# Question #fb528

##### 1 Answer

For ideal gases,

Recall that

In an open container, we should recognize the condition of **constant pressure** (the phrase "open to the air", or "coffee-cup calorimeter", imply constant pressure as well). As a result, we can use this equation:

#DeltaH = DeltaE + Delta(PV)#

#= DeltaE + PDeltaV + cancel(VDeltaP + DeltaPDeltaV)#

#= DeltaE + PDeltaV#

where the result is something you should recognize (it was given in your textbook). Note that

Your two reactions were:

#"H"_2(g) + "Br"_2(g) -> 2"HBr"(g)# #" "" "" "" "" "" "color(red)((1))#

#"C"(s) + 2"H"_2"O"(g) -> 2"H"_2(g) + "CO"_2(g)# #" "" "color(red)((2))#

If we assume ideal gases, then in

Note that the change in volume due to gas formation is **significantly more** than due to liquid formation, for instance (gases take up way more space, since they generally have a density over 1000 times as small as liquids or solids).

In this approximation, *nonnegative in volume*, since

Therefore,