A-level Chemsistry- Hess' Law

Calculate the standard enthalpy change for the following reaction using the enthalpy changes given
SO ₂(g)+2H ₂S(g)------------>3S(s)+2H ₂O(l)
ΔH°c: S(s): -297KJmol-1
ΔH°f: H ₂O (l):-286 KJ mol-1
H ₂S (g):-20 KJ mol-1

Reply 1

TSR Jessica

Sorry you've not had any responses about this. :frown:

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Here's a link to our subject forum which should help get you more responses if you post there. :redface:

Reply 2

JIMBO789

I could give you the exact answer, but it's against community guidelines. It's not actually Hess' law. You have to use the sum of all the moles of products-the sum of all the moles of reactants

Reply 3

he3456

Original post by JIMBO789

I could give you the exact answer, but it's against community guidelines. It's not actually Hess' law. You have to use the sum of all the moles of products-the sum of all the moles of reactants

I know the exact answer but I just don't know how to get to it

Reply 4

JIMBO789

Original post by he3456

I know the exact answer but I just don't know how to get to it

You weren't given the SO2's standard enthalpy?

Reply 5

username3249896

Draw a Hess cycle

SO ₂(g)+2H ₂S(g)------------>3S(s)+2H ₂O(l)

write S + 2H2 + O2
draw an arrow from S + 2H2 + O2 to the right hand side labelled as 2ΔH°f H ₂O
draw an arrow from S + 2H2 + O2 to the right hand side labelled as ΔH°c S(s) + 2ΔH°f(H2S)

hence ΔH°c[S(s)] + 2ΔH°f(H2S) + ΔH_r = 2ΔH°f(H₂O)
solve for ΔHr = -235 kJ/mol