HELP

[gabrielle__]

I was doing a bit of Chem revision today and came accross something that confused me a little bit.
A question on one of my revision sheets asked:

In an experiment, some methane and steam are mixed in a 1L vessel at 1030K wiht a nickel catalyst and equilbrium is reached.

CH_4(g) + H₂O_(g) (double headed arrow) CO_(g) + 3H_2(g) Delta H = +206kJ/mol

a) More CO is added. Will the temperature in the vessel rise, fall or remain constant? Why?
Ans: The temperature will rise and the reaction will tend to go to the left to reduce the number of CO molecules. Left is exothermic (energy prodcued)


My confusion:
It states that K is only dependent on Temperature, however in this case it states that increasing the amount of a product will effect the temperature. From this it can be said that any other factor that effects equilibruim will have a change in temperature and hence, K will also change.

So if in an exam or SAC and question asked: How does "increasing the pressure" effect the value of K?
and I were to right "It has no effect as K is only dependent on temperature", will I be wrong?

HELPPPP ME.

[Collin Li]

How does "increasing the pressure" effect the value of K?

When you are asked this, you assume everything else stays constant. So even though it will cause an energy flow in or out of the system, that is not necessarily an increase in temperature. "Heat" is not necessarily an increase in temperature, because I might simply allow that heat to escape (via cooling or heatsink).

So no, even though the position of equilibrium changes, would not change, by assumption.

In VCE, you don't really do this rigorously. If it was done properly, you'd hear terms like:

- adiabatic (no heat flow across the system, i.e: if heat is released, temperature goes up)
- isothermal (no temperature change)

Those aren't mutually exclusive, by the way.

[Collin Li]

I agree that the two questions have different assumptions attached. It's unfortunate that the VCE course does not spell these out explicitly in the question.

In the first one (where T changes), we are assuming an adiabatic system (no heat flows in or out of the system), so the heat generated will lead to an increase in temperature.

In the second one (where T doesn't change), we are assuming an isothermal system. Any heat generated or lost by the system will be compensated by an equal and opposite heat flow, so that the temperature remains the same (for example, a reaction vessel that is in contact with the atmosphere, will go back to room temperature).

[gabrielle__]

So which assumption do I use?

[gabrielle__]

....In an exam?

[Collin Li]

Typically, for VCE questions, you'll assume the isothermal case (keep temperature constant), unless it implies otherwise. The first question was worded in a way that suggested the reaction vessel was adiabatic (heat is trapped in the system).

[gabrielle__]

ohkay!
thanks for your help xoxo