mCAT clarification minus or not? (1 Viewer)

[enigma_1]

In molar heat of combustion, which one do we use?

Why are there 2 ???????? Confusing much

 

[sabz1996]

There aren't two. The proper formula has a negative which is used to show whether or not the reaction is exothermic (releases heat) or endothermic. If the reaction is an exothermic one, the negative sign will remain at the end once you have finished the calculation and it is there to show that heat was released. On the other hand, for endothermic reactions, once you have finished the calculation, you will end up having a positive number which shows that heat was absorbed.

I think in many multiple choice questions, they ignore the negative sign and just have the number which can confuse people but I think to be on the safe side, you should leave it in short answers because I remember looking at the marking guidelines for a question and they actually took a mark off for not showing it was an exothermic reaction. (i.e. leaving the negative sign)

 

[enigma_1]

uhh ok thank you. Can you tell me, using the negative sign for the formula

How do you know if it's exo or endo? 3 years and I still dunno

 

[nerdasdasd]

uhh ok thank you. Can you tell me, using the negative sign for the formula

How do you know if it's exo or endo? 3 years and I still dunno

If the heat is on the right of the equation, then it is exo, (think of it like.. Chucking heat away).

 

[someth1ng]

uhh ok thank you. Can you tell me, using the negative sign for the formula

How do you know if it's exo or endo? 3 years and I still dunno

It really depends on the reaction. For example, combustion reactions would be exothermic.

It's more important to understand what causes some reactions to be exothermic while other reactions are endothermic and it's to do with bond energies. Each bond has a specific bond energy which is negative and denotes how much energy is required to break that bond (energy is NOT stored in a bond). If the products have more bond energy (more negative), energy must have been released during the reaction and if the reactants have more bond energy, energy must have been absorbed during the reaction.

 

[mrpotatoed]

q=mcat is used to calculate the amount of heat absorbed by the CALORIMETER

H=-mcat is used to calculate the change in enthalpy of the substance realeasing / absorbing heat.

Yes there are two, not much of a difference... you can use one and change the sign depending on what you are finding.

 

[Queenroot]

Strong bump

 

[dan964]

For molar heat of combustion
Q=mcΔT (Q is generally in kJ or J)
ΔH = -mcΔT (ΔH is almost always in kJ/mol)

Say the molar heat of combustion is ΔH=-10kJ/mol
you can write it as
"molar heat of combustion is 10kJ/mol" (preferred)
however if using ΔH, must be negative.

 

[enigma_1]

q=mcat is used to calculate the amount of heat absorbed by the CALORIMETER

H=-mcat is used to calculate the change in enthalpy of the substance realeasing / absorbing heat.

Yes there are two, not much of a difference... you can use one and change the sign depending on what you are finding.

For molar heat of combustion
Q=mcΔT (Q is generally in kJ or J)
ΔH = -mcΔT (ΔH is almost always in kJ/mol)

Say the molar heat of combustion is ΔH=-10kJ/mol
you can write it as
"molar heat of combustion is 10kJ/mol" (preferred)
however if using ΔH, must be negative.

Thanks

 

[enigma_1]

Strong bump

Haha this was one of those things "they" confused us with

 

[D94]

From a thermodynamics view, in a closed system, what we have is Q-W=∆U. If there is work done onto the system or work done by the system, Q=∆U+W=∆H=m(c_p)∆T. But if there is no work transfer, Q=∆U=m(c_v)∆T.
So it's important to know if there is work transfer since that affects the specific heat capacity.

Whether it's minus or positive is purely dependent on whether heat is going into the system or heat is going out of the system. Heat in is positive, heat out is negative.

 

[D94]

For molar heat of combustion
Q=mcΔT (Q is generally in kJ or J)
ΔH = -mcΔT (ΔH is almost always in kJ/mol)

Say the molar heat of combustion is ΔH=-10kJ/mol
you can write it as
"molar heat of combustion is 10kJ/mol" (preferred)
however if using ΔH, must be negative.

Since when? The base units for enthalpy is the Joule. If your specific heat capacity is in a per mol basis, then sure, enthalpy will be in a per mol basis, but it would no longer be written as H, but rather, H with a bar on top.

(Or does HSC Chemistry not distinguish between the two?)

 

[anomalousdecay]

Since when? The base units for enthalpy is the Joule. If your specific heat capacity is in a per mol basis, then sure, enthalpy will be in a per mol basis, but it would no longer be written as H, but rather, H with a bar on top.

(Or does HSC Chemistry not distinguish between the two?)

Can't remember any of this too well, but looking at the units, H should be in units for Energy. To get a unit of Energy per mole you need to multiply by Avogadro's number.

But yeah there is no H bar in HSC Chemistry from what I recall.