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Need help for chemistry!! Below is the question:Gold is ab extremely unreactive metal,...

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india123 | Student, Grade 9 | (Level 2) eNoter

Posted May 9, 2010 at 11:49 AM via web

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Need help for chemistry!! Below is the question:

Gold is ab extremely unreactive metal, Gold does react, however, with aqua regia (a mixture of concentrated nitric acid, HNO3(aq) and hydrochloric acid, HCl(aq)). The complex ion AuCl4, as well as N2 and H2O are formed. This reaction is always carried out with aqua regia in excess. Why would a chemist not have the gold in excess?

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hnystrom | High School Teacher | (Level 1) Assistant Educator

Posted July 10, 2010 at 8:56 PM (Answer #1)

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Aqua regia is called "noble water" becaise it dissolved gold, a "noble" metal that otherwise would not react with other substances.  The proportion of HCl to HNO3 can vary.  To answer your question, we need a balanced equation.  First writing down an equation from the information given we get:

Au + HCl + HNO3 > AuCl4 + H20 + N2 (unbalanced)

Balancing it we get:

Au + 4HCl + 2HNO3 > AuCl4 + 3H2O + N2

We need a lot of acid in order to dissolve just a little bit of gold!  One mole of gold needs 4 moles of HCl and 2 moles of HNO3.  We had better have the acid in excess in order to dissolve the gold otherwise the reaction would quickly come to halt because either the HCl or the HNO3 would be used up.  Ideally we want to make our aqua regia with the proportion of HCl to HNO3 of 2:1.  If there is more HNO3 to HCl this could also limit the reaction further because the HCl would get used up faster.

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