Question

What are ΔE, q, w and ΔH for the evaporation of 18.3 g of Br2(l) at 298 K and 1 atm?

Answer 1

Calculate the delta H of the evaporation of Br2 (l), which is a liquid at room temperature.

  The equation for the formation of FeBr3(s) is:

Fe(s) + 3/2 Br2(l) --> FeBr3(s) DH = -269 kJ

For your second equation, you are given:

2 Fe(s) + 3 Br2(g) --> 2 FeBr3(s) DH = -631 kJ

evaporation of bromine

Br2(l) --> Br2(g) and its DH.

So, you should halve the second equation and flip it over.

That changes the sign of DH and give you:

FeBr3(s) --> Fe(s) + 3/2 Br2(g) DH = +315.5 kJ

Now, if you just add that to the first equation and cancel everything that appears on both sides,

you are left with your desired equation.

When you add the two DH values you have:

Br2(l) --> Br2(g) DH = +47

molar heat of evaporation of bromine formula:

Heat = mass x molar heat capacity