How does enthelpy change as the reaction proceeds?

In summary: My concept of enthalpy change is that the change is based on limits that approach zero. Something like the heat evolved/absorbed per second (but instead of second its a infinitely small).In summary, the change in enthalpy is based on the limits of the reaction and approaches zero as the reaction proceeds.
  • #36
Chestermiller said:
From my post #28, for a single reversible reaction occurring in a reactor held at constant temperature, the equation for the variation of the mixture enthalpy H with time is given by:

$$\frac{dH}{dt}=V(r_fΔH_R-r_rΔH_R)=V(r_f-r_r)ΔH_R$$

where V is the reactor volume, rf is the rate of the forward reaction, rr is the rate of the reverse reaction and ΔHR is the molar Heat of Reaction (which is independent of concentration for an ideal solution). So,while the enthalpy of the reaction mixture is changing with time, the molar Heat of Reaction is constant.

Chet

But wouldn't the rate of the forward and backwards reaction also change with time?
DrDu said:
... and ##r=r_f-r_r## is the net rate of the reaction.
It seems like you and Chestermiller have a different concepts on ΔH. So I'm a bit confused. Is ΔH fixed or does it change during the reaction?
 
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  • #37
sgstudent said:
It seems like you and Chestermiller have a different concepts on ΔH. So I'm a bit confused. Is ΔH fixed or does it change during the reaction?
No, our concepts are quite coincident.
##\Delta H## changes with reaction are mostly negligible for all practical purposes.
 
  • #38
sgstudent said:
But wouldn't the rate of the forward and backwards reaction also change with time?
Sure. But, for an ideal gas mixture or an ideal liquid solution, ΔHR doesn't change.

So what if the forward and backward reaction rates change? That's a separate factor from the ΔHR.

Chet
 

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