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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: caleb on June 16, 2005, 10:21:52 PM

Title: calculating the value of q
Post by: caleb on June 16, 2005, 10:21:52 PM

Can anyone give me a hand .first off what excatly is q .


hydrogen peroxide can decompose to water and oxygen by the following reaction.

2H2O2  ---  2H2O+O2        deltaH=-196kj .
the problem ask you to calculate the value of q when 5.00g of H2O2 decomposes at constant pressure.


the answer is -14.4kj i just need to know how they got that .thanks

Title: Re:calculating the value of q
Post by: arnyk on June 16, 2005, 11:02:46 PM

Q usually stands for Heat Energy.  So the reaction is exothermic, determine the the amount of heat released if 5 grams of H2O2 were to react.

Title: Re:calculating the value of q
Post by: Donaldson Tan on June 17, 2005, 10:21:25 AM

consider a chemical reaction as an open system

dH = Q + V.dp

under constant pressure, dp = 0, so V.dp = 0

=> Q = dH = -196kJ

Title: Re:calculating the value of q
Post by: arnyk on June 17, 2005, 03:28:43 PM

Ok I just thought of a simpler way.  I think that you can simply use the ratio between the energy values and the moles.

2 mol of H2O2 will result in q = -196 kJ.

Therefore, what will q be when there are "x" mol of H2O2.  Calculate "x" moles using the mass of H2O2 given.  Then use the ratio.

Pretty easy actually, there's probably a proper way to do it. But this way is faster. :p