[thuanthuan]

Hello eveyone,
I have a question for you guys about using heat of formation of a compound to predict the reaction
By definition, heat of formation of a compound gives the number of calories evolved (dH<0) or absorbed (dH>0) when a mole of the compound is formed by direct union of its elements. . Unless otherwise indicated, the product's in solid state.(Everyone's already known this )
The Barron's SAT II Chemistry says that "If the heat of formation is a large number preceded by a minus sign, the combination is likely to occur spontaneously and the reaction is exothermic. If, on the other hand, the number is small and negative or is positive, heat will be needed to get the reaction to proceed at any noticeable rate."
Can you explain why ?

I think a lot about it but I can't find out the answer. I'm very bad at Chemistry so I need your help. I appreciate every idea from you guys.
My English is also very bad, so I'm sorry for any ambiguous phrase that I've written. Thank you very much

[thuanthuan]

Hello eveyone,
I have a question for you guys about using heat of formation of a compound to predict the reaction
By definition, heat of formation of a compound gives the number of calories evolved (dH<0) or absorbed (dH>0) when a mole of the compound is formed by direct union of its elements. . Unless otherwise indicated, the product's in solid state.(Everyone's already known this )
The Barron's SAT II Chemistry says that "If the heat of formation is a large number preceded by a minus sign, the combination is likely to occur spontaneously and the reaction is exothermic. If, on the other hand, the number is small and negative or is positive, heat will be needed to get the reaction to proceed at any noticeable rate."
Can you explain why ?

I think a lot about it but I can't find out the answer. I'm very bad at Chemistry so I need your help. I appreciate every idea from you guys.
My English is also very bad, so I'm sorry for any ambiguous phrase that I've written. Thank you very much

Oh my God!
Maybe I've post my question in the wrong box. I thought there's sub-forum in "High school Chemistry" ...
Anyway, I hope the mod will move it into the appropriate box, if needed. Thank you.

[thuanthuan]

Is there a general rule that if dH is high negative number then the reaction will occur more easily ? Will it occur spontaneously ?
So is it possible for the reaction with dH>0 to occur ? The answer is yes, I'm pretty sure, but heat must be returned to the reactants. Is there a difference between the purposes of giving heat to the reactants (as mentioned above) and heating a substance in its oxidation (since it cannot burn in room temperature) ? Both reactions require heat.
That's what I don't understand.

[Yggdrasil]

The basic idea is that things in nature like to have a low amount of potential energy.  For example, a ball that is high in the air has a high amount of gravitational potential energy.  That ball (unless supported) will fall, releasing that potential energy and moving to a state of lower potential energy.  Chemicals behave in the same way.  Enthalpy is a measure of chemical potential energy.  An exothermic reaction releases potential energy so that the products have a lower potential energy than the reactants.  Conversely, an endothermic reaction raises the potential energy of the products relative to the reactants.

So, to answer your question, highly exothermic reactions are likely to be spontaneous because going from a state of high potential energy to a state of low potential energy is favorable.

However, potential energy is not the only factor involved in determining whether a reaction is spontaneous or not.  In addition to minimizing potential energy, nature also tends toward maximizing the entropy (disorder) of the system.  Endothermic processes can and do happen because the end result increases entropy.  For example, when you dissolve certain salts in water, the solution becomes colder, indicating that the dissolution of the salts was endothermic.  However, the dissolution also breaks apart a highly ordered crystal lattice, causing a large increase in entropy.  This increase in entropy dirves the dissolution.

You can use the following equation to determine whether a reaction will occur sponaneously:

ΔG = ΔH - TΔS

where ΔH is the change in enthalpy, T is the temperature, ΔS is the change in entropy, and ΔG is the change in a quantitiy called Gibbs Free Energy.  If ΔG < 0 then the process is spontaneous.  If ΔG > 0 then the process is not spontaneous (unless work is done on the system by an outside force).

[note: minimizing the potential energy of the system and maximizing the entropy actually turn out to do the same thing (maximize the number of ways that you can distribute energy throughout the system), but you learn this in very advanced physical chemistry courses].

[thuanthuan]

The basic idea is that things in nature like to have a low amount of potential energy.  For example, a ball that is high in the air has a high amount of gravitational potential energy.  That ball (unless supported) will fall, releasing that potential energy and moving to a state of lower potential energy.  Chemicals behave in the same way.  Enthalpy is a measure of chemical potential energy.  An exothermic reaction releases potential energy so that the products have a lower potential energy than the reactants.  Conversely, an endothermic reaction raises the potential energy of the products relative to the reactants.

So, to answer your question, highly exothermic reactions are likely to be spontaneous because going from a state of high potential energy to a state of low potential energy is favorable.

However, potential energy is not the only factor involved in determining whether a reaction is spontaneous or not.  In addition to minimizing potential energy, nature also tends toward maximizing the entropy (disorder) of the system.  Endothermic processes can and do happen because the end result increases entropy.  For example, when you dissolve certain salts in water, the solution becomes colder, indicating that the dissolution of the salts was endothermic.  However, the dissolution also breaks apart a highly ordered crystal lattice, causing a large increase in entropy.  This increase in entropy dirves the dissolution.

You can use the following equation to determine whether a reaction will occur sponaneously:

ΔG = ΔH - TΔS

where ΔH is the change in enthalpy, T is the temperature, ΔS is the change in entropy, and ΔG is the change in a quantitiy called Gibbs Free Energy.  If ΔG < 0 then the process is spontaneous.  If ΔG > 0 then the process is not spontaneous (unless work is done on the system by an outside force).

[note: minimizing the potential energy of the system and maximizing the entropy actually turn out to do the same thing (maximize the number of ways that you can distribute energy throughout the system), but you learn this in very advanced physical chemistry courses].

Assume that we have the reversible reaction
   a A (gas) + b B (gas) + ..  <->  m M (gas) + n N (gas) + ... + HEAT

where a+b+c+... < m+n+p+... ( ΔS>0)
(equilibrium is reached)

if we increase temperature, according to Le Chatelier principle, the reverse reaction will be favored, reducing temperature.

but if we increase temperature, then according to the equation above, ΔG = ΔH - TΔS, because ΔS>0 then ΔG will change from 0 (initial equilibrium) to be more negative, so the forward reaction will be favored. Something is violating Le Chatelier principle ?
Two opposing impact of the two driving force ?? What will happen if we increase temperature ?

[thuanthuan]

Any idea about my question