How does enthelpy change as the reaction proceeds?

[sgstudent]

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, r_f is the rate of the forward reaction, r_r is the rate of the reverse reaction and ΔH_R 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?

... 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?

 

[DrDu]

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.

 

[Chestermiller]

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, ΔH_R doesn't change.

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

Chet