exo or endo? (1 Viewer)

[cwag]

is it possible to determine whether a certain reaction is exothermic or endothermic when just given the equation....like is there some rule? or is it just something we just know?.... ***without obviously been given something like + 92kJ at the end...just a normal equation...eg is CO_(g) + Cl_2(g) --> COCl_2(g) exothermic or endothermic?

 

[tommykins]

i don't think so, most of the time you should know whether it's exo or endo. often they ask on equilibirum q's or something like that.

 

[danz90]

Well... as a rule of thumb... exothermic reactions involve the formation of bonds ( when you break bonds, you absorb energy from surroundings to break those bonds... thus such a reaction would be endothermic).

Endothermic reactions usually involve the breaking of bonds, such as the decomposition of a compound - that would be endothermic.

Generally, if there is greater energy released from the formation of bonds, than is absorbed for the breaking of bonds when rearrangement of atoms occurs in chemical reactions... then the reaction is exothermic. vice versa for endothermic reactions.

The formation of Phosgene, the reaction you gave in your post, is EXOTHERMIC, since bonds are being formed between the Carbon Monoxide and Chlorine gas.
However, usually the questions given in HSC papers don't require you to determine whether an unseen (for you) reaction is exo or endothermic - usually if this sort of question comes - it would be for haber process, which is obviously exothermic (syllabus content).
Also, for industrial chemistry, you could be aksed to determined if a particular reaction is exo or endo - then you would need a graph or table or information showing trends with K-constant in relation to temperature (since only temp changes K-constant).

Hope that helps.

 

[cwag]

ahh thanks that does help alot actually... so for say 4FeS₂ + 11O₂ --> 2Fe₂O₃ + 8SO₂ this would be endothermic because bonds are broken between Fe and S? and say an obvious exothermic reaction like the combustion of Ethanol because of the formation of CO₂ and H₂O? is that right?

Yea, even though they may not ask you indirectly, there have been questions asking how yeild could be increased...and if nature of reaction is known then we can talk about temperature as well. Also iv seen a qusetion asking you to explain why the ionisation of sulfuric acid is exothermic....but this isn't in our course ay?

 

[danz90]

Actually the oxidation/combustion of Iron Sulfide is exothermic... because there is more energy released in the formation of bonds with S and O2 as well as the iron and oxygen atoms to form iron(III) oxide, than is absorbed to break the bonds of FeS2 and O2.
Aren't all combustions exothermic anyway?

I believe that the Prelim Energy topic involves knowledge about exothermic and endothermic reactions and why they are endo or exo etc.

It would be very uncommon or strange if we got a question in the Industrial Chem part asking us why ionisation of sulfuric acid is exothermic.. but I guess all you would say is that a coordinate covalent bond is formed between the H+ provided by the H2SO4 in solution, and the H20 molecules.. hence since a bond is formed, energy is released.. etc

 

[cwag]

ahh yes of course...stupid me......alright thanks for all ur help

 

[Trebla]

is it possible to determine whether a certain reaction is exothermic or endothermic when just given the equation....like is there some rule? or is it just something we just know?.... ***without obviously been given something like + 92kJ at the end...just a normal equation...eg is CO_(g) + Cl_2(g) --> COCl_2(g) exothermic or endothermic?

Within the constraints of the HSC course, it's probably just memorising whether they're exothermic or endothermic, but there is a reason behind them.

For example, the ionisation of sulfuric acid is exothermic. Here's a very crude explanation of it, using a bit of university level organic chemistry (though it's not beyond you) and without using actual numbers. If you think about it, it should make a bit of sense.

When you draw the structure of sulfuric acid, it's a central sulfur atom with two hydroxides and two double bonded oxygens attached. (see http://en.wikipedia.org/wiki/Sulfuric_acid)

Now, we know that oxygen is highly electronegative. So looking at the hydroxy groups, the O atom has a δ^- due to it sucking electrons from the H atom, making H a δ⁺. Therefore, the covalent bond in the O-H is 'susceptible' to breakage since the sharing of electrons is not quite even. If there was enough energy available, then the O would completely suck out the covalent bond electrons and breaking off with the H, leaving a H⁺.

However, it does not do that spontaneously (i.e. pure sulfuric acid won't just drop off its protons naturally) since there isn't enough energy in the surroundings to promote it. Thus, we need to input energy in to make it happen. When we do that, the O-H covalent bond absorbs this energy to break. However, since the covalent bond is very fragile (due to electrons favouring the O atom), it doesn't require a lot of energy to do so. So there is little energy absorbed to go from H₂SO₄ to HSO₄^- and H⁺.

This energy comes from collisions with water molecules when you react molten sulfuric acid with water. So, once you add water, you immediately gain a lot of H⁺ ions. If you draw the Lewis electron dot structure of water, you'll notice that there are lone electron pairs on the oxygen atom. These lone electron pairs (as well as the δ^- of the O) attach the H⁺ very easily to form a coordinate covalent bond due to electrostatic attraction. This process of bond formation releases a lot of energy, because it doesn't need any more input energy to happen and there is too much energy in the surroundings (it's pretty much spontaneous). So there is a lot of energy released to form H₃O⁺.

Put that all, together: little energy absorbed to break bonds and a lot of energy released to make bonds. The net effect is that energy is released, thus it is exothermic.

This doesn't actually happen in separate events, it all happens at once. The water molecule actually attaches itself to the H on the sulfuric acid like a leech, before breaking the H off from the sulfuric acid molecule.

This isn't the complete picture though, there are other factors to consider, but that crude explanation does make a bit of sense in a way....lol

 

[henry08]

No, you will be given if the reaction is endothermic or exothermic.