chemical energetics mcq.

• Bigcable22

  4 Jan 09, 06:36

  

   

   

• eagle

  4 Jan 09, 08:33

  One tip:

  Last time for F maths, when I encountered TYS questions I don't know, I go to the nearest friendly popular bookshop and flipped open their redspot TYS to read the answer to that particular question

• UltimaOnline

  4 Jan 09, 10:11

  

   

  Below the equation in the question, and in the center, write Ca(s) + O2(g) + 2H2(g). You thus have a Hess Law energetics cycle diagram in which to solve for enthalpy of formation of Ca(OH)2(s). The only other information you need, would be the "enthalpy of formation of H2O (ie. 2H2 + O2 --> 2H2O)" which also happens to be an equivalent process (and hence equivalent enthalpy value) to "enthalpy of combustion of H2 (ie. 2H2 + O2 --> 2H2O)".

   

• Bigcable22

  5 Jan 09, 18:33

  thanks to the answers to previous question. And btw the nearest popular bookstore is a bus interchange away..

   

  i got a new question..its for part d.) drawiing the diagram..

   

  i know that enthalpy change of soln= enthalpy change of hydration - L.E. but how do i represent it in this diagram..? omg i forgotten even a simple question like this..

  

• UltimaOnline

  5 Jan 09, 18:48

  Originally posted by Bigcable22:

  thanks to the answers to previous question. And btw the nearest popular bookstore is a bus interchange away..

   

  i got a new question..its for part d.) drawiing the diagram..

   

  i know that enthalpy change of soln= enthalpy change of hydration - L.E. but how do i represent it in this diagram..? omg i forgotten even a simple question like this..

  

   

  >>> i know that enthalpy change of soln= enthalpy change of hydration - L.E. but how do i represent it in this diagram..?  <<<

   

  That's a lousy formula, which most JC students dogmatically memorize without understanding.

   

  A better formula, which makes obvious sense, and one which would immediately self-explain how to draw the Hess Law energetics cycle diagram, would be :

  Solution Enthalpy = Lattice Dissociation Enthalpy* + Solvation Enthalpy (when the solvent is water, solvation enthalpy is called "Hydration Enthalpy", for obvious reasons).

   

  Afterall (ie. the reason why this formula is superior and intuitive), the 1st step is the endothermic process of investing energy to overcome the electrostatic attraction between the oppositely charged ions, in order to produce the gaseous cations and anions. Thereafter, the 2nd step is the exothermic process in which ion-dipole interactions or 'bonds' are formed, between the ions and solvent molecules (which are usually polar water molecules). Hence the formula makes perfect sense, and the Hess Law energetics cycle diagram becomes self-explanatory.

   

  (*Lattice Dissociation Enthalpy is endothermic; Lattice (association) Enthalpy is exothermic. Same magnitude, since we're talking about the same process of overcoming-vs-forming a lattice formation, but in reverse to each other.)

   

• Bigcable22

  5 Jan 09, 19:23

  er is the answer diagram like this? i put a minus sign infront of L.E. ? 

  

• UltimaOnline

  5 Jan 09, 19:34

  Originally posted by Bigcable22:

  er is the answer diagram like this? i put a minus sign infront of L.E. ? 

  

  
  Yes, your diagram is correct.

• Bigcable22

  5 Jan 09, 19:34

  Thanks.

• UltimaOnline

  5 Jan 09, 19:41

  (moved to a separate thread)