March 29, 2024, 02:57:41 AM
Forum Rules: Read This Before Posting


Topic: calculating the value of q  (Read 13362 times)

0 Members and 1 Guest are viewing this topic.

caleb

  • Guest
calculating the value of q
« 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

arnyk

  • Guest
Re:calculating the value of q
« Reply #1 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.
« Last Edit: June 16, 2005, 11:04:40 PM by arnyk »

Offline Donaldson Tan

  • Editor, New Asia Republic
  • Retired Staff
  • Sr. Member
  • *
  • Posts: 3177
  • Mole Snacks: +261/-13
  • Gender: Male
    • New Asia Republic
Re:calculating the value of q
« Reply #2 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
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

arnyk

  • Guest
Re:calculating the value of q
« Reply #3 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

Sponsored Links