## constant pressure

$\Delta U=q+w$

Clement Ng
Posts: 73
Joined: Sat Jul 22, 2017 3:00 am
Been upvoted: 3 times

### constant pressure

I understand that at constant pressure, q= delta H, but do you use enthalpy of the reaction or enthalpy of formation?

Christine Wastila 1H
Posts: 61
Joined: Fri Sep 29, 2017 7:04 am

### Re: constant pressure

That statement refers to the enthalpy of a reaction.

Justin Chang 2K
Posts: 53
Joined: Fri Sep 29, 2017 7:04 am

### Re: constant pressure

Yes. As long as you know the reaction is performed under constant pressure (if it's in an open beaker) then q=deltaH(rxn)

MSkye Goldwater 2K
Posts: 19
Joined: Fri Sep 29, 2017 7:05 am

### Re: constant pressure

Would you have to take into account the number of moles of gas in relation to the balanced equation when solving for q ?

Matthew Lin 2C
Posts: 30
Joined: Fri Sep 29, 2017 7:06 am

### Re: constant pressure

MSkye Goldwater 2K wrote:Would you have to take into account the number of moles of gas in relation to the balanced equation when solving for q ?

Yes it depends on how many moles the question says are reacting. For example, if the question says to find q when 3 moles of C2H6 are reacting, but your balance equation has only 1 mole of C2H6 reacting, then you must multiply the delta H of the reaction by 3 to find q.

Emma Ward 2C
Posts: 32
Joined: Fri Sep 29, 2017 7:05 am

### Re: constant pressure

as previously mentioned, it is important to note that the units for delta H are kj/mole. Therefore, when determining the enthalpy for a specific molecule or compound in a reaction, you need to take into account the stoichiometric coefficient to determine the enthalpy for that specific molecule or compound.