Question

Hydrogen ( H2 ) reacts with bromine ( Br2 ) to produce hydrogen bromide (HBr) gas as shown in the equation below.

mc021-1.jpg

Standard Enthalpies of Formation
Substance
DHf (kJ/mol)
Br2 (l)
0.0
Br2 (g)
30.907
HBr (g)
–36.4
HCl (g)
–92.307
HI (g)
26.48
H2 (g)
0.0
I2 (g)
62.438

Based on the equation and the information in the table, what is the enthalpy of the reaction?
–134.6 kJ
–103.7 kJ
103.7 kJ
134.6 kJ

Answer 1

Use ΔH°rxn = ΣΔHf°(products) − ΣΔHf°(reactants).

Reaction: H2(g) + Br2(g) → 2 HBr(g)

ΔH° = 2(−36.4) − [0.0 + 30.907] = −72.8 − 30.907 = −103.707 ≈ −103.7 kJ

Answer: −103.7 kJ.