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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: lou_skywalker on October 17, 2009, 03:04:08 PM

Title: Calculate ΔHrxn from two equations?
Post by: lou_skywalker on October 17, 2009, 03:04:08 PM

Calculate ΔHrxn for the reaction below.
Ca(s) + 1/2 O2(g) + CO2(g)  CaCO3(s)

You are given the following set of reactions.
Ca(s) + 1/2 O2(g)  CaO(s) ΔH = -635.1 kJ
CaCO3(s)  CaO(s) + CO2(g) ΔH = 178.3 kJ

Title: Re: Calculate ΔHrxn from two equations?
Post by: UG on October 18, 2009, 02:44:50 AM

You gotta flip the second equation:

CaO(s) + CO2(g)  :rarrow: CaCO3(s) ΔH = -178.3 kJ

Then the other elements will cancel out

Ca(s) + 1/2 O2(g)  :rarrow: CaO(s)
CaO(s) + CO2(g)  :rarrow: CaCO3(s)
Add up the two numbers and wha la.

Title: Re: Calculate ΔHrxn from two equations?
Post by: lou_skywalker on October 18, 2009, 03:45:31 PM

ok got it thnx!