Another AP CHEM question

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Discus: SAT/ACT Tests and Test Preparation: October 2003 Archive: Another AP CHEM question
By Mo222 (Mo222) on Saturday, October 04, 2003 - 01:17 pm: Edit

Okay I know this seems like an easy Hess's Law problem but I can't seem to get it right:

To produce silicon, used for semiconductors, from sand, (SiO2), a reaction is used which can be broken down into three steps:

SiO2(s) + 2C(s) --> Si(s) + 2CO(g) /\H = 689.9 kJ
Si(s) + 2Cl2(g) --> SiCl4 (g) /\H = -657.0 kJ
SiCl4(g) + 2Mg(s) --> 2MgCl2(s) + Si(s) /\H = -625.6 kJ

Write the thermochemical equation for the overall reaction for the formation of silicon from silicon dioxide; CO and MgCl2 are byproducts. What is the /\H for the formation of 1 mol of silicon ? Is the reaction exothermic or endothermic?

I know this problem seems long, but I am really confused and would really appreciate help.

By Mo222 (Mo222) on Saturday, October 04, 2003 - 05:21 pm: Edit


By Mo222 (Mo222) on Saturday, October 04, 2003 - 07:51 pm: Edit


By Mo222 (Mo222) on Saturday, October 04, 2003 - 10:10 pm: Edit

This question is not possible. Obviously silicon is not one of the reactants, since thats what your trying to get, but if you reverse that reaction the SiCl4 s will not cross out.... so confusing.

Please can someone help me w/these problems?

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