Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Oxyale on May 06, 2006, 03:35:08 AM
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Hey guys, I'm kinda stuck. It's to do with one of Le Chatelier's principles (i.e. if the temperature of a reaction mixture at equilibrium is increased, the equilibrium moves in the direction which absorbs heat). What I don't understand is how you are able to determine which (forward) reactions are endothermic and which are exothermic.
In my textbook, there is an example saying that (note : i'm using '=' as the reversiblity symbol)
2 SO2(g) + O2 (g) = 2 SO3 (g) is exothermic in the forward direction. Why is this? Is it possible to simply look at an equation and determine whether it's forward/reverse reaction is endothermic/exothermic, without the change in enthalpy being given?
Also, there is text in my textbook that says..
Exothermic reactions move left as the temperature increases; endothermic ones move to the right.
...which is nothing short of confusing ???.
Thanks.
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Is it possible to simply look at an equation and determine whether it's forward/reverse reaction is endothermic/exothermic, without the change in enthalpy being given?
In general - no.
Exothermic reactions move left as the temperature increases; endothermic ones move to the right.
That's Le Chatelier's principle you have quoted earlier, just told other way. If you take exothermic reaction and heat the vessel equilibrium will shift to the left - to reactants side - to absorb heat. If you cool it - equilibrium will shift to the right, to products side.
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In my textbook, there is an example saying that (note : i'm using '=' as the reversiblity symbol)
2 SO2(g) + O2 (g) = 2 SO3 (g) is exothermic in the forward direction. Why is this? Is it possible to simply look at an equation and determine whether it's forward/reverse reaction is endothermic/exothermic, without the change in enthalpy being given?
You sound extremely confused. The reversible sign does not represent that the forward reaction is exothermic. The enthalpy change is given to you so that you can identify which of the forward and backward reactions is exothermic and endothermic.
1. 2 SO2(g) + O2 (g) = 2 SO3 (g) dH < 0
2. 2 SO3 (g) = 2 SO2 (g) + O2 (g) dH > 0
Equation (1) simply means the forward reaction is exothermic and the backward reaction is endothermic.
Equation (2) simply means the forward reaction is endothermic and the backward reaction is exothermic.
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Exothermic reactions move left as the temperature increases; endothermic ones move to the right.
when heat is a product the reaction is exothermic so adding heat is going to cause the reaction to shift back to reactants. If heat is a reactant, adding heat will shift the reaction to favor products. it is basic equilibria.