You are Here: Home

# Enthalpy Question watch

1. Hi
i have been asked to calculate the standard enthalpies of formatino of sulphur dioxide, on burning in excess oxygen under standard conditions 1g of sulphur evolves 9.28KJ.

2. 1/2S2 + O2 -> SO2
Number of mole of SO2 = 2nS2 = 2*1/64 = 1/32.
So dH = -9.28/(1/32) = -9.82*32 kJ/mol (because it's exothermic reaction)
3. The enthalpy of formation of SO2 concerns the reaction
(1/8)S8 + O2 --> SO2
while the enthalpy of combusion of sulphur concerns the reaction
S8 + 8O2 --> 8SO2

The two reactions are in a 1:8 ratio, so will their enthalpies. So if you find how many moles of S8 you have, you then know the combustion enthalpy for that many moles. From there, you can find the combustion enthalpy for one mole of S8. Then convert that by the correct ratio for the enthalpy of formation of SO2, and you've got your enthalpy of formation
4. Yeah, sorry... if sulphur is in solid state, it's S8, and mass is 1g.
If Sulphur is in gas state, it should be volume of gas.
It'd better if we solve like that
number of moles of SO2 = number of moles of S = 1/32.
So the results are the same, that makes me confused ....
Ah, right ... if S2 is used instead of S8, energy released maybe smaller (different condition)

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 29, 2005
Today on TSR

### University open days

Wed, 21 Nov '18
• Buckinghamshire New University
Wed, 21 Nov '18
• Heriot-Watt University