[ultrashogun]

In my chem book it says the following reguarding the reaction C + 2H2 ---> CH4, The reaction has deltaS = +24,1kJ and deltaH = -74,8kJ, so at STP deltaG = -50,7kJ.

Now it say the following : "The reaction is exothermic, with a variation of enthalpy equal to -78,k kJ. Part of this energy is used to cause more order into the system (diminishing the entropy). 24.1 kJ are used for this, resulting in only 50,7Kj disposable for doing work"

This confuses me because I imagine that if deltaH is - 74,8kJ then thats how much heat the system emmits, so what is keeping me from using that heat?

In my mind I contrast this with the reaction NH4NO3(s) ---> NH4NO3(aq) which has a positive deltaH, but a larger TdeltaS. It cools the surrounding by means of its positive deltaH, yet happens spontaneously due to a negative deltaG.

In the second example I understand that the surrounding actually "feels" the deltaH, the deltaG actually only being a measure of if iz happens or not. In the first example it seems that the universe "feels" the deltaG, because part of the deltaH is "used" to create order.

Thanks for your *delete me*!

[Sikhandar]

In my chem book it says the following reguarding the reaction C + 2H2 ---> CH4, The reaction has deltaS = +24,1kJ and deltaH = -74,8kJ, so at STP deltaG = -50,7kJ.

Now it say the following : "The reaction is exothermic, with a variation of enthalpy equal to -78,k kJ. Part of this energy is used to cause more order into the system (diminishing the entropy). 24.1 kJ are used for this, resulting in only 50,7Kj disposable for doing work"

This confuses me because I imagine that if deltaH is - 74,8kJ then thats how much heat the system emmits, so what is keeping me from using that heat?

No the system does not emmit 74,8 kJ, the system must rearrange in a definite configuration written in the products part of the reaction; in doing this, it emmits 50,7 kJ. The system MUST use the 24.1 kJ to reach the products "configuration" (more order!)

In my mind I contrast this with the reaction NH4NO3(s) ---> NH4NO3(aq) which has a positive deltaH, but a larger TdeltaS. It cools the surrounding by means of its positive deltaH, yet happens spontaneously due to a negative deltaG.

In the second example I understand that the surrounding actually "feels" the deltaH, the deltaG actually only being a measure of if iz happens or not. In the first example it seems that the universe "feels" the deltaG, because part of the deltaH is "used" to create order.

Thanks for your *delete me*!

No! The delta G is an index of where your system is going, imagine it as a gradient of a plane of a ball on the top of a hill. If this gradient is negative, the ball will go somewhere else, releasing heat in doing this; if this gradient is positive, the ball won't go in any place. Remember that a system at the equilibrium has delta G = 0, the system is not going anywhere. The entropy and the enthaply are details of the system, G has been invented to check where the system is going. Ok?

[ultrashogun]

I understand deltaG, the confusion is just that in the one example he tells me that the system emmits deltaH - TdeltaS kJ and in the other example it seems as if the system absorbs deltaH kJ.

BTW, are you from Italy?

[Yggdrasil]

The system will emit delta(H) amount of heat, but delta(G) defines the amount of work one can extract from the system.  This distinction comes about because not all of the energy from heat can be converted to work.

[Sikhandar]

The system will emit delta(H) amount of heat, but delta(G) defines the amount of work one can extract from the system.  This distinction comes about because not all of the energy from heat can be converted to work.

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[ultrashogun]

Thank you, that helps me.