How to calculate enthalpy change? I just don't get it, please help :(

I have a major chemistry exam in 3 weeks and i'm starting to freak out :\

Practice question:

What is the enthalpy change for the following reaction?

SiCl4(l) + 2H2(g) --> Si(s) + 4HCl(g)

Compound..............Delta-f-H* / kJ mol-1
SiCl4(l.....................-687
HCl(g).....................-92

Any help as to how to work this out is greatly appreciated.

The enthalpy change for a reaction can be calculated by adding up the enthalpy of formation for all the product molecules and subtracting the enthalpy of formation of each reactant molecule.

The enthalpy of formation for a compound is the enthalpy change when one mole of the compound is produced from its elements in their standard states. You have been given the values for the enthalpy of formation for SiCl4 and HCl. Since Si(s) and H2(g) are elements in their standard state, their enthalpy of formation values are 0.

For you equation:

del H = delHf of Si + 4 x delHf of HCl - delHf of SiCl4 - 2 x delHf of H2

(the 4 x and 2 x are because there are 4 moles of HCl and 2 moles of H2 in the equation.)

del H = 0.0 + 4(-92) - (-687) - 2(0.0)

delH = (-368 kJ) - (-687 kJ) = 329 kJ

Did that help?

oops......you are right about 319. That is what my calculator said, I typed it in wrong.

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