Hess's Law Urgent!

Me14

There is no Oxygen present in the reactants so how can you make CO2 and H2O?

(edited 11 years ago)

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Reply 1

Spoogle

Original post by Me14

There is no Oxygen present in the reactants so how can you make CO2 and H2O?

They're combustion values, ie, the values are the energy released when you react the carbon with oxygen to get CO2.

Reply 2

amd1

You add oxygen to the reactants and the products to get CO2 and H20. It's only there hypothetically for the purpose of the cycle to work out the values, it's not part of the reaction.

Reply 3

Me14

Original post by amd1

You add oxygen to the reactants and the products to get CO2 and H20. It's only there hypothetically for the purpose of the cycle to work out the values, it's not part of the reaction.

So every time carbon and hydrogen are in the reactants, the products will always be CO2 and H2O? As in, I'm meant to assume Oxygen will be present?

(edited 11 years ago)

Reply 4

Goods

The enthalpy changes of combustion (the numbers quoted) are the energy in joules given out when a substance is burnt in excess oxygen so you down need to add it in as its implied and you don't need it for the reaction you are actually trying to work out the enthalpy change of so they don't put it in.

Reply 5

Me14

Also,

Reaction:

2CO + O2 -> 2CO2

ΔH_f/kJ
mol^–1 : CO = -11.525, CO2 = -393.509

2(-110.525) + 2(-393.509) = -221.05 + (-787.018)

What do I do after this point because the answer shows to be – 566.0 kJ mol^–1
but I got 566, why is there a negative?

(edited 11 years ago)