A few chem questions unclear to me...

[YT2095]

here`s a question that has me a little puzzled, if a catalyst is to lower the activation energy (I think we all agree on that) then how come a catalyst is used in some reactions that need to take place in a cold environment?

 

I`ll not mention any specifics, but surely the same reaction at room temp without the catalyst would be logicaly just as good?

 

why add something to "speed it up" only to slow it down?

[woelen]

here`s a question that has me a little puzzled' date=' if a catalyst is to lower the activation energy (I think we all agree on that) then how come a catalyst is used in some reactions that need to take place in a cold environment?

 

I`ll not mention any specifics, but surely the same reaction at room temp without the catalyst would be logicaly just as good?

 

why add something to "speed it up" only to slow it down?[/quote']

Adding a catalyst is not completely the same as raising the temperature. It might be that the reaction you mention has more side-reactions at room temp and that for that reason the reaction must proceed at lower temperature. The addition of the catalyst, combined with the lower temperature hence may result in a cleaner reaction.

 

In terms of kinetics, the mechanistic pathway for the reaction may differ considerably from that of the catalysed reaction at lower temperature. Some pathways may lead to a single endproduct, while other pathways lead to multiple products (the path has branches) and hence to lower yield.

 

Yet another reason may be the shifting of equlibria as function of temperature. A catalyst does NOT affect the position of an equilibrium, it only affects the speed at which this equilibrium is reached. Change of temperature does affect the equilibrium.

So, in your reaction, the location of the equilibrium may be more at the side of the wanted products at low temperatures.

[YT2095]

yeah, that makes sense, because without the cooling you get a Dimer as opposed to the Trimer which is the desired product.

[DQW]

An excellent analogy. Thanks for the effort. I hope the OP gains an intuitive feel for reaction energetics from it.

In practice, hence, you could say that the reaction 2H2O + O2 --> H2O2 does not occur, theoretically speaking there might be a few molecules which change into H2O2, but the number of them will be so low, that in practice it will not be detected.

That's exactly what I was talking about. The probability goes like [imath]e^{-E_a/RT} [/imath], but even a very large activation only creates a very small probability.

 

So, when asking whether a reaction will happen, it is important to understand what one means by 'happen'. Typically, though, this is taken to mean an equilibrium constant that is noticably larger than 1 or a free energy change that is significantly negative (of course, one follows from the other).

[DQW]

A catalyst does effect the amount of energy needed for a reaction by lowering the activation energy.

 

~Scott

Lowering the activation energy does not affect the reaction enthalpy.

[brokenbin]

1. to test for water, or water vapour, use cobalt chloride paper, which turns pink in the presense of water. anything can have vapour, such as alcohol, and it is not water. electrolysis just in accurate, you'd never know what gas you have produced.

 

2. no idea

[albertlee]

thanks woelen

 

redox

acid/base

coordination

precipitation

 

are those all types of reaction??

 

what is coordination??

 

I thought all reactions are redox, because one reactant receives electrons and the other looses them??

[albertlee]

1. to test for water' date=' or water vapour, use cobalt chloride paper, which turns pink in the presense of water. anything can have vapour, such as alcohol, and it is not water. electrolysis just in accurate, you'd never know what gas you have produced.

[/quote']

 

Why??

 

Secondly, is H2O2 pH 7???

[DQW]

I thought all reactions are redox, because one reactant receives electrons and the other looses them??

Quick counterexample : [imath]NaOH + HCl \longrightarrow NaCl + H_2O [/imath]

[albertlee]

oh.

 

and btw, is electrolysis a type of reaction?? is that called decomposition??

 

More over, how do you determine whether a salt is insoluble?? ie, the reaction is either solely neutralisation (acid + base + soluble salt) or neutralisation+percipitation(acid + base + insoluble salt)??

 

Albert

[YT2095]

there are other reaction that will lead to insoluble salt, that require neither acid or base reactions, ie/ Silver Nitrate and Sodium Chloride

I`m sure there`s also one for Barium Sulphate too (and one or two others).

 

it`s also important to remember that Insoluble salts are always insoluble in water, they just don`t dissolve very well, and a gram may need a kilolitre of water to dissolve

[albertlee]

there are other reaction that will lead to insoluble salt' date=' that require neither acid or base reactions, ie/ Silver Nitrate and Sodium Chloride

I`m sure there`s also one for Barium Sulphate too (and one or two others).

 

it`s also important to remember that Insoluble salts are always insoluble in water, they just don`t dissolve very well, and a gram may need a kilolitre of water to dissolve [/quote']

 

1, since the percipitations of acid + alkali is also called neutralisation. What do those ones called according to YT that involve not acid/alkali??

 

2, do you mean even insoluble salt 'can' be dissolved, but with large quantity of water?

 

btw, what I ask is why some salts are insoluble, whereas some aren't.

 

Albert

[DQW]

Albert, you are asking a lot of good, yet very specific, questions in several dispersed areas of general/physical chemistry. While a forum like this can answer these questions individually, it can not help you get a ground-up understanding of the basic concepts that will clarify several of these doubts with a single fell swoop. The underlying principles are what you want to learn, and the best way to do that (outside of classes) is from a good book. I recommend Physical Chemistry, by Atkins. That is one book you will not regret buying - but at least check it out at your library.

[DQW]

1' date=' since the percipitations of acid + alkali is also called neutralisation. What do those ones called according to YT that involve not acid/alkali??

 

2, do you mean even insoluble salt 'can' be dissolved, but with large quantity of water?

 

btw, what I ask is why some salts are insoluble, whereas some aren't.

 

Albert[/quote']

1. double decomposition

2. yes, in principle, you can dissolve a large fraction (not completely, ever) of any salt with sufficient solvent

[woelen]

thanks woelen

 

redox

acid/base

coordination

precipitation

 

are those all types of reaction??

 

what is coordination??

 

I thought all reactions are redox' date=' because one reactant receives electrons and the other looses them??[/quote']

This list was not intended as a complete list, it lists a few very common types of reactions, but there are reactions, which cannot easily be placed in one of these groups.

A very energetic type: nuclear reactions (MUCH MUCH more energetic than redox reactions).

Equlibrium reaction: N2O4 <---> 2NO2.

And many others can be added....

 

A classical definition of coordination reaction is that one entity possessing lone pairs (the entity being either a molecule or an ion) attaches to another entity and shares the lone pair with that entity. A lone pair is a pair of electrons, that are not used in bonding inside a certain entity.

 

An example:

 

NH3: N has 5 electrons in its outer shell. Three of these are used for bonding with the H atoms, two electrons form a so-called lone pair. This lone pair can be shared with certain metal ions. A very well known complex of ammonia is the deep royal blue [Cu(NH3)4](2+) ion. The NH3 molecules are (loosely) connected to the copper ion. Unfortunately I cannot make drawings here, but try to imagine the copper ion in the center and the three NH3 molecules around it, with the N's faced towards the copper ion and the H's pointing outwards.

 

In this complex, NH3 is called the ligand, and the copper ion is called the core atom or central atom.

 

Many entities have lone pairs somewhere in the molecule or ion and hence, coordination complexes can be formed with many many entities.

 

 

 

One way to recognize complexes is to take the oxidation number of the central entity (usually being a single atom) and look at the number of ligands. If the number of ligands (counting multidentate ligands once for each coordination) exceeds the oxidation number of the central atom, then the whole system can be regarded as a complex. Examples:

 

TiF6(2-): The titanium core has oxidation state +4, there are 6 fluoro ligands, so this is a complex.

 

CrO4(2-): The chromium core has oxidation state +6, there are 4 oxo-ligands. So in the classical sense, this would not be considered a complex.

 

[Cr(C2O4)3](3-): The chromium core has oxidation state +3, there are three bidentate oxalato ligands, making a total of 6 coordinations. The number of coordinations is larger than 3, so this can be considered a complex.

 

This 'rule' is very nice and I have tried many examples and this really works. It nicely classifies between what one naturally would call a complex and what one naturally would call molecules or ions with covalent bonds inside.

 

 

In modern chemistry, often the concept of complex is extended to many more compounds. E.g. the molecule H2 can coordinate to certain other molecules, forming very weird types of bonds, which do not fit in the model, as described above.

 

The chemistry of complexes is a world on its own. If you really want to know more about that, then buy a good book on the subject, but it is not the subject to start with. First try to get a sound basis on subjects like types of bonding, the role of electrons in general, covalent bonds, lone pairs and electrostatics.

[Primarygun]

It might be, but only with considerable input of energy. As I explained in my lengthy post, a catalyst does not affect the energy released (or needed) for a reaction.

Thanks

[EL]

A catalyst is a substance that accelerates the rate of a chemical reaction without itself being transformed or consumed by the reaction; that is it participates in the reaction but is neither a chemical reactant nor a chemical product.

 

Major catalysis groups

* Converter catalysis

* Coordination catalysis

* Enzymes - biocatalysts

* Nanomaterial based surface catalysis

 

The major symbolic action of catalysts C.

A + C → AC (1)

B + AC → AB + C (2)

A + B + C → AB + C

 

The net enthalpy of C is supposed to be zero but this is not true for the rest of the reactants.

In exothermic reactions a catalyst may trigger a spontaneous reaction.

In endothermic reactions, the addition of energy from an external source is channelled through the catalyst in catalyzed reactions.

[albertlee]

Do bond energies tell how energetic the reactants are??

 

Or are they irrevalent??

 

 

ps, dont panic, the thread will stop not so long.

[EL]

Do bond energies tell how energetic the reactants are??

Or are they irrevalent??

 

I am sorry to tell you that your question is incomprehensible.

There is no meaning for "How energetic reactants are?"

The reason is that the bond energy is there after reactants react.

As for "how do they react", has no standard reference to tell us how "energetic" they are.

Chemical reactions are so diverse that it would be seriously funny to even think that there is such a thing as a standard behaviour of reactants.

Sorry again for my harsh words.

[DQW]

Bond energies (when interpreted in the context of the reaction conditions, molecular geometry, etc.) can give you a good idea about the reactivity of a particular compound to other specified compounds under a given set of conditions (temperature, pressure, etc). Also, you can calculate bond energies from enthalpies and vice versa. So, it is possible to determine the thermicity of a reaction given all the bond energies, but that's about it.

[EL]

I am sorry but that does not make sense either.

I could not answer him saying "irrelevant" because it is relevant, but in a different way than he is expecting.

Reactivity though has nothing to do with the final bond energy and I am not even aware of any equation that links both, so post one if you have one.

[woelen]

Do bond energies tell how energetic the reactants are??

 

Or are they irrevalent??

 

 

ps' date=' dont panic, the thread will stop not so long. [/quote']

I would say that a reactant is not energetic, but a reaction. If a compound with a high bond energy (lots of potential energy) is transformed into another compound, also with a lot of potential energy, then the reaction does not release much energy in the form of radiation and/or heat. It is the difference between reactants and products, which matters. In terms of the hill/marble: If you go from a very high hill to a somewhat lower, but still very high hill, then only a small amount of kinetic energy is released.

 

Of course, as EL stated, the nature of the bonds does matter. It tells something about the potentials of a compound. Even then, things may be misleading. E.g., a persulfate (a very strong oxidizer, redox potential from persulfate to sulfate is appr. 2 Volts, which is high) contains a lot of potential energy. But, the compound also reacts quite sluggishly (has to do with kinetics, reaction pathway), such that its reactions usually do not appear to be very energetic. A lot of energy is released, but it is released slowly. On the other hand, the compound can achieve very unusual things, such as formation of Ag(3+) and nickel (IV) compounds, see one of my experiments on

http://81.207.88.128/science/chem/exps/Ni+persulfate/index.html which potentially are very energetic.

[DQW]

Reactivity though has nothing to do with the final bond energy and I am not even aware of any equation that links both, so post one if you have one.

Who said anything about final bond energies ? And I didn't know that "reactivity" was a physical quantity that could be described by an equation.

 

From the reactant as well as product bond energies, one can calculate the reaction enthalpy. From a knowledge of the physical states, concentrations and other conditions, you can determine the entropies of the various species and hence the entropy change for the reaction. From the enthalpy, entropy and temperature, you can find the free energy change for the reaction (and hence the equilibrium constant at that temperature). This essentially tells you how much the reactants want to react under the given conditions.

 

How fast the reaction proceeds depends on concentrations and the rate constant, which in turn can be determined (in theory) from the Arrhenius factor (molecular geometries, steric effects, molecularity, etc.), the activation energy (which essentially, can be found from bond energies - as well as other relevant energies, such as solvation, lattice, ionization, etc. - and a knowledge of the reaction mechanism) and the reaction temperature.

[EL]

DQW,

I understand what you are saying.

However, let me give you an example to show you that the relevancy is neither directly proportional nor inversely proportional as the relation between the energy of a bond and the reactants activity-coefficient at a specific temperature and concentration.

A simplest example should be HCl and NaOH.

HCl, at 25 decrees C., have the activity-coefficient varying between 0.796 at 0.1 molality to 0.809 at 1.0 molality.

NaOH, at 25 decrees C., have the activity-coefficient varying between 0.766 at 0.1 molality to 0.678 at 1.0 molality.

NaCl ionic bond and H2O hydrogen bond are (at 298 K) has a dissociation energy of 412.1 kJ/mol and 427.6 kJ/mol, respectively.

So now you have the unrelated data. What on earth can you conclude from one to give you information about the other?

The bond strength is obviously a constant, while the activity-coefficient is obviously a variant.

In other words, regardless of the conditions that causes the variance of activity (reactivity) the products have a constant bond strength per ambient condition. So what is it that you were implying to be a correlation?

[albertlee]

If the energy level decreases in product from the reactants, it is considered to be endothermic, right??