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Hess Law help

Can someone please explain part b to me

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For each equation construct a Hess's Law 'triangle'. You have the enthaplies of formation for the reactants and products in each case, so write the (balanced) elements below each equation.
Use Hess's Law to work out the enthalpy of combustion via the enthalpies of formation for the reactants and products.

Reply 2

charco

Study Forum Helper

Original post by jhuk

Can someone please explain part b to me

Write out the equation for enthalpy of combustion of each compound.

Use

(sum of) formation enthalpy of products - (sum of) formation enthalpy of reactant = enthalpy of reaction.

Reply 3

jhuk

I dont understand how to construct the triangle, I have balanced all the equations

Reply 4

ChemistryWebsite

Original post by jhuk

I dont understand how to construct the triangle, I have balanced all the equations

You construct like the example.

(You should probably include the state symbols, unlike my example!)

Thank you

Original post by jhuk

Thank you

You're welcome!

Hopefully when you did the Hess's triangle for hydrogen you noticed a relationship;
A relationship between the enthalpy of combustion and the enthalpy of formation of water.
Why is that relationship there? Because hydrogen (and the oxygen required in combustion) are elements, so their enthalpy of formation is zero.