Question

If the total change in heat energy (∆H) is 400 J for a chemical reaction, then the reaction is

endothermic

exothermic

abiding by the conservation of energy

Answer 1

since most things do in classical science abide by the last one then so, presumably, will this one ?

Answer 2

I think it's C, but only because exothermic and endothermic relate to energy entering or leaving the system.

Answer 3

Basic facts -->
∆H>0 Reaction is Endothermic
∆H<0 Reaction is Exothermic

∆H= Sum of Products ∆Hf- ∆Hf Sum of Reactants

So:

If the ∆H is positive --> means that the system absorbed heat (aka heat is on reactant side of equation)

If the ∆H is negative --> means that the system gave off heat (aka heat is on product side of equation)

Answer 4

I think it's C because most things end up being C ... it's one of the "catch all" principles of science. Albeit that it should "really" be MASS-ENERGY in view of relativity. But, I guess that here we're talking about very very low energies (probably not in the MeV region), so, there's no need to invoke it.
Yes indeed, there are the exo/endo thermicitities as well, but C catches all that. If the option WEREN'T in the q, then it wold be a different matter.

Answer 5

@osprey but the reaction is clearly endothermic?

Answer 6

yes and abides be the conservation of energy. It's the wording of the q that I don't like. And, no, I'm not blaming the poster, 'cos I haven't seen, and am an unlikely to see, the original q.

Answer 7

Oh XD yeah OP is nowhere to be seen but...
#3 is obviously true tooo... so yeah weird that they would put that as one of the options hahaha

Answer 8

@osprey

Answer 9

@sunnnystrong in one sense the question's answer is so "obvious" that ... but as I say, can't see the original q. Ho hum ...

Answer 10

@Monpon123 In my mind, all things "abide by the conservation of energy" so any answer that challenges this would need extraordinary proof.

An exothermic reaction is a chemical reaction that releases energy by light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation: reactants → products + energy.

Since heat is produced (delta heat is +) I can't avoid the answer:

exothermic AND abiding by the conservation of energy.

Answer 11

@sunnnystrong
If (bad) memory serves me ...

Enthalpy is a function of state.
so it only depends on the starting and finishing values to calculate the change. A bit like grav pot energy in mechanics, I think.
First law of thermo ...
dQ=dU+dW isn't a function of state

dQ=dU+PdV hydrostatic system

dH=dQ+VdP

Question, now I relook at it, is is H ENTHALPY or is it something else ... ie is it "heat energy". Good old thermo ... so many variables whizzing around.

Answer 12

@bugman954 ...
no if the change in enthalpy is positive ... that means that the system absorbed energy from the surroundings there fore is endothermic

Answer 13

@osprey...
yeah.
idk, this is CLEARLY an endothermic reaction is clearly abides by the conservation of energy.

XD

Answer 14

@sunnnystrong point granted, delta H was what was stated and I took H for Heat because the words Heat energy were in the question, This seems to be what @osprey is saying to. I was unaware that H was also the standard symbol for Enthalpy. If this is the context, I stand corrected still think it a combination of "endothermic" AND "abiding by the conservation of energy"

Answer 15

@bugman954 what made me think of enthalpy was both the delta and the H. Since this is a chemical reaction (whatever the reaction actually is) then it looks like thermochemistry. In other words, heat is being produced by a molecular/atomic rearrangement of bits, bonds breaking, bonds making, pressures changing, volumes changing etc.
All of that seems to be summarised by enthalpy, or so my rust laden brain tells me.

I think that the below stuff is right ...
As an aside, I draw attention to "thermochemistry" to compare it to my old favourite which is "radiochemistry". This is when a piece of radioactive substance is assayed for its contents. I believe that it was this that lead Professor Lisa Meitner and Professor Otto Hahn to think/believe/conjecture that they had split the atomic nucleus way back in the 1930s. Professor Hahn got a Nobel prize for this work, whilst Professor Meitner seems to have been .... A similar fate that seems to have befallen Mileva Maric-Einstein, wife to Albert.

Some time ago, I tried to find the Nobel citation for the splitting of the nucleus, and failed. I smiled wryly when I realised why I'd failed. It seems that it wasn't awarded for physics, it was awarded for chemistry - radio chemistry.