printed directly from BOS for this year) shows the the formula for measuring heat as
h=-mc:delta:t

Why the minus sign i asked my teacher and being the poor teacher she is, gave me all this mumbo jumbo and told the class to move on. The HSC syllabus says minus when it should infact be a plus.

Any way to alter this because she has threatened that if i dont use the minus sign in an exam, i will lose marks for it. (Note: Whole Chemistry Faculty in our school is not flash).

 

[nit]

The -ve sign is present due to the fact that there are two aspects of the universe - the system and the surroundings basically. The system is the region of the universe of interest - basically in each of these calorimetry experiments it is the substance whose heat of reaction you are investigating. The surroundings is the region that heats up or cools down as a result of the exothermicity or endothermicity of the reaction occurring within the system. So an increase in the temperature of the surroundings implies heat was lost from the system and that the reaction that occurred was exothermic (hence -ve dH), and vice versa.

 

[Trebla]

It's a minority of the Prelim Chemistry Syllabus but the Syllabus i have got from my teacher (i think printed directly from BOS for this year) shows the the formula for measuring heat as
h=-mc:delta:t

Why the minus sign i asked my teacher and being the poor teacher she is, gave me all this mumbo jumbo and told the class to move on. The HSC syllabus says minus when it should infact be a plus.

Any way to alter this because she has threatened that if i dont use the minus sign in an exam, i will lose marks for it. (Note: Whole Chemistry Faculty in our school is not flash).

I think you have confused yourself.
Here is what I think:
The formula for the AMOUNT OF HEAT RELEASED OR ABSORBED is:
q = mCΔt

the formula in the syllabus is
ΔH = - mCΔt

q: is the amount of heat (can be absorbed or released)
ΔH: is change in enthalpy or HEAT ABSORBED(it is not heat released)

The formula: q = mCΔt
This measures the heat in general. It does not matter whether it is released or absorbed. The answer should be positive all the time.

The formula:
ΔH = - mCΔt
This measures the amount of heat absorbed. The negative is there is distinguish between exothermic and endothermic reactions. I think the ΔH concerns with the heat within the particles. Therefore when you sub in some numbers, the negative shows the amount of heat lost within the particle and released to the surrounding environment. It is negative, because positive is defined as heat abosrbed and having a negative amount heat absorbed means there is release of heat (exothermic).

Anyway, that's what I think.
People, correct me if I'm wrong...

 

[insert-username]

Anyway, that's what I think.
People, correct me if I'm wrong...

You're correct. The negative simply changes the equation from heat in general to heat absorbed. You shouldn't need to get too worked up over this - your teacher won't be marking your HSC paper.


I_F

 

[munkaii]

Yep i picked it up.

My teachers excuse was quite blatant but now I understand.

Though with q, i believe the value can be less than zero if its an exothermic reaction as heat is being released from the particles to the surroundings? Then again, i rely moreso on results of an experiment that the positive and negative signs to determine whether reactions are exo or endothermic.