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Topic: What exactly makes something a precipitation reaction?  (Read 574 times)

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Offline Kalya

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What exactly makes something a precipitation reaction?
« on: August 20, 2020, 05:38:27 AM »
A learnt a definition that said "A precipitation reaction is a type of chemical reaction, in which two soluble salts in an aqueous solution combine and one of the products in an insoluble salt called a precipitate". Thought it was neat, then I was reading an article and it said this was a precipitation reaction:

2AgNO3(aq) + Cu(s) →Cu(NO3)2(aq) + 2Ag(s)

According to the above definition, we need two soluble salts, copper is not a soluble salt (right?) I mean it's solid for one thing and I learnt that a salt is an ionic compound that consists of a cation and an anion which obviously, pure copper is not.

So how exactly can you categorizes something as a precipitation reaction, thank you.
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Re: What exactly makes something a precipitation reaction?
« Reply #1 on: August 20, 2020, 06:33:39 AM »
I wouldn't call the reaction you listed "precipitation", but that's a bit gray area. I have a feeling that the classification is treated a bit differently in different languages, can be wrong though.

It is a classic example of a single displacement reaction (but that's a separate classification).
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Offline jeffmoonchop

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Re: What exactly makes something a precipitation reaction?
« Reply #2 on: August 20, 2020, 05:27:19 PM »
I dont think it has to be salts. I think it can be any reaction where the reactants are soluble and the products are solid. This can also depend on the solubility of the reactants and products in the solvent used. For example a reaction in water may precipitate because the product solubility in water is low, whereas if the solvent is changed the product solubility may be much higher, in that case the product may not precipitate.

A method of producing poymorphs of crystals is changing solvents, so you can control the supersaturation to produce metastable polymorphs based on solubility. Polymorphs can differ in solubility so would crystalise at different supersaturation.

Offline AWK

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Re: What exactly makes something a precipitation reaction?
« Reply #3 on: August 21, 2020, 04:07:01 AM »
A method of producing poymorphs of crystals is changing solvents, so you can control the supersaturation to produce metastable polymorphs based on solubility. Polymorphs can differ in solubility so would crystalise at different supersaturation.
You can also obtain polymorphs without changing the solvent - the time of crystallization is decisive.

IUPAC
Precipitation (in chemistry)

Sedimentation of a solid material (precipitate) from a liquid solution in which the material is present in
amounts greater than its solubility in the liquid.

The liquid solution is important

The reaction of eg H2SO4 with BaCl2 is also precipitation reaction
similarly - thermal decomposition of calcium bicarbonate solution
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Re: What exactly makes something a precipitation reaction?
« Reply #4 on: August 21, 2020, 04:32:57 AM »
The problem here is whether we consider process in which one solid is replaced with another "precipitation", or do we expect the solution to be solid free before the reaction.

This can be a language thing, "precipitate" has a different range of meanings in English than in other languages (for example it is perfectly correct to say that someone "precipitated the crisis", that wouldn't make sense in Polish). These are subtle things, which sometimes lead to a bit incompatible ways of understanding some terms between people coming from different languages/cultures.
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Offline AWK

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Re: What exactly makes something a precipitation reaction?
« Reply #5 on: August 21, 2020, 08:13:50 AM »
I disagree with Borek. The IUPAC (the chemical Vatican for faithful chemists) does not specify the term "precipitation reaction". It defines sedimentation (synonymous with precipitation) and a chemical reaction.
https://goldbook.iupac.org/terms/view/P04795
https://goldbook.iupac.org/terms/view/C01033
In this case, the meaning of the term "precipitation reaction" is determined by the simple combination of both IUPAC defined terms.

And the linguistic subtleties in different languages ​​are irrelevant - the definitive IUPAC language is English.

The authors of various textbooks and guides for different levels of chemistry education use different abbreviations and simplifications and are entitled to do so, but should also make the reader aware that their definitions have a limited scope of application.
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Re: What exactly makes something a precipitation reaction?
« Reply #6 on: August 21, 2020, 09:10:07 AM »
In this case, the meaning of the term "precipitation reaction" is determined by the simple combination of both IUPAC defined terms.

It is not clear to me what you disagree with. Is the reaction mentioned by the OP a precipitation reaction, or not?
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Offline AWK

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Re: What exactly makes something a precipitation reaction?
« Reply #7 on: August 21, 2020, 09:39:15 AM »
In this case, the meaning of the term "precipitation reaction" is determined by the simple combination of both IUPAC defined terms.

It is not clear to me what you disagree with. Is the reaction mentioned by the OP a precipitation reaction, or not?

If you write a publication or book and define the precipitation reaction in this way (and the publisher accepts it), then in this document it will be the definition of the precipitation reaction.
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Re: What exactly makes something a precipitation reaction?
« Reply #8 on: August 21, 2020, 01:45:56 PM »
You are still avoiding answering the question: do you consider the example given by OP a precipitation reaction, or not?
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Offline AWK

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Re: What exactly makes something a precipitation reaction?
« Reply #9 on: August 21, 2020, 02:20:19 PM »
Precipitation in a heterogeneous process is called cementation.
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Re: What exactly makes something a precipitation reaction?
« Reply #10 on: August 21, 2020, 05:23:32 PM »
You are still evading a simple yes/no answer.

Plus, wikipedia says "cementation is a type of precipitation". Not too reliable source, but at least it shows some consider this type of reaction "precipitation".
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